A Comparison of Hybrid-4DEnVar and Hourly Hybrid-3DEnVar assimilation of Tail Doppler Radar observations on the prediction of rapidly evolving Hurricanes
Beginning the summer of 2017 the National Weather Service upgraded the operational HWRF to continuously cycled GSI based, 6-hourly, Hybrid-3DEnVar data assimilation (Lu et al 2017). During rapidly evolving conditions or when observations are non-uniformly distributed over the 6-hour window, 6 hourly Hybrid-3DEnVar can have difficulties resulting from not accounting for the temporal evolution of the background error covariances. The lack of temporal evolution of error covariances can degrade the analyses and subsequent forecasts. These degradations can accumulate in further DA cycles.
Two methods of reducing errors introduced by rapidly changing conditions are considered in this study: 1) Hybrid 4DEnVar, and 2) Hybrid 3DEnVar with hourly frequency (hourly-3DEnVar). In 4DEnVar, the temporal evolution of error covariances is incorporated by the use of 4-dimensional ensemble perturbations with implicit linearity assumption during the minimizations. However, non-linear error growth within a 6-hour window can still pose difficulties. Hourly-3DEnVar assimilates observations in one hour windows instead of one longer six hour window, allowing the error covariances to change each hour. However, frequent interruption of the model could introduce additional instabilities. While previous work has been done to evaluate different data assimilation frequencies and methods on thunderstorms, little work has been done comparing these two methods in the case of a hurricane. The goal of this study is to evaluate the performance of the hourly-3DEnVar and 4DEnVar hybrid data assimilation systems in the HWRF model assimilating inner core Tail Doppler Radar (TDR) data to understand the benefits of each method in hurricane forecasting.
Experiments are conducted for cycles in which TDR data is available during Hurricane Edouard (2014) and Hurricane Irma (2017). Edouard became a tropical storm on Sep. 12 and remained tropical until Sep. 19, reaching a peak intensity of 105 knots on Sep 16, becoming a category 3 hurricane. TDR data is available from 4 flights over 5 DA cycles from September 15-17. Irma became a tropical storm on Oct. 30, before rapidly strengthening into a category 3 hurricane on Sep. 1. After a period without significant intensity change, intensification resumed and Irma made landfall in Barbuda as a 155kt Category 5, then made several more landfalls prior to making landfall in Cuba as a Category 5 storm. Irma turned northward before making final landfall over southwest Florida as a category 2 hurricane.
A control run is performed using 6-hourly Hybrid-3DEnVar. 4DEnVar and hourly-3DEnVar experiments are run for each DA cycle where TDR data is available. All experiments start from the same the background produced by the control run, except where consecutive DA cycles have TDR data available, in which case the first cycle uses the background from the control and then is continuously cycled. Analyses and forecasts are verified against best track, tcvitals, satellite, and TDR observations to try and explain differences we see in these systems.