Cloud Vertical Structures Study By Combined Active And Passive Satellite Data

Clouds are an important modulator of the energy budget of the planet and play a critical role in the atmospheric energy cycle, water vapor cycle as well as earth climate system. The variations in cloud vertical structures can alter the atmospheric circulation by affecting the vertical gradients of radiative heating/cooling and radiative effect of cloud at TOA and surface. Thus, cloud vertical structures are a very important factor for studying climate change. Focused on the significance of the cloud vertical structures research, the regional and seasonal variations of cloud vertical distribution over East Asia are analysed. By combining the advantages of active and passive satellite sensors, we also quantify the cloud radiative effect differences between multi-layered and single-layer clouds. Additionally, a novel method, which is not confined to daytime conditions and a single layer cloud structure, is developed based on CALIPSO level 1 attenuated backscatter profile data for deriving the mean extinction coefficient of water droplets close to cloud top. The main conclusions are as following:Based on the Level 2₀5km cloud data of lidar that is carried on CALIPSO, we studied the cloud vertical distribution over East Asia （18°N-53°N,74°E-144°E）. The results show that the multi-layered cloud fraction of East Asia is 43.6%,29.6%, 21.1% and 33.3% are for Summer, Autumn, Winter and Spring, respectively. Two-layered clouds account for the largest proportion in multi-layered cloud systems. Altitude of cloud layer also changes with territories apparently, besides obvious seasonal variation. Altitudes of cloud top and cloud base for single-layered clouds, two-layered clouds, three-layered clouds shows:top layer of three-layered cloud is the highest layer, the second is top layer of two-layered cloud. Mean thickness of cloud layer and the separation distance between two consecutive layers in a multi-layered cloud system have not remarkable seasonal and regional variation. The averaged thickness of cloud layer is ranging between 0.9km and 2km. Moreover, the distance between two consecutive layers is 0.35 km that accounts for almost 50%, the 15% probability is for the distance about 1.45 km. But the probability decreases with the distance between two consecutive layers increasing.The instantaneous cloud radiative effect （CRE） induced by multi-layered （ML） and single-layer （SL） clouds is also estimated by analyzing data collected by the CALIPSO, CloudSat, and CERES missions from March 2007 through February 2008. The CRE differences between ML and SL clouds at the top of the atmosphere （TOA） and at the surface were quantified. The zonal mean shortwave （SW） CRE differences between the ML and SL clouds at the TOA and surface were positive at most latitudes, peaking at 120 Wm-2 in the tropics and dropping to-30 Wm-2 at higher latitudes. This indicated that the ML clouds usually reflected less sunlight at the TOA and transmitted more to the surface than the SL clouds, due to their higher cloud top heights. The zonal mean longwave （LW） CRE differences between ML and SL clouds at the TOA and surface were relatively small, ranging from-30 to 30 Wm-2. This showed that the ML clouds tended to cool the atmosphere in the tropics and warm it elsewhere when compared to SL clouds.In addition, a new method is developed based on CALIPSO level 1 attenuated backscatter profile data for deriving the mean extinction coefficient of water droplets close to cloud top. The method is applicable to low level （cloud top<2 km）, opaque water clouds in which the lidar signal is completely attenuated beyond about 100m of penetration into the cloud. The photo multiplier tubes （PMTs） of the 532 nm detectors （parallel and perpendicular polarizations） of the CALIOP both exhibit a non-ideal recovery of the lidar signal after striking a strongly backscattering target （such as water cloud or surface）. Therefore, the effects of any transient responses of CALIOP on the attenuated backscatter profile of the water cloud must first be removed in order to obtain a reliable （validated） attenuated backscatter profile. Then, the slope of the exponential decay of the validated water cloud attenuated backscatter profile, and the multiple scattering factor are used for deriving the mean extinction coefficient of low-level water cloud droplets close to cloud top. This novel method was evaluated and compared with the previous method which combined the cloud effective radius （3.7μm） reported by MODIS with the lidar depolarization ratios measured by CALIPSO to estimate the mean extinction coefficient. Statistical results show that the extinction coefficients derived by the new method based on CALIOP alone agree reasonably well with those obtained in the previous study using combined CALIOP and MODIS data. The mean absclure relative difference in extinction coefficient is about 13.4%. An important advantage of the new method is that it can be used to derive the extinction coefficient also during night time, and it is also applicable when multi-layered clouds are present. Overall, the stratocumulus dominated regions experience larger day-night differences which are all negative and seasonal. However, a contrary tendency consisted in the global mean values. The global mean cloud water extinction coefficients during different seasons range from 26 to 30 km-1, and the differences between day and night time are all positive and small （about 1-2km-1）. In addition, the global mean layer-integrated depolarization ratios of liquid water clouds during different seasons range from 0.2 to 0.23, and the differences between day and night also are small, about 0.01.