DEVELOPMENT AND PROPAGATION OF PRECIPITATION CORES ON COLD FRONTS

The structure and propagation of precipitation cores on a variety of c old fronts are described. The shapes of the cores, and their orientati ons with respect to the synoptic-scale front, are not always uniform e ither between fronts or within the same front. Interactions between co res depend on their proximity and relative strength. Large gaps betwee n precipitation cores move along the cold front at a slower speed than the cores and affect the evolution of the precipitation cores. The sp acing of precipitation cores, the height of the head of the density cu rrent associated with the cold front, and the strength of the cold fro nt are positively correlated with precipitation strength. This suggest s that interactions between cores are strongly influenced by precipita tion. Current theories for precipitation cores on narrow cold-frontal rainbands cannot account for these observations. A new mechanism is pr oposed based on a positive feedback between the diabatic cooling produ ced by the melting and evaporation of precipitation, the head of the d ensity current, and the strength of the front.