A Systematic Assessment of the Overall Dropsonde Impact during the 2017-20 Hurricane Seasons Using the Basin-Scale HWRF

This study comprehensively assesses the overall impact of dropsondes on tropical cyclone (TC) forecasts. We compare two experiments to quantify dropsonde impact: one that assimilated and another that denied dropsonde ob-servations. These experiments used a basin-scale, multistorm configuration of the Hurricane Weather Research and Fore-casting Model (HWRF) and covered active North Atlantic basin periods during the 2017-20 hurricane seasons. The importance of a sufficiently large sample size as well as thoroughly understanding the error distribution by stratifying re-sults are highlighted by this work. Overall, dropsondes directly improved forecasts during sampled periods and indirectly impacted forecasts during unsampled periods. Benefits for forecasts of track, intensity, and outer wind radii were more pro-nounced during sampled periods. The forecast improvements of outer wind radii were most notable given the impact that TC size has on TC-hazards forecasts. Additionally, robustly observing the inner-and near-core region was necessary for hurricane-force wind radii forecasts. Yet, these benefits were heavily dependent on the data assimilation (DA) system qual-ity. More specifically, dropsondes only improved forecasts when the analysis used mesoscale error covariance derived from a cycled HWRF ensemble, suggesting that it is a vital DA component. Further, while forecast improvements were found regardless of initial classification and in steady-state TCs, TCs undergoing an intensity change had diminished benefits. The diminished benefits during intensity change probably reflect continued DA deficiencies. Thus, improving DA system qual-ity and observing system limitations would likely enhance dropsonde impacts.SIGNIFICANCE STATEMENT: This study uses a regional hurricane model to conduct the most comprehensive as-sessment of the impact of dropsondes on tropical cyclone (TC) forecasts to date. The main finding is that dropsondes can improve many aspects of TC forecasts if their data are assimilated with sufficiently advanced assimilation techni-ques. Particularly notable is the impact of dropsondes on TC outer-wind-radii forecasts, since improving those forecasts leads to more effective TC-hazard forecasts.