Validation of the Hurricane Imaging Radiometer Forward Radiative Transfer Model for a Convective Rain Event

Abstract

The airborne Hurricane Imaging Radiometer (HIRAD) was developed to remotely sense hurricane surface wind speed (WS) and rain rate (RR) from a high-altitude aircraft. The approach was to obtain simultaneous brightness temperature measurements over a wide frequency range to independently retrieve the WS and RR. In the absence of rain, the WS retrieval has been robust; however, for moderate to high rain rates, the joint WS/RR retrieval has not been successful. The objective of this paper was to resolve this issue by developing an improved forward radiative transfer model (RTM) for the HIRAD cross-track viewing geometry, with separated upwelling and specularly reflected downwelling atmospheric paths. Furthermore, this paper presents empirical results from an unplanned opportunity that occurred when HIRAD measured brightness temperatures over an intense tropical squall line, which was simultaneously observed by a ground based NEXRAD (Next Generation Weather Radar) radar. The independently derived NEXRAD RR created the simultaneous 3D rain field “surface truth”, which was used as an input to the RTM to generate HIRAD modeled brightness temperatures. This paper presents favorable results of comparisons of theoretical and the simultaneous, collocated HIRAD brightness temperature measurements that validate the accuracy of this new HIRAD RTM.

Validation of the High-Altitude Wind and Rain Airborne Profiler during the Tampa Bay Rain Experiment

“…The individual remote sensing datasets (NEXRAD and ASCAT) were near-simultaneous and provided overlapping spatial coverage, and the in situ buoy measurements provided ocean surface wind speed (WS) and wind direction (WD) time series that span the HIWRAP observation period. Previously, results were reported for the TBRE concerning another GH remote sensor named the Hurricane Imaging Radiometer (HIRAD) that used NEXRAD rain measurements for calibration [3]. This present work significantly expands the scope of that previous paper by including the HIWRAP remote sensor and by focusing on the validation of the HIWRAP geophysical retrievals of rain rate and wind speed.…”

“…However, this paper does not present hurricane measurements; rather, it deals with a serendipitous opportunity that occurred during a return flight from hurricane observations in the western Gulf of Mexico during the early hours of 16 September 2013, which is hereafter referred to as the Tampa Bay Rain Experiment (TBRE) [3]. At this time, a fastmoving tropical squall line appeared on the real-time National Weather Service meteorological radar network that was directly ahead of the projected GH flight path near the northern Florida coast.…”

Experimental Proof of Concept for the Hurricane Imaging Radiometer (HIRAD) Measurement of a Hurricane Ocean Surface Wind Speed Field

“…Further, during the Tampa Bay Rain Experiment, the HIRAD forward radiative transfer model was experimentally validated, and the measured and modeled T B at the top of the atmosphere during heavy convective rain were in good agreement over the entire swath [16]. So, the HIRAD-retrieved path average rain rate is robust; however, the associated WS is not, because the WS contribution to the total brightness temperature is relatively small.…”

“…The HIRAD brightness temperatures, at the aperture of the antenna, are modeled by the RTM described in [16]. The major component of this RTM is the HIRAD ocean emissivity model [17], which covers frequencies from 4-7 GHz, incidence angles from nadir to 75 • , and ocean surface wind speeds from 0-70 m/s.…”