Evaluation of analog-based post-processing with a special focus on strong wind episodes

The error reduction of NWP wind speed forecast is a rather expected effect when an analog-based statistical post-processing method (ABM) is applied. The efficiency of ABM, naturally, directly depends on the exact setup. For instance, the wind direction NWP is proven to be the most important predictor variable in the analog search process, in addition to the wind speed itself. The properly weighted additional predictor variables improve results even further, especially in complex terrain. Additionally, since the post-processing methods can be prone to underestimate the intensity of extreme events, the correction for high wind speed is implemented as a valuable tool, affecting the ABM when the raw NWP forecast exceeds a certain value.

After several upgrades were recently made in the operational DHMZ suite, these upgraded ABM forecasts are thoroughly evaluated, taking into consideration the overall performance. Since wind speed often reaches gale values during the bora and jugo wind episodes in the coastal region of Croatia, the effect on rare event forecasts is also considered, especially in complex terrain prone to high wind speed (e.g., bora wind).

Results show that the error, in general, is larger in complex terrain prone to high-wind speed than in less complex continental areas, and the reduction of error by ABM is thus more pronounced. In addition to improvement for the climatologically more common events, it can also be achieved for rare events in some cases as well. This is especially the case after applying the statistical correction which enlarges the frequency of such forecasts. The large errors occurred mainly due to short-term weakening of the wind speed within the strong wind episode, or because of the time delay of the beginning or the ending of the forecasted strong wind episode.