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

ChEAS meeting: May 14, 2007 Overview Motivation Parameterization Experiment Setup Results Conclusions Future Work

ChEAS meeting: May 14, 2007 Motivation A 20% error in Z i produces a 20% error in CO 2 tendency Z i is very difficult to determine accurately in mesoscale models because of the coarse resolution Z i is the depth of the PBL

ChEAS meeting: May 14, 2007 SAM model Courtesy Tak Yamaguchi White = pos buoyant Red = neg buoyant Large-Eddy Simulation: Morning Mixed-Layer Development

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 CO 2 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 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 Z i and CO 2 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