Estimation of Daily Area-Average Rainfall during the CaPE Experiment in Central Florida

Abstract

The principal component of the water cycle over land areas is precipitation. Knowledge of the accuracy of areal precipitation estimates, therefore, is imperative. The manifold observations of hydrometeorological quantities in the Convection and Precipitation/Electrification Experiment (CaPE) that took place in central Florida during the summer of 1991 have provided an opportunity to examine the various water fluxes. This paper deals with only one of them, the daily area-average rainfall as derived from rain gauges. The theory for spatial sampling errors for random gauge design is extended to include rain gauge bias and random errors. The requirement for randomly located gauges turns out to be quite well met with the approximately 100 gauges available each day over the 17 300-km2 CaPE study area. Rain gauge bias, due mainly to wind effects, is estimated to be 6% based on a previous study and random measurement error is calculated from seven pairs of collocated gauges. For the category of daily area-average rainfall less than 1 mm, the standard error was found to be 0.27 mm or a 53% variation with respect to the category mean. For the highest rainfall category, 11?15 mm, the standard error was 1.49 mm or 11%. For this application, the standard error is essentially a consequence of the limited number of gauges. Given that the rain gauges are randomly distributed and arithmetic averaging is used to estimate the daily area-average rainfall, application of the appropriate standard error provides a useful measure of the best accuracy achievable in assessing the daily water budget of this area.