The Influences Of Dust Aerosol On Cloud Properities And Cloud Raditative Forcing Over Arid And Semi-arid Region Of Northwest China

The climate system is an extreme complex multi-circles system. There are lots of factors affecting the climate change. The radiative forcings by aerosols and clouds are recognized as the two important contributors for climate change, due to the complicated cloud-aerosol interaction and the lack of data linked to their physical optical characteristics. The means of satellite remote sensing can make up the insufficiency of observations. However, quantitive study of these issues depends on accurate retrieval of cloud parameters from remote sensing data of satellite. " Clouds and the Earth's Radiant Energy System (CERES)" , one of the important part of the Earth observation system (EOS), provides the surface and the top radiances of atmosphere, reflectivity and instantaneous flux, as well as the clear sky and cloud parameters in satellite scanning scope. The most recent data of Clouds and the Earth's Radiant Energy System (CERES) Single Scanner Footprint (SSF) Terra Moderate-resolution Imaging Spectroradiometer (MODIS) Edition 2B dataset with high spectral and spatial resolution were used. This dataset provides the most accurate cloud property and radiative parameters, due to a new angular distribution model. The cloud properties parameters, such as cloud amounts, water cloud particle effectiveradius (r_e), ice cloud particle effective diameter (D_e), optical thickness (Ï„), cloud water path(WP) and so on, are retrieved by using this dataset over arid and semi-arid region of Northwest China in 2001.Firstly, cloud property parameters were compared among months of 2001. Then the smallest one chosen and compared with the one obtained in the Yangtze River delta area at the same time. Three cases were analysed in the conditiond of monthly, dusty days, dust-free days in April 2001. Seasonal variation of cloud radiative forcing over Lanzhou region was researched with the radiative transfer modle Streamer, as well as the temperature changes caused by cloud radiative forcing. The main purpose of this work is to accumulate some experience and conduct some exploration for further study of the cloud-aerosol interaction and the impact of the climate system. The main results are as follows:1.The results of comparation of the cloud properties indicate that: 1) The values of cloud properties parameters, including cloud particle size, water path and optical thickness in April 2001, are relatively small in all of the year over arid and semi-arid region of Northwest China. 2) The values of cloud property parameters (cloud particle size, water path) over arid and semi-arid region of Northwest China are smaller than that over Yangtze River delta area in April 2001. 3) In April, the minimum cloud properties parameters (cloud particle size, water path and optical thickness) are in dusty days, and the maximum ones are in dust-free days, the median ones are in other days else. Conversely, the minimum cloud efficient temperature is in dust-free days, and the maximum is in dusty days. Thus the dust aerosols may alter the cloud properties by decreasing the cloud size, water path and optical thickness, as well as increasing the cloud efficient temperature.2. The cloud radiative forcing was calculated with Streamer at each altitude in atmosphere in April. The weakest cooling effect is in dusty days, and its value isbetween -115^W/m2 and -137^W/m2 The monthly average cooling effect followed, with the value from -115^W/m2 to-137^W/m2 . The strongest one is in dust-free days, and the value is between -177^W/m2 and -193^W/m2. Threfore the dust aerosols reduce the radiative forcing of clouds and restrain the cooling effect of cloud.3. The cloud radiative forcing in each season were modeled with the mid-latitude winter and summer standard profile. The net cloud radiative forcings are all negative in seasons. The maximum appeared in summer, followed by spring, autumn again. The minimum is in winter. Such change is consistent with the change of cloud amount. The heat rate and cool rate of radiation forcing, due to cloud, the change of shortwave heating rate and longwave cooling rate in summer, are greater than that in winter. That is because the water content in summer is greater than that in winter.