Role of Land-Atmosphere Interactions on Convection Initiation and Precipitation over the Southern Great Plains

Numerical simulations using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) examine the impact of land-vegetation processes on convective initiation for the International H2O Project 2002 case study period 24-25 May 2002. For the control run COAMPS is configured with the WRF (Weather Research and Forecasting model) version of the Noah land surface model (LSM) and initialized using a high-resolution land-surface data assimilation system (HRLDAS). Physically consistent surface fields are ensured by an 18-month spin-up time for HRLDAS, and physically consistent mesoscale fields are ensured by a 2-day data-assimilation spin-up for COAMPS. Partially because of the spin-up procedure, the control run replicates the major mesoscale features of the cold front that moved across Kansas and Oklahoma during the case study time and the dryline that moved across the Texas and Oklahoma Panhandles, albeit with a 2-3 hour delay in convective initiation. Three sensitivity simulations are performed to assess the impact of land-vegetative processes on the modeled pre- and post-storm environment by: (1) replacing the Noah LSM with a simple slab soil model, (2) adding a photosynthesis, canopy resistance/transpiration scheme (the Gas Exchange/photosynthesis-based evapotranspiration Model, GEM) to the Noah LSM, and (3) replacing the HRLDAS soil moisture with the National Centers for Environmental Prediction (NCEP) 40-km Eta Data Assimilation (EDAS) operational soil fields. The location and timing of the front and convection and the structure of the dryline prove to be sensitive to land-vegetative processes. For this case the control and GEM simulations agree best with observations. The GEM run provides the strongest coupling between the surface, vegetation and atmosphere, a reflection of the importance of evapotranspiration and soil moisture and its responsiveness to environmental characteristics. The sensitivity of the synoptically forced strong convection to land surface processes indicates that such enhancements are important and need to be included in weather forecasting models, particularly for severe storm forecasting where local scale information is important. Additional studies with different synoptic conditions, storm characteristics, as well as surface conditions are recommended.