Uncertainties of ground-based microwave radiometer retrievals in zenith and off-zenith observations under snow conditions

摘要

This paper is to investigate the uncertainties of microwaveradiometer (MWR) retrievals in snow conditions and also explore thediscrepancies of MWR retrievals in zenith and off-zenith observations. TheMWR retrievals were averaged in a ±15 min period centered atsounding times of 00:00 and 12:00 UTC and compared with radiosondeobservations (RAOBs). In general, the MWR retrievals have a bettercorrelation with RAOB profiles in off-zenith observations than in zenithobservations, and the biases (MWR observations minus RAOBs) and root mean squareerrors (RMSEs) between MWR and RAOB are also clearly reduced in off-zenithobservations. The biases of temperature, relative humidity, and vapor densitydecrease from 4.6 K, 9 %, and 1.43 g m in zenith observations to−0.6 K, −2 %, and 0.10 g m in off-zenith observations, respectively.The discrepancies between MWR retrievals and RAOB profiles byaltitude present the same situation. Cases studies show that the impact ofsnow on accuracies of MWR retrievals is more serious in heavy snowfall thanin light snowfall, but off-zenith observation can mitigate the impact ofsnowfall. The MWR measurements become less accurate in snowfall mainlydue to the retrieval algorithm, which does not consider the effect of snow,and the accumulated snow on the top of the radome increases the signal noise ofMWR measurements. As the snowfall drops away by gravity on the sides of theradome, the off-zenith observations are more representative of theatmospheric conditions for RAOBs.