Error Statistics of VPR Corrections in Stratiform Precipitation

Abstract

Abstract Errors in surface rainfall estimates that are caused by ignoring the vertical profile of reflectivity (VPR) and range effects have been assessed by simulating how fine-resolution 3D reflectivity measurements at close ranges are sampled by the radar at various ranges and heights. Uncorrected and corrected accumulations from 33 events of mainly stratiform precipitation, with a recognizable melting layer for over 250 h, have been generated using two basic procedures: (a) the “near range” or “inner” VPR and (b) the intensity-dependent “climatological” VPR. The root-mean-square (rms) error structure has been derived as a function of height and range, for accumulations ranging from 5 min to 2 h, for various brightband heights and verification areas. However, it is the errors along the lowest default height that are most relevant. The stratification of the results by the height of the bright band is essential to understand the influence of the bright band with range. The largest errors (>100% at near ranges without correction) are encountered with lower and stronger bright bands. After correction, errors of less than 20% can be achieved with method “a” but only over large verification areas (>100 km2), with long accumulation intervals (>45 min), with bright bands that are relatively high (>2.5 km), and for ranges within ∼130 km. The climatological correction yields errors that are roughly 2 times as large. The results with the inner VPR method can only be obtained by assuming conditions of spatial homogeneity in the VPR structure of the rainfall fields. Simulations of the VPR variability have indicated that larger errors are to be expected in real-time operations, particularly when measurements are made inside the bright band. The magnitude of these errors may approach those of a “realistic climatological” correction that incorporates some uncertainty in the brightband height.