HSRL-2 Retrievals of Ocean Surface Wind Speeds

Abstract

Abstract. This study introduces and evaluates ocean surface wind speed retrieval capabilities of the High Spectral Resolution Lidar – generation 2 (HSRL-2) instrument through comparison with wind speed data collected by National Center for Atmospheric Research (NCAR) Airborne Vertical Atmospheric Profiling System (AVAPS) dropsondes. Wind speed is derived from HSRL-2 measurements of the transmitted laser’s specular reflection off the ocean surface. The magnitude of the surface reflectivity is determined by the surface’s wave-slope variance, which is driven by surface winds. The assessment relies on the multi-year airborne data set collected as part of NASA’s Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) campaign, where HSRL-2 retrievals and AVAPS dropsonde measurements of surface wind speeds were horizontally synchronized owing to their joint deployment on one of two aircraft used during the mission. A total of 577 collocated HSRL-2 - dropsonde surface wind speed data points over the northwest Atlantic Ocean are used for this study. Treating the dropsonde wind speeds as truth, it is found that, through two established wind speed – wave-slope parameterizations, the HSRL-2 wind speed retrievals have small errors (0.15 m s−1 ± 1.80 m s−1 and 0.62 m s−1 ± 1.70 m s−1) and high correlation coefficients (0.89 and 0.88) with dropsonde wind speed measurements. Also, HSRL-2 wind speed error is higher in winter than in summer due at least partly to the higher frequency of low wind speeds and reduced cloud fraction in summer. Two research flights from 28 August 2020 and 1 March 2020 serve as detailed case studies to show the success of the collocation method based on ACTIVATE’s spatial-coordination strategy and how HSRL-2 wind speed retrievals can enhance science-oriented studies such as those related to cloud evolution and general air-sea interaction. Another case flight examined from 11 January 2022 demonstrates the challenge of conducting HSRL-2 wind speed retrievals in high cloud fraction conditions. Overall, this study highlights the airborne HSRL-2’s ability to retrieve surface wind speeds with accuracy as well as the potential of using dropsondes to validate aircraft instrument data sets within a field campaign.