Radar calibration by gage, disdrometer, and polarimetry: Theoretical limit caused by the variability of drop size distribution and application to fast scanning operational radar data

A long time record of drop size distributions (DSDs) is used to evaluate the effect of the DSD variability on the accuracy of radar adjustment by comparison with a rain gage on a daily basis. Radar and gage measurements are simulated from DSDs. When a single R–Z relationship is used in the adjustment of the radar as a hydrological instrument, a standard deviation of fractional error of ∼28% is expected. This uncertainty is related to the DSD variability in time. A calibration of reflectivity can be done if a disdrometer is available. This disdrometric radar calibration is not affected by the DSD variability. Thus, the uncertainty that is expected in the radar adjustment with a gage is eliminated. Some uncertainty in radar-disdrometer comparison due to the difference in sampling volumes is minimized by applying a sequential intensity filtering technique (SIFT). Good correlations between radar and disdrometric reflectivities indicate that this could be an excellent way of calibrating radar on a daily basis when a disdrometer is located at close range (less than 30 km) from radar. Furthermore, the consistency of independent checks of radar calibration error with different disdrometers and polarimetry validates the argument that the radar-disdrometer comparison can be used as a tool for absolute radar calibration. The information from the McGill operational S-band polarimetric radar is also used to calibrate radar. This method is based on the fact that the specific differential phase shift ( K DP) or differential phase shift ( Φ DP) between the horizontal and vertical polarized beams is immune to the radar calibration error whereas the reflectivity is affected by the calibration error. Due to the variability of DSDs only, the uncertainty in polarimetric calibration is the standard deviation of 1 dB with a single parameter K DP and reduces to 0.5 dB when the differential reflectivity ( Z DR) is added as well. To guarantee the stability of this calibration method, data longer than at least an hour is necessary to calculate the calibration error for the fast scanning McGill operational polarimetric radar and contamination by bright band or snow should be avoided. The sensitivity of this calibration method with respect to the drop deformation is tested.