Quantitative precipitation forecasting is currently limited by the paucity of observations on sufficiently fine temporal and spatial scales. In particular, convective storms have been observed to develop in regions of strong and rapidly evolving moisture gradients that vary spatially on sub-meso γ scales (2-5 km). Therefore, measurements of water vapor aloft with high time resolution and sufficient spatial resolution have the potential to improve forecast skill for the initiation of convective storms. Such measurements may be used for assimilation into and validation of numerical weather prediction (NWP) models.

Currently, water vapor density profiles are obtained in-situ using probes on radiosondes and remotely using lidars, GPS ground and satellite networks and a relatively small number of radiometers on-board satellites. In-situ radiosonde measurements have excellent vertical resolution but are severely limited in temporal and spatial coverage. In addition, the duration of each measurement is 45-60 minutes from the ground to the tropopause. Differential-absorption lidars measure water vapor with comparable resolution to that of radiosondes during only clear-sky conditions from a very limited number of sites. Tomographic inversion applied to ground-based measurements of GPS wet delay is expected to yield 0.5-1 km vertical resolution at 30-minute intervals with poor horizontal resolution, on the order of 50 km. The COSMIC satellite network in low earth orbit (LEO) provides measurements with 0.1-0.5 km vertical resolution on 30-minute intervals, but has poor horizontal resolution, on the order of 200 km. Radiometers in orbit provide either decent profiles with mesoscale horizontal resolution with long repeat times using microwave from LEO or 2 km vertical and 50 km horizontal resolution nearly continuously using infrared from GEO.

Measurements from a ground-based network of coordinated, scanning microwave radiometers are expected to provide 0.5-1 km resolution both vertically and horizontally with 15-minute temporal sampling. To this end, the Compact Microwave Radiometer for Humidity profiling (CMR-H) was developed at the Microwave Systems Laboratory at Colorado State University. The use of MMIC technology and unique packaging yields a light-weight, inexpensive and low-power consumption instrument that is highly suitable for deployment in a network of remote sensors.

In such a remote sensor network, multiple radiometers provide different perspectives on the same volume. The network design is based on an optimal hexagonal topology, in which each sensor “node” scans its “domain” using 10 elevation and 10 azimuthal angles within about 10 minutes, shorter than most convective time scales. For water vapor on the spatial scales of the retrieval, all looks within these 10 minutes are considered to be simultaneous. Finally, a linearized form of the radiative transfer equation is used with optimal estimation and Kalman filtering to provide an algebraic tomographic reconstruction of the 3-D water vapor field.

An Observation System Simulation Experiment (OSSE) was performed in which synthetic examples of retrievals using a network of radiometers were compared with results from the Weather Research Forecasting (WRF) model at a grid scale of 500 m. Results will be shown to predict the expected resolution and accuracy of water vapor retrievals from a microwave radiometer network. These are believed to be sufficient for validation of and assimilation into future generations of NWP models.