A New Look at the Melting Layer

Abstract

The authors derive a relationship between the vertical Doppler spectrum of the rain just below the radar bright band and that of the snow just above. It neglects vertical air motions and assumes that each snowflake simply melts to form a raindrop of the same mass, disregarding other possible effects such as aggregation to form larger particles or breakup to create smaller ones. The relationship shows that, regardless of the dependence of particle fallspeed on size, the product of the equivalent reflectivity factor and the mean Doppler velocity of the snow is proportional to the same product for the rain, with a constant proportionality factor of 0.23, which equals the ratio of the dielectric factors of ice and water. Observed values of the reflectivity and mean Doppler velocity above and below the melting layer sometimes agree with this theoretical prediction but more often deviate from it in ways that may be interpreted as indicating the predominance of either aggregation or breakup processes. The data suggest that aggregation is occurring much of the time in the melting layer but that breakup effects become dominant in heavy precipitation. The analysis is extended by assuming relations between particle size and fallspeed for rain and snow. This enables the comparison of measured spectra with those derived theoretically. A simple allowance for aggregation or breakup in the spectral transformation from snow to rain is found to give improved spectral agreement in cases where these effects are indicated.