Search for: All records

Abstract. Most available verification metrics for ensemble forecasts focus on univariate quantities. That is, they assess whether the ensemble provides anadequate representation of the forecast uncertainty about the quantity of interest at a particular location and time. For spatially indexed ensemble forecasts, however, it is also important that forecast fields reproduce the spatial structure of the observed field and represent the uncertainty about spatial properties such as the size of the area for which heavy precipitation, high winds, critical fire weather conditions, etc., areexpected. In this article we study the properties of the fraction of threshold exceedance (FTE) histogram, a new diagnostic tool designed forspatially indexed ensemble forecast fields. Defined as the fraction of grid points where a prescribed threshold is exceeded, the FTE is calculated for the verification field and separately for each ensemble member. It yields a projection of a – possibly high-dimensional – multivariatequantity onto a univariate quantity that can be studied with standard tools like verification rank histograms. This projection is appealing since itreflects a spatial property that is intuitive and directly relevant in applications, though it is not obvious whether the FTE is sufficientlysensitive to misrepresentation of spatial structure in the ensemble. In a comprehensive simulation study we find that departures from uniformity ofthe FTE histograms can indeed be related to forecast ensembles with biased spatial variability and that these histograms detect shortcomings in the spatial structure of ensemble forecast fields that are not obvious by eye. For demonstration, FTE histograms are applied in the context of spatiallydownscaled ensemble precipitation forecast fields from NOAA's Global Ensemble Forecast System.