2000 Volume 78 Issue 1 Pages 1-12
Nonconvective partial condensation in a coarse grid box of low-resolution climate models has been parameterized in terms of a heavy-tailed probability density function of “liquid water” relative humidity. In our diagnostic scheme, temperatures of the cloud and clear-air parts in a model grid box are assumed to be equal, so that a strong buoyant force does not act on the clouds. When the present diagnostic scheme is adopted, nonconvective stratiform clouds begin to form at grid-averaged relative humidifies lower than 70%.
Numerical experiments on the baroclinic wave growth in a midlatitude β-plane channel have shown that our scheme brings earlier occurrence and larger amount of precipitation than the “all-or-nothing” scheme or the Le Treut-Li (1991) scheme. Moreover, it has been indicated that our scheme produces nonconvective precipitation not only along the warm and cold frontal regions, but also in the warm sector of an occluding extratropical cyclone.