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 aircra ft over the arctic region on May 20, 1998, were used to estimate precipitab le water (PW) in the range less than or equal to 0.60 g/cm(2). The approach is a modified version of the recent work by Miao [1], which utilized the r adiometric measurements at 150, 183.3 +/- 3, and 183.3 +/- 7 GHz of the SSM /T-2 sensor to retrieve PW over the antarctic region. However, Miao has imp licitly 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 pr ovided a robust retrieval of PW, while allowing for a surface emissivity th at varied linearly with frequency, The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations, Th e differences between the retrieved and calculated values are not more than +/-0.02 g/cm(2), which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band natu re of the 183.3 GHz measurements in the raditive transfer calculations for development of the retrieval algorithm, The PW values estimated from the al gorithm developed from single side band, 183.3 GHz radiative transfer calcu lations could be in error by as much as +/-0.10 g/cm(2). Finally, the effec t of surface temperature variations is shown to introduce only a small erro r in the estimation of PW.