| Clouds can change the radiation budget of the Earth-atmospheric system and so play a critical role in driving atmospheric circulation and regulating the Earth climate and the water cycle process. It is important to the research of climate and water resources to know the climatological characteristics and global variability of cloud properties and to be able to relate them to the radiation budget and the water cycle process. To quantitily study these issues depends on accurate retrieval of cloud parameters from satellite remote sensing data. Therefore, in this thesis the temporal and spatial characteristics of cloud parameters, the relation between clouds and climate and water cycle, the cloud radiative effects on the Earth-atmospheric system over northwestern China were studied, and also an experimental study of cloud retrieval using the multichannel remote sensing data of MODIS with the radiation transfer model and look-up-table method was conducted.The dataset used in this thesis is International Satellite Cloud Climatology Project (ISCCP) D2 which has the best continuity, longest time coverage and the most coherent calibration among the present international cloud climatic datasets. The most recent data of Clouds and the Earth's Radiant Energy System (CERES) Single Scanner Footprint (SSF) Aqua MODerate-resolution Imaging Spectroradiometer (MODIS) Edition IB data with high spectral and spatial resolution were also used in which broadband shortwave and longwave radiance measurements are matched to simultaneous retrievals of cloud properties from the MODIS and a new angular distributive model is employed. Thus this dataset provides the most accurate cloud property and radiative parameters than ever before.In this study the parameters of total cloud, low-, middle-, high-level and 15 different types of clouds were analyzed and compared with respect three climatic regions and four typical geo-topographic regions for the first time so that a better understanding of temporal and spatial distribution of cloud properties over northwestern China has been obtained, which has enriched climatological researchabout cloud properties in our country. The findings show that the regional average total cloud amount, optical thickness and water path are between 52.5%-58.3%, 2.6-6.6 and 44.9-77.6 g/m2, respectively. The cloud water resources at the Asian monsoon influence region with its fringe area are richest. The cloud amount of cirrus is the most among the 15 types that reaches about 20% over every climatic region;The water content of ice nimbostratus, ice stratus and deep convective cloud are the highest, which regional means of water path are between 400.8~437.9g/m2. The spatial distributive characteristics is that cloud amount of liquid altostratus, liquid nimbostratus, ice altostratus, ice nimbostratus, cirrus, cirrostratus and deep convective cloud have a similar pattern to the total cloud amount, middle-level cloud amount, optical thickness and water path for total cloud. Their highest-level values are over Tianshan Mountains, Kunlun Mountains, Qilian Mountains, and southern Shanxi and/or southern Gansu, while the lowest-level values are located over Tarim Basin, the western desert of Inner Mongolia and the northwestern part of Loess Plateau. The amounts of total cloud and most specific types of clouds in summer and spring are significantly more than those in autumn and winter. During March~July the mean amounts of total cloud for the westerly wind climatic region and the plateau climatic region are at the highest level for all year while this time coverage expands to August and September for the Asian monsoon influence region with its fringe area. The optical thickness and water path for total cloud both show an increasing tendency over the 15 years while the cloud amount does not rise.The results above indicate that the variation of cloud properties is highly dependent on topography, and also geography. Therefore, four specific regions associated with different topographic and geographic conditions were defined to characterize the seasonal and annual variations in cloud properties over northwestern China. The regions are the Asia monsoon influence region with its fringe region, the Tianshan Mountains, the Qilian Mountains and the Taklimakan Desert. The results show that, at these four regions, the maxima of cloud amount and optical thickness occur in summer and the minima occur in winter. Generally, cloud particle size in summer is the smallest and the greatest in winter. The variation of cloud propertymeasurements with the different regions is quite large. The largest difference of yearly mean amounts for higher layer cloud occurs between the Qilian Mountains and Taklimakan Desert, which is 16.4%. The differences of yearly mean amounts for total cloud and lower layer cloud between the Asia monsoon influence region with its fringe region and the Taklimakan Desert reach 27.6% and 19.5% respectively. The differences of yearly mean optical thickness for higher layer and lower layer cloud between these two places is 8.7 and 7.5 respectively. The measurements of cloud amounts and optical thickness over the two mountainous regions are between the values over those two regions above. The seasonal means of water cloud particle radius over the four regions are between 7.5-15.S\xm and of ice cloud particle effective diameter are between 33.3~51.6um. The particle size is the smallest over Taklimakan Desert and is close to the measurement over the Asia monsoon influence region with its fringe region. The largest is over either of the two mountains.In this study, the relations of the parameters such as cloud amount and water path for different cloud types to the precipitation and temperature were discussed with respect to the three different climatic regions. The results indicate that, in the three regions, the amounts of low-middle-level clouds increase and the amounts of high-level cloud reduce with the increasing of the temperature. But the variation ranges are different. The low-middle-level cloud amounts increase about 0.4%~ 1.8%/°C and the high-level cloud amounts reduced about 1.3%~6.0% /°C. A new viewpoint is presented here concerning the phenomenon of increasing warmth and humidity over northwestern China since 1980's: during the nearly 20 years, the high-level cloud amounts significantly decreased while low-middle-level cloud amounts significantly increased at all of the three climatic regions. A process of the interaction between cloud and precipitation and temperature probably exists in northwestern China that the increase of surface temperature enhances evaporation and speeds the circulation of humidity, thus causing the low-middle-level cloud amount and precipitation to increase and the high-level cloud amount to decrease, which would reduce the surface temperature or decelerate the rising of it as a result. But this result is preliminary and further research using more data is needed.In this thesis, also for the first time, the seasonal and annual variation of cloud radiative forcing with respect to the four typical regions in northwestern China was studied and the influences of cloud parameters on radiative forcing were discussed that has improved the understanding of local cloud radiative forcing characteristics. The results show that clouds have a significant impact on the northwestern China's radiation budget. For all seasons clouds have a net cooling effect over all four regions. The cloud radiative forcing varied remarkably not only with seasonal change but also with change of topographic and geographic conditions. In general, the greatest magnitude of net cloud radiative forcing occurs in summer which reaches -158.2w/m2 over the Asia monsoon influence region with its fringe region, and -120.7 w/m , -115.2 w/m2 and -29.0 w/m2 respectively over Qilian, Tianshan mountains and Taklimakan desert;the smallest occurs in winter which reaches -75.2w/m2 over the Asia monsoon influence region with its fringe region, -19.5 w/m2 over Qilian and -2.7 w/m2 over Tianshan but the smallest occurs in autumn over Taklimakan Desert which is -7.8 w/m2. The results also reveal that cloud radiative forcing varies rather notably with the different regions. For instance, for the same season the largest difference of net cloud radiative forcing is 129w/m2 that occurs between the monsoon influence region with its fringe region and the Taklimakan Desert in summer. The yearly means of shortwave, longwave and net cloud radiative effects on the monsoon influence region all are the strongest of the four egions which reach -171.2 w/m2, 48.9 w/m2 and -122.3 w/m2;and are the smallest over the Taklimakan Desert which is -54.7 w/m2,ry ry37.3 w/m and-17.4 w/m respectively.In the last part of this thesis, based on the research of the advanced cloud retrieval methods overseas, the parameters such as cloud optical thickness, cloud top temperature, water path, water particle radius and ice particle effective diameter are retrieved through Look-up-table method using 0.65-, 3.75-, 11.0- and 12.0-um radiances data observed by MODIS on Aqua. Referring to the method of Minnis et al.(l993a, 1998), firstly the database of the cloud microphysical models and the corresponding optical properties was established to provide the approximation of cloud phase, shape and particle size distribution in real atmosphere;then the effects ofcloud parameters on absorption, reflection, transmission and emmitation of radiation were simulated using the Adding-doubling radiation thansfer model and the look-up-table was built to reflect the dependence of radiation field on the cloud parameter;when using the look-up-table, the simulated signals were compared with the signals observed by satellite to find the closest match between them, thus the cloud parameters were retrieved from the cloud models with their optical properties which corresponded to the simulated multichannal signals. Three retrieval experiments were conducted which focused on the Qilian Mountains, Tianshan Mountains and Taklimakan Desert. The results were compared with CERES SSF data and the errors were estimated by taking the SSF data as the true value. Among the three cases, generally the errors of all parameters for Qilian case are the smallest. The average absolute errors for all pixels of every parameter are all below 0.1. The relative errors of all parameters are very close to zero except that of ice particle effective diameter which is 1%;The average absolute errors of Tianshan case are between -0.42 ~ 0.20 and the relative errors are between -0. 77%~0. 84%, which. are both small. The errors of Taklimakan case are the greatest, the average absolute errors are between -2.46 ~ 2.41. The relative errors are between -5%~ 1% except the optical depth for ice cloud is 8%. In some words, the experimental results are very close to CERES SSF data that shows the four-channel-combined method of retrieval proposed by this thesis is feasible. This achievement has not yet been found in domestic research community and has provided a helpful experience for the retrieval of cloud properties under the bright background that has laid a foundation for the cloud property retrieval from FY satellites in the future.
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