Uncertainties of ground-based microwave radiometer retrievals in zenith and off-zenith observations under snow conditions
2016
This paper is to investigate the uncertainties of microwave
radiometer (MWR) retrievals in snow conditions and also explore the
discrepancies of MWR retrievals in zenith and off-zenith observations. The
MWR retrievals were averaged in a ±15 min period centered at
sounding times of 00:00 and 12:00 UTC and compared with radiosonde
observations (RAOBs). In general, the MWR retrievals have a better
correlation with RAOB profiles in off-zenith observations than in zenith
observations, and the biases (MWR observations minus RAOBs) and root mean square
errors (RMSEs) between MWR and RAOB are also clearly reduced in off-zenith
observations. The biases of temperature, relative humidity, and vapor density
decrease from 4.6 K, 9 %, and 1.43 g m−3 in zenith observations to
−0.6 K, −2 %, and 0.10 g m−3 in off-zenith observations, respectively.
The discrepancies between MWR retrievals and RAOB profiles by
altitude present the same situation. Cases studies show that the impact of
snow on accuracies of MWR retrievals is more serious in heavy snowfall than
in light snowfall, but off-zenith observation can mitigate the impact of
snowfall. The MWR measurements become less accurate in snowfall mainly
due 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 of
MWR measurements. As the snowfall drops away by gravity on the sides of the
radome, the off-zenith observations are more representative of the
atmospheric conditions for RAOBs.
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