The effect of time steps and time-scales on parametrization suites

The problem of intense, truncation-scale storms that form in high-resolution versions of the Community Atmosphere Model Version 4 (CAM4) is studied. These storms are characterized by extreme vertical motion and heavy precipitation. This problem arises when some individual parametrizations do not produce an atmospheric-like state because they are restrained by the time-scales assumed in their formulation; other unconstrained parametrizations that follow then work in unintended ways. The behaviour of the moist parametrization components is examined in CAM4 for one typical, strong cell. At T340 spectral truncation with a 5 min time step, the deep and shallow convection parametrizations do not remove instabilities and supersaturation because they have time-scales of 1 h and 30 min, respectively. Then the prognostic cloud-water scheme, which is not constrained by a time-scale, does remove supersaturation. That local release of latent heat drives very strong vertical motion and horizontal convergence, which transports even more water vapour into the column, exacerbating the problem. Two simple model problems are introduced that illustrate the ramifications of the time-scale and time-step mismatch. When either the time-scales are shortened or the time step is lengthened, the convection parametrizations are more active and strong storms do not form. Copyright (c) 2012 Royal Meteorological Society