Aircraft Observations of Boundary Layer Rolls off the Coast of California

Abstract

Large-scale horizontal rolls can have a significant influence on turbulent transport across the atmospheric boundary layer. The formation and maintenance of such rolls is dependent on the thermal and dynamic stability of the boundary layer (BL). The authors present aircraft observations of boundary layers, both with and without roll circulations, off the coast of California. The contribution of the rolls to the turbulent fluxes of heat, moisture, and momentum, and the variances of the three velocity components are determined for four cases. The fractional roll contributions to the u and w variances, and the sensible heat and along-wind momentum fluxes, show a near linear increase with altitude, from less than 10% at 30 m to more than 70% at the top of the BL. The variance in υ and crosswind momentum flux are more scattered, although the variance shows a slight increase with altitude from about 40% to 60%. The latent heat flux also shows a great deal of scatter, especially in the lower third of the BL where the total flux is small; above this, values range between about 40% and 85% but show no clear trends. A stability parameter in the form of a bulk Richardson Ri number is calculated for each of 13 profiles through the boundary layer; it is found that the Richardson number successfully identifies those cases where rolls are present, and its value corresponds to some extent with the strength of the rolls. Values close to zero correspond to cases with well-defined rolls; for 0.1 < Ri < 0.25 rolls are found to exist, but they tend to be weak and patchy; and no rolls are found where Ri is greater than the critical value of approximately 0.25. Reynolds numbers are calculated from a number of different definitions and indicate the dynamic instability of the shear dominated boundary layers.