Oceanic LES Simulations to Interpret and Synthesize Turbulence Measurements Obtained During CBLAST-Low

Abstract

LONG-TERM GOALSTo understand the effects of upper-ocean turbulent processes on air-sea interaction and obtain improved parameterization of these processes for use in coastal and large-scale ocean models. OBJECTIVESThis proposal has two primary objectives: (1) to improve understanding of air-sea interaction in fetchlimited shallow seas; (2) interpret observational data collected during CBLAST-Low. APPROACHBecause of limited wind fetch and shallow water depth, air-sea interaction in coastal oceans is very different from that in the open ocean. Surface waves in coastal oceans often have a sea state far from the fully-developed sea. As shown in recent observations (Donelan et al., 1993;Garrett, 1997), drag coefficient is a strong function of wave age. At the same wind speed, the air-sea momentum flux in growing seas may be significantly larger than that in fully-developed seas. Wave spectrum in growing seas is narrowly peaked so that the Stokes drift current is different. These differences in wind stress and wave field can cause dramatic changes in turbulence characteristics in the ocean surface layer. The Air-Sea Interaction Tower (ASIT) constructed during the CBLAST program provides an ideal platform to observe and monitor air-sea interaction processes in the coastal ocean. Our modeling work is intended to facilitate the interpretations of the CBLAST measurements and improve our understanding of air-sea interaction in fetch-limited shallow seas.Surface forcing conditions during CBLAST-Low experiment varied from negligible wind stress and strong thermal forcing to medium wind speeds where wave breaking and Langmuir circulations are expected to play a role in air-sea exchange processes (Plueddemann, 2004). The majority of winds were from the SW quadrant at 3-7 m/s, whereas the weakest winds were from the SE. The strongest winds were from the NW, but these events were short lived compared to those from the SW. Varying wind directions relative to Martha's Vineyard create a complex wave field at the observational site, which may include growing, short-fetch wind waves and decaying wind waves along with persistent swell from the south. To interpret the observational data collected under these complex wind and wave conditions, we have identified a few "key" scenarios/events and conducted simulation studies using the LES model. They include (1) steady wind and growing/decaying waves; (2) steady wind and swells; 1