MODELING OF TRADE-WIND CUMULI WITH A LOW-ORDER TURBULENCE MODEL - TOWARD A UNIFIED DESCRIPTION OF CU AND SC CLOUDS IN METEOROLOGICAL MODELS

A simple method is proposed to extend a low-order turbulence scheme in cluding a subgrid-scale cloudiness scheme to represent not only noncon vective (stratiform) cloudiness and turbulence but also shallow, nonpr ecipitating cumulus convection by utilizing an appropriate subgrid-sca le distribution function. A simple approach is chosen that avoids the knowledge of the skewness parameter. All cloud water-related variables (cloud water content, partial cloudiness, liquid water flux) are comp uted by interpolating linearly as a function of the saturation deficit between two limit cases: the stratocumulus case, which can be well re presented by a Gaussian distribution function, and the trade wind cumu lus case, characterized by a positively skewed distribution function w ith a skewness of 2. Comparisons of the scheme with a third-order turb ulence scheme and large-eddy simulations (LES) for the Puerto Rico Fie ld Experiment show satisfying results. It is shown that the liquid wat er Bur term, which is strongly dependent on the distribution chosen, i s the crucial parameter of the scheme.