Since the first visible satellite image revealed the face of the Earth, the usefulness of satellite observations in cloud studies has been obvious. However, visible and then infrared measurements only provide information on height and reflectance of cloud tops. How to relate the cloud tops to the surface precipitation is a big challenge. For example, in the left figure, in some regions, cold infrared brightness temperature are related very well to the surface rain indicated by radar echoes. However, there are many cases for which no surface rain is associated with cold clouds. By comparing the TRMM VIRS and PR observations, techniques of retrieving surface rainfall from infrared observations may be validated and adjusted.
Clouds play an vital role in the global radiation budget. It is difficult but also important to quantify the role of the cirrus clouds. By comparing the infrared observations from VIRS to the precipitation observations from the PR, we can improve our understanding about the generation of cirrus clouds from the deep convection in the tropics.
Here we summarized the fractional occurrences of cold infrared brightness temperature compared with the occurrence of precipitation. The seasonal and diurnal variations of occurrence of clouds with cold infrared brightness temperatures are generated using a decade (1998-2007) of TRMM PR data. These occurrences can be easily derived from University of Utah TRMM database Level-3 products (detail see data description
Fractional occurrence of cold VIRS infrared (10.8 micron) TB
Importance of precipitation estimates from infrared brightness temperatures Even though there is good ground precipitation radar coverage over a few parts of the world, most of the world lacks these facilities.? Even current spaceborne microwave observations cannot provide real time observations globally because all of them are onboard polar orbiting satellites. However, infrared and visible radiometers on board geostationary satellites can provide near real time observations with global coverage. Then the rainfall retrieved from infrared, trained with the help of passive microwave algorithms, can be used to fill in the gaps when other observations are not available. TRMM 3B42 and GPCP (Huffman et al 2001) are outstanding examples.
Cloud observations from other techniques There are many other ways to detect clouds from space including cloud radar on CloudSat for most clouds and lidar on Calipso for thin clouds and aerosols. These instruments provide detailed information of clouds with very high vertical resolution. There is great potential for many valuable scientific studies by combining their observations with those from instruments on other A-Train satellites, including ?MODIS, MLS, AIRS etc. However, all the above measurements are onboard A-Train satellites that obtain data only near 1:30 AM/PM local time globally. It is known that there are strong diurnal variations in radiation and clouds, varying with location and meteorological regime. So the representativeness of these observations to? the remainder of the 24 hours is an important question. Using TRMM observed diurnal cycles of precipitation and clouds, this database may help to answer this question (Liu and Zipser 2008, Liu et al 2008).