A NEW LOOK AT THE MELTING LAYER

The authors derive a relationship between the vertical Doppler spectru m of the rain just below the radar bright band and that of the snow ju st above. It neglects vertical air motions and assumes that each snowf lake simply melts to form a raindrop of the same mass, disregarding ot her possible effects such as aggregation to form larger particles or b reakup to create smaller ones. The relationship shows that, regardless of the dependence of particle fallspeed on size, the product of the e quivalent reflectivity factor and the mean Doppler velocity of the sno w is proportional to the same product for the rain, with a constant pr oportionality factor of 0.23, which equals the ratio of the dielectric factors of ice and water. Observed values of the reflectivity and mea n Doppler velocity above and below the melting layer sometimes agree w ith this theoretical prediction but more often deviate from it in ways that may be interpreted as indicating the predominance of either aggr egation or breakup processes. The data suggest that aggregation is occ urring much of the time in the melting layer but that breakup effects become dominant in heavy precipitation. The analysis is extended by as suming relations between particle size and fallspeed for rain and snow . This enables the comparison of measured spectra with those derived t heoretically. A simple allowance for aggregation or breakup in the spe ctral transformation from snow to rain is found to give improved spect ral agreement in cases where these effects are indicated.