A study of tropical cirrus: Multisensor algorithm development for space-based observation and the role of cirrus in regulating the humidity of the tropical upper troposphere

Cirrus clouds, through their radiative properties which are determined by their microphysical properties and macroscopic structure, have a strong radiative effect in the climate system. A suite of algorithms prepared for the A-Train are developed to derive cirrus cloud properties from observational data. Algorithms convert the data streams from the individual multiple remote sensors into layer-averaged cirrus bulk microphysical properties. Due to the wide range of cirrus properties that can be expected in the global upper troposphere, the algorithms are capable of treating cirrus layers that range from very tenuous and below the detection threshold of cloud radars, to layers that can cause significant lidar attenuation. Two field cases from the CRYSTAL-FACE campaign are used to illustrate the requirements for an algorithm suite and multiple remote sensors and to demonstrate the implementation of the algorithms. The algorithms are also compared to validated ground-based retrievals.; Water vapor is the principal contributor to the earth's greenhouse effect due to the dominance of infrared absorption. Motivated by the coverage difference between cirrus clouds and convection systems, the relationship between upper-tropospheric humidity (UTH) and cirrus is explored to explain the role of cirrus to moisten the upper troposphere in the tropical region. Using satellite observations, we show that UTH is highly correlated with cirrus-cloud coverage, especially in the regions without frequent and widespread active convection. The intra annual variations of the high cloud coverage and the UTH averaged between 5°S and 5°N agree well with the MJO structure, which indicates that the UTH must be primarily controlled by the large-scale dynamics. The UTH tendency change with the presence of cirrus is explored from a Lagrangian perspective. The results show that thicker cirrus is associated with higher moistening rates than thin cirrus and clear-sky conditions, and the periods of cirrus decay are related to enhanced moistening of the upper troposphere compared with clear-sky conditions. However, the large scatter and the large range of humidities indicate that cirrus ice water content (IWC) alone can not explain the variability of tropical moisture. So, the presence of cirrus clouds perturbs the UTH in relatively small scale. It is also shown by a model simulation that cirrus clouds, through their radiative effects, cause redistribution of the upper-tropospheric water vapor resulting in a moistening of the cloudy columns.