A modeling study of the effects of baroclinicity on the structure of the interacting atmospheric and oceanic boundary layers

Le Ngoc Ly, Eugene S. Takle
1988 Journal of Geophysical Research  
The effects of baroclinicity on the air and ocean boundary layers under conditions for strong dynamical (compared to thermodynamic) forcing are studied by use of a numerical model of air-sea interaction, which consists of a closed system of equations including equations of motion, turbulent kinetic energy, turbulent exchange coefficient, local turbulent length scale, and assumptions of fixed stratification and baro½linicity in both the atmosphere and ocean. Baro½linicity is incorporated into
more » ... equations of motion by specifying horizontal gradients of air temperature in the atmosphere and seawater density in the ocean. Experiments were conducted to determine the effects of different magnitudes and directions of baroclinicity and of atmospheric stratification on the dynamical and turbulent structure of the interacting boundary layers. The results of the simulations demonstrate that certain levels of baroclinicity produce double maxima in the K profiles in the atmosphere and ocean. Baro½linic effects change the dominant components of the turbulent kinetic energy in both air and sea boundary layers from shear production and dissipation for dimensionless heights and depths of less than 0.1 (about 20% of the height or depth of the boundary layer at zero surface heat flux) to shear production and buoyant destruction for dimensionless heights and depths greater than 0.1. The results show that the most significant effects of baroclinicity in the air and sea boundary layers are the increases in turbulent exchange coefficient, turbulent kinetic energy budget, shear stresses, and dimensionless wind and windinduced current in the regions of the boundary layers far from the interface. The results of the simulations also show that for fixed stratification and baroclinicity, surface quantities (e.g., friction velocity, drag coefficient, and geostrophic drag coefficient) are affected more by surface heat flux than by barodinicity, whereas the opposite is true for characteristics of the whole boundary layer (e.g., boundary layer height and angle between the geostrophic wind and surface stress). Our results show good agreement with the few observations that have been taken where baroclinicity has been reported. 1. 1986; Tarnopolski and Shnaydrnan, 1984; Yeh, 1973, 1974; Laikhtrnan, 1970]. However, horizontal gradients of air temperature and seawater density always are present in their respective boundary layers. Early studies [Vorobyev, 1969; Wippermann and Yordanov, 1972; Voltsinger et al., 1973; Arya and Wyngaard, 1975] and more recent reports [Russell and Takle, 1985a, b; Stubley and Rooney, 1986] have suggested a significant role of baroclinicity, but these results are only for the atmosphere and do not consider the coupled boundary layers. In this paper we examine, by use of a numerical model, the effects of horizontal temperature gradients on properties of the atmospheric and oceanic boundary layers for conditions of strong dynamical (as compared with thermodynamic) forcing. The main objective of the model is to determine the effects of Copyright 1988 by the American Geophysical Union. Paper number 8C0111. 0148-0227/88/008C-0111 $05.00 8203 the primary flow on the air-sea interaction processes, so the effects of secondary flows (e.g., horizontal rolls as discussed by Brown and Liu [-1982] and Brown [-1981]) are not included in the model. Baroclinic conditions commonly exist in both media, so we examined how baroclinicity in one medium affects its own properties as well as properties of the other medium. The specific properties we studied include the turbulence kinetic energy, the turbulence exchange coefficient, boundary layer depth, angular difference between the surface wind and the geostrophic wind, friction velocity, drag coefficient, roughness height, and shear stress. Few measurements exist for comparison with model-derived results, but where available, observations are compared with our calculations. 2. GOVERNING EQUATIONS AND BOUNDARY CONDITIONS The present work uses the well-known equations of motion for planetary boundary layers of the atmosphere and ocean. Governing equations for the model include equations of motion, turbulent kinetic energy equations, turbulent exchange coefficient equations, equations for atmospheric and oceanic stratification, and equations for baroclinicity in both media. Such a system of equations can be written in the same form for the atmospheric and oceanic boundary layers [Ly, 1986]. The steady state momentum equations for a horizontally homogeneous boundary layers (i = 1 for the atmosphere and i = 2 for the ocean)can be written as
doi:10.1029/jc093ic07p08203 fatcat:kmyk5wfvwzap7osn44lbyyyjje