ESTIMATION OF WATER CLOUD PROPERTIES FROM SATELLITE MICROWAVE, INFRARED AND VISIBLE MEASUREMENTS IN OCEANIC ENVIRONMENTS 1 - MICROWAVE BRIGHTNESS TEMPERATURE SIMULATIONS

Theoretical calculations are used to examine the spectral characterist ics of SSM/I (special sensor microwave/imager) brightness temperature (Tb) values for non-precipitating clouds over oceans. It was found tha t liquid water path (LWP) and the cloud water temperature (Tw) could b e derived simultaneously with a technique using the SSM/I 37-GHz and 8 5-GHz brightness temperatures. Uncertainties in column water vapor (CW V) are the most important error sources in the estimation of LWP and T w, while ice particles smaller than 100 mu m in nonprecipitating cloud s have a very weak effect (< 1 K) on the Tb values at the relevant SSM /I frequencies. When all SSM/I instrument noise and error sources asso ciated with sea surface temperature, wind speed, and CWV are considere d, the biases in LWP from current microwave methods are very small (le ss than or equal to 0.01 mm) and the standard deviations vary from 0.0 2 to 0.04 mm. The Tw bias and standard deviation decrease with increas ing LWP from about 6 and 8 K, respectively, for clouds with low LWP to < 1 K for LWP > 0.4 mm. For most marine stratocumulus clouds (LWP sim ilar to 0.1 to 0.2 mm) the Tw bias and standard deviation are about 2 and 4 K, respectively, resulting in cloud height errors of similar to 1 to 2 km. The method should yield an improvement in the accuracy of r etrieved LWP because it more closely approximates cloud temperature th an previous techniques. To use the radiative transfer results, it is n ecessary to normalize or calibrate them to the observations. This rela tive calibration using 22-GHz brightness temperatures reveals differen ces of 2.86 K and -1.93 K for the 37-GHz horizontal and 85-GHz vertica l channels, respectively, between the SSM/I observations and the model simulations. In multilayered cloud conditions, this new microwave ana lysis method, when combined with infrared data should make it possible to determine cloud temperature for an upperlevel ice cloud from the i nfrared brightness temperatures while simultaneously deriving Tw and L WP for the lower liquid water cloud with the microwave data.