云滴谱分布对FY-4A/AGRI水云光学厚度与有效粒子半径反演的影响

Effect of Cloud Droplet Spectrum Distribution on Retrievals of Water Cloud Optical Thickness and Effective Particle Radius by AGRI Onboard FY-4A Satellite

为了定量评估对数正态谱分布假设对水云光学厚度(COT)与有效粒子半径(Re)反演的影响,利用欧洲中期数值天气预报中心建立的RTTOV(Radiative Transfer for TIROS Operational Vertical Sounder)模式,对比模拟了基于对数正态谱和修正Gamma谱两种云滴谱分布下FY-4A/AGRI(Advanced Geosynchronous Radiation Imager)的第2、5通道液态水云的反射率,分析了这两个谱分布假设条件下水云反射率随COT以及Re的变化特征。在此基础上,建立了两种谱分布条件下的COT和Re查算表,并基于2020年夏季的一个初生对流云个例,定量分析了云滴谱分布类型对云参数反演结果的影响。结果表明,在第2通道,两个云滴谱类型假设下计算的反射率仅有0.1%~2%的差异,但在第5通道,采用修正Gamma云滴谱计算的反射率比采用对数正态云滴谱计算的反射率低10%~20%。反演结果表明,采用对数正态云滴谱反演的有效粒子半径Re比采用修正Gamma云滴谱反演的Re整体偏大,前者反演的Re集中在15~35μm,而后者反演的Re集中在10~30μm。采用两种云滴谱反演的COT的空间一致性良好,相差-2%~5%。

In order to quantitatively evaluate the effect of log-normal spectral distribution assumption on retrieval of cloud optical thickness(COT) and effective particle radius(Re), the reflectance of water clouds with log-normal spectrum and modified Gamma spectrum was simulated for the 2 nd and 5 th channels of the AGRI(Advanced Geosynchronous Radiation Imager) onboard FY-4 A satellite by the RTTOV(Radiative Transfer for the TIROS Operational Vertical Sounder) model developed by the European Center for Medium-Range Numerical Weather Forecasts. The variation characteristics of water cloud reflectivity with COT and Re under these spectral distribution assumptions were analyzed. On this basis, the look-up tables of COT and Re under two kinds of spectral distribution conditions are established, and the influence of cloud droplet spectral distribution type on cloud parameter inversion results is quantitatively analyzed based on a primary convective cloud in summer of 2020. The results indicate that for the 2 nd channel, reflectance difference under the assumption of two cloud droplet spectra is within 0.1%--2%, whereas for the 5 th channel, the reflectance calculated with the modified Gamma cloud droplet spectrum is 10%--20% lower than that calculated with the log-normal cloud droplet spectrum. The retrival results show that the effective particle radius r;retrieved with the log-normal cloud droplet spectrum is larger than that retrieved with the modified Gamma cloud droplet spectrum. The former is concentrated in 15--35 μm, while the latter is within 10--30 μm. The differences of COT retrieved with the two cloud drop spectra are relatively small, ranging from-2% to 5%.