Passive Microwave Split-Step Retrievals of the Vertical Structure of Condensed Water and Water Vapor in Deep Convective Clouds

Today’s constellation of passive microwave radiometers in low-Earth orbit includes two broad categories of instruments: the “opaque-channel” sounders with channels near different gas absorption lines (making them particularly useful to estimate quantities associated with these gases, namely temperature and relative humidity) and the “window-channel” imagers with channels away from the absorption lines (so that their sensitivity to the amount of condensed water can be more easily quantified). The millimeter-wavelength channels of today’s radiometers are mostly near the oxygen absorption line at 118.75 GHz or the water vapor absorption line at 183.31 GHz. While this suggests that the absorption would limit their utility over clouds, the reverse is actually true. The central question is: to what extent can the high-frequency radiances be used to estimate the vertical structure, however coarsely, over the convective clouds; and conversely can one predict the mm-wavelength radiances from simulated clouds.

Analyses of satellite data, and of cloud-resolving model simulations, have laid the foundation for the estimation of the vertical structure information from the radiometers in the Global Precipitation Measurement (GPM) mission constellation including the GPM Microwave Imager’s 6 higher-frequency channels. Indeed,

1) the high-frequency channels are actually at least as sensitive to the condensed mass in the upper levels as to precipitation at the surface (witness the cold temperatures measured over the deepest portions of the clouds in the figure to the right);

2) the analyses demonstrate the ability to estimate cloud-top heights (or rather condensed-water heights) for different condensed-mass thresholds, e.g. three thresholds which would produce three heights, as well as the first two vertical principal components (PCs) of condensed mass, and the total Condensed Water Path;

3) while these variables are mutually correlated, it takes at least 3 PCs of brightness temperature to capture 99% of the variability for any given radiometer, so there are at least three independent pieces of information in the radiances from any one beam – these different pieces of information are useful for different interpretations of the condensed water within the column sensed by the beam.

The approach starts with a first step that exploits the empirical relation between the average structure of condensed water and the radiances, followed by a second step based on cloud-resolving simulations to capture the signature of the water vapor in the cloud. The results over organized storms illustrate the effectiveness of the approach.