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 midtropospheric 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-rainwater conversion rate. These improvements acted to (i) trigger mesoscale convective systems in unfavorable environmental conditions to enhance total rainfall amount, (ii) lower cloud base and increase cloud depth to increase low-level clouds and reduce surface shortwave radiation, (iii) suppress penetrative cumuli from shallow boundary layers to remedy the overestimation of precipitation frequency, and (iv) increase water detrainment to form sufficient cir-rus 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.

Automated summary

By improving the cumulus parameterization, we have successfully addressed the warm and dry summer biases in the central United States in climate models. These improvements better represent the lifting effect of small-scale rising motions associated with Great Plains low-level jets and midtropospheric perturbations, constraint the cumulus entrainment rate depending on the boundary layer depth, and adjust the cloud-to-rainwater conversion rate according to temperature. These improvements trigger mesoscale convective systems, lower cloud base and increase cloud depth, suppress penetrative cumuli, and increase water detrainment, resulting in consistently heavier precipitation, colder temperature, and a realistic atmospheric energy balance, essentially eliminating the warm and dry biases in the central United States.