Showing papers by "David A. Short published in 1993"
TL;DR: In this article, the impact of RRD variations on the threshold method is shown in an analysis of 1-min rainrate data from a network of tipping-bucket gauges in Darwin, Australia.
Abstract: The threshold method for estimation of area-average rain rate relies on determination of the fractional area where rain rate exceeds a preset level of intensity. Previous studies have shown that the optimal threshold level depends on the climatological rain-rate distribution (RRD). It has also been noted, however, that the climatological RRD may be composed of an aggregate of distributions, one for each of several distinctly different synoptic conditions, each having its own optimal threshold. In this study, the impact of RRD variations on the threshold method is shown in an analysis of 1-min rainrate data from a network of tipping-bucket gauges in Darwin, Australia. Data are analyzed for two distinct regimes: the premonsoon environment, having isolated intense thunderstorms, and the active monsoon rains, having organized convective cell clusters that generate large areas of stratiform rain. It is found that a threshold of 10 mm/h results in the same threshold coefficient for both regimes, suggesting an alternative definition of optimal threshold as that which is least sensitive to distribution variations. The observed behavior of the threshold coefficient is well simulated by assumption of lognormal distributions with different scale parameters and same shape parameters.
32 citations
TL;DR: Optimal thresholds for gamma and inverse Gaussian distributions are derived and compared and suggest that the optimal threshold can be understood as arising due to sampling variations, from snapshot to snapshot, of a parent rain-rate distribution.
Abstract: Optimization of the threshold method, achieved by determination of the threshold that maximizes the correlation between an area-average rain-rate moment and the area coverage of rain rates exceeding the threshold, is demonstrated empirically and theoretically. Empirical results for a sequence of GATE radar snapshots show optimal thresholds of 5 and 27 mm/h for the first and second moments, respectively. Theoretical optimization of the threshold method by the maximum-likelihood approach of Kedem and Pavlopoulos (1991) predicts optimal thresholds near 5 and 26 mm/h for lognormally distributed rain rates with GATE-like parameters. The agreement between theory and observations suggests that the optimal threshold can be understood as arising due to sampling variations, from snapshot to snapshot, of a parent rain-rate distribution. Optimal thresholds for gamma and inverse Gaussian distributions are also derived and compared.
26 citations
TL;DR: In this paper, a statistical setup based on the mixed distribution of rain rate provides the quadratic relationship between the rain rate variance and the probability that rain rate exceeds a fixed threshold level.
Abstract: A statistical setup based on the mixed distribution of rain rate provides the quadratic relationship between the rain rate variance and the probability that rain rate exceeds a fixed threshold level. The paper proposes singleand double-threshold methods for the quadratic estimation of the areaaverage rain rate variance. An argument leads to a statistical explanation for choosing optimal thresholds. Empirically determined single optimal thresholds are compared with those for lognormal, gamma and inverse Gaussian distributions. Empirical results for the GATE I data set show an optimal threshold of 31mm/hr for the singlethreshold method, while the lognormal, gamma and inverse Gaussian distributions respectively give optimal thresholds of 42.4, 24.8 and 27.4mm/hr, derived from the proposed criterion. Illustration indicates that the single-threshold method which uses a single optimal threshold is as effective as the double-threshold method which uses a pair of optimal thresholds for linearly estimating the area-average rain rate first and second moments.
15 citations
2 citations