CoAuthors

Frances Achorn (NOAA Ocean Prediction Center, USA); Alejandro Egido (NOAA Laboratory for Satellite Altimetry, USA); Eric Leuliette (NOAA Laboratory for Satellite Altimetry, USA); Graham Quartly (Plymouth Marine Laboratory, UK); Joseph Sienkiewicz (NOAA Ocean Prediction Center, USA)

The U. S. National Oceanic and Atmospheric Administration (NOAA)'s Ocean Prediction Center (OPC) provides hazardous marine weather warnings and forecasts for much of the north Atlantic and Pacific Oceans. Satellite altimeters are one of the few ways to measure the extreme significant wave height (SWH) and near-surface wind speed (WS) values that may occur in tropical and extra-tropical cyclonic storms. Data from CryoSat-2, Jason-2, Jason-3, SARAL, Sentinel-3A and Sentinel-3B are routinely displayed on the workstations of OPC forecasters. Data with less than 2 hours latency are used to verify the performance of forecast models and to make decisions about issuing warnings. For this purpose, forecasters want to see extreme values, if they are reliable, and therefore the standard editing procedures applied to altimetric sea surface height for assimilation into ocean models are not used in forecasting marine weather.

We find that altimeters seem to be measuring extreme SWH reliably in a variety of storms we have observed. However, during the 2018 North Atlantic hurricane season, Hurricanes Florence and Michael occasionally caused the Jason-2 altimeter to estimate truly spurious wave heights exceeding 20 m SWH. Inspection of the altimeter's radar waveforms and derived retracking parameters seems to show Jason-2 losing track of the radar echo soon after the extreme SWH values are estimated. In the case of Michael, the altimeter data were close enough to shore that we can compare them with ground-based radar measurements of rain intensity. It appears that the extreme SWH values occur as the altimeter flies through extreme rainfalls.

During its primary mission Jason-2 flew an exact-repeat orbit along a known ground track, and its radar measurement was controlled by the DIODE/DEM ("open loop tracking") system. Since July 2017 Jason-2 has been on a geodetic mission orbit, over which it does not use DIODE/DEM but strives to follow the sea surface automatically ("closed loop tracking"). We think that extreme rain intensity causes the Jason-2 altimeter to lose track of the sea surface, producing the spurious SWH values. We are developing a new editing scheme specific to the Jason-2 GM data that will allow large but likely correct SWH values to pass through to OPC forecasters, while editing SWH values that are likely due to the altimeter's loss of track.

The contents of this abstract and presentation are solely the findings and opinions of the authors and should not be construed as a statement of official policy or position of NOAA or the United States Government.