Effects Of Aerosols On Microphysical Characteristics Of Precipitation

The impact of aerosols on clouds and precipitation is full of complexity and uncertainty,which is one of the focuses of cloud microphysics research.As one of the most abundant and widely distributed aerosols in the troposphere,dust aerosols act as condensation nucleus to alter the life cycle of clouds and precipitation,thus affecting weather and climate.Therefore,the effects of dust and anthropogenic aerosol on precipitation are studied in detail in this paper.Firstly,the seasonal,annual and diurnal variations of the microphysical parameters of precipitation are analyzed using the minute by minute raindrop size distribution（DSD）data in Pudong,Shanghai from January 1,2021 to December 31,2022.Then,air quality data,ERA5 reanalysis released by the European Centre for Medium Range Weather Forecasts（ECMWF）and MERRA-2 reanalysis released by NASA were then used for analysis.Taking multiple precipitation episodes（605 samples）in Pudong,Shanghai,China,during two mega-dust storms in March 2021 as an example,we attempt to reveal the influence of dust aerosol on microphysical characteristics of weak precipitation.Finally,we analyze the DSD characteristics of precipitation under the circumstance of high concentration anthropogenic aerosol.Finally,the DSD characteristics of 259 raindrop spectrum samples under high anthropogenic aerosol pollution scenario are analyzed.The main conclusions are as follows:（1）The average raindrop spectrum of convective precipitation is obviously higher than that of stratiform precipitation.The average raindrop spectrum of convective precipitation is unimodal,and the peak diameter is about 0.56 mm.Under different rain intensity classes,the mean value of mean mass weighted diameter（D_m）increased with increasing rain intensity,while the mean value of normalized intercept parameter(Log₁₀N_w)increased first and then decreased.（2）For the precipitation contributions of precipitation particles with different diameters,the main contributors of convective precipitation intensity are medium and large raindrops,while stratiform precipitation is small and medium raindrops.With the increase of rain intensity,the peak pattern of normalized rainfall intensity contribution(R（D）^norm)distribution changes double-peak to three-peak,and the peak diameter becomes larger.In weak precipitation events,small raindrops contribute more to precipitation,while in heavy precipitation events,large raindrops contribute more to precipitation.（3）Dust aerosols may inhibit weak precipitation.In details,the mean D_m of dust-carrying precipitation is much smaller than the weak precipitation without dust aerosols,while the mean Log₁₀N_w of dust carrying precipitation is greater than the dust-free precipitation.Moreover,compared with dust-free precipitation events,the mean concentration of small raindrops in dust-carrying precipitation events increases by 13.8%,while that of large raindrops decreases by 19.2%,indicating that dust aerosol may lead to a significant increase（decrease）in the number concentration of small（large）raindrops.This may be because the increase of dust aerosol concentration leads to the increase of the number concentration of small cloud droplets and the decrease of the collision effect of cloud droplets,which leads to the significant decrease of the number concentration of large raindrops and the inhibition of precipitation efficiency,thus inhibiting the weak precipitation.（4）The increase of anthropogenic aerosol concentration is conducive to the convective precipitation to produce larger particle size and more large raindrop concentration,which promotes the development of convective precipitation.Under the condition of high pollution,the number concentration of large raindrops in convective precipitation increases significantly,the cumulative precipitation increases,and the peak time of daily precipitation is delayed,while the number concentration of small raindrops in stratiform precipitation decreases significantly.In addition,compared with clean conditions,the mean concentration of small raindrops of stratiform precipitation decreases by 63.5%under high pollution conditions,while the mean concentration of large raindrops of convective precipitation increases by 8.04times.This further indicates that the high concentration of aerosol is conducive to promoting the development of convective precipitation,while inhibiting the stratiform precipitation and delaying the occurrence of convective precipitation.