Articles | Volume 10, issue 1
Atmos. Meas. Tech., 10, 155–165, 2017
Atmos. Meas. Tech., 10, 155–165, 2017

Research article 11 Jan 2017

Research article | 11 Jan 2017

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

Wengang Zhang1, Guirong Xu1, Yuanyuan Liu2, Guopao Yan3, Dejun Li4, and Shengbo Wang5 Wengang Zhang et al.
  • 1Hubei Key Laboratory for Heavy Rain Monitoring and Warning Research, Institute of Heavy Rain, China Meteorological Administration, Wuhan, China
  • 2Hubei Meteorological Information and Technological Support Center, Wuhan, China
  • 3Wuhan Meteorological Bureau of Hubei Province, Wuhan, China
  • 4Hubei Meteorological Service Center, Wuhan, China
  • 5Mewbourne School of Petroleum & Geological Engineering, University of Oklahoma, Norman, USA

Abstract. 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.

Short summary
A comparison between a microwave radiometer and radiosonde is carried out, and performances of zenith and off-zenith observations during snowfall are shown. In off-zenith observations, the effect of snow is obviously mitigated, and the deviation between microwave radiometer and radiosonde is small. With the aid of off-zenith observation, reliable thermodynamic atmospheric profiles can be collected, and those will be useful for the analysis and forecasting of severe convective weather.