Validation of the Aircraft-Based Significant Wave Height Measurements within Tropical Cyclones

Two instruments on board NOAA’s P-3 aircraft routinely measure significant waves heights (SWH) within Tropical Cyclones (TCs): The Ka-/Ku-band Interferometric Altimeter (KaIA) and the Wide Swath Radar Altimeter (WSRA). The KaIA instrument is a nadir-pointing radar altimeter capable of measuring the first three Doppler moments with a range gate spacing of 5 cm. It was developed to provide centimetric precision altimetry at Ka-band as part of a NOAA SBIR project for the Marine Debris Small Object Mapping Radar System (DSOM) and was installed on the NOAA P-3 Hurricane Hunter research airplane in the winter of 2019. It has flown every hurricane season since, as well as the annual ocean winds winter experiment conducted in either the North Atlantic or North Pacific to research extratropical cyclones. During the 2021 Atlantic hurricane season, a Ku-band channel was added to KaIA. For the 2023 winter deployment, one of the interferometric antennas at Ka-band was replaced with a wide-swath antenna that is oriented with the swath cross-track. It can be mechanically adjusted to one of several boresight angles from nadir to 25 degrees and is now being evaluated for synthetic aperture radar (SAR) capabilities. Starting from its initial deployment in 2019, we have provided near-real-time (NRT) significant wave height (SWH) retrievals from the Ka-band channel on our website (https://manati.star.nesdis.noaa.gov/datasets/AircraftData.php). The KaIA SWH processor was recently modified to improve performance in the extremes. The SWH retrievals from previous years have been reprocessed and their performance will be presented here.

The Wide Swath Radar Altimeter (WSRA) instrument operates at Ku-band. Its digital beam-forming capabilities that generate 80 narrow beams over 30° off nadir make it capable of providing both SWH and directional wave spectra observations. ProSensing designed WSRA and generates retrievals from the instrument’s data in NRT.

Analysis of the KaIA SWH retrievals from the past several years in comparison to models, satellites, and buoys have revealed what appears to be a saturation effect above 10m SWH, which does not appear to occur with the WSRA SWH retrievals. Subsequent analysis suggests that the averaging scheme implemented in the KaIA retrieval processor is not accurately capturing the largest wavelengths, resulting in false saturation in the retrievals. As mentioned above, the KaIA SWH retrievals from past seasons have recently been reprocessed with a new averaging scheme and we have seen some promising results that validate this hypothesis.

The initial statistics performed on KaIA and WSRA show similarities but also some significant differences. It is of paramount importance to understand these differences and overall sensor performance considering that data is regularly distributed in near real time to the National Hurricane Center. In this presentation we will assess performance of both WSRA and KaIA observations utilizing wave models, satellites, buoys, saildrones, and wave drifters. We examine these data by year and other relevant ocean parameters to evaluate if there are additional sources of bias in the SWH retrievals from either sensor.