The warm surface air temperature (T2m) bias over mid-latitude continents during the summertime is a long-standing issue for many weather and climate models. Such warm bias can happen within a few days of model integrations. The warm bias involves interactions of precipitation, clouds, and radiation processes on the surface energy and water budgets, which determines the temperature simulations near the surface. The issue is important because such bias can affect the projections of future temperature changes over land. In this presentation, we will demonstrate the impact of land surface parameterizations, specifically the bare-ground resistance, stomatal conductance and leaf optical properties, on the simulated surface heat fluxes and T2m over the central U.S. during the summertime through both short-term hindcasts and long-term climate simulations. Our preliminary results suggest that T2m bias is much smaller over the central U.S. with the increased bare-ground resistance to evaporation and the increased minimum stomatal conductance. The improvement is mainly from the increased canopy transpiration compared to the default model version, which leads to a larger evaporative fraction and colder T2m.

(This study is funded by the RGCM and ASR programs of the U.S. Department of Energy as part of the Cloud-Associated Parameterizations Testbed. This work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-755320)