Aircraft Measurements of a Low-Level, Three-Layer Structure Offshore of Pt. Buchon, CA and the Possible Role of Kelvin-Helmholtz Instability

Particularly strong, low-level northerly winds along the coast of southern California on 24 May 2012 were measured by the University of Wyoming King Air research aircraft during the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE). Observations were gathered by in situ probes and the Wyoming Cloud Lidar with upward and downward pointing beams. The fast flow is bounded laterally by the coastal topography and vertically by a pronounced temperature inversion separating the cool, moist air in the boundary layer with the warm, dry air aloft. Many studies have investigated the response of a two-layer flow to changes in the coastline by invoking hydraulic theory, which explains the essential characteristics. Thus, most of the attention has been given to changes of the boundary layer depth and the attendant wind.

Processes just above the boundary layer are also important, and observations on this day provide some evidence of Kelvin-Helmholtz instability above the boundary layer that may be responsible for modifying the ideal two-layer structure of the lower atmosphere. Observations of two examples north (upstream) of a major bend at Point Conception are shown. The first example is north of Point Buchon where the coastal boundary is fairly linear and unbroken. The wind has little to no cross-shore component. The measurements suggest that there is a superadiabatic layer in the process of overturning that is generating vertical mixing near the top of the inversion layer (400-550 m). The second example is above an expansion fan by Point Buchon where wind shear increases and stability decreases, which would both act to lower the Richardson number below the critical threshold. There is clear a secondary well-mixed layer as detected by the Wyoming Cloud Lidar that is bounded by two narrow layers of higher stability, which marks the marine boundary layer below and the free troposphere above, which is confirmed by the in situ probes. While the limited observations are not completely conclusive, it appears that the structure of the lower atmosphere is consistent with an environment after the overturning of Kelvin-Helmholtz instability.