Frictional decoupling and the inertial oscillation in stable marine atmospheric boundary layers

The vertical structure of the Marine Atmospheric Boundary Layer (MABL), up to a height of 600m, was studied using remote and in situ instrumentation on Nantucket Island during the CBLAST-Low field experiment in the summer of 2003. An Acoustic Sounder (SODAR) system was deployed at the CBLAST Nantucket site that measured the vertical profiles of the horizontal wind speed and direction, the echo strength, the three wind components and their variances, the momentum fluxes ( and ) and the atmospheric stability at 30 minutes intervals with a vertical resolution of 40 m. Concurrent measurements of mean and turbulence field from a 20-m mast were also made to yield momentum, heat, and water vapor fluxes and the basic meteorological parameters. Rawinsondes were also launched at the site four to six times per day. The vertical wind profile measured by SODAR revealed the presence of a Low-Level Jet (LLJ) between 150 and 200m above the ground that last for several hours at night. The vertical profiles of the echo strength (analogue to the temperature structure parameter CT2) and the stability class has shown that the MABL is characterized by very stable atmospheric conditions at the first 150-200m followed by slightly stable to neutral conditions at higher levels. The corresponding profiles of the potential temperature, and the bulk Richardson number (Ri), estimated from the rawinsondes data, confirmed the existence of an intense ground based inversion (with mean gradient Äè/Äz =2oK/100m) up to 150-200m height followed by slightly stable conditions. This strong stability of the MABL lower layer leads to a frictional decoupling over the sea that could be associated with the development of an inertial oscillation of the wind vector. This mechanism is confirmed by the hodographs of the wind near the LLJ core, where the wind turned relative to the geostrophic wind with a period between fifteen to seventeen hours. This period is consistent with that estimated from the theory of inertial oscillations. Results from analyses of SODAR and other relevant measurements on the vertical structure of the marine boundary layer, the LLJ, and the inertial oscillation of the wind vector under different meteorological conditions will be presented and discussed. We will also examine the capability of COAMPS in simulating the inertial oscillations in similar conditions.