Statistics of the temperature and its derivatives at the surface of a wind-driven air-water interface

The statistics of the temperature and its spatial derivatives at a wind‐driven air‐water interface were obtained from a comprehensive data set of high resolution infrared imagery for wind speeds ranging from 2 ms−1 to 10 ms−1. We focus our effort on considerations of the anisotropy, symmetry, and intermittency of the surface turbulence. The analysis reveals that the root‐mean‐square surface temperature, when made nondimensional by using the surface heat flux and friction velocity, is nearly independent of Richardson number (Ri, defined in section 2). In addition, the derivatives of the thermal field appear also to converge to a limiting value at low Ri. The skewness of the temperature field, though slightly positive for the lowest wind speed (2 ms−1), is otherwise negative. On the other hand, the skewness of the derivative of the temperature field in the along‐wind direction is strictly positive, while the skewness in the cross‐wind direction is essentially zero, owing to the spanwise symmetry of the flow. This has the consequence that wind direction can be estimated by computing the skewness of the directional derivative of the temperature field. The flatness of the temperature field is observed to be near the Gaussian value of 3 throughout the wind speed range, while the along‐wind and cross‐wind derivatives show non‐Gaussian behavior, indicating the presence of intermittency in the thermal fields at small scales. All probability density functions of the temperature derivatives are seen to have Gaussian cores, with distinct exponential tails. Key Points Skewness of directional derivative of surface temperature related to wind speed Surface renewal model of Garbe et al. [2004] fits experimental data quite well Along‐wind and cross‐wind derivatives indicate intermittency in thermal fields