Representing convective organization in prediction models by a hybrid approach

The mesoscale organization of precipitating convection is highly relevant to next-generation global numerical weather prediction models, which will have an intermediate horizontal resolution (grid spacing about 10 km). A primary issue is how to represent dynamical mechanisms that are conspicuously absent from contemporary convective parameterizations. A hybrid parameterization of mesoscale convection is developed, consisting of convective parameterization and explicit convectively driven circulations. This kind of problem is addressed for warm-season convection over the continental United States, although it is argued to have more general application. A hierarchical strategy is adopted: cloud-system-resolving model simulations represent the mesoscale dynamics of convective organization explicitly and intermediate resolution simulations involve the hybrid approach. Numerically simulated systems are physically interpreted by a mechanistic dynamical model of organized propagating convection. This model is a formal basis for approximating mesoscale convective organization (stratiform heating and mesoscale downdraft) by a first-baroclinic heating couplet. The hybrid strategy is implemented using a predictor-corrector strategy. Explicit dynamics is the predictor and the first-baroclinic heating couplet the corrector. The corrector strengthens the systematically weak mesoscale downdrafts that occur at intermediate resolution. When introduced to the Betts-Miller-Janjic convective parameterization, this new hybrid approach represents the propagation and dynamical structure of organized precipitating systems. Therefore, the predictor-corrector hybrid approach is an elementary practical framework for representing organized convection in models of intermediate resolution.