Retrieval of Surface-layer Refractivity using the CSU-CHILL Radar

The surface-layer refractivity, i.e., the refractive index of air near the earth´s surface, can be retrieved from radar using a technique developed by Frederic Fabry, et al. In warm weather, the equation for the index of refraction humidity dominates over temperature and pressure, which has a significant impact on the phase of propagating electromagnetic waves from a radar. Thus, the refractive index can be measured by observing the phase change between any two stationary ground targets along a radial from the radar at roughly the same ground level. This index, in turn, can be used to estimate the water vapor near the surface. With this data, the evolution of the near- surface boundary layer moisture field leading up to convective storm initiation and storm evolution can be detected to enhance quantitative precipitation forecasts. Stationary ground targets are those that return strong radar echoes and do not produce rapid phase changes under slowly varying humidity levels, unlike vegetation, for example, that adds a significant random component to the measured phase as it moves in the wind. The calibration stage, which determines the stationary targets and reference phase, is a critical step, currently requiring manual selection of scans, ideally under conditions of uniform humidity in the radar coverage space. This procedure was recently performed using the dual-polarized CSU-CHILL S-band radar to estimate the refractivity. Prior to this experiment, only single polarization had been used for this estimation.