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.