Variability of ocean skin temperature from airborne infrared imagery during CBLAST-Low

Airborne infrared (IR) remote sensing techniques have been shown to quickly characterize the spatial and temporal scales of ocean skin temperature as well as a wide variety of processes that are important to the variability of air-sea fluxes of heat, mass, and momentum.  Measurements of ocean skin temperature variability were made during the main field campaigns of the CBLAST-Low (Coupled Boundary Layers, Air-Sea Transfer in Low Winds) experiment in August/September 2002 and July/August 2003 off the south coast of Martha’s Vineyard.  We flew a state-of-the-art, high spatial resolution, dual up- and down-looking longwave IR imaging system that included in-flight calibration capability aboard a Cessna Skymaster.  High-quality daytime measurements were achieved since the longwave imager minimized solar contamination of the IR imagery.  The downward-looking IR imager had a spatial resolution of less than 1 m and temperature resolution of less than 0.02°C.  Regional maps of the CBLAST study site using low-noise, high-resolution spatial series of skin temperature show that diurnal warming and tidal advection/mixing control the scales of SST (mean of 2°C 10km^-1) that regulate the heat flux variability (mean of 60 W m^-2 10km^-1).  Fine-scale snapshot imagery of ocean skin temperature with a spatial coverage of roughly 444 m by 555 m elucidate a variety of mechanisms related to atmospheric and sub-surface phenomena that produce horizontal variability over a wide range of scales that decreased with increasing wind speed.  Under low wind-speed conditions (0 to 2.5 m s^-1), the IR imagery shows high temperature variability on scales of O(1 m to 100 m), including extensive regions (O(1 km)) of closely-spaced (O(10 m)) successive sharp coherent temperature ramps of O(1ºC) that coincide with ubiquitous visible surface slicks parallel to the fronts.  During moderate winds (2.5 to 5 m s^-1), we observed distinct row/streak structures in the IR imagery that were aligned with the wind and were likely the surface manifestation of Langmuir circulation cells.  The horizontal spacing of these features increased from 10 m up to 50 m with the increase in fetch offshore and coincided with wind-aligned surface slicks and bubbles visible in the video.  For wind speeds greater than 5 m s^-1, the data show significantly less temperature variability with a high incidence of breaking waves. Comparisons of the IR measurements to in-situ sea-surface microlayer characteristics and a suite of moored, drifting, and towed ocean measurements are used to investigate the mechanisms that affect the spatial and temporal scales of ocean skin temperature variability.