Thursday, 12 November 2009
Clouds are considered one of the largest uncertainties in global climate models. The question is not just how many clouds are in the sky, but what is their composition and location that determine what impact clouds have on climate. It has been reported that climate models can vary by a factor of three, depending on how clouds are represented in the model. The optical thickness, altitude, phase, and composition of the cloud all play a crucial role in determining its effectiveness at reflecting shortwave or trapping longwave radiation. This research aims to look at the identification of subvisual cirrus clouds (SVC) using remote sensing techniques. These ice clouds tend to transmit and forward scatter the incoming solar radiation. They also act to absorb and therefore reduce, the infrared radiation, which would otherwise exit our atmosphere. These processes play an important role in our weather and radiative climate as their variations modify the radiation balance of our planet. The objective of this study is to validate a technique for detecting the infrared radiative properties of SVC from Earth orbiting satellites. This study utilizes lidar, multi-spectral visible and infrared, and hyperspectral infrared measurements from the CALIPSO, MODIS and AIRS satellite instruments concurrently. An analysis of the radiative heating impact of SVC was determined by comparing clear and cloudy fields of view through radiative transfer calculations for typical atmospheric conditions to help identify and quantify the cloud optical properties. Initial findings indicate that the majority of SVC and thin cirrus clouds are located in the tropics with low effective emissivity and optical depth values while lower level clouds are observed more frequently with higher emissivity values.
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