<title>Remote sensing of clouds and water vapor by microwave limb sounding</title>

Abstract

Passive radiometers at 100-700 GHz offer great potential for remote sensing of cirrus clouds and upper-tropospheric water vapor from space because radiation at these frequencies can penetrate and interact with cirrus clouds without being cut off at/near cloud surface. This paper focuses mostly on satellite microwave observations from UARS MLS at 186.5 and 203.2 GHz. Advantages of limb-viewing geometry include better vertical resolution and less contamination of Earth's surface conditions since the limb background radiance depends only on pointing and water vapor abundance. MLS radiance measurements show that water vapor and cloud signals are separable with frequencies of different sensitivities to water vapor. The cloud-induced radiances can be used to directly retrieve ice water content at 14- 18km altitudes if the ice particles are smaller than 100 microns. Model simulations at these frequencies are also presented for various cloud types and heights. With the modified MLS radiative transfer mode, we are able to simulate limb and nadir radiances at all frequencies between 10 and 1000GHz for both cloudy and clear atmospheres. Our sensitivity studies show that brightness temperature depression at the lowest MLS tangent height can be used to infer cloud height and ice content in the upper troposphere. With more channels near 122, 240, and 640 GHz, we find that the chance of separating cirrus cloud and water vapor is greatly enhanced, and these radiometers are to be flown as the future MLS on board NASA CHEM-1 spacecraft.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.