The Impact of NASA Global Hawk Unmanned Aircraft Observations on HEDAS Surface Wind Analyses

The Hurricane Ensemble Data Assimilation System (HEDAS) was developed at the Hurricane Research Division (HRD) of NOAA to incorporate observations taken within the tropical cyclone inner core into a high resolution, regional hurricane model. The primary application has been to provide a realistic initial condition for the Hurricane Weather Research and Forecasting (HWRF) model. However, the analyses produced using HEDAS are also capable of providing forecast centers with a detailed estimate of the tropical cyclone surface wind field. These surface wind analyses can be a valuable tool for anticipating the strength, structure and extent of potentially damaging winds a location might experience from a landfalling tropical cyclone.

HEDAS currently assimilates ground based rawindsonde, dropwindsonde, flight level and tail Doppler radar observations obtained using NOAA and U.S. Air Force aircraft as well as remotely sensed temperature and humidity retrievals. HEDAS is also capable of assimilating observations taken using NASA's Global Hawk (GH) unmanned aircraft, including GPS dropwindsonde, High Altitude MMIC Sounding Radiometer (HAMSR) and Scanning High Resolution Inferometer Sounder (S-HIS) profiles. The use of Global Hawk to obtain both in situ and remotely sensed observations of the tropical cyclone and the surrounding environment has been supported by the Sensing Hazards with Operational Unmanned Technology (SHOUT) program since 2015 and previously during the Hurricane and Severe Strom Sentinel (HS3) campaign (2012-2014). The GH data are expected to enhance traditional observations in key areas by providing information about atmospheric conditions above the typical altitude of reconnaissance aircraft and across the TC vortex at times when there may not be a satellite overpass.

The quality of ocean surface wind analyses produced using HEDAS will be assessed for a number of cases where GH observations are available. Analyses where only traditional observations are assimilated and ones which also include the GH will be compared in order to determine the impact of the additional data. Analyses for some land falling hurricane cases will also be evaluated, since the greatest value of surface wind analyses is for times when a storm is approaching land. The quality of these analyses relative to those for the open ocean cases are then compared in order to determine whether GH data might have a similar impact for land falling storms.

Since there is a limited number of cases for which GH observation are available, the value of these data toward improving surface analyses will also be tested within an observing system simulation experiment (OSSE). For this experiment, ocean surface wind observations such as those obtained using the HIWRAP instrument will be simulated from a “nature run” and assimilated into the HWRF model using HEDAS. The value of such an experiment is that the true state of the wind field is given by the nature run, so that the impact of assimilating the simulated observations can be exactly quantified.