Sea surface wind speed estimation from space-based lidar measurements

Y Hu,C Hostetler,M Cisewski,B Lin,M Vaughan,C Weimer,D Wu,A Omar,J Pelon,W Sun,K Stamnes,M Santa-Maria,D Winker,D Flittner,C Trepte,P Yang,G Gibson

doi:10.5194/acp-8-3593-2008

Abstract

Abstract. Global satellite observations of lidar backscatter measurements acquired by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission and collocated sea surface wind speed data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), are used to investigate the relation between wind driven wave slope variance and sea surface wind speed. The new slope variance – wind speed relation established from this study is similar to the linear relation from Cox-Munk (1954) and the log-linear relation from Wu (1990) for wind speed larger than 7 m/s and 13.3 m/s, respectively. For wind speed less than 7 m/s, the slope variance is proportional to the square root of the wind speed, assuming a two dimensional isotropic Gaussian wave slope distribution. This slope variance – wind speed relation becomes linear if a one dimensional Gaussian wave slope distribution and linear slope variance – wind speed relation are assumed. Contributions from whitecaps and subsurface backscattering are effectively removed by using 532 nm lidar depolarization measurements. This new slope variance – wind speed relation is used to derive sea surface wind speed from CALIPSO single shot lidar measurements (70 m spot size), after correcting for atmospheric attenuation. The CALIPSO wind speed result agrees with the collocated AMSR-E wind speed, with 1.2 m/s rms error. Ocean surface with lowest atmospheric loading and moderate wind speed (7–9 m/s) is used as target for lidar calibration correction.

Highlights

It has been over half a century since Cox and Munk (1954) introduced the Gaussian distribution relation between sea surface wind and the slopes of wind driven waves
The wind speed – wave slope variance relation derived from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) clear sky sea surface lidar backscatter and AMSR-E wind speed data, as described in Eq (5), can be applied for measuring ocean surface wind speed using spacebased lidar measurements, wherever/whenever the attenuation of the atmosphere can be assessed with sufficient confidence
Using the collocated CALIPSO sea surface lidar backscatter measurements and the wind speeds reported in the AMSR-E data products, we have studied the relationship between wave slope variance and surface wind speed on a global scale

Summary

Introduction

It has been over half a century since Cox and Munk (1954) introduced the Gaussian distribution relation between sea surface wind and the slopes of wind driven waves. In this study we introduce the following relation between wave slope variance and wind speed, based on comparison between CALIPSO lidar sea surface backscatter (γ ) and collocated AMSR-E wind speed measurements: CALIPSO: σ. The relation between wind speed and wave slope variance can be assessed on a global scale using the collocated wind speed measurements from AMSR-E and the variance of the wave slope distribution estimated from CALIPSO lidar sea surface integrated backscatter coefficient using Eq (4). The wave slope variance σ 2– wind speed U relations from Cox-Munk, Wu and the best fit from the CALIPSO/AMSR-E data are plotted in the lower panel of Fig. 2 as the yellow, green and black curves, with the y-axis as wind speed and xaxis as the inverse of lidar backscatter, 1/γ , after whitecap correction. Studying wind and slope variance relation using combined wind/lidar measurements can avoid uncertainty associated cloud/aerosol contaminaiton (Flamant et al, 1998)

Improving calibration with ocean surface lidar backscatter

Findings

Summary and discussion