Estimates of Hurricane Wind Speed Measurement Accuracy using the Airborne Hurricane Imaging Radiometer

The measurement of peak winds in hurricanes is critical to classification of hurricane intensity. The Hurricane Imaging Radiometer (HIRAD) is an airborne passive microwave remote sensor that has been developed to retrieve ocean surface wind speed and rain rate within tropical cyclones through category-5 intensities. HIRAD is based on the existing technique of the C-band Stepped Frequency Microwave Radiometer (SFMR), which operates from NOAA and Air Force reconnaissance aircraft to measure along track surface winds and rain rate. HIRAD adds the capability for cross-track observations by using synthetic thinned array radiometry technology, resulting in 2 – 5 km cross-track resolution over a swath width of approximately 70 km, when operating from a high-altitude aircraft (60,000 feet) above the storm.

This paper presents results from research conducted at the Central Florida Remote Sensing Laboratory, to estimate the HIRAD wind speed and rain rate measurement performance in hurricanes under realistic operating conditions. These computer simulations use the MM5 numerical hurricane model as the “nature run” to provide realistic 3D environmental parameters (rain, water vapor, clouds and surface winds) from which wide-swath brightness temperature measurements are calculated using an advanced radiative transfer model (RTM). This RTM includes a rain model for the hurricane environment and an ocean surface emissivity model for hurricane force winds over incidence angles from nadir to > 60°. Simulations were done for typical high altitude “Figure-4” flight tracks on a global Hawk unmanned aircraft, and include realistic sources of errors, such as antenna pattern and cross-polarization effects, which are expected in the actual observations.

Using these simulated HIRAD measurements with errors, Monte Carlo retrievals of wind speed and rain rate were performed using databases of sea surface temperatures and climatological hurricane atmospheric parameters. Examples of retrieved hurricane wind speed and rain rate images are presented, and comparisons of these retrieved parameters with the nature run are made. Statistical analysis are performed and rms measurement errors are characterized over a broad range of wind and rain conditions and as a function of path length over the full HIRAD swath. These results will be used as error estimates for HIRAD performance in future numerical Observing System Simulation Experiments (OSSEs).