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Caren Marzban, Scott Sandgathe, and Hilary Lyons

Abstract

Recently, an object-oriented verification scheme was developed for assessing errors in forecasts of spatial fields. The main goal of the scheme was to allow the automatic and objective evaluation of a large number of forecasts. However, processing speed was an obstacle. Here, it is shown that the methodology can be revised to increase efficiency, allowing for the evaluation of 32 days of reflectivity forecasts from three different mesoscale numerical weather prediction model formulations. It is demonstrated that the methodology can address not only spatial errors, but also intensity and timing errors. The results of the verification are compared with those performed by a human expert.

For the case when the analysis involves only spatial information (and not intensity), although there exist variations from day to day, it is found that the three model formulations perform comparably, over the 32 days examined and across a wide range of spatial scales. However, the higher-resolution model formulation appears to have a slight edge over the other two; the statistical significance of that conclusion is weak but nontrivial. When intensity is included in the analysis, it is found that these conclusions are generally unaffected. As for timing errors, although for specific dates a model may have different timing errors on different spatial scales, over the 32-day period the three models are mostly “on time.” Moreover, although the method is nonsubjective, its results are shown to be consistent with an expert’s analysis of the 32 forecasts. This conclusion is tentative because of the focused nature of the data, spanning only one season in one year. But the proposed methodology now allows for the verification of many more forecasts.

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Caren Marzban, Scott Sandgathe, Hilary Lyons, and Nicholas Lederer

Abstract

Three spatial verification techniques are applied to three datasets. The datasets consist of a mixture of real and artificial forecasts, and corresponding observations, designed to aid in better understanding the effects of global (i.e., across the entire field) displacement and intensity errors. The three verification techniques, each based on well-known statistical methods, have little in common and, so, present different facets of forecast quality. It is shown that a verification method based on cluster analysis can identify “objects” in a forecast and an observation field, thereby allowing for object-oriented verification in the sense that it considers displacement, missed forecasts, and false alarms. A second method compares the observed and forecast fields, not in terms of the objects within them, but in terms of the covariance structure of the fields, as summarized by their variogram. The last method addresses the agreement between the two fields by inferring the function that maps one to the other. The map—generally called optical flow—provides a (visual) summary of the “difference” between the two fields. A further summary measure of that map is found to yield useful information on the distortion error in the forecasts.

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