Trade-off between vertical resolution and accuracy in water vapor retrievals from ground-based microwave brightness temperature measurements

Abstract

Thermodynamic properties of the troposphere, particularly water vapor content and temperature, change in response to physical mechanisms, including frictional drag, evaporation, transpiration, heat transfer, pollutant emission and flow modification due to terrain. The planetary boundary layer (PBL) is characterized by a greater rate of change in its thermodynamic state than higher tropospheric altitudes. Such changes in the PBL typically occur on time scales of less than one hour; whereas the upper troposphere exhibits much longer time constants. Large horizontal gradients in vertical wind speed and steep vertical gradients in water vapor and temperature in the PBL result in high-impact weather, including severe thunderstorms. Observation of these gradients in the PBL with improved vertical resolution is important for improvement of weather prediction. Additionally high vertical resolution and accuracy of measured thermodynamic profiles, especially water vapor and temperature, are important for initialization of numerical weather prediction models. Satellite remote sensing in the visible, infrared and microwave bands provides qualitative and quantitative measurements of many atmospheric properties, including cloud cover, precipitation, liquid water content and precipitable water vapor in the atmosphere above the PBL. However, its ability to characterize thermodynamic properties of the PBL is limited by the confounding factors of ground emission in microwave channels and of cloud cover in visible and IR channels, as well as limitations in the vertical resolution of the remote sensing instruments onboard the satellite. Ground-based microwave radiometers are routinely used to estimate thermodynamic profiles, but the accuracy and resolution of vertical profiles may be improperly estimated. Here a new technique has been used to improve the vertical resolution of retrieved water vapor density profiles, based on the design of the Compact Microwave Radiometer for Humidity Profiling (CMR-H) [1]. The CMR-H operates at four frequencies near the weak water vapor absorption line, namely 22.12, 22.67, 23.25, and 24.5 GHz.