Study On Microphysical Characteristics And Precipitation Particle Spectrum Evolution Of Stratocumulus Clouds Over North China

Stratocumulus mixed cloud is the main precipitation cloud system in north China.Based on the microphysical observation data of stratocumulus,the study on the evolution of stratocumulus microstructure and spectral parameters is of great practical significance to improve the scientific understanding of stratocumulus precipitation mechanism,the development of cloud physics parameterization scheme in numerical model,and the quantitative inversion of cloud microstructure by radar and satellite.In this paper,through the analysis of airborne microphysics data of a stratocumulus cloud in 2019,it is found that there are significant differences in the microphysics characteristics of different cloud areas,and the spectral distribution and evolution of particles in the melt layer are significantly changed compared with those above the melt layer.The classification numerical simulation results reappear the melting law of ice particles in the melt layer.In order to further study the vertical evolution characteristics of microphysical properties of different stratocumulus cloud regions,further analysis of 8 vertical detection data of two stratocumulus processes on May 22,2017and May 21,2018 showed that the microphysical properties of stratocumulus cloud regions and accumulative cloud regions,and clouds below and above the melting layer were significantly different.Specific understanding is as follows:（1）The research found that ice particles in some cloud areas can grow through the Begeron process,except for the rime attached to cold water and the growth of ice particles.Under the zero layer,the melting rate of ice precipitation particles in the high relative humidity zone（RH≥95%）is faster than that in the low relative humidity zone（RH<95%）.In the low relative humidity zone,the melt water on the surface of particles evaporates and absorbs latent heat,which reduces the ambient temperature and slows the melting rate of particles.Numerical studies show that the HUJI classification scheme can describe the melting process of particles.The results of observation and simulation show that the melting rate of large size ice particles is faster than that of medium size particles.The intercept of the mean spectrum of precipitation particles in the classification scheme is consistent with the observed data,but the slope is larger than the observed results.（2）The study of microphysical characteristics of different cloud areas of stratified mixed clouds showed that the number of particles in convective cells was relatively high,and the peak diameter of precipitation particles concentrated on small to medium sized particles.Better rime growth conditions make there are more large particles in the convective cell,the spectrum of precipitation particles is wider,and the strong gravity drag of large particles makes the downdraft in the convective cell stronger than that in the stratiform cloud area.In the stratiform cloud region,the peak diameter of particles tends to be medium diameter particles with decreasing height.The height of the precipitation particles in the convective cell with bimodal distribution and the height of the zero layer are higher than those in the stratiform cloud area.Below zero layer,particle spectrum fitting parameters in convection cell intercept₀,slopeand total particle concentrationis greater than the stratiform cloud and the total number of particles concentration area.It is found that the ice particles have begun to melt in the negative temperature layer near the zero layer,and there are microphysical processes such as melting,sublimation and bonding in the melting layer.In each detection process,isothermal layer formed by the melting of ice particles has been observed.（3）By studying the evolution characteristics of precipitation particle spectrum parameters in a total of 10 vertical cloud microphysical detection,the evolution law of straculus cloud precipitation particle spectrum intercept with temperature,the correlation between particle spectrum intercept and slope,and the maximum particle size and slope suitable for North China were obtained.It is worth noting that the zero layer below the relationship between the precipitation particle spectrum parameter₀ andsignificantly different from zero layer.