Better use of ground targets for radar refractivity retrievals

Radar refractivity retrievals can provide valuable information of near surface moisture. These retrievals are based on the fact that the phase of a stationary target changes with the refractivity index of air along the radar beam path. To obtain a good quality refractivity field at a given height above the terrain, it is necessary to know the vertical gradient of refractivity between the radar and the target. Determining the vertical gradient of refractivity requires reliable point-like ground targets.

First, we focus on point-like ground targets selection. Data is collected with high temporal resolution at successive low elevation scans in order to depict the phase and power patterns of ground targets along a few azimuths and elevations. These patterns are related to the target's properties (height, size and etc.) and the radar antenna pattern.

Second, a new method for determining the vertical profile of refractivity is proposed based on the power pattern of selected point-like targets. The temporal evolution of the power of these targets records the diurnal variation associated with the propagation conditions. The estimation of vertical profile of refractivity is qualitatively consistent with in-situ tower observations. The vertical profile of refractivity data helps reduce the spatial error of refractivity retrievals, especially in complex topography.

In addition, we characterized the antenna phase pattern. While usually neglected in radar applications, the phase pattern can guide target selection and scanning strategy and thus may impact refractivity retrievals.