Numerical Simulations Of The Effect Of Cloud Droplet Concentration And Autoconversion Parameterizations On Strong Convective Precipitation

Cloud drop number concentration and autoconversion process of cloud drop to rain drop will effect the formation and growth of particles both in liquid phase and ice phase in a mixed phase convective cloud, consequently influences numerical simulation and prediction deep convective precipitation. In this paper, a deep convective weather process during Meiyu period in Jiangsu province, on July 5th of the year 2013 is chosen, the concentration of cloud droplets in micro-physical scheme of WRF model is calculated by using local actual CCN activation spectrum parameter, and a study in the influences of cloud droplets concentration and autoconversion parameterizations in deep convective precipitation is carried on.The number concentration of cloud condensation nuclei(CCN) was observed using a CCNC in Nanjing from April to December 2013. The fitted CCN spectra for various weather conditions are obtained using the expression N=CSk, C is the highest and reach 13085 cm-3 on haze days, but fall to 8054 cm-3 after rain, all of them exhibit a continental type. CCN number concentration is affected by meteorological elements, weather conditions, aerosol source emissions, physical and chemical properties of aerosol. Based on observations, a relationship between the CCN number concentration and aerosol number concentration is established and shows a good correlation. C value, winter>spring>autumn> summer.Simulate the strong convective weather process in Jiangsu province on July 5 th 2013.The cloud base C value is calculated by the local CCN activation spectrum parameter and CCN number concentration distribution rule and it is about 4000 cm-3.During this weather process, the Northeast cold vortex appears in high altitude and gradually moves eastward, in the low altitude, cold air from south and warm moist air from southwest intersects in Jianghuai region and a shear line forms at altitude 850 hPa, and this is the main weather system.The melting of the graupel and hail particles constitute an important source of this precipitation.Make a cloud droplets concentration sensibility experiment of above-mentioned deep convective case in Jiangsu, results show that, under the condition of high background cloud droplets concentration, the increase of cloud droplets concentration may cause delay in precipitation beginning time, a lower total precipitation, and a lower rain rate maximum; the starting time of precipitation is late under the condition of abnormal low background cloud droplets concentration; under the condition of high background cloud droplets concentration, there can save a large number of ice crystals in the late time of precipitation process, ice crystal is good for the growth of hail particles, and this leads to the melting hail particles become an important source of low-level rain in the late rain of the precipitation, so late precipitation enhancement in the high cloud droplets concentration experiment.Use the below three schemes in the autoconversion of cloud drop to rain drop process and simulate this precipitation weather, they are Kessler scheme, Ziegler scheme, and Ferrier scheme.Results show that, in Kessler scheme, the simulation of antecedent precipitation distinctly high. In Ziegler scheme and Ferrier scheme, the model cloud droplets concentration is set up based on local cloud droplets concentration, in the late precipitation there remains more ice crystal particles in the cloud because of high cloud droplets concentration, so stronger late precipitation appears in both Ziegler scheme and Ferrier scheme.