Analysis On The Contribution Of Edge Conglutination Between Particles In Stratus Cloud To Radar Reflectivity Factor

Radar bright band is not only an important symbol to the dissipation of thunderstorm, but also one of the main criteria in distinguishing freezing rain. And the band may lead to overestimation of precipitation when weather radar detects stratiform precipitation(including cumulus-stratus mixed precipitation).But quantitative research on the characteristics and mechanisms of radar bright band are relatively rare. Therefore, it is of great significance in further research on radar bright band.Taking two small, spherical particles for example, the backscattering cross-section as a whole during the stage of edge conglutination between them would be greatest as compared with that before they contact with each other and that after they fuse and coalesce into a single spherical particle, especially the conglutination and coalescence process starts from two small, spherical ice particles. The paper studies the contribution of this phenomenon to radar bright band. DDA is used to calculate σDDA, the backscattering cross-section of the edge-conglutinated particles. It has been found that σDDA in case the conglutination is along the direction of radar wave polarization may be 1.5 times greater than σmieequal, the backscattering cross-section of the equal volume, spherical particle at a certain size though the ratio may decrease to even less than 1 and fluctuate as the particle size increases because of resonance for large particles.Applying these results into simulating radar bright band, it has been shown that, for a normal intense cloud echo such as 37.69dBZ (5874mm6/m3), radar reflectivity factor can increase by ～4.09dBZ in case the conglutination percentage p=33%,～5.18dBZ in case p=50%, and ～7.48BZ in case p=100%, respectively, while the increase is only ～3.82dBZ if the conglutination is not considered. Therefore, conglutination on the edge of two spherical particles along radar wave polarization direction can effectively increase the radar reflectivity factor and benefit the formation of bright band.In this study, the microwave physics parameters in different period of one stratiform clouds precipitation processes detected by X-band Doppler Radar at July 13,2012 were inversed by SimRAD software containing the scattering calculation of edge conglutination between particles.During the inversion process, the input parameters of SimRAD were modified until the output profile was most consistent with the observation profile. Then the input parameters were deemed to be the parameters of the hydrometers in real cloud. The results represents that that the wider (narrower), brighter (darker) bright band with a melting layer in which the vertical distributions of the melting particles were wider(narrower), the mean diameters of the melting particles were lager(smaller) and the amount of the melting particles were larger(smaller).