Retrieval of precipitable water vapor by the millimeter-wave imaging radiometer in the arctic region during FIRE-ACE

Millimeter-wave radiometric measurements obtained from the NASA ER-2 aircraft over the arctic region on May 20, 1998, were used to estimate precipitable water (PW) in the range/spl les/0.60 g/cm/sup 2/. The approach is a modified version of the recent work by J. Miao (1998), which utilized the radiometric measurements at 150, 183.3/spl plusmn/3, and 183.3/spl plusmn/7 GHz of the SSM/T-2 sensor to retrieve PW over the antarctic region. However, Miao has implicitly assumed a surface emissivity that is frequency independent over the 150-183 GHz range. This assumption turns out not to be a good one based on the airborne measurements described below and the errors introduced in the PW estimation were substantial in many cases. It is shown below that four-frequency radiometric measurements in the frequency range of 150-220 GHz provided a robust retrieval of PW, while allowing for a surface emissivity that varied linearly with frequency. The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations. The differences between the retrieved and calculated values are not more than /spl plusmn/0.02 g/cm/sup 2/, which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band nature of the 183.3 GHz measurements in the radiative transfer calculations for development of the retrieval algorithm. The PW values estimated from the algorithm developed from single side band, 183.3 GHz radiative transfer calculations could be in error by as much as /spl plusmn/0.10 g/cm/sup 2/. Finally, the effect of surface temperature variations is shown to introduce only a small error in the estimation of PW.