Effects of surface heat and moisture exchange on ARW-WRF warm-season precipitation forecasts over the central United States

The evolution of the daytime planetary boundary layer (PBL) and its association with warm-season precipitation is strongly impacted by land-atmosphere heat and moisture exchange (hereafter surface exchange). However, substantial uncertainty exists in the parameterization of surface exchange in numerical weather prediction (NWP) models. In the current study, we examine 0-24-h convection-permitting forecasts with different surface exchange strengths for a six-day period during the International H₂O Project (IHOP_2002). Results indicate sensitivity in the timing of simulated afternoon convection initiation and subsequent precipitation amounts to variations in surface exchange strength. Convection initiation in simulations with weak surface exchange was delayed by 2-3 h compared to simulations with strong surface exchange, and area-averaged total precipitation amounts were less by up to a factor of two. Over the western high plains (105°-100°W longitude), where deep convection is locally generated, simulations using a formulation for surface exchange that varied with vegetation category (height) produced area-averaged diurnal cycles of forecasted precipitation amounts in better agreement with observations than simulations that used the current ARW-WRF formulation. Parcel theory is used to diagnose mechanisms by which differences in surface exchange influence convection initiation in individual case studies. The more rapid initiation in simulations with strong surface exchange results from a more rapid removal of negative buoyancy beneath the level of free convection, which arises primarily from greater PBL warming.