The Property Differences Between Precipitating Clouds And Non-precipitating Clouds Over Global Tropics Based On ISCCP & TRMM Measurements

Along with the application of various advanced meteorological satellite platforms, the role played by clouds and precipitation in climate system will be revealed step by step. In virtue of combined measurements based upon multichannel, multi-instrument, and multi-perspective observations, clouds and precipitation has been an active research field in atmospheric remote sensing and climate change. In this study, by using TRMM PR and VIRS measurements that provide reliable precipitation detection and refined radiative information from cloud top, together with ISCCP datasets, the cloud climatology was investigated within a long time and a global scale. Particularly, the differences of radiative signals on visible and infrared channels, macro- and micro- physical properties, as well as climatological features between precipitating clouds (PCs) and non-precipitating clouds (N-PCs) were emphasized. The preliminary results on three aspects are presented as following.(1) Climatological features of global cloudsThe ISCCP D2 dataset was used to analyze the spatial distribution of cloud amount (CA) and cloud optical properties. The diurnal, seasonal, interannual and interdecadal variations of these cloud parameters were also examined. The results obtained will be beneficial for the further analysis on individual cloud types.It was found that low-clouds mostly occur over southeastern Pacific and northeastern Pacific near by the western coast of America, which is related to the lower sea surface temperature (SST) of local region. Intertropical Convergence Zone (ITCZ), South Pacific Convergence Zone (SPCZ), and the midlatitude low-pressure zone are all covered by quite more high-clouds due to strong convection in these areas. There is hardly seasonal variation of CA especially for those over ocean, whereas the diurnal cycle is significant and shows notable difference between maritime and terrestrial clouds. A slightly decreasing trend was found for total clouds, low-clouds and high-clouds, while there is a small increase in middle-clouds during the past two decades. Mediterranean Sea, Australia, central and southern Africa exhibit low cloud optical thickness (COT), which is corresponding to the low CA. The most ample cloud water path (CWP) occurs over Mainland of China and adjacent oceanic areas, as well as coastal regions of western South America. COT and CWP are found to be quite steady in various seasons but show weak increase on a long-time scale.(2) Characteristics of visible and infrared radiation from PCsFocusing on a set of precipitation samples including summer frontal and typhoon cloud systems, the radiative characteristics of visible (VIS), near infrared (NIR) and infrared (IR) channel from both PCs and N-PCs were examined, and various shortwave and longwave index for precipitation detection were evaluated. Attention was also paid to the structure of PCs in the typical mesoscale precipitation system.Results indicate 0.63μm reflectance (RF1) and 10.8μm Brightness Temperature (TB4) both display acceptable discriminability from PCs to N-PCs. Among several threshold methods for identifying PCs, RF1, RF1/RF2 and RF1&TB4 show approvable performance. It was found that the combined usage of RF1 and TB4 is the optimal way to detect PCs, which lead to a rather low False Alarm Ratio (FAR) and retain a higher Probability of Detection (POD). For weak and moderate precipitation, an approximately linear relationship between surface rain rate and RF1 (or TB4) was retrieved by using statistics, which shows to be representative for various precipitation systems. Within the specific precipitation system considered in this study, mean rain rate that could be obtained from above relationship is about 3.3mm/h, slightly higher than that assigned in GOES Precipitation Index (GPI) algorithm. A new technique called Equivalent Radius (ER) for acquiring the effective horizontal scale of PCs was introduced, which is similar to Correlation-Scale Length (CSL) method and is not affected by the truncation error due to the finite swath width. The ER technique was applied to the frontal and typhoon systems, in which the result is proved to be satisfied. The vertical extent of precipitating clouds was investigated by means of difference between rain top altitude and cloud top altitude, in which a negative correlation lying between the height-difference index and surface rain rate is quantificationally revealed.(3) Difference of cloud properties between PCs and N-PCsThe radiative characteristics of tropical PCs were further examined on climate scale. Based upon the retrieved results, PCs' cloud properties were analyzed on their spatial and temporal variations and were compared with those of N-PCs. A frequency distribution in ISCCP standard cloud types was investigated for both PCs and N-PCs.In comparison with the radiation background composed of Clear-sky and N-PCs, PCs increase the mean RF1 by 100% and reduce the TB4 by about 30K. As for the broadband albedo in shortwave, it is found to be increased 25% by PCs. The occurrence frequency of Clear-sky, N-PCs and PCs over global tropics is 48.1%, 47.9% and 4.0%, respectively. The CA of PCs is a bit higher over ocean than over land and the spatial distribution of PCs is nearly identical with high-clouds of N-PCs. The diurnal variation of PCs is generally vigorous than that of N-PCs, while showing a similar phase pattern with N-PCs over ocean and over land, respectively. The global mean of PCs' COT, higher than 60, is about 10 times that of N-PCs. The ice-water ratio is totally different for PCs and N-PCs, and averaged cloud droplet Effective Radius (Re) of PCs is larger than that of N-PCs by 10μm, associated with a relatively higher Cloud Water Path (CWP) that is about 3 times that of N-PCs. According to ISCCP cloud classification, cumulus, stratocumulus, altocumulus and cirrus are revealed to be the four primary cloud categories of N-PCs. However, besides deep convective clouds and nimbostratus, cirrostratus also shows a high probability for generating precipitation, indicating an inapposite denomination to this type of clouds. which was introduced by ISCCP scheme.