Analog-based post-processing of the operational temperature NWP in Croatia

NWP models have been a staple of modern weather forecasting for a long time. Although their skill is steadily improving, their limitations are still notable. An additional forecast improvement obtained by applying statistical post-processing for the locations where the measurements are available is thus a natural next step. Such an example of various NWPs improvements is achieved by the analog method.  For that reason, a potential post-processing contribution of the analog method is analyzed by verifying forecasts against the temperature measurements across the Republic of Croatia.

 

The raw NWP model used in this work is the ALADIN model with a 4 km horizontal resolution. The analyzed analog-based method includes weight optimization and the correction for more extreme values. Weight optimization aims to improve the process of similarity search by determining the ideal weight for each predictor variable, and prevents overfitting. The correction for more extreme values tends to improve forecasts on both ends of the temperature distribution by adding the correction factor if the starting NWP forecast exceeds the before-determined threshold.

 

The verification of the hourly temperature forecasts is performed using both the continuous and categorical approaches. In the categorical approach, the analysis is performed for both common and more extreme events. Considering the sensitivity of forecast accuracy on the terrain type, verification is also performed for different groups of stations. Results reveal the benefits of using the more advanced version of the analog method. Moreover, such an advanced analog method forecast outperforms raw NWP in most cases. When results are compared depending on the location, it is shown that the greater post-processing improvement can be seen in the continental than in the coastal area of Croatia.

 

Considering that the minimal and maximal daily values are a pragmatic way to provide forecasters with a quick and summarized overview, such forecasts are additionally examined. In addition to the statistical verification approach, a few interesting cases were also singled-out and analyzed more thoroughly. Finally, the amplitude of diurnal temperature variation seems to be more realistic for the analogs during heat or cold waves.