New Metric for Assessing Model Simulations of Convective Systems May Improve Projection of Future Precipitation Trends
Although organized propagating storms known as mesoscale convective systems (MCSs) can bring up to 60% of summer rainfall to the central United States, they are not well simulated by most global climate models. One limitation to evaluating and improving model simulations of MCSs is the lack of a clear metric for identifying them in simulations and observations. Now, a new empirical orthogonal function based index has been developed to identify MCS events based on the strength, time, and location of maximum convection. The index is applied to systematically compare different versions of the Community Atmosphere Model (CAM) against observations. The model simulations include several versions of a more computationally expensive model, the ‘super-parameterization,’ in which a high-resolution two-dimensional cloud resolving model is run within each grid box of the climate model. The results show that nocturnal, eastward propagating convection is a robust effect of the super-parameterization versions of the model, but some details are sensitive to the model version used. In an early version of the super-parameterization, convective MCS anomalies are unrealistically large scale and concentrated, while surface precipitation is too weak. These aspects of the MCS signal are improved in the latest version of the super-parameterization, which uses more complex treatments of cloud microphysics. Future work will apply this index to study what physics introduced by the super-parameterization (i.e., sub-grid scale wind shear, memory, and different triggering of convection) favors MCS development and propagation. The ability to more accurately simulate MCSs in climate models will enable improved projections of future precipitation trends.
Kooperman, G. J., M. S. Pritchard, and R. C. J. Somerville. 2013. “Robustness and Sensitivities of Central U.S. Summer Convection in the Super-Parameterized CAM: Multi-Model Intercomparison with a New Regional EOF Index,” Geophysical Research Letters 40 , 3287–91. DOI: 10.1002/grl.50597.