Assessing the quality of humidity measurements from operational radiosonde sensors

Friday, 19 December 2014
Phillip A Arkin, Univ of MD-ESSIC, College Park, MD, United States, Isaac Moradi, Cooperative Institute for Climate and Satellites University of Maryland, College Park, MD, United States and Ralph R Ferraro, Univ Maryland-ESSIC/CICS and NOAA/NESDIS, College Park, MD, United States
The quality of humidity measurements from global operational radiosonde sensors in upper, middle, and lower troposphere for the period 2000–2011 were investigated using satellite observations from three microwave water vapor channels. We first simulated the satellite brightness temperatures using radiosonde profiles and a radiative transfer model then carefully collocated simulated and observed satellite brightness temperatures. We partitioned the data into daytime and nighttime to evaluate the daytime radiation bias then separately evaluated the statistics for each sensor type. The error characteristics among different radiosondes vary significantly, which largely reflects the differences in sensor type. These differences are more evident in the mid-upper troposphere than in the lower troposphere, mainly because some of the sensors stop responding to tropospheric humidity somewhere in the upper or even in the middle troposphere.
In the upper troposphere, most sensors have a dry bias but Russian sensors and a few other sensors including GZZ2, VZB2, and RS80H have a wet bias. In middle troposphere, Russian sensors still have a wet bias but all other sensors have a dry bias. All sensors, including Russian sensors, have a dry bias in lower troposphere. The systematic and random errors generally decrease from upper to lower troposphere. Sensors from China, India, Russia, and the U.S. have a large random error in upper troposphere, which indicates that these sensors are not suitable for upper tropospheric studies as they fail to respond to humidity changes in the upper and even middle troposphere. Overall, Vaisala sensors perform better than other sensors throughout the troposphere exhibiting the smallest systematic and random errors. Because of the large differences between different radiosonde humidity sensors, it is important for long-term trend studies to only use data measured using a single type of sensor at any given station. If multiple sensor types are used then it is necessary to consider the bias between sensor types and its possible dependence on humidity and temperature.