Sensitivity Analysis of Assimilating Doppler Radar Radial Winds Within the Inner- and Outer-Core Regions of Tropical Cyclones

The assimilation of Doppler radar radial wind observations is important for improving tropical cyclone (TC) prediction. However, the specific impacts and contributions of these observations over different areas are still unclear. In this study, the impact of assimilating radar radial winds within the inner and outer-cores of TCs is evaluated. For TC Mujigae (2015), analyses suggest that although the outer-core observations better improve the upper-level outflow of the TC, the inner-core observations contribute more to enhancing tangential wind and updraft near the eyewall as well as correcting the TC size, warm core structure, and asymmetrical eyewall convection. The improvements conferred by the assimilating the total observations in the analysis and forecasting of TC tracks and intensities are mainly contributed by the inner-core observations. This is probably because: (a) the model has a higher deficiency in simulating the TC inner-core structure, which leads to a larger background deviation relative to the observations and the assimilation increment contributed by the inner-core observations; (b) as determined by the stronger nonlinearity of the physical processes, the ensemble spread (i.e., model uncertainty) around the inner core is larger, resulting in a more distinct impact of data assimilation. The significant improvements in the inner-core observations are further consolidated by analyses of two other TCs of different sizes and intensities, in which the number of inner-core observations accounts for only 1/3 of the total on average. The TC inner-core area should be a major consideration of future TC observation projects.Plain Language Summary This study analyzes the individual impact and contribution of the radar radial wind observation within the inner-and outer-cores of tropical cyclones (TCs) to its analysis and forecasting. Regarding the case study of TC Mujigae (2015), the inner-core observations contribute more to enhancing tangential wind and updraft near the eyewall as well as correcting the TC size, warm core structure, and asymmetrical eyewall convection. The improvements of radar data assimilation to TC track and intensity are found to mainly come from the assimilation of inner-core observations. This may come from two aspects: (a) the model has a higher deficiency in simulating the precise structure of TC inner-core, which leads to a larger assimilation increment by inner-core observation. (b) the physical processes of the TC inner-core region have a stronger nonlinearity, which makes a larger uncertainty in the model background, resulting in a more distinct impact of data assimilation. These results are further validated by two other TCs of different sizes and intensities. The TC inner-core area should be a major consideration of future TC observation projects.