A new intensity-scale approach for the verification of spatial precipitation forecasts

A new intensity-scale method for verifying spatial precipitation forecasts is introduced. The technique provides a way of evaluating the forecast skill as a function of precipitation rate intensity and spatial scale of the error. Six selected case studies of the UK Met Office now-casting system NIMROD are used to illustrate the method. The forecasts are assessed using the Mean Squared Error (MSE) skill score of binary images, obtained from the forecasts and analyses by thresholding at different precipitation rate intensities. The skill score is decomposed on different spatial scales using a two-dimensional discrete Haar wavelet decomposition of binary error images. The forecast skill can then be evaluated in terms of precipitation rate intensity and spatial scale. The technique reveals that loss of forecast skill in NIMROD is predominantly due to small spatial scale (< 40 km) errors of more intense events. The technique is capable of isolating specific intensity-scale errors for individual cases. As an example, in one of the case studies the displacement error of an incorrectly advected storm is well detected by a minimum negative skill score occurring at the 160 km spatial scale for thresholds between 112 and 4 mm/h.