Impact of the ASCAT scatterometer winds on the quality of HIRLAM analysis in case of severe storms

The impact of the Advanced Scatterometer (ASCAT) data assimilation on the quality of HIRLAM analysis is assessed in cases of rapidly developing severe storms of 2013. The HIRLAM quality is analysed for two observing system experiments: with and without the ASCAT data assimilation. Mainly impact on the model analysis output is evaluated. Marine observations of 10-m wind speed and mean sea level pressure are used as measures of quality. The results show that depending on ASCAT data coverage in the HIRLAM domain and temporal availability of the data at the assimilation time moment, the impact may be either more or less accurate. It is also detected that some narrow places of the Baltic Sea (Bothnian Bay, Gulf of Finland) are not affected by the ASCAT data assimilation. According to the ASCAT Wind Product specification, ASCAT measurements near the shoreline are usually flagged as land contaminated. The ASCAT winds in these areas are not admitted to the analysis after the procedure of the HIRLAM quality control, most likely due to the proximity to the land. The use of the ASCAT Coastal Wind Product in the future may enlarge the ASCAT data coverage in these areas. In addition, some weaknesses of the ASCAT data assimilation were detected in the study raising the question of the optimal ASCAT data usage. Further attempts to improve the quality of the HIRLAM analyses are expected in the ASCAT data thinning before assimilation or by reducing time differences between the HIRLAM analyses.