Analysis on the microphysical characteristics of weak precipitation process observation of a stratiform cloud in Sichuan Basin in autumn

Using observation data from a Ka-band millimeter wave cloud radar, a raindrop spectrometer, a L-band Basin in autumn is analyzed, especially focusing on its macro and microstructure characteristics and raindrop spectrum distribution characteristics. The melting layer (radar echo bright band) is analyzed in detail. The results are as follows. (1) There were convective bubbles in the cloud system. The rainrate was mostly below 2 mm·h^-1, and the peak precipitation had a good correspondence with the convective bubbles. (2) The cloud system before precipitation was mainly stratiform with the cloud top about 3 km and the cloud base about 1.5 km. Cloud drops were mainly liquid particles with small particle size and in single phase state. The cloud system during the precipitation development period was dominated by stratiform clouds, accompanied by convective bubbles. The stratified cloud top was about 4 km, and the convective bubble cloud top in the cloud system reached up to 10 km. The radar echoes reached the ground and ground precipitation formed. There was a clear bright band (melting layer) phenomenon in the cloud system. Above the melting layer were ice crystals, graupels, super-cooled water, etc. In the melting layer clouds were consisted of mixed-phase particles. Below the melting layer there were raindrops, super-cooled water, drizzles, etc. The cloud system during the precipitation dissipation period became double-layer. The lower layer was dominated by stratocumulus clouds, with cloud tops below 4 km, dominated by liquid particles with small particle sizes and in single phase state. The upper layer was dominated by altocumulus clouds, the cloud base was about 5 km, and the cloud top extended above 8km, which was dominated by ice particles with small particle size and in single phase state. (3) The cloud drop spectra in the early precipitation period and the development period showed single peak structure, mainly small particles. The spectra were narrow. The spectra width was between 0.312 mm and 2.375 mm. The peaks were all located at 0.437 mm. The cloud drop spectra during the precipitation dissipation period were bimodal, with peaks located at 0.437 mm and 1.375 mm. The raindrop spectra was broader and number concentration was larger during the precipitation development period. (4) The thickness of the bright band zone of the stratified precipitation cloud system was 200~600 m, and the thickness of the bright band zone was different for different radar parameters. The depolarization ratio is most sensitive to the bright band layer, and the bright band information appears on its vertical profile first, which is an important indicator of the phase change of particles. The thickness of the bright band is highly correlated with the difference in radial velocity, spectral width, and depolarization ratio, but has little relation with the difference in echo intensity.