Impact of Assimilating Surface Wind Fields from the Hurricane Imaging Radiometer (HIRAD) on Analyses and Forecasts of Tropical Cyclones

The Hurricane Imaging Radiometer (HIRAD) was developed for use on the Global hawk unmanned aircraft and is based on Stepped Frequency Microwave Radiometer (SFMR) technology which has been well tested on NOAA and USAF aircraft. This instrument has the advantage that it can be deployed on unmanned aircraft (UAS) to obtain surface wind observations in situations when no conventional aircraft are available or within range and provide information when the Tropical Cyclone (TC) is located outside of satellite orbital paths.

The potential benefit of these data for producing 3-dimensional analyses of the tropical cyclone vortex for model initialization is tested within an Observing System Experiment (OSE). The HIRAD surface winds are assimilated into the Hurricane Weather Research and forecasting model (HWRF) using the Hurricane Ensemble Data Assimilation System (HEDAS) for the case of hurricane Joaquin 2015. The WB-57 aircraft, which deploys instruments designed for the Global Hawk UAS and flies at a similar altitude, flew 2 missions into Joaquin as part of the Tropical Cyclone Intensity experiment (TCI). Observations were obtained from HIRAD and Yankee, high altitude, dropwindsonde instruments. This is an ideal case for demonstrating the potential value of these data since very few conventional observations were available. Parallel experiments are conducted, one in which only conventional observation types are assimilated, another which also assimilates HIRAD data, and a third where the high altitude dropwindsonde profiles are also assimilated. The impact of HIRAD observations on the analyses, which provide the model initial condition, and the resulting forecasts are evaluated in terms of initial vortex structure and track and intensity forecast.  We also explore how quality control and filtering of HIRAD observations might aid in the assimilation of these data The influence of assimilating dropsonde profiles, in addition to HIRAD, is assessed in a similar manner. These experiments are conducted with support from the Sensing Hazards with operational unmanned technology (SHOUT) program in an effort to determine optimal sampling strategies for UAS.