Improving radar refractivity retrieval by considering the change in the refractivity profile and the varying altitudes of ground targets

Accurate radar refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or studying boundary layer and convection processes. However, the technique as originally developed makes some simplistic assumptions about the heights of ground targets (H_T) and the vertical gradient of refractivity (dN/dh). In reality, the field of target phases used for refractivity retrieval is noisy because of varying terrain and introduces estimation biases. To obtain a refractivity map at a constant height above terrain, a 2D horizontal refractivity field at the radar height must be computed and corrected for altitude using an average dN/dh. This is achieved by theoretically clarifying the interpretation of the measured phase considering the varying H_T and the temporal change of dN/dh. Evolving dN/dh causes systematic refractivity biases, as it affects the beam trajectory, the associated target range, and the refractivity field sampled between selected targets of different heights. To determine H_T and dN/dh changes, a twofold approach is proposed: first, H_T can be reasonably inferred based on terrain height; then, a new method of dN/dh estimation is devised by using the property of the returned powers of a pointlike target at successive antenna elevations. The dN/dh obtained shows skill based on in situ tower observation. As a result, the data quality of the retrieved refractivity may be improved with the newly added information of dN/dh and H_T.