Regional Climatic Model RegCM User Manual - gforge
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1. The Abdus Salam International Centre for Theoretical Physics Strada Costiera 11 I 34151 Trieste Italy Earth System Physics Section ESP Regional Climatic Model RegCM User Manual Version 4 3 Nellie Elguindi Xunqiang Bi Filippo Giorgi Badrinath Nagarajan Jeremy Pal Fabien Solmon Sara Rauscher Ashraf Zakey Travis O Brien and Graziano Giuliani Trieste Italy January 2013 Acknowledgements This paper is dedicated to those that have contributed to the growth of RegCM system over the past 20 years the members 800 of the RegCNET and the ICTP Contents 1 The REGional Climate Model RegCM 2 Description PI y A A E A 2 27 Model components 2 a coe ee ts id a A eee ee TE ed o re Geo em 2 3 The RegCM Model Horizontal and Vertical Grid o o e 2 4 Map Projections and Map Scale Factors o o e 3 Model Physics S21 Dynamics Av E A PRE RE OEA AAN A E EA 3 1 1 Horizontal Momentum Equations 2 0 00000000 3 1 2 Continuity and Sigmadot G Equations 2 o o ee ee 3 1 3 Thermodynamic Equation and Equation for Omega 0 o 3 14 Hydrostatic Equation z en een A A A Be E aes 3 2 Physics parametrizations iros a le a wa Ea da 3 2 1 Radiation Scheme eiii rl ed eee e a Fee bah le E 3 2 2 Land Surface Models csc o a ee ho Se e ee ee a A 3 2 3 Planetary Boundary Layer Scheme o e 3 2 4 Convective Precipitatio2. 1996 It includes 18 spectral intervals from 0 2 to 5 um The cloud scattering and absorption parameterization follow that of Slingo 1989 whereby the optical properties of the cloud droplets extinction optical depth single scattering albedo and asymmetry parameter are expressed in terms of the cloud liquid water content and an effective droplet radius When cumulus clouds are formed the gridpoint fractional cloud cover is such that the total cover for the column extending from the model computed cloud base level to the cloud top level calculated assuming random overlap is a function of horizontal gridpoint spacing The thickness of the cloud layer is assumed to be equal to that of the model layer and a different cloud water content is specified for middle and low clouds 3 2 2 Land Surface Models BATS default BATS is a surface package designed to describe the role of vegetation and interactive soil moisture in modifying the surface atmosphere exchanges of momentum energy and water vapor see Dickinson et al 1993 for details The model has a vegetation layer a snow layer a surface soil layer 10 cm thick or root zone layer 1 2 m thick and a third deep soil layer 3 m thick Prognostic equations are solved for the soil layer temperatures using a generalization of the force restore method of Deardoff 1978 The temperature of the canopy and canopy foilage is calculated diagnostically via an energy balance formulation including sensib
3. 14 2h Jo The diffusivity of scalar quantities and momentum at a given height z are given as Ky m z 1 z Shm z V2e The TKE budget equation is solved at each time step according to equation 3 15 where the shear frequency Oz Following Grenier and Bretherton 2001 the TKE diffusivity Ke is set as 5 times the eddy diffusivity Km The Sp 4 qu 2 which is the balance of buoyancy B shear S transport T and dissipation D terms The correction factors are called the stability functions Spm which are defined in Galperin et al 1988 21 RegCM dynamical core has been modified to account for horizontal transport 1 e advection and diffusion of TKE when the UW model is active sa ONAK ESK e 3 15a ala RT a eg T mee 0 B S T D 3 15b Ot IBL The UW model treats TKE and diffusivity at the surface and the PBL top specially At the surface TKE is diagnosed as ey Bu2 where B is a constant At the PBL top the temperature inversion diffusivity for all quantities is set as Ky weAjnyz The entrainment flux which uses the Turner Deardorff formulation is set as We A where A is the entrainment efficiency U is a scale velocity and R is a bulk Richardson number The UW model specifies the bulk Richardson number as R E with U ein and L l as the master length scale It is assumed that the PBL does not entrain or detrain TKE The UW model accounts for the production of turbulence b
4. Climate 6 1993 30 Holtslag A A M E I F de Bruijn and H L Pan A high resolution air mass transformation model for short range weather forecasting Mon Wea Rev 118 1561 1575 1990 Hostetler S W G T Bates and F Giorgi Interactive nesting of a lake thermal model within a regional climate model for climate change studies Geophysical Research 98 5045 5057 1993 Hsie E Y R A Anthes and D Keyser Numerical simulation of frontogenisis in a moist atmosphere J Atmos Sci 41 2581 2594 1984 Kiehl J T J J Hack G B Bonan B A Boville B P Breigleb D Williamson and P Rasch Description of the ncar community climate model ccm3 Tech Rep NCAR TN 420 STR National Center for Atmospheric Research 1996 Kueppers L et al Seasonal temperature response to land use change in the western united states Global and Planetary Change 60 2008 Laurent B B Marticorena G Bergametti J Leon and N Mahowald Modeling mineral dust emissions from the sahara desert using new surface properties and soil database Journal of Geophysical Research 113 414218 2008 Lawrence P and T Chase Representing a new MODIS consistent land surface in the Community Land Model CLM3 0 J Geophys Res 112 g01023 2007 O Brien T A P Y Chuang L C Sloan I C Faloona and D L Rossiter Coupling a new turbulence parametrization to regcm adds realistic stratocumulus clouds Geoscientifi
5. Land Model version 0 CLM2 Community Land Model version 2 CLM3 Community Land Model version 3 CMAP CPC Merged Analysis of Precipitation CRU Climate Research Unit CPC Climate Prediction Center ECMWF European Centre for Medium Range Weather Forecasts ERA40 ECMWF 40 year Reanalysis ESMF Earth System Modeling Framework ESP Earth Systems Physics FAO Food and Agriculture Organization of the United Nations fvGCM NASA Data Assimilation Office atmospheric finite volume general circulation model GLCC Global Land Cover Characterization GCM General Circulation Model HadAM3H Hadley Centre Atmospheric Model version 3H ICTP Abdus Salam International Centre for Theoretical Physics IPCC Intergovernmental Panel on Climate Change IBIS Integrated Blosphere Simulator LAT leaf area index LAMs limited area models LBCs lateral boundary conditions MC2 Mesoscale Compressible Community model MIT Massachusetts Institute of Technology MM4 Mesoscale Model version 4 MMS Mesoscale Model version 5 33 MERCURE Modelling European Regional Climate Understanding and Reducing Errors NNRP NCEP NCAR Reanalysis Product NNRP1 NCEP NCAR Reanalysis Product version 1 NNRP2 NCEP NCAR Reanalysis Product version 2 NCAR National Center for Atmospheric Research NCEP National Centers for Environmental Prediction PBL planetary boundary layer PC Personal Computer PIRCS Project to Intercompare Regional Climate Simulations PFT plant functional type PSU Penn
6. across this network currently subscribed by over 750 participants can communicate through an email list and via regular scientific workshops and they have been essential for the evaluation and sequential improvements of the model Since the release of RegCM3 described by Pal et al 2007 the model has undergone a substantial evolution both in terms of software code and physics representations and this has lead to the development of a fourth version of the model RegCM4 which was released by the ICTP in June 2010 as a prototype version RegCM4 0 and in May 2011 as a first complete version RegCM4 1 The purpose of this Manual is to provide a basic reference for RegCM4 with a description of the model with a special accent to the improvements recently introduced Compared to previous versions RegCM4 includes new land surface planetary boundary layer and air sea flux schemes a mixed convection and tropical band configuration modifications to the pre existing radiative transfer and boundary layer schemes and a full upgrade of the model code towards improved flexibility portability and user friendliness The model can be interactively coupled to a 1D lake model a simplified aerosol scheme including OC BC SO4 dust and sea spray and a gas phase chemistry module CBM Z Overall RegCM4 shows an improved performance in several respects compared to previous versions although further testing by the user community is needed to fully explore its s
7. as part of the Community Climate System Model CCSM described in detail in Collins et al 2006 CLM version 3 5 was coupled to RegCM for a more detailed land surface description option CLM contains five possible snow layers with an additional representation of trace snow and ten unevenly spaced soil layers with explicit solutions of temperature liquid water and ice water in each layer To account for land surface complexity within a climate model grid cell CLM uses a tile or mosaic approach 17 Table 3 1 Land Cover Vegetation classes 1 Crop mixed farming 2 Short grass 3 Evergreen needleleaf tree 4 Deciduous needleleaf tree 5 Deciduous broadleaf tree 6 Evergreen broadleaf tree 7 Tall grass 8 Desert 9 Tundra 10 Irrigated Crop 11 Semi desert 12 Ice cap glacier 13 Bog or marsh 14 Inland water 15 Ocean 16 Evergreen shrub 17 Deciduous shrub 18 Mixed Woodland 19 Forest Field mosaic 20 Water and Land mixture to capture surface heterogeneity Each CLM gridcell contains up to four different land cover types glacier wetland lake and vegetated where the vegetated fraction can be further divided into 17 different plant functional types Hydrological and energy balance equations are solved for each land cover type and aggregated back to the gridcell level A detailed discussion of CLM version 3 implemented in RegCM3 and comparative analysis of land surface parameterization options is presented in Steiner e
8. first version developed in the late eighties RegCM1 Dickinson et al 1989 Giorgi 1990 to later versions in the early nineties RegCM2 Giorgi et al 1993b Giorgi et al 1993c late nineties RegCM2 5 Giorgi and Mearns 1999 and 2000s RegCM3 Pal et al 2000 The RegCM has been the first limited area model developed for long term regional climate simulation it has participated to numerous regional model intercomparison projects and it has been applied by a large community for a wide range of regional climate studies from process studies to paleo climate and future climate projections Giorgi and Mearns 1999 Giorgi et al 2006 The RegCM system is a community model and in particular it is designed for use by a varied community composed by scientists in industrialized countries as well as developing nations Pal et al 2007 As such it is designed to be a public open source user friendly and portable code that can be applied to any region of the World It is supported through the Regional Climate research NETwork or RegCNET a widespread network of scientists coordinated by the Earth System Physics section of the Abdus Salam International Centre for Theoretical Physics Abdus Salam International Centre for Theoretical Physics ICTP being the foster the growth of advanced studies and research in developing countries one of the main aims of the ICTP The home of the model is http users ictp it Reg CNET Scientists
9. heat of vaporization For further details on the calculation of these parameters refer to Zeng et al 1998 3 2 7 Prognostic Sea Surface Skin Temperature Scheme By default in RegCM sea surface temperatures SST are prescribed every six hours from temporally interpolated weekly or monthly SST products These products which are produced from satellite retrievals and in situ measurements are representative of the mean temperature in the top few meters of the ocean However the actual SST can differ significantly from this mean temperature due to the cool skin and warm layer effects described by Fairall et al 1996 To improve the calculation of diurnal fluxes over the ocean the prognostic SST scheme described by Zeng 2005 was implemented in RegCM4 The scheme is based on a two layer one dimensional heat transfer model with the top layer representing the upper few millimeters of the ocean which is cooled by net longwave radiation loss and surface fluxes The bottom layer is three meters thick it is warmer by solar radiation and exchanges heat with the top layer This diurnal SST scheme appears to provide significant although not major effects on the model climatology mostly over tropical oceans for example the Indian ocean and it is now used as default in RegCM4 3 2 8 Pressure Gradient Scheme Two options are available for calculating the pressure gradient force The normal way uses the full fields The other way is the hydrostatic deduct
10. hydrology part 1 Effects of temperature and water vapor disaggregation Journal of Hydrometeorology 4 317 333 2003b Giorgi F J S Pal X Bi L Sloan N Elguindi and F Solmon Introduction to the tac special issue The regcnet network Theoretical and Applied Climatology 86 1 4 2006 Grell G Prognostic evaluation of assumptions used by cumulus parameterizations Mon Wea Rev 121 764 787 1993 Grell G A J Dudhia and D R Stauffer Description of the fifth generation Penn State NCAR Mesoscale Model MMS Tech Rep TN 398 STR NCAR Boulder Colorado pp 121 1994 Grenier H and C S Bretherton A moist pbl parameterization for large scale models and its application to subtropical cloud topped marine boundary layers Monthly Weather Review 129 357 377 2001 Giittler I C Brankovic T A O Brien E Coppola B Grisogono and F Giorgi Sensitivity of regional climate model RegCM4 to planetary boundary layer scheme 2013 Hack J J B A Boville B P Briegleb J T Kiehl P J Rasch and D L Williamson Description of the ncar community climate model ccm2 Tech Rep NCAR TN 382 STR National Center for Atmospheric Research 1993 Henderson Sellers B Calculating the surface energy balance for lake and reservoir modeling A review Rev Geophys 24 3 625 649 1986 Holtslag A A M and B A Boville Local versus nonlocal boundary layer diffusion in a global climate model J
11. in the model from the vertical integral of Equation 3 3 15 1 f2 0pt gt 0p u m dp v m o oh E ne J a do 3 5 where 0 is a dummy variable of integration and o 0 0 3 1 3 Thermodynamic Equation and Equation for Omega 0 The thermodynamic equation is op T Op uT p vT p T PT lt a DAL y OT N OR ES y ot ox dy 00 RT O po CpmlO P Past Cpm FgT FyT 3 6 where Cpm is the specific heat for moist air at constant pressure Q is the diabatic heating FT represents the effect of horizontal diffusion FyT represents the effect of vertical mixing and dry convective adjustment and is f dp ie 7 O p6 o0 di 3 7 where E m ae v En 3 8 The expression for Cpm Cp 1 0 8q where cp is the specific heat at constant pressure for dry air and q is the mixing ratio of water vapor 3 1 4 Hydrostatic Equation The hydrostatic equation is used to compute the geopotential heights from the virtual temperature T db 3 9 din o pi p oe 1 RT ed 1 9 where T T 1 0 608qy q qc and q are the water vapor cloud water or ice and rain water or snow mixing ratios 16 3 2 Physics parametrizations 3 2 1 Radiation Scheme RegCM4 uses the radiation scheme of the NCAR CCM3 which is described in Kiehl et al 1996 Briefly the solar component which accounts for the effect of O3 H20 CO and O follows the 6 Eddington approximation of Kiehl et al
12. levels to the coordinate system of RegCM is also performed surfaces near the ground closely follow the terrain and the higher level surfaces tend to approximate isobaric surfaces Since the vertical and horizontal resolution and domain size can vary the modeling package programs employ 11 Nile ho Nil 3 _m ll AEE A I GG W gh eee TT ee a Sn u v T q p 4 0 3 a 5 0 4 el 55 0 5 0 6 T g 0 7 9 0 78 10 0 84 11 0 89 12 0 93 13 0 96 16 1 00 par ATR Figure 2 1 Schematic representation of the vertical structure of the model This example is for 16 vertical layers Dashed lines denote half sigma levels solid lines denote full sigma levels Adapted from the PSU NCAR Mesoscale Modeling System Tutorial Class Notes and User s Guide parameterized dimensions requiring a variable amount of core memory and the requisite hard disk storage amount is varied accordingly 2 3 The RegCM Model Horizontal and Vertical Grid It is useful to first introduce the model s grid configuration The modeling system usually gets and analyzes its data on pressure surfaces but these have to be interpolated to the model s vertical coordinate before input to the model The vertical coordinate is terrain following Figure 2 1 meaning that the lower grid levels follow the terrain while the upper surface is flatter Intermediate levels progressively flatten as the pressure decreases toward the top of the model A dimensionless co
13. 5 5 Light sensitivity factor m W7 0 02 0 02 0 06 0 06 0 06 0 06 0 02 0 02 0 02 0 02 0 02 0 02 0 02 0 02 0 02 0 02 0 02 0 06 0 02 0 02 Upper soil layer depth mm 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Root zone soil layer depth mm 1000 1000 1500 1500 2000 1500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 2000 2000 2000 Depth of total soil mm 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 Soil texture type 6 6 6 6 7 8 6 3 6 6 5 12 6 6 6 6 5 6 6 0 Soil color type 5 3 4 4 4 4 4 1 3 3 2 1 5 5 5 4 3 4 4 0 Vegetation albedo for wavelengths lt 0 7 um 0 10 0 10 0 05 0 05 0 08 0 04 0 08 0 20 0 10 0 08 0 17 0 80 0 06 0 07 0 07 0 05 0 08 0 06 0 06 0 06 Vegetation albedo for wavelengths gt 0 7 um 0 30 030 0 23 0 23 0 28 0 20 030 040 0 30 0 28 0 34 0 60 0 18 0 20 0 20 0 23 0 28 0 24 0 18 0 18 Table 3 3 Resolution for CLM input parameters Input data Grid Spacing Lon range Lat range Glacier 0 05 x 0 05 179 975 89 975 Lake 0 05 x 0 05 179 975 89 975 Wetland 0 05 x 0 05 179 975 89 975 Land fraction 0 05 x 0 05 179 975 89 975 LAI SAI 0 5 x 0 5 179 75 89 75 PFT 0 5 x 0 5 179 75 89 75 Soil color 0 05 x 0 05 179 975 89 975 Soil texture 0 05 x 0 05 179 975 89 975 Max sat area 0 5 x 0 5 179 75 89 75 3 2 3 Planetary Boundary Layer Scheme Holtslag PBL The Holtslag planetary bo
14. a fraction of the condensed moisture forms precipitation while theremaining fraction forms the cloud The cloud is assumed to mix withthe air from the environment according to a uniform spectrum ofmixtures that ascend or descend to their respective levels of neutralbuoyancy The mixing entrainment and detrainment rates are functionsof the vertical gradients of buoyancy in clouds The fraction of thetotal cloud base mass flux that mixes with its environment at eachlevel is proportional to the undiluted buoyancy rate of change withaltitude The cloud base upward mass flux is relaxed towards thesub cloud layer quasi equilibrium In addition to a more physical representation of convection the MIT Emanuel scheme offers several advantages compared to theother RegCM4 convection options For instance it includes aformulation of the auto conversion of cloud water into precipitationinside cumulus clouds and ice processes are accounted for by allowingthe auto conversion threshold water content to be temperaturedependent Additionally the precipitation is added to a single hydrostatic unsaturated downdraft that transports heat and water Lastly the MIT Emanuel scheme considers the transport of passive tracers The MIT scheme is the most complex of the three and also includes a number of parameters that can be used to optimize the model performance in different climate regimes Differently from the Grell scheme however test experiments did not identify a single
15. aft evaporation and is the fraction of updraft condensation that re evaporates in the downdraft B depends on the wind shear and typically varies between 0 3 and 0 5 Rainfall is given by PY HKmy 1 B 3 20 Heating and moistening in the Grell scheme are determined both by the mass fluxes and the detrainment at the cloud top and bottom In addition the cooling effect of moist downdrafts is included Due to the simplistic nature of the Grell scheme several closure assumptions can be adopted RegCM4 s earlier version directly implements the quasi equilibrium assumption of AS74 It assumes that convective clouds stabilize the environment as fast as non convective processes destabilize it as follows ABE ABE mp NAN 3 21 where ABE is the buoyant energy available for convection ABE is the amount of buoyant energy available for convection in addition to the buoyant energy generated by some of the non convective processes during the time interval Ar and NA is the rate of change of ABE per unit mp The difference ABE ABE can be thought of as the rate of destabilization over time At ABE is computed from the current fields plus the future tendencies resulting from the advection of heat and moisture and the dry adiabatic adjustment 23 In the latest RegCM4 version by default we use a stability based closure assumption the FC80 type closure assumption that is commonly implemented in GCMs and RCMs In this closure it is a
16. arameterization of warm cloud microphysical conversion processes Atmos Res 33 193 206 1994 Bretherton C S J McCaa and H Grenier A new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i Description and Id results Monthly Weather Review 132 864 882 2004 Briegleb B P Delta eddington approximation for solar radiation in the ncar community climate model J Geophys Res 97 7603 7612 1992 Collins W D et al The Community Climate System Model version 3 CCSM3 Journal of Climate 19 2122 2143 2006 Deardoff J W Efficient prediction of ground surface temperature and moisture with inclusion of a layer of vegetation J Geophys Res 83 1889 1903 1978 Dickinson R E Climate Processes and Climate Sensitivity chap Modeling evapotranspiration processes for three dimensional global climate models pp 52 72 American Geophysical Union 1984 Dickinson R E P J Kennedy A Henderson Sellers and M Wilson Biosphere atmosphere transfer scheme bats for the ncar community climate model Tech Rep NCARE TN 275 STR National Center for Atmospheric Research 1986 Dickinson R E R M Errico F Giorgi and G T Bates A regional climate model for the western United States Climatic Change 15 383 422 1989 Dickinson R E A Henderson Sellers and P J Kennedy Biosphere atmosphere transfer scheme bats version le
17. as coupled to the ncar community climate model Tech rep National Center for Atmospheric Research 1993 Emanuel K A A scheme for representing cumulus convection in large scale models J Atmos Sci 48 21 2313 2335 1991 Emanuel K A and M Zivkovic Rothman Development and evaluation of a convection scheme for use in climate models J Atmos Sci 56 1766 1782 1999 Fairall C E Bradley J Godfrey G Wick J Edson and G Young Cool skin and warm layer effects on sea surface temperature Journal of Geophysical Research 101 1295 1308 1996 Fritsch J M and C F Chappell Numerical prediction of convectively driven mesoscale pressure systems part i Convective parameterization J Atmos Sci 37 1722 1733 1980 Galperin B L H Kantha S Hassid and A Rosati A quasi equilibrium turbulent energy model for geophysical flows Journal of the Atmospheric Sciences 45 1 55 62 1988 29 Giorgi F Two dimensional simulations of possible mesoscale effects of nuclear war fires J Geophys Res 94 1127 1144 1989 Giorgi F Simulation of regional climate using a limited area model nested in a general circulation model J Climate 3 941 963 1990 Giorgi F and G T Bates The climatological skill of a regional model over complex terrain Mon Wea Rev 117 2325 2347 1989 Giorgi F and M R Marinucci Validation of a regional atmospheric model over europe Sensitivity of wintertime and su
18. b saturated conditions and conversion into rain via a bulk autoconversion term Prognosed cloud water variable is directly used in the cloud radiation calculations and not diagnosed in terms of the local relative humidity adding a very important and far reaching element of interaction between the simulated hydrologic cycle and energy budget calculations The solar spectrum optical properties are based on the cloud liquid water path which is in turn based on the cloud liquid water amount prognostically calculated by the model cloud fractional cover which is calculated diagnostically as a function of relative humidity and effective cloud droplet radius which is parameterized as a function of temperature and land sea mask for liquid water and as a function of height for ice phase In addition the scheme diagnostically calculates a fraction of cloud ice as a function of temperature In the infrared spectrum the cloud emissivity is calculated as a function of cloud liquid ice water path and cloud infrared absorption cross sections depending on effective radii for the liquid and ice phase One of the problems in this formulation is that the scheme uses the cloud fractional cover to produce grid box mean cloud properties which are then treated as if the entire grid box was covered by an effectively thinner cloud layer However because of the non linear nature of radiative transfer this approach tends to produce a grayer mean grid box than if separate clou
19. c Model Development 5 4 989 1008 doi 10 5194 gmd 5 989 2012 2012 Oleson K e a Technical description of the Community Land Model CLM Tech Rep Technical Note NCAR TN 461 STR NCAR 2004 Oleson K W et al Improvements to the Community Land Model and their impact on the hydrological cycle Journal of Geophysical Research Biogeosciences 113 G1 2008 OBrien T A L C Sloan P Y Chuang I C Faloona and J A Johnstone Multidecadal simulation of coastal fog with a regional climate model Climate Dynamics pp 1 12 doi 10 1007 s00382 012 1486 x 2012 Pal J S E E Small and E A B Eltahir Simulation of regional scale water and energy budgets Representation of subgrid cloud and precipitation processes within RegCM J Geophys Res Atmospheres 105 D24 29 579 29 594 2000 Pal J S F Giorgi X Bi et al The ICTP RegCM3 and RegCNET Regional climate modeling for the developing world Bull Amer Meteor Soc 88 1395 1409 2007 Patterson J C and P F Hamblin Thermal simulation of a lake with winter ice cover Limn Oceanography 33 323 338 1988 Slingo J M A gcm parameterization for the shortwave radiative properties of water clouds J Atmos Sci 46 1419 1427 1989 Small E E and L C Sloan Simulating the water balance of the aral sea with a coupled regional climate lake model J Geophys Res 104 6583 6602 1999 Solmon F M Mallet N Elguindi F Giorgi A Zak
20. dy and clear sky fractional fluxes were calculated By taking advantage of the fact that the scheme also calculates clear sky fluxes for diagnostic purposes in RegCM4 we modified this radiative cloud representation by first calculating the total cloud cover at a given grid point and then calculating the surface fluxes separately for the cloudy and clear sky portions of the grid box The total cloud cover at a model grid box is given by a value intermediate between that obtained using the random overlap assumption which maximizes cloud cover and that given by the largest cloud cover found in any single layer of the column overlying the grid box which implies a full overlap and it is thus is a minimum estimate of total cloud cover This modification thus accounts for the occurrence of fractional clear sky at a given grid box leading to more realistic grid box average surface radiative fluxes in fractional cloudy conditions A large scale cloud and precipitation scheme which accounts for the subgrid scale variability of clouds Pal et al 2000 parameterizations for ocean surface fluxes Zeng et al 1998 and multiple cumulus convection scheme Anthes 1977 Grell 1993 Emanuel 1991 Emanuel and Zivkovic Rothman 1999 are the same as in RegCM3 but a new mixed scheme Grell Emanuel is introduced it allows the user to select one of the two schemes in function of the ocean land mask The other main development compared to RegCM3 concerns the a
21. e shape of small areas so that dx dy everywhere but the grid length varies across the domain to allow a representation of a spherical surface on a plane surface Map scale factors need to be accounted for in the model equations wherever horizontal gradients are used 14 Chapter 3 Model Physics 3 1 Dynamics The model dynamic equations and numerical discretization are described by Grell et al 1994 3 1 1 Horizontal Momentum Equations Opus Op unjm _ p vu m Op uG d ox dy do Al RT op 0 m P F p o ox Ox Opv 4 dp uv m dp vv m op v A ax dy do fp v Fyut Fyu 3 1 R ap A 2 fp u Fyv Fy 32 aa cae EA oo where u and v are the eastward and northward components of velocity T is virtual temperature is geopotential height f is the coriolis parameter R is the gas constant for dry air m is the map scale factor for either the Polar Stereographic Lambert Conformal or Mercator map projections 6 a and Fy and Fy represent the effects of horizontal and vertical diffusion and p ps pr 3 1 2 Continuity and Sigmadot 5 Equations Op o0ptu m dp v m op 6 bE ne de Jo 85 The vertical integral of Equation 3 3 is used to compute the temporal variation of the surface pressure in the model op Yf 1 0ptu m op v m W m El A do 8 4 After calculation of the surface pressure tendency ee the vertical velocity in sigma coordinates 6 is computed at each level
22. ensitivities and range of applications The RegCM is available on the World Wide Web thanks to the Democritos Italy CNR group at https eforge escience lab org gf project regem Chapter 2 Description 2 1 History The idea that limited area models LAMs could be used for regional studies was originally proposed by Dickinson et al 1989 and Giorgi 1990 This idea was based on the concept of one way nesting in which large scale meteorological fields from General Circulation Model GCM runs provide initial and time dependent meteorological lateral boundary conditions LBCs for high resolution Regional Climate Model RCM simulations with no feedback from the RCM to the driving GCM The first generation NCAR RegCM was built upon the National Center for Atmospheric Research NCAR Pennsylvania State University PSU Mesoscale Model version 4 MM4 in the late 1980s Dickinson et al 1989 Giorgi 1989 The dynamical component of the model originated from the MM4 which is a compressible finite difference model with hydrostatic balance and vertical o coordinates Later the use of a split explicit time integration scheme was added along with an algorithm for reducing horizontal diffusion in the presence of steep topographical gradients Giorgi et al 1993a b As a result the dynamical core of the RegCM is similar to that of the hydrostatic version of Mesoscale Model version 5 MM5 Grell et al 1994 the RegCM4 is thus a hydrosta
23. erosol radiative transfer calculations In RegCM3 the aerosol radiative forcing was based on three dimensional fields produced by the aerosol model and included only scattering and absorption in the shortwave spectrum see Giorgi et al 2002 In RegCM4 we added the contribution of the infrared spectrum following Solmon et al 2008 This is especially important for relatively large dust and sea salt particles and it is calculated by introducing an aerosol infrared emissivity calculated as a function of aerosol path and absorption cross section estimated from aerosol size distribution and long wave refractive indices Long wave diffusion which could be relevant for larger dust particles is not treated as part of this scheme The mosaic type parameterization of subgrid scale heterogeneity in topography and land use Giorgi et al 2003b allows finer surface resolution in the Biosphere Atmosphere Transfer Scheme version le BATS1e 2 2 Model components The RegCM modeling system has four components Terrain ICBC RegCM and Postprocessor Terrain and ICBC are the two components of RegCM preprocessor Terrestrial variables including elevation landuse and sea surface temperature and three dimensional isobaric meteorological data are horizontally interpolated from a latitude longitude mesh to a high resolution domain on either a Rotated and Normal Mercator Lambert Conformal or Polar Stereographic projection Vertical interpolation from pressure
24. ey and A Konare Dust aerosol impact on regional precipitation over western africa mechanisms and sensitivity to absorption properties Geophysical Research Letters 35 124705 2008 Steiner A L J S Pal S A Rauscher J L Bell N S Diffenbaugh A Boone L C Sloan and F Giorgi Land surface coupling in regional climate simulations of the West African monsoon Climate Dynamics 33 6 869 892 2009 Sundqvist H E Berge and J E Kristjansson The effects of domain choice on summer precipitation simulation and sensitivity in a regional climate model J Climate 11 2698 2712 1989 31 Tiedtke M A comprehensive mass flux scheme for cumulus parameterization on large scale models Mon Wea Rev 117 1779 1800 1989 Zeng X A prognostic scheme of sea surface skin temperature for modeling and data assimilation Geophysical Research Letters 32 114605 2005 Zeng X M Zhao and R E Dickinson Intercomparison of bulk aerodynamic algoriths for the computation of sea surface fluxes using toga coare and tao data J Climate 11 2628 2644 1998 32 BATS Biosphere Atmosphere Transfer Scheme BATSle Biosphere Atmosphere Transfer Scheme version le CAM Community Atmosphere Model CAPE convective available potential energy CCM Community Climate Model CCM1 Community Climate Model version 1 CCM2 Community Climate Model version 2 CCM3 Community Climate Model version 3 CLM Community Land Surface Model CLMO0 Common
25. fications have been made to BATSin order to account for the subgrid variability of topography and landcover using a mosaic type approach Giorgi et al 2003a Thismodification adopts a regular fine scale surface subgrid for eachcoarse model grid cell Meteorological variables are disaggregatedfrom the coarse grid to the fine grid based on the elevationdifferences The BATS calculations are then performed separatelyfor each subgrid cell and surface fluxes are reaggregated onto thecoarse grid cell for input to the atmospheric model This parameterization showed a marked improvement in the representation ofthe surface hydrological cycle in mountainous regions Giorgi et al 2003a As a first augmentation in REGional Climate Model version 4 RegCM4 two new land use types were added to BATS to represent urban and sub urban environments Urban development not only modifies the surface albedo and alters the surface energy balance but also creates impervious surfaces with large effects on runoff and evapotranspiration These effects can be described by modifying relevant properties of the land surface types in the BATS package such as maximum vegetation cover roughness length albedo and soil characteristics For this purpose we implemented the parameters proposed in Table 1 of Kueppers et al 2008 CLM optional The Community Land Model CLM Oleson et al 2008 is the land surface model developed by the National Center of Atmospheric Research NCAR
26. h land and ocean surface types With this approach a weighted average of necessary surface variables was calculated for land ocean gridcells using the land fraction input dataset This method provides a better representation of coastlines using the high resolution land fraction data described in Table 3 3 For a more detailed description of CLM physics parameterizations see Oleson 2004 18 61 Table 3 2 BATS vegetation land cover Parameter Land Cover Vegetation Type 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Max fractional vegetation cover 0 85 080 0 80 0 80 0 80 090 0 80 0 00 0 60 080 0 35 0 00 0 80 0 00 0 00 0 80 0 80 0 80 0 80 0 80 Difference between max fractional vegetation cover and cover at269K 0 6 0 1 0 1 0 3 0 5 0 3 0 0 0 2 0 6 0 1 0 0 0 4 0 0 0 0 0 2 0 3 0 2 0 4 0 4 Roughness length m 0 08 0 05 1 00 1 00 080 2 00 010 005 0 04 0 06 010 0 01 0 03 0 0004 0 0004 0 10 0 10 0 80 0 3 0 3 Displacement height m 0 0 0 0 9 0 9 0 0 0 18 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Min stomatal resistence s m 45 60 80 80 120 60 60 200 80 45 150 200 45 200 200 80 120 100 120 120 Max Leaf Area Index 6 2 6 6 6 6 6 0 6 6 6 0 6 0 0 6 6 6 6 6 Min Leaf Area Index 0 5 0 5 5 1 1 gt 0 5 0 0 5 0 5 0 5 0 0 5 0 0 5 1 3 0 5 0 5 Stem dead matter area index 0 5 4 0 2 0 2 0 2 0 2 0 2 0 0 5 0 5 2 0 2 0 2 0 2 0 2 0 2 0 2 0 2 0 2 0 2 0 2 0 Inverse square root of leaf dimension m 10 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
27. h latitude winter conditions and allows the model to better capture surface inversions These modifications have thus been incorporated as default in the RegCM4 code The UW Turbulence Closure Model As an alternative to the Holtslag PBL the University of Washington turbulence closure model Grenier and Bretherton 2001 Bretherton et al 2004 has been coupled to RegCM The development of this coupling and its validation for western North America is described by O Brien et al 2012 and validation over Europe is described by Giittler et al 2013 This parameterization was originally implemented to allow RegCM to simulate stratocumulus and coastal fog O Brien et al 2012 OBrien et al 2012 The UW model is a 1 5 order local down gradient diffusion parametrization It will be referred to as a PBL model but it has capabilities that allow it to calculate vertical fluxes out side of the PBL as well as within Bretherton et al 2004 refers to it as a moist turbulence parametrization As with other 1 order models such as the Holtslag model the UW model parameterizes turbulent fluxes as the product of a diffusivity and a gradient In contrast to 1 order models however the model prognostically determines the turbulent kinetic energy TKE also referred to as e and it uses TKE to define the diffusivites As with the Holtslag mode diffusivity is defined as the product of a length scale and a velocity scale though the velocity sca
28. ion scheme which makes use of a perturbation temperature In this scheme extra smoothing on the top is done in order to reduce errors related to the PGF calculation 3 2 9 Lake Model The lake model developed by Hostetler et al 1993 can be interactively coupled to the atmospheric model In the lake model fluxes of heat moisture and momentum are calculated based on meteorological inputs and the lake surface temperature and albedo Heat is transferred vertically between lake model layers by eddy and convective mixing Ice and snow may cover part or all of the lake surface In the lake model the prognostic equation for temperature is oT T sE ke t km d2 3 31 where T is the temperature of the lake layer and ke and km are the eddy and molecular diffusivities respectively The parameterization of Henderson Sellers 1986 is used to calculate ke and km is set to a constant value of 39 x 1077 m s7 except under ice and at the deepest points in the lake Sensible and latent heat fluxes from the lake are calculated using the BATS parameterizations Dickinson et al 1993 The bulk aerodynamic formulations for latent heat flux and sensible heat flux F are as follows Fa PaCpVa qs qa 3 32 Fy PaCpCpVa Ts gt Ta 3 33 where the subscripts s and a refer to surface and air respectively pa is the density of air V4 is the wind speed Cp q is specific humidity and T is temperature The momentum drag coefficient C
29. le radiative and latent heat fluxes The soil hydrology calculations include predictive equations for the water content of the soil layers These equations account for precipitation snowmelt canopy foiliage drip evapotranspiration surface runoff infiltration below the root zone and diffusive exchange of water between soil layers The soil water movement formulation is obtained from a fit to results from a high resolution soil model Dickinson 1984 and the surface runoff rates are expressed as functions of the precipitation rates and the degree of soil water saturation Snow depth is prognostically calculated from snowfall snowmelt and sublimation Precipitation is assumed to fall in the form of snow if the temperature of the lowest model level is below 271 K Sensible heat water vapor and momentum fluxes at the surface are calculated using a standard surface drag coefficient formulation based on surface layer similarity theory The drag coefficient depends on the surface roughness length and on the atmospheric stability in the surface layer The surface evapotranspiration rates depend on the availability of soil water Biosphere Atmosphere Transfer Scheme BATS has 20 vegetation types Table 3 2 soil textures ranging from coarse sand to intermediate loam to fine clay and different soil colors light to dark for the soil albedo calculations These are described in Dickinson et al 1986 In the latest release version additional modi
30. le is defined as the square root of local TKE rather than the convective velocity scale The length scale is the UW model s master length scale either xz or l kz 1 kz A this can be set in the RegCM configuration file multiplied by a correction factor that depends on local stability and the velocity scale is the square root of twice the TKE The boundary layer height in the UW model is defined as the first level where the expression N 2 1 2 where N is the Brunt V is l frequency N a 2 exceeds half of the negative of its layer mean value Since the flux of buoyancy b can be written as wb Kn 2 and it can be shown that N A N can be viewed as being proportional to the local buoyancy flux in the UW model In this interpretation this condition for PBL top or the top of any turbulent layer can be approximately viewed as a condition that the buoyancy flux anywhere in the interior of a convective layer not be more negative than 0 5 of the layer mean buoyancy flux Bretherton et al 2004 In other words for an unstable PBL the PBL ends approximately when the virtual potential temperature profile becomes so stable that the buoyancy flux is opposite to and half as strong as the mean buoyancy flux below This condition for the height of the PBL can be encapsulated in the following implicit equation where the z and h values are restricted to lie on the model s vertical grid 11 f N MP h 53 N z I z dz 3
31. mmertime simulations to selected physics parameterizations and lower boundary conditions Quart J Roy Meteor Soc 117 1171 1206 1991 Giorgi F and L O Mearns Introduction to special section Regional climate modeling revisited J Geophys Res 104 6335 6352 1999 Giorgi F G T Bates and S J Nieman The multi year surface climatology of a regional atmospheric model over the western united states J Climate 6 75 95 1993a Giorgi F M R Marinucci and G T Bates Development of a second generation regional climate model regem2 1 Boundary layer and radiative transfer processes Mon Wea Rev 121 2794 2813 1993b Giorgi F M R Marinucci G T Bates and G DeCanio Development of a second generation regional climate model regcm2 ii Convective processes and assimilation of lateral boundary conditions Mon Wea Rev 121 2814 2832 1993c Giorgi F X Q Bi and Y Qian Radiative forcing and regional climatic effects of anthropogenic aerosols over East Asia A regional coupled climate chemistry aerosols model study J Geophys Res 107 2002 Giorgi F X Q Bi and Y Qian Indirect vs direct effects of anthropogenic sulfate on the climate of east asia as simulated with a regional coupled climate chemistry aerosol model Climatic Change 58 345 376 2003a Giorgi F R Francisco and J S Pal Effects of a subgrid scale topography and land use scheme on the simulation of surface climate and
32. n Schemes 0 00 0 000000004 3 2 5 Large Scale Precipitation Scheme 0 0 0 0 000000004 3 2 6 Ocean flux Parameterization 2 2 0 00 0 2 000000 002 eee 3 2 7 Prognostic Sea Surface Skin Temperature Scheme o o o 3 2 8 Pressure Gradient Scheme 20 0 000 eee eee eee 3 29 Lake Mod l t ica e Be oath eee See eee aid Ba tds 3 2 10 Aerosols and Dust Chemistry Model o o o eee 4 Future Developments 4 1 Tiedtke convection scheme a 4 2 Semi Lagrangian dynamic Core 4 3 Non Hydrostatic COTE ms ee e 10 10 11 12 13 15 15 15 15 16 16 17 17 20 22 24 25 26 26 26 27 List of Figures 2 1 Schematic representation of the vertical structure of the model This example is for 16 vertical layers Dashed lines denote half sigma levels solid lines denote full sigma levels Adapted from the PSU NCAR Mesoscale Modeling System Tutorial Class Notes and User s Guide 12 2 2 Schematic representation showing the horizontal Arakawa B grid staggering of the dot and cross Prid pomis i gaa Ben beg At aoe has a St Sapa le ge RS Att Sa el ee as 13 List of Tables 3 1 Land Cover Vegetation classes 3 2 BATS vegetation land cover 3 3 Resolution for CLM input parameters Chapter 1 The RegCM The RegCM is a regional climate model developed throughout the years with a wide base of model users It has evolved from the
33. ng one of three schemes 1 Modified Kuo scheme Anthes 1977 2 Grell scheme Grell 1993 and 3 MIT Emanuel scheme Emanuel 1991 Emanuel and Zivkovic Rothman 1999 In addition the Grell parameterization is implemented using one of two closure assumptions 1 the Arakawa and Schubert closure Grell et al 1994 and 2 the Fritsch and Chappell closure Fritsch and Chappell 1980 hereafter refered to as AS74 and FC80 respectively 1 Kuo Scheme Convective activity in the Kuo scheme is initiated when the moisture convergence M in a column exceeds a given threshold and the vertical sounding is convectively unstable A fraction of the moisture convergence B moistens the column and the rest is converted into rainfall PU according to the following relation The entrainment efficiency is partially determined by the mixture of clear and cloudy air that happens at the inversion top Grenier and Bretherton 2001 takes special care to develop a parametrization for A that includes evaporative enhancement effects for cases when a cloudy clear mixture of air is more dense than its surroundings 22 PY M 1 B 3 17 B is a function of the average relative humidity RH of the sounding as follows 1 0 otherwise 3 18 B 2 1 RH RH gt 05 Note that the moisture convergence term includes only the advective tendencies for water vapor However evapotranspiration from the previous time step is indirectly included in M since it tends t
34. nted by the dashed lines in Figure 2 1 Vertical velocity is carried at the full levels solid lines In defining the sigma levels it is the full levels that are listed including levels at 0 and 1 The number of model layers is therefore always one less than the number of full sigma levels The finite differencing in the model is of course crucially dependent upon the grid staggering wherever gradients or averaging are represented terms in the equation 2 4 Map Projections and Map Scale Factors The modeling system has a choice of four map projections Lambert Conformal is suitable for mid latitudes Polar Stereographic for high latitudes Normal Mercator for low latitudes and Rotated Mercator for extra choice The 13 x and y directions in the model do not correspond to west east and north south except for the Normal Mercator projection and therefore the observed wind generally has to be rotated to the model grid and the model u and v components need to be rotated before comparison with observations These transformations are accounted for in the model pre processors that provide data on the model grid Please note that model output of u and v components raw or postprocessed should be rotated to a lat lon grid before comparing to observations The map scale factor m 1s defined by m distance on grid actual distance on earth and its value is usually close to one varying with latitude The projections in the model preserve th
35. o moisten the lower atmosphere Hence as the evapotranspiration increases more and more of it is converted into rainfall assuming the column is unstable The latent heating resulting from condensation is distributed between the cloud top and bottom by a function that allocates the maximum heating to the upper portion of the cloud layer To eliminate numerical point storms a horizontal diffusion term and a time release constant are included so that the redistributions of moisture and the latent heat release are not performed instantaneously Giorgi and Bates 1989 Giorgi and Marinucci 1991 Grell Scheme The Grell scheme Grell 1993 similar to the AS74 parameterization considers clouds as two steady state circulations an updraft and a downdraft No direct mixing occurs between the cloudy air and the environmental air except at the top and bottom of the circulations The mass flux is constant with height and no entrainment or detrainment occurs along the cloud edges The originating levels of the updraft and downdraft are given by the levels of maximum and minimum moist static energy respectively The Grell scheme is activated when a lifted parcel attains moist convection Condensation in the updraft is calculated by lifting a saturated parcel The downdraft mass flux m depends on the updraft mass flux mp according to the following relation mo Pizy 3 19 h where is the normalized updraft condensation Jy is the normalized downdr
36. oltslag and Boville 1993 for a more detailed description Compared to other schemes this formulation tends to produce relatively strong and often excessive turbulent vertical transfer For example after extensive testing we found excessive vertical transfer of moisture in the model resulting in low moisture amounts near the surface and excessive moisture near the PBL top 3 13 20 Therefore in order to ameliorate this problem the countergradient term for water vapor was removed in RegCM4 Another problem of the Holtslag scheme at least in our implementation is an excessive vertical transport of heat moisture and momentum in very stable conditions such as during the winter in northern hemisphere high latitude regions For example we found that in such conditions the scheme fails to simulate near surface temperature inversions This in turn leads to large warm winter biases even 10 degrees over regions such as Northern Siberia and Northern Canada As an ad hoc fix to address this problem in RegCM4 we implemented the following modification to the scheme e We first define very stable conditions within the Holtslag parameterization as conditions in which the ratio of the height from the surface over the Monin Obhukov length is lower than 0 1 e When such conditions are found we set to O the eddy diffusivity and counter gradient terms for all variables Preliminary tests showed that this modification reduces the warm bias in hig
37. ordinate is used to define the model levels where p is the pressure p is a specified constant top pressure p is the surface pressure ae p Pr 2 1 Ps Pr It can be seen from the equation and Figure 2 1 that is zero at the top and one at the surface and each model level is defined by a value of o The model vertical resolution is defined by a list of values between zero and one 12 1Y 1 Y IX 1 1 J gt 1 4X Figure 2 2 Schematic representation showing the horizontal Arakawa B grid staggering of the dot and cross grid points that do not necessarily have to be evenly spaced Commonly the resolution in the boundary layer is much finer than above and the number of levels may vary upon the user demand The horizontal grid has an Arakawa Lamb B staggering of the velocity variables with respect to the scalar variables This is shown in Figure 2 2 where it can be seen that the scalars T q p etc are defined at the center of the grid box while the eastward u and northward v velocity components are collocated at the corners The center points of grid squares will be referred to as cross points and the corner points are dot points Hence horizontal velocity is defined at dot points Data is input to the model the preprocessors do the necessary interpolation to assure consistency with the grid All the above variables are defined in the middle of each model vertical layer referred to as half levels and represe
38. p depends on roughness length and the surface bulk Richardson number Under ice free conditions the lake surface albedo is calculated as a function of solar zenith angle Henderson Sellers 1986 Longwave radiation emitted from the lake is calculated according to the Stefan Boltzmann law The lake model uses the partial ice cover scheme of Patterson and Hamblin 1988 to represent the different heat and moisture exchanges between open water and ice surfaces and the atmosphere and to calculate the surface energy of lake ice and overlying snow For further details refer to Hostetler et al 1993 and Small and Sloan 1999 26 3 2 10 Aerosols and Dust Chemistry Model The representation of dust emission processes is a key element in a dust model and depends on the wind conditions the soil characteristics and the particle size Following Laurent et al 2008 and Alfaro and Gomes 2001 here the dust emission calculation is based on parameterizations of soil aggregate saltation and sandblasting processes The main steps in this calculation are The specification of soil aggregate size distribution for each model grid cell the calculation of a threshold friction velocity leading to erosion and saltation processes the calculation of the horizontal saltating soil aggregate mass flux and finally the calculation of the vertical transportable dust particle mass flux generated by the saltating aggregates In relation to the BATS interface these paramete
39. parameter to which the model is most sensitive A major augmentation in RegCM4 compared to previous versions of the model is the capability of running different convection schemes over land and ocean a configuration which we refer to as mixed convection Extensive test experiments showed that different schemes have different performance over different regions and in particular over land vs ocean areas For example the MIT scheme tends to produce excessive precipitation over land areas especially through the occurrence of very intense individual precipitation events In other words once the scheme is activated it becomes difficult to decelerate Conversely we found that the Grell scheme tends to produce excessively weak precipitation over tropical oceans These preliminary tests suggested that a mixed convection approach by which for example the MIT scheme is used over oceans and the Grell scheme over land might be the most suitable option to pursue and therefore this option was added to the model 3 2 5 Large Scale Precipitation Scheme Subgrid Explicit Moisture Scheme SUBEX is used to handle nonconvective clouds and precipitation resolved by the model This is one of the new components of the model SUBEX accounts for the subgrid variability in 24 clouds by linking the average grid cell relative humidity to the cloud fraction and cloud water following the work of Sundqvist et al 1989 The fraction of the grid cell covered by clo
40. re detailed description of SUBEX and a list of the parameter values refer to Pal et al 2000 Traditionally REGional Climate Model version 3 RegCM3 has shown a tendency to produce excessive precipitation especially at high resolutions and optimizations of the in cloud liquid water threshold for the activation of the autoconversion term Qcth and the rate of sub cloud evaporation Cevap parameters have proven effective in ameliorating this problem greater values of Oth and Cevap lead to decreased precipitation amounts 3 2 6 Ocean flux Parameterization BATS uses standard Monin Obukhov similarity relations to compute the fluxes with no special treatment of convective and very stable conditions In addition the roughness length is set to a constant i e it is not a function of wind and stability The Zeng scheme describes all stability conditions and includes a gustiness velocity to account for the additional flux induced by boundary layer scale variability Sensible heat SH latent heat LH and momentum t fluxes between the sea surface and lower atmosphere are calculated using the following bulk aerodynamic algorithms T paux ux uy 2 Ju 3 28 SH PgCpatlr 9x 3 29 LH PaL Usqu 3 30 25 where uy and uy are mean wind components u is the frictional wind velocity 0 is the temperature scaling parameter qx is the specific humidity scaling parameter p4 is air density Cpa is specific heat of air and Le is the latent
41. rizations become effective in the model for cells dominated by desert and semi desert land cover 27 Chapter 4 Future Developments We have lot of exciting plans for future model improvements some of which are in a already mature stage and under testing with some published results whereas others are done only on the paper in a whishlist for next years Nevertheless we want to share this with users to have hints and encourage contributions Some of the development results ideas are listed below in a time to market order 4 1 Tiedtke convection scheme Adrian Tompkins is developing an adaptation of the ECMWF 38R2 Tiedtke 1989 cumulus convection scheme for the RegCM model 4 2 Semi Lagrangian dynamic core A semi Lagrangian advection scheme for the water vapor and advection tracers will allow a different timestep for the transport schemes which should result in a performance prize 4 3 Non Hydrostatic core We want to implement the non hydrostatic core to allow physical downscaling of large scale model simulation under the 20 kilometers limit of the hydrostatic model 28 Bibliography Alfaro S C and L Gomes Modeling mineral aerosol production by wind erosion Emission intensities and aerosol size distributions in source areas Journal of Geophysical Research 106 d16 2001 Anthes R A A cumulus parameterization scheme utilizing a one dimensional cloud model Mon Wea Rev 105 270 286 1977 Beheng K D A p
42. ssumed that convection removes the ABE over a given time scale as follows ABE 22 NAT ee Mp where T is the ABE removal time scale The fundamental difference between the two assumptions is that the AS74 closure assumption relates the convective fluxes and rainfall to the tendencies in the state of the atmosphere while the FC80 closure assumption relates the convective fluxes to the degree of instability in the atmosphere Both schemes achieve a Statistical equilibrium between convection and the large scale processes A number of parameters present in the scheme can be used to optimize its performance and Giorgi et al 1993c discusses a wide range of sensitivity experiments We found that the parameter to which the scheme is most sensitive is by and large the fraction of precipitation evaporated in the downdraft Peff with values from 0 to 1 which essentially measures the precipitation efficiency Larger values of Peff lead to reduced precipitation 3 MIT Emanuel scheme More detailed descriptions can be found in Emanuel 1991 andEmanuel and Zivkovic Rothman 1999 The scheme assumes that the mixing in clouds ishighly episodic and inhomogeneous as opposed to a continuousentraining plume and considers convective fluxes based on anidealized model of sub cloud scale updrafts and downdrafts Convection is triggered when the level of neutral buoyancy is greaterthan the cloud base level Between these two levels air is liftedand
43. sylvania State University PWC Physics of Weather and Climate RCM Regional Climate Model RegCM REGional Climate Model RegCM1 REGional Climate Model version 1 RegCM2 REGional Climate Model version 2 RegCM2 5 REGional Climate Model version 2 5 RegCM3 REGional Climate Model version 3 RegCM4 REGional Climate Model version 4 RegCNET REGional Climate Research NETwork RMIP Regional Climate Model Intercomparison Project ROMS Regional Oceanic Modeling System SIMEX the Simple EXplicit moisture scheme SST sea surface temperature SUBEX the SUB grid EXplicit moisture scheme USGS United States Geological Survey JJA June July and August JJAS June July August and September JFM January February and March 34
44. t al 2009 Since CLM was developed for the global scale several input files and processes were modified to make it more appropriate for regional simulations including 1 the use of high resolution input data 2 soil moisture initialization and 3 and an improved treatment of grid cells along coastlines For the model input data CLM requires several time invariant surface input parameters soil color soil texture percent cover of each land surface type leaf and stem area indices maximum saturation fraction and land fraction Lawrence and Chase 2007 Table 3 3 shows the resolution for each input parameter used at the regional scale in RegCM CLM compared to resolutions typically used for global simulations The resolution of surface input parameters was increased for several parameters to capture surface heterogeneity when interpolating to the regional climate grid Similar to Lawrence and Chase 2007 the number of soil colors was extended from 8 to 20 classes to resolve regional variations The second modification was to update the soil moisture initialization based on a climatological soil moisture average Giorgi and Bates 1989 over the use of constant soil moisture content throughout the grid generally used for global CLM By using a climatological average for soil moisture model spin up time is reduced with regards to deeper soil layers The third modification to the CLM is the inclusion of a mosaic approach for gridcells that contain bot
45. tic compressible sigma p vertical coordinate model run on an Arakawa B grid in which wind and thermodynamical variables are horizontally staggered using a time splitting explicit integration scheme in which the two fastest gravity modes are first separated from the model solution and then integrated with smaller time steps For application of the MM4 to climate studies a number of physics parameterizations were replaced mostly in the areas of radiative transfer and land surface physics which led to the first generation RegCM Dickinson et al 1989 Giorgi 1990 The first generation RegCM included the Biosphere Atmosphere Transfer Scheme BATS Dickinson et al 1986 for surface process representation the radiative transfer scheme of the Community Climate Model version 1 CCM1 a medium resolution local planetary boundary layer scheme the Kuo type cumulus convection scheme of Anthes 1977 and the explicit moisture scheme of Hsie et al 1984 A first major upgrade of the model physics and numerical schemes was documented by Giorgi et al 1993a b and resulted in a second generation RegCM hereafter referred to as REGional Climate Model version 2 RegCM2 The physics of RegCM2 was based on that of the NCAR Community Climate Model version 2 CCM2 Hack et al 1993 and the mesoscale model MM5 Grell et al 1994 In particular the CCM2 radiative transfer package Briegleb 1992 was used for radiation calculations the non local boundar
46. uds FC is determined by V RA max a RHA nin where RH pin is the relative humidity threshold at which clouds begin to form and RAinax is the relative humidity where FC reaches unity FC is assumed to be zero when RH is less than RH nin and unity when RH is greater than RA max gt Precipitation P forms when the cloud water content exceeds the autoconversion threshold Q according to the following relation P Cpp Qe FC Q FC 3 24 where 1 C p can be considered the characteristic time for which cloud droplets are converted to raindrops The threshold is obtained by scaling the median cloud liquid water content equation according to the following m C80 Peer 3 25 where T is temperature in degrees Celsius and Cacs is the autoconversion scale factor Precipitation is assumed to fall instantaneously SUBEX also includes simple formulations for raindrop accretion and evaporation The formulation for the accretion of cloud droplets by falling rain droplets is based on the work of Beheng 1994 and is as follows Pace T CaccQPsum 3 26 where Pycc is the amount of accreted cloud water Caco is the accretion rate coefficient and Pym is the accumulated precipitation from above falling through the cloud Precipitation evaporation is based on the work of Sundqvist et al 1989 and is as follows Pevap Cevap 1 z RDP sum 3 27 where Peyap is the amount of evaporated precipitation and Cevap is the rate coefficient For a mo
47. undary layer scheme developed by Holtslag et al 1990 is based on a nonlocal diffusion concept that takes into account countergradient fluxes resulting from large scale eddies in an unstable well mixed atmosphere The vertical eddy flux within the PBL is given by 3 10 where Y is a countergradient transport term describing nonlocal transport due to dry deep convection The eddy diffusivity is given by the nonlocal formulation z2 K bwc 1 3 where k is the von Karman constant w is a turbulent convective velocity that depends on the friction velocity height and the Monin Obhukov length and A is the PBL height The countergradient term for temperature and water vapor is given by 3 11 9 wh Ye C 3 12 where C is a constant equal to 8 5 and b lt is the surface temperature or water vapor flux Equation 3 12 is applied between the top of the PBL and the top of the surface layer which is assumed to be equal to 0 1h Outside this region and for momentum Y is assumed to be equal to 0 For the calculation of the eddy diffusivity and countergradient terms the PBL height is diagnostically computed from y Ricr u h v h 3 0 0 11 9s where u h v h and O are the wind components and the virtual potential temperature at the PBL height g is gravity Ricr is the critical bulk Richardson number and 8 is an appropriate temperature of are near the surface Refer to Holtslag et al 1990 and H
48. y cloud top radiative cooling which is a critical difference from the Holtslag PBL If a turbulent layer e g the PBL is cloud topped then a term is added to the TKE budget equation de RD Eon lin Where AF is the jump in long wave flux at cloud top This term is crucial for ensuring that turbulence is produced in the otherwise stable regions where stratocumulus exist The UW model is written specifically to deal with moist thermodynamic processes i e mixing between clear and cloudy air its core prognostic equations are written to predict liquid water potential temperature 0 total water mixing ratio O and momentum u The use of these variables ensures that enthalpy and water are explicitly conserved when mixing between clear and cloudy parcels of air care has to be taken otherwise when using 8 and q to ensure conservation in this situation At each model timestep the UW model does the following determines the boundary layer height h calculates the surface TKE predicts the change in TKE due to PBL processes determines the diffusivities at each height and predicts the change in each prognostic quantity due to vertical convergence of turbulent fluxes The full set of equations that the UW PBL model solves at each time step including equations 3 14 and 3 15 follows Me esn OV 3 16a r lA 3 160 Ql IMITA 3 160 Ma VTA 3 164 3 2 4 Convective Precipitation Schemes Convective precipitation is computed usi
49. y layer scheme of Holtslag et al 1990 replaced the older local scheme the mass flux cumulus cloud scheme of Grell 1993 was added as an option and the latest version of BATS1E Dickinson et al 1993 was included in the model In the last few years some new physics schemes have become available for use in the RegCM mostly based on physics schemes of the latest version of the Community Climate Model CCM Community Climate Model version 3 CCM3 Kiehl et al 1996 First the CCM2 radiative transfer package has been replaced by that of the CCM3 In the CCM2 package the effects of H20 03 O2 CO2 and clouds were accounted for by the model Solar radiative transfer was treated with a 6 Eddington approach and cloud radiation depended on three cloud parameters the cloud fractional cover the cloud liquid water content and the cloud effective droplet radius The CCM3 scheme retains the same structure as that of the CCM2 but it includes new features such as the effect of additional greenhouse gases NO2 CH4 CFCs atmospheric aerosols and cloud ice Scattering and absorption of 10 solar radiation by aerosols are also included based on the aerosol optical properties Absorption Coefficient and Single Scattering Albedo A simplified explicit moisture scheme Hsie et al 1984 is included where only a prognostic equation for cloud water is used which accounts for cloud water formation advection and mixing by turbulence re evaporation in su
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