EnKF OSSE Experiments analyzing the impact of HIWRAP data on Hurricane Karl

This study utilizes ensemble Kalman filter (EnKF) observation system simulation experiments (OSSEs) to analyze the impact of assimilating radial velocity observations with the high-altitude imaging wind and rain airborne radar (HIWRAP) into Hurricane Karl. HIWRAP is a new observing system mounted upon the NASA Global Hawk unmanned aerial vehicle platform, which has a cruising altitude of about 17-18 km. The radar uses conical scans with incidence angles of 30 and 40 degrees and produces data over a 30-40-km wide swath. The Global Hawk has the benefit of much longer range and on-station capabilities than conventional aircraft, which gives it the ability to observe tropical cyclones from a unique perspective. Thus, the Global Hawk is a desirable addition to other operational platforms, and quantification of the impact of these new observations is needed.

To set up the experiment, a high-resolution mesoscale ensemble is first created using the Weather Research and Forecasting (WRF) model at a convective permitting resolution. From this ensemble, the ‘truth' is selected as the member that best represents Hurricane Karl for it's nearly 36-h track across the Bay of Campeche. Simulated data is then obtained from this simulation and assimilated with an EnKF into the remaining 30 members of the ensemble. Experiments increase in complexity from a single pass through the storm to multiple passes. The impact of estimating fall speed is also examined.

Results are examined to determine the impact of assimilating such data. To do this, ensemble forecast initialized from the EnKF analyses and perturbations are performed and compared to the pure ensemble forecast with no assimilation. In addition, these results are compared with the impact of assimilating traditional airborne data (i.e., that obtained from the radars mounted upon NOAA's P3 aircraft).