Investigation of passive atmospheric sounding using millimeter and submillimeter wavelength channels

SUMMARY OF ACTIVITIESThe goal of this investigation is to develop satellite-based observational techniques for high-resolution imaging of precipitation and sounding of atmospheric cloud ice and water vapor usingpassive microwave radiometers operating in the frequency range from 89 to approximately 600 GHz.This goal is being achieved by radiative transfer modeling at millimeter-wave and submillimeter-wave frequencies and by the development and operation of an airborne Millimeter-wave ImagingRadiometer (MIR) with channels at 89, 150, 183,220, and 325 GHz [Racette et al., 1996]. To date,unique millimeter- and submillimeter-wave imagery of the atmosphere has been obtained using theNASA Millimeter-wave Imaging Radiometer (MIR). The MIR was developed jointly byNASA/GSFC and Georgia Tech, and has channels at 89, 150, 183+1, +3, +7, 220, and 325+1, _+3,•+9 GHz. These channels sample the radiation field at several of the most important MMW andSMMW tropospheric absorption lines and windows. The MIR imagery over convective stormcellshas been used to illustrate the potentially useful cloud and water vapor sensing and stormcellmapping capabilities of SMMW channels [Gasiewski et al., 1994]. The radiometric data has alsobeen used to analyze radiative transfer model discrepancies caused by water vapor errors inradiosondes [Jackson and Gasiewski, 1995]. A synopsis of activities undertaken during the periodfrom July 1, 1994 to June 30, 1996 follows.1. MIR Data Processing. Nine-channel radiometric imagery has been obtained using the MIR onthe NASA ER-2 during the Convection and Moisture Experiment (CAMEX September-October,1993 and CAMEX2, August-September, 1995), the Tropical Ocean Global Atmosphere/CoupledOcean-Atmosphere Response Experiment (TOGA/COARE, January-March 1993), and relatedaircraft deployments. A key task in this investigation has been the optimal calibration, archiving,and interpretation of the multichannel imagery. Progress has been made in the first and last of thesetasks (calibration and interpretation). Data archiving is currently underway, and has been delayedpending final calibration.Calibration of radiometric data from instruments such as the MIR has been most commonlyaccomplished using a linear infinite impulse response (IlR) filter operating on single-scan gain andoffset estimates. While this method is quick it does not properly filter the large spurious gain andoffset jumps and spikes that can impair the interpretation of the radiometric data. Moreover, the timeconstant and order of the IIR filter are often chosen by qualitative guess. In order to account fornonstationary features such as jumps and spikes and to provide the optimal form for the gain andoffset calibration filter, a nonlinear multidimensional Wiener filter was developed for radiometercalibration [Sharpe et al., 1995]. The filter first searches for nonstationary jumps and spikes in theraw (single-scan) gain and offset data, then removes them. Jump heights are estimated and storedin a table; spikes are replaced by a local moving average. The resulting gain and offset streamconsists of stationary data. A joint gain/offset (2-dimensional) Wiener filter is subsequentlyconstructed using autocorrelation estimates of the gain and offset processes. After filtering, the gainand offset jumps are replaced at their original locations.The performance of the Wiener filter can be judged by analyzing the variance of MIR clear-airbrightness temperature fluctuations for calibration both with the Wiener filter and with a standardIIR filter (Figure 1). In the clear-air case the brightness fluctuations are primarily caused by2