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Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling System. Erica McGrath-Spangler Dept. of Atmospheric Science Colorado State University ChEAS May 14, 2007. Acknowledgements: Scott Denning, Kathy Corbin, Ian Baker. Overview. Motivation
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Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling System Erica McGrath-Spangler Dept. of Atmospheric Science Colorado State University ChEAS May 14, 2007 Acknowledgements: Scott Denning, Kathy Corbin, Ian Baker
Overview • Motivation • Parameterization • Experiment Setup • Results • Conclusions • Future Work ChEAS meeting: May 14, 2007
Motivation • A 20% error in Zi produces a 20% error in CO2 tendency • Zi is very difficult to determine accurately in mesoscale models because of the coarse resolution Zi is the depth of the PBL ChEAS meeting: May 14, 2007
Large-Eddy Simulation: Morning Mixed-Layer Development White = pos buoyant Red = neg buoyant SAM model Courtesy Tak Yamaguchi ChEAS meeting: May 14, 2007
Mesoscale Models • Mesoscale models can’t resolve overshooting thermals because of grid spacing • Process is not currently parameterized in RAMS ChEAS meeting: May 14, 2007
Mixing at the top of the PBL • At the top of the boundary layer, the Richardson number is very large ( ) • Since the mixing coefficient is inversely proportional to the Richardson number, the mixing is ~ 0 within the capping inversion • Very difficult to initiate growth of the boundary layer • RAMS does not include any process to initiate mixing ChEAS meeting: May 14, 2007
Closure Assumption • Heat flux at the boundary layer top is negatively proportional to the surface heat flux • Mixes warm, dry free tropospheric air into the PBL and cool, moist boundary layer air into the capping inversion ChEAS meeting: May 14, 2007
Also mix the three wind components, TKE, and CO2 concentration • The tendencies from entrainment mixing are the quantities themselves times the mass flux divided by density and the layer thickness Units of kg m-2 s-1 ChEAS meeting: May 14, 2007
RAMS setup • RAMS version 5.04 modified to BRAMS version 2.0 • 42 vertical levels starting at 15m and vertically stretched by ~1.1 up to 6600m • Includes a shallow convection parameterization • Use Mellor and Yamada (1982) closure option for vertical diffusion • Smagorinsky (1963) used for horizontal diffusion • Coupled to SiB version 3 ChEAS meeting: May 14, 2007
Idealized simulation • Cyclic lateral boundary conditions • No weather systems can be horizontally advected into the system • Initialized horizontally homogeneously from a dry sounding • Homogeneous surface • Flat topography at sea level • Vegetation is C3 broadleaf and needleleaf trees • Loam soil type • FPAR = 0.8 • LAI = 4.0 ChEAS meeting: May 14, 2007
Conclusions • In nature, overshooting thermals warm, dry, and deepen the PBL • Mesoscale models don’t include overshooting thermals • I’ve introduced a parameterization into RAMS that accounts for this process • Hope to be able to better simulate Zi and CO2 concentrations ChEAS meeting: May 14, 2007
Future Work • Compare mesoscale simulations to an LES run of RAMS and to observations • Both with and without the parameterization included • Parameterization also affects surface temperature and dew point that are observed • Assimilate those variables in order to better determine a value for the tunable parameter ChEAS meeting: May 14, 2007
Thanks ChEAS meeting: May 14, 2007
