Understanding and Reducing Warm and Dry Summer Biases in the Central United States: Improving Cumulus Parameterization


Most climate models still suffer large warm and dry summer biases in the Central United States (CUS). As a solution, we improved cumulus parameterization to represent 1) the lifting effect of small-scale rising motions associated with Great Plains low-level jets and mid-tropospheric perturbations by defining the cloud base at the level of condensation, 2) the constraint of the cumulus entrainment rate depending on the boundary layer depth, and 3) the temperature-dependent cloud to rain water conversion rate. These improvements acted to i) trigger mesoscale convective systems in unfavorable environmental conditions so to enhance total rainfall amount, ii) lower cloud base and increase cloud depth so to increase low-level clouds and reduce surface shortwave radiation, iii) suppress penetrative cumuli from shallow boundary layers so to remedy the overestimation of precipitation frequency, and increase water detrainment so to form sufficient cirrus clouds and balanced outgoing longwave radiation. Much of these effects were nonlocal and nonlinear, where more frequent but weaker convective rainfall led to stronger (and sometimes more frequent) large-scale precipitation remotely. Together they produced consistently heavier precipitation and colder temperature with a realistic atmospheric energy balance, essentially eliminating the CUS warm and dry biases through robust physical mechanisms.