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

Abstract

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.

Comparative Study of Cloud Liquid Water and Rain Liquid Water Obtained From Microwave Radiometer and Micro Rain Radar Observations Over Central China During the Monsoon

“…Combined with ambient temperature, pressure, and relative humidity from the meteorological sensors attached to MWR, atmospheric profiles (temperature, vapor density, and relative humidity) are retrieved from these brightness temperatures. Moreover, an infrared radiation thermometer equipped on MWR measures sky infrared temperature at one zenith infrared (9.6–11.5 μm) channel and the cloud base height is obtained from the MWR‐retrieved temperature profile (Zhang et al, 2017). Atmospheric profiles derived from MWR have a temporal resolution of ∼3 min and vertical resolutions of 50 m from the surface to 0.5 km, 100 m from 0.5 to 2.0 km, and 250 m from 2.0 to 10.0 km.…”

“…Besides, the rain drops deposited on the radome will also affect the measurements of MWR (Battaglia et al, 2010; Cadeddu et al, 2017). The influence of the second factor can be decreased significantly using the off‐zenith observation method under precipitation and snow conditions (Xu et al, 2014; Zhang et al, 2017), although, unfortunately, the off‐zenith observation method is not employed on the MWR at XN. Considering that WH is within 100 km far away from XN and both zenith and off‐zenith observation methods are employed on the MWR of WH, the MWR observations of WH from June to July in 2018 are used to compare CLWC profiles obtained by the zenith and off‐zenith observation methods under different sky conditions.…”

A Review of Remote Sensing of Atmospheric Profiles and Cloud Properties by Ground-Based Microwave Radiometers in Central China

“…According to a case study under a rainy and snowy sky in Wuhan, the signal saturation phenomenon of brightness temperature at K and V bands in the zenith method can be effectively eliminated in the off-zenith method [35]. Moreover, Zhang et al (2017) [36] compared MWR retrievals in zenith and off-zenith methods with radiosondes during three snow cases in 2014 in Wuhan. The results show that MWR retrievals in the off-zenith method obviously agree better with radiosondes than those in the zenith method (Table 5).…”

“…Moreover, the comparison between MWR retrievals and radiosondes at each profile height shows that at most profile heights, the MWR retrieval accuracy is improved in the off-zenith method compared to that in the zenith method. Therefore, according to the studies of [22,35,36], the off-zenith method can improve MWR retrieval accuracy under precipitation and snowy skies in central China. The off-zenith method can also reduce the impact of snow on MWR retrieval accuracy.…”

“…Comparative results between radiosondes and MWR retrievals in zenith and off-zenith methods for all profile heights during three snow cases from 2014 in Wuhan[36].…”