Monitoring middle and upper tropospheric humidity from satellite microwave sensors: Applications for tropical cyclone studies and climate monitoring

Mid-tropospheric humidity is widely recognized to be an important variable in determining the development and intensification of tropical cyclones. However a clear understanding of its role is limited, in part, by the lack of observations with sufficient coverage and accuracy to adequately quantify the concentrations of water vapor in the mid-troposphere. In-situ measurements offer limited coverage and global assimilation products of free tropospheric humidity are notoriously unreliable. Fortunately, the water vapor channels located at 183.31±1 GHz and 183.31±3 GHz are sensitive to upper and middle tropospheric relative humidity and much less prone to the clear-sky sampling bias than infrared sensors, making them a valuable source of information on free tropospheric water vapor.

To make 183 GHz water vapor channel measurements more suitable for both diagnostics studies of tropical cyclone development and long-term climate monitoring, we identify biases and spurious trends in the measurements and document necessary correction procedures. A limb correction method removes the scan angle dependence of the cloud-filtered radiances. Biases due to the difference in local observation time between satellites and spurious trends associated with satellite orbital drift are then diagnosed and adjusted using synthetic radiances from reanalysis products. These correction procedures significantly improve the consistency and quantitative agreement between microwave satellite records of upper and middle tropospheric water vapor. Applications of these products to the analysis of tropical cyclone development and long-term monitoring of the tropics will be presented at the conference.