Estimation of wind friction velocity and direction at the ocean surface from physical models and space‐borne radar scatterometer measurements

Abstract

Abstract. This paper presents a new technique for estimating the wind friction velocity at the ocean surface from C-band radar scatterometer measurements. This technique uses physical models of ocean surface waves and electromagnetic backscattering from a rough surface at intermediate angles of incidence to generate predictions of the normalized radar cross section (NRCS, or rr ø) of the ocean surface for a given wind friction velocity and observational geometry. The ocean spectral model used in this technique has been developed specifically for this application. It combines in situ wave measurements at low wave numbers with the Phillips [1985] equilibrium spectral model. This choice of ocean wave model is supported by a set of open ocean wave measurements summarized in this paper. A suite of models, derived from both in situ and remote measurements of the sea surface, is used to characterize the directional spreading of ocean waves relative to the wind direction. The resulting two-dimensional ocean wave spectra are used with a composite surface model to predict radar backscattering from the ocean surface at Cband. These radar cross-section predictions are combined with ERS-1 scatterometer measurements in a cost function minimization scheme to yield estimates of the friction velocity vector at the ocean surface. We present examples of this technique and compare friction velocity retrievals obtained via this scheme with buoy-based measurements under a variety of wind and wave conditions. On the basis of the analysis of a limited number of cases, this technique yielded friction velocity estimates for which the magnitude was within 22% and the direction was within _+25 ø . Given that scientific applications require magnitude estimates within 10-15% and directional estimates within _+20 ø of in situ measurements, these preliminary results suggest that this is a promising approach to wind retrieval. Introduction

Wave‐turbulence interaction and its induced mixing in the upper ocean

“…According to the previous investigations, such as McWilliams et al [1997], Teixeira and Belcher [2002], Kantha and Clayson [2004], and Ardhuin and Jenkins [2006], the TKE dissipation rate w induced by wave‐turbulence interaction can be expressed as where u ′ and w ′ are horizontal and vertical velocity fluctuations, respectively, and z is the vertical coordinate with an origin at the mean sea level and positive upward. In the upper ocean, − ∼ u * 2 , in which u * = is the friction velocity in water, τ 0 is the surface wind stress, and ρ is the density of the seawater [ Mellor and Yamada , 1982; Lettvin and Vesecky , 2001]. The Stokes drift induced by surface waves can be expressed as where u s 0 (= c ( Ak ) 2 ) is the magnitude of the Stokes drift at the surface ( z = 0), c is the phase speed, A (= H s /2) is the wave amplitude, H s is the significant wave height, k (= 2 π / L ) is the wave number, and L is the wavelength.…”

“…where u′ and w′ are horizontal and vertical velocity fluctuations, respectively, and z is the vertical coordinate with an origin at the mean sea level and positive upward. In the upper ocean, −u′w′ ∼ u * 2 , in which u * = ffiffiffiffiffiffiffiffiffi 0 = p is the friction velocity in water, t 0 is the surface wind stress, and r is the density of the seawater [Mellor and Yamada, 1982;Lettvin and Vesecky, 2001]. The Stokes drift induced by surface waves can be expressed as…”

The FuGas 2.5 Updated for the Effects of Surface Turbulence on the Transfer Velocity of Gases at the Atmosphere–Ocean Interface

“…The FuGas allows u * to be estimated from the uw and vw wind components measured by the EC method. One frequent option is Equation (2a) [26,52,66,67,95,100,104,105]. The u', v' and w' are the longitudinal, lateral and vertical wind fluctuations, with the overbars corresponding to the bin averaged second order crossed central moments.…”

An improvement on the gas transfer velocity model with application to scatterometer data

“…However, this physical variable is difficult to measure and efforts have been made to obtain it through the use of satellites (LETTVIN;VESECKY, 2001). The u * is related to the wind speed measured 10 meters above the ocean surface (u 10 ) and the latter is used to calculate the gas transfer coefficient k. This approach is justified because there is a much larger number of measurements of u 10 from buoys and ships available in comparison with u * .…”