The paper theoretically explains some of the microphysical phenomena observed in Mei-Yu. In the melting layer the theory shows that most of the changes from the top to the bottom of the melting layer in the particle number density and the particle size distribution slope can be explained by assuming a constant precipitation rate and increasing particle fall speed due to melting. However, the theory implies that some aggregation is going on during melting. In the warm-rain region, binary interaction of coalescence and breakup are the prominent microphysical processes. In general, coalescence dominates over breakup. But in these cases the two processes will approximately arrive at equilibrium when the precipitation reaches the Earth's surface. ¿ American Geophysical Union 1990