Observations of Large Wind Shear above the Marine Boundary Layer near Point Buchon, California

• Published in: Journal of the atmospheric sciences Vol. 73; no. 8; pp. 3059 - 3077
• Main Authors: Rahn, David A, Parish, Thomas R, Leon, David
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.08.2016
• Subjects: Aerosols; Air; Air temperature; Aircraft; Atmosphere; Atmospheric boundary layer; Boundary layers; Case studies; Coasts; Dry air; Gravitational waves; Gravity waves; Hydraulics; Kelvin-Helmholtz instability; Kinematics; Lower atmosphere; Mixed layer; Offshore; Research aircraft; Richardson number; Strong winds; Temperature inversion; Temperature inversions; Topography; Troposphere; Turbulence; Velocity; Wind; Wind shear; Winds; Southern California; United States > US; California; Santa Barbara California
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-15-0363.1

Abstract Particularly strong winds along the coast of Southern California on 24 May 2012 were measured by the Wyoming King Air research aircraft during the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE). The fast flow is bounded laterally by the coastal topography and vertically by a pronounced temperature inversion separating the cool, moist air in the marine boundary layer (MBL) from the warm, dry air aloft. Many studies have investigated the response of this two-layer flow to changes in the coastline by invoking hydraulic theory, which explains the essential characteristics including changes in MBL depth and the attendant wind. Processes occurring just above the MBL are important to the low-level thermodynamic and kinematic structure. Observations on this day demonstrate how the large shear above the MBL can impact the lower atmosphere. A typical two-layer system was observed north of Point Buchon, which was supercritical. Around Point Buchon, the depth of the MBL decreased and wind increased, characteristic of an expansion fan. As a result, the Richardson number becomes reduced and favors shear instability that breaks down into turbulence. Observations indicate that a secondary well-mixed layer develops above the MBL that is bounded by narrow layers of high stability separating the secondary layer from the MBL below and the free troposphere above. It is hypothesized that the secondary layer develops as a result of Kelvin–Helmholtz instability, although more targeted observations are needed to confirm or reject that hypothesis.