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Measurement of Ocean Surface Turbulence Generated by Rain Using the Controlled Flux Method
[24-May-17] Asher, W., Drushka, K., Jessup, A., Thompson, E., and Clark, D.
Presented at the Global Ocean Salinity and the Water Cycle Workshop
Freshwater deposited on the sea surface as rainfall can produce a stable near-surface layer of fresher water with a lifetime of O(1–10) hours and a depth of O(1) meter. One-dimensional turbulence models show that the magnitude of the salinity decrease at the surface is a function of wind speed and rain rate, and that near surface turbulence is a critical factor in the formation and evolution of a fresh lens. Although it is known that rain generates surface turbulence, the role this turbulence plays in mixing the freshwater downwards is not well understood. Measurements of surface mixing in a rain simulator suggest that turbulence generated by raindrops impacting the water surface only mixes the top few centimeters, an order of magnitude shallower than the observed rain stratification. Whether or not rain-generated turbulence is relevant in terms of the formation or evolution of a fresh lens is an open question due to lack of field measurements of surface turbulence in rain. Direct measurement of surface turbulence and vertical profiles of salinity during rain are needed to understand whether the turbulence generated by rain in the upper few centimeters has an impact on fresh lenses generated by rain. During the 2016 SPURS-2 field experiment in the eastern equatorial Pacific Ocean, the controlled flux technique (CFT) was used to infer surface turbulence before, during, and after rainstorms. CFT uses a carbon dioxide laser to heat a small patch of the ocean surface by a few degrees Celsius. An infrared imager then tracks the temperature decay of the patch, and the measured rate of the temperature decay can be related to the turbulence dissipation rate at the ocean surface. When combined with concurrent microstructure measurements made at a depth of 0.3 m, the contribution of rain to the dissipation rate at the ocean surface can be estimated. In this paper, preliminary results from the SPURS-2 CFT measurements will be presented to show the effect of rain on turbulence dissipation at the ocean surface. This data will be used along with concurrent measurements of salinity and temperature profiles in the upper meter of the ocean to better understand the generation and evolution of fresh lenses generated by rain.

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