Remote sensing QPE uncertainties associated with sub-pixel rainfall variation

Rain gauge networks are used to calibrate and validate quantitative precipitation estimation (QPE) methods based on remote sensing, which may be used as data sources for hydrologic models. The typical approach is to adjust (calibrate) or compare (validate) the rainfall in the QPE pixel with the rain gauge located within the pixel. The QPE result represents a mean rainfall over the pixel area, whereas the rainfall from the gauge represents a point, although it is normally assumed to represent some area. In some cases the QPE pixel area may be millions of square meter in size. We hypothesize that many rain gauge networks in environments similar to this study (i.e., tropical coastal), which provide only one rain gauge per remote sensing pixel, may lead to error when used to calibrate/validate QPE methods, and that consequently these errors may be propagated throughout the hydrologic models. The objective of this paper is to describe a ground-truth rain gauge network located in western Puerto Rico which will be available to test our hypothesis. In this paper we discuss results from the rain gauge network, but do not present any QPE validation results. In addition to being valuable for validating satellite and radar QPE data, the rain gauge network is being used to test and calibrate atmospheric simulation models and to gain a better understanding of the sea breeze effect and its influence on rainfall. In this study, 62 storms were evaluated between August 2006 and August 2007. The area covered by the rain gauge network was limited to a single GOES-12 pixel (4 km × 4 km). Five-minute and total storm rainfall amounts were spatially variable at the sub-pixel scale. Average storm rainfall from more than a quarter (27%) of the 3,627 rain gauge-pairs evaluated were significantly different at the 5% of significance level, indicating significant rainfall variation at the sub-pixel scale. The majority of storms during the study period were locally formed by sea breezes and heating, although the 27% of gauges whose average rainfall amounts were significantly different could not be correlated with any single type of storm.