Ground-Based Microwave Radiometric Observations of Precipitable Water Vapor: A Comparison with Ground Truth from Two Radiosonde Observing Systems

Abstract

Dual-channel microwave radiometric measurements of precipitable water vapor are compared with values determined from two types of radiosondes. The first type is used in conventional soundings taken by the National Weather Service. The second is used by the CLASS system, as operated by the National Center for Atmospheric Research. The standard deviations of the two comparisons are nearly equal, being about 0.1 cm, but statistically significant biases occur between the radiometer and the radiosondes. A bias of 0.162 cm is present between radiometer and NWS values during the day and 0.075 cm during the night. The comparison shows that significant differences exist between the radiometer and the NWS moisture soundings when the relative humidity drops below 20 percent for pressures greater than 500 hPa. When this situation occurs, the NWS soundings contain a default dewpoint depression value of 30°C. After such data are removed from the comparisons, agreement between radiometer and NWS radiosonde data is excellent.