Quadrant analysis of the scalar and momentum fluxes in the marine Atmospheric Surface Layer

Sonic anemometer measurements of the wind speed and the temperature at two different levels (10m and 20m height), along with humidity measurements at 20 m height, in the marine atmospheric surface layer, at Nantucket Island, Massachusetts, all sampled at 20Hz, are used for the calculation of both momentum (u'w' and v'w' ) and scalar fluxes (w'T' and w'q' ), under varying stability and wind speed conditions. These measurements were conducted at a distance of 90 meters from the shoreline over relatively flat terrain, in the framework of the Coupled Boundary Layer Air – Sea Transfer, Low Wind Project (CBLAST-Low), during the 2003 experimental campaign. Ten minutes averages of the three wind components (u, v and w ), calculated in the sonic coordinate system, for a 28 days time period, were utilized in order to correct the data for sonic alignment errors. This correction was made in the sense of eliminating the dependence of w from both u and v . Furthermore, the non-stationary records, corresponding to 1 hour time period, were excluded. The data concerning the marine surface layer was carefully separated from the whole data set. It should be mentioned that an Internal Boundary Layer (IBL) is expected to develop reaching the depth of 10 meters, in most of the cases. Thus, marine and IBL influenced data for the 20m height was defined through a detailed examination of the measured momentum, heat and humidity flux, and the stability parameter (z/L) time series at both levels (10 and 20m). Both scalar and momentum fluxes for the marine sector have been analyzed using the quadrant analysis. The relative importance of the dominant, short–lived turbulent events under varying meteorological conditions is revealed. Regarding the momentum flux, stress fractions for organized motions (ejections and sweeps) are enhanced for decreasing time fractions, while the uncorrelated motions (inward and outward interactions) exhibit the opposite behavior. Ejections prevail against sweeps for low wind speed and neutral or unstable conditions. Exuberance and Time Exuberance are enhanced for increasing stability and low drag coefficient values, while they are independent of the wind speed. Regarding the scalar fluxes, the general characteristics and main difference compared to the momentum fluxes are also presented and discussed.