APRAMVI And Its Application To The Aerosol Indirect Effect

Up to now, there are so many satellites having the capabilities to simultaneously observe various variables related to the Earth-Atmosphere System, with kinds of sensors in high-performance. Based on these united observations, it can be beneficial in the development and the improvement of multi-variables retrieval algorithm, in turn the investigation on the climate change associated to the variation of the variables and the relationship to each other. In this study, by using the combined measurements from TRMM TMI, VIRS, as well as PR, an integrated algorithm - APRAMVI (All Parameters Retrieval Algorithm for combined Microwave and Visible/Infrared measurements) is proposed to simultaneously retrieve surface and atmospheric variables including the sea surface temperature (SST), wind (U), columnar water vapor (CWV), cloud liquid water path (LWP), cloud temperature (Tc), cloud geometric depth (Dc), cloud height (Hc), cloud effective droplet radius (Re), cloudoptical thickness (τ_c), and so on, over ocean in the absence of rain. Moreover, anapplication of this algorithm to explore the aerosol indirect effect over the Yellow Sea region is presented.1. APRAMVIThere are four sub-algorithms in APRAMVI, which are dependent on each other in retrieval step, but different in retrieval technique.(1) Environmental variables (SST, U and CWV) sub-algorithm for TMI measurements: Applying an inherent log-linear relationship between the microwave brightness temperature (T_B) and variables including CWV, SST, U, LWP and Tc, the TMI T_Bs on five channels are log-linearly combined to retrieve environmental variables. For the validation, the retrievals are compared to the ground-based measurements and the other TMI-based retrieval product, respectively. The results show that there are small differences between the retrievals and ground-based measurements, e.g., the mean bias (MB) and root mean square error (RMS) between CWV retrievals and radiosonde observations are 0.435 kg m^-2 and 2.593 kg m^-2, while MB and RMS between U (SST) retrievals and buoy observations are -0.075 m s^-1 (0.116K) and 0.672 m s^-1(0.665K), respectively. On the other hand, a good agreement is also shown when global grid-to-grid (0.25°) comparing with the other TMI-based retrieval product (RSS TMI product), where there is the mostly unbiased Gaussian probability distribution of the difference between them with maximum frequency near zero. Moreover, for various other variables, the variation of MB or RMS statistics is small, which suggests that the uncertainties related to other variables on any retrieved variable are eliminated in current sub-algorithm, except that there is significant error in retrieved SST with high wind speed. Finally, the results from monthly global distribution indicate the rationality of current sub-algorithm, due to low differences over mostly global ocean area with compared to other climatic dataset.(2) Cloud variables (LWP and Tc) sub-algorithm for TMI measurements: Based on the above log-linear relationship and environmental retrievals, a published LWP retrieval frame is corrected and improved to simultaneously retrieve LWP_m (subscript m indicates LWP obtained from TMI measurements)and Tc with only TMI measurements as the input data. The quantitative validation shows that, with respect to cloud presence, the retrieved LWP_m is consistent to the retrieved LWP_s (subscript m indicates LWP obtained from VIRS measurements) by Visible/Infrared sub-algorithm (See subsequent paragraph about it), whether the pattern or values. Specially, the MB about 0.03 kg m^-2 (LWP_m is low) is in agreement with the generally intrinsic difference between them; the difference about 7 K between retrieved Tc and SST is coherent with actual atmosphere. For cloud absence, only 20% of clear-sky pixels are misjudged as cloud pixels by current sub-algorithm, where the average value is less than 0.01 kg m^-2; the grid-to-grid comparison with RSS TMI product on global scale indicates the slight difference of 0.007 kg m^-2 between them.(3) Cloud variables (Dc and Hc) sub-algorithm for combined TMI and VIRS measurements: Based on a simple cloud model, the conjunction of above retrieved SST and Tc for TMI measurements and cloud top temperature detected by VIRS is used to retrieve Dc and Hc. Case study results show that Dc and Hc of multi-layer ice-water mixed cloud are larger than single-layer water cloud, which qualitatively suggests the rationality and accuracy of current sub-algorithm.(4) Cloud optical variables (Re,τ_c and LWP_s) sub-algorithm for VIRS measurements: Based on the separate dependence of VIRS visible and near-infrared channel onτ_c and Re, combination of these two channels is used to simultaneously retrieveτ_c and Re. The retrieval technology for this sub-algorithm is derived from published references.2. The aerosol indirect effect over the Yellow Sea regionFirst, the aerosol characteristics in middle-eastern China, such as aerosol optical depth, aerosol type, mass concentration and fine aerosol fraction, are investigated by using the MODIS aerosol product from Terra and Aqua satellites. The results indicate that there is high aerosol concentration in this region during Spring and Summer. Specially, the largest aerosol optical depth locates the region near the Yellow Sea, due to aerosol accumulation effect with the prevailing west-south wind direction in East Asia summer monsoon period. Similarly, it is possible that there is obvious aerosol indirect effect over the Yellow Sea. As a result, the aerosol-cloud relationship is studied by using retrieved cloud parameters derived from APRAMVI for TRMM measurements and aerosol optical depth in MODIS product. The cases statistics results show that, the second aerosol indirect effect is obvious over the Yellow Sea region, whether for water clouds or for all non-precipitation clouds. However, the first aerosol indirect effect can not be observed by satellite, though there is significant negative correlation between Re and aerosol optical depth. Further investigation on the relationship between Re and cloud thermodynamic parameters suggests that the influence of thermodynamics on Re exceeds that of aerosol, so that the first aerosol indirect effect is obscure in this particular region.