STRATIFORM PRECIPITATION IN REGIONS OF CONVECTION - A METEOROLOGICAL PARADOX

It was once generally thought that stratiform precipitation was someth ing occurring primarily, if not exclusively, in middle latitudes-in ba roclinic cyclones and fronts. Early radar observations in the Tropics, however, showed large radar echoes composed of convective rain alongs ide stratiform precipitation, with the stratiform echoes covering grea t areas and accounting for a large portion of the tropical rainfall. T hese observations seemed paradoxical, since stratiform precipitation s hould not have been occurring in the Tropics, where baroclinic cyclone s do not occur. Instead it was falling from convection-generated cloud s, generally thought to be too violent to be compatible with the layer ed, gently settling behavior of stratiform precipitation. In meteorolo gy, convection is a dynamic concept; specifically, it is the rapid, ef ficient, vigorous overturning of the atmosphere required to neutralize an unstable vertical distribution of moist static energy. Most clouds in the Tropics are convection-generated cumulonimbus. These cumulonim bus clouds contain an evolving pattern of newer and older precipitatio n. The young portions of the cumulonimbus are too violent to produce s tratiform precipitation. In young, vigorous convective regions of the cumulonimbus, precipitation particles increase their mass by collectio n of cloud water, and the particles fall out in heavy showers, which a ppear on radar as vertically oriented convective ''cells.'' In regions of older convection, however, the vertical air motions are generally weaker, and the precipitation particles drift downward, with the parti cles increasing their mass by vapor diffusion. In these regions the ra dar echoes are stratiform, and typically these echoes occur adjacent t o regions of younger convective showers. Thus, the stratiform and conv ective precipitation both occur within the same complex of convection- generated cumulonimbus cloud. The feedbacks of the apparent heat sourc e and moisture sink of tropical cumulonimbus convection to the large-s cale dynamics of the atmosphere are distinctly separable by precipitat ion region. The part of the atmospheric response deriving from the are as of young, vigorous convective cells is two layered, with air conver ging into the active convection at low levels and diverging aloft. The older, weaker intermediary and stratiform precipitation areas induce a three-layered response, in which environmental air converges into th e weak precipitation area at midlevels and diverges from it at lower a nd upper levels. If global precipitation data, such as that to be prov ided by the Tropical Rainfall Measuring Mission, are to be used to val idate the heating patterns predicted by climate and general circulatio n models, algorithms must be applied to the precipitation data that wi ll identify the two principal modes of heating, by separating the conv ective component of the precipitation from the remainder.