The Transition to Strong Convection

Abstract

Recent work has shown that observations of tropical precipitation conform to properties associated with critical phenomena in other systems. Here some of these universal properties are used to probe the physics of tropical convection empirically, providing potential tests for models and parameterizations. The power-law pickup of ensemble average precipitation as a function of column water vapor w occurs above a critical value wc whose temperature dependence is determined for layer-integrated tropospheric temperature or saturation value. This dependence differs from the simplest expectations based on column saturation. Rescaling w by wc permits a collapse of precipitation-related statistics to similar functional dependence for all temperatures. The sharp precipitation variance peak at wc, obtained without detailed vertical structure information, appears consistent with arguments that onset requires a deep moist layer. Sea surface temperature (SST) is found not to have a significant effect on the precipitation pickup. The effect of SST on the climatological precipitation occurs via the frequency of occurrence of w values as the system spends a larger fraction of time near criticality over regions of warm SST. Near and above criticality, where most precipitation occurs, the w distribution is highly constrained by the interaction with convection, with a characteristic sharp drop at criticality. For precipitating points, the distribution has a Gaussian core with an approximately exponential tail akin to forced advection?diffusion problems. The long tail above wc, implying relatively frequent strong events, remains similar through the range of tropospheric temperature and SST spanning tropical large-scale conditions. A simple empirical closure illustrates time decay properties.