The Structure of the Unstable Marine Boundary Layer Viewed by Lidar and Aircraft Observations

Abstract

The combination of vertical lidar and in situ meteorological observations from two aircraft provide an unprecedented view of the marine atmospheric boundary layer (MABL) during a cold air outbreak. To a first approximation, the lidar reflectivity is associated with the concentration of sea salt aerosols. Across the capping inversion, the lidar reflectivity contours approximate isentropes and streamlines thereby defining the inversion. Within the mixed layer, high reflectivity cores are associated with updrafts carrying aerosol-rich air upward and conversely. These effects are enhanced by increasing humidity in updraft and decreasing humidity in downdrafts that operate to increase and decrease aerosol sizes. Narrow high reflectivity columns extend upward from the ocean indicating that organized flow exists all the way to the surface. Entrainment across the inversion is manifested by small scale perturbations (?200?500 m) superimposed upon the large scale ( generally, this is on the upshear side of the, convective. domes where Kelvin-Helmholtz instability is triggered by local compression of the inversion, The MABL on 20 January 1983 is highly organized. The organization takes the form of 1?2 km scale roll vortices and corresponding undulations of the inversion with amplitude of 150?200 m peak to trough. The roll circulation is very strong with up and downdrafts of 2?4 in s?-1 at the 210 m level. The axes of the rolls are essentially north-south along the direction of the strong northerly low-level winds. The rising arm of the roll coincides with a column of high lidar reflectivity and with the updraft which transport aerosols, moisture, and heat up from the surface. The presence of the rolls, driven mainly by the combination of strong transverse sheer and buoyancy, serves to produce low-level convergence which concentrates the small-scale buoyant eddies to form a single well-ordered updraft in the manner previously postulated by LeMone. The fluxes measured by the covariance method in the undulating inversion are unreliable because of the sensitivity to detrending and inadequate sampling of the exchanges across the interfaces of the dames and troughs. The partitioning method of Wilczak and Businger provides improved insight as to the mechanisms responsible for the downward flux in the inversion. However, unlike Wilczak and Businger, who find the downward flux dominated by cold updrafts we find that it is due mainly to the entrainment of warm eddies which are then transported downward by the larger-scale roll circulations on the downshear side of the domes.