三参数云微物理方案中气溶胶谱函数对云滴谱影响的数值模拟研究

观测和分档方案的数值模拟都证明气溶胶的谱分布特征对云滴谱的演变有直接影响继而作用于降水的发展。目前广泛使用的总体双参数云滴谱方案因为表征云滴谱的预报量不足,在凝结过程中云滴谱呈不正常的拓宽现象。因此在参数化方案中,气溶胶谱对云滴谱的影响未有明确结论。中科学院大气物理研究所(IAP)云降水物理与强风暴重点实验室(LACS)新研发的三参数方案(IAP-LACS)通过增加的预报量克服了云滴谱的拓宽问题,提高了云滴谱模拟的准确性。为了研究在参数化方案中气溶胶谱分布特征对云滴谱的影响,本文采用新方案进行WRF(Weather Research and Forecasting mode)大涡理想性试验,验证了新方案中气溶胶对数正态谱函数中数浓度、几何半径和标准差3个参量对云滴谱演变的影响。针对3个参量的敏感性试验表明新的气溶胶活化方案和三参数云滴凝结增长方案能够描述气溶胶谱对云滴谱演变的影响规律:气溶胶数浓度对云滴谱影响最显著,数浓度越高活化生成的云滴数量越多,云滴半径越小,云滴谱趋向窄谱,气溶胶数浓度低时,云滴数量少、半径大。较大的几何半径使气溶胶谱向大粒径移动,导致大云滴生成,标准差对云滴谱的影响最不显著。

Numerical Simulation Research on the Effects of the Size Distribution of Aerosols on the Droplet Spectrum with a Newly Developed Triple-Moment Bulk Scheme

Both observations and numerical simulations with bin microphysics indicated that aerosol concentrations and size distributions play an important role in cloud droplet spectrum evolution and precipitation formation. With limited prognostic variables, current bulk microphysics parameterization cannot simulate the cloud droplet spectrum evolution properly because of the abnormal broadening problem during condensation. No studies of the effect of the size distribution of aerosols on cloud droplet spectra with bulk scheme simulations have been conducted. The newly developed triple-moment bulk scheme includes an additional spectrum shape parameter and overcomes the abnormal broadening problem, which can be better applied to simulate the main characteristics of cloud droplet spectrum evolution than double-moment schemes. To analyze the effect of the size distribution of aerosols on the cloud droplet spectrum evolution using triple-moment microphysical bulk scheme IAP-LACS which has been developed by Key Laboratory of Cloud-Precipitation Physics and Severe Storms (LACS) of Insitute of Atmospheric Physics (IAP). This work focuses on the effects of three parameters of the size distribution of aerosols (i.e., number concentration, geometric radius, and standard deviation) on the cloud droplet spectrum evolution using WRF-LES ideal simulations. The results of the numerical sensitivity tests for the three parameters show that the newly developed triple-moment water vapor growth scheme coupled with the explicit aerosol activation process is a powerful tool to simulate the effect of the size distribution of aerosols on the cloud droplet spectrum evolution. Aerosol number concentration significantly affects the shape of the cloud droplet spectrum. A high aerosol number concentration results in a narrow droplet spectrum with activated droplets and small average size. By contrast, a low aerosol number concentration produces less droplets but with large radius. Enlarging the geometric radius, which means moving the aerosol spectrum toward a large particle size, leads to cloud droplets with a large size. Notably, the standard deviation plays a less important role in the cloud droplet spectrum than the aerosol number concentration and geometric radius.