Retrieval of 3-D Water Vapor Field Using a Network of Scanning Compact Microwave Radiometers

Quantitative precipitation forecasting is currently limited by the paucity of observations of thermodynamic variables in the troposphere, including water vapor. Specifically, measurements of 3-D water vapor fields are needed at sub-meso-gamma scales in pre- storm conditions. This can be achieved using a network of remote sensors to retrieve the water vapor field with high spatial and temporal resolution. Such measurements may be used for assimilation into and validation of numerical weather prediction (NWP) models. Conventional measurements of water vapor density profiles are obtained using in-situ probes on-board weather balloons, including radiosondes. Remote sensing techniques to retrieve moisture profiles include ground-based networks receiving Global Navigation Satellite Systems (GNSS) signals, including GPS, and GPS receivers aboard the COSMIC satellite constellation for atmospheric occultation. These methods provide measurements with high vertical resolution but with coarse horizontal resolution. Differential Absorption Lidars (DIAL) can retrieve water vapor with comparable resolution to that of radiosonde observations. However, these lidars are expensive, and their operation is limited to clear-sky conditions due to the high opacity of clouds at optical wavelengths. Inversion of brightness temperatures measured by upward- looking, ground-based microwave radiometers allows the estimation of vertical profiles with high temporal resolution in both clear and cloudy conditions. However, assimilation of retrieved 3-D water vapor fields with improved spatial coverage into NWP models in pre-storm conditions has the potential for substantial impact on numerical weather prediction of convective storm activity. Measurements using a network of multi-frequency microwave radiometers can provide the necessary information to retrieve the 3-D distribution of water vapor in the troposphere.