The diurnal wind variation in a variable eddy viscosity semi-geostrophic Ekman boundary-layer model: Analytical study |
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| Authors: | Yi Zhang Zhe-Min Tan |
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| Institution: | (1) The Key Laboratory of Mesoscale Severe Weather/MOE, Department of Atmospheric Sciences, Nanjing University, Nanjing, P. R. China, CN |
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| Abstract: | Summary ?A time-dependent semi-geostrophic Ekman boundary-layer model (SG), including slowly varying eddy diffusivity with height
and inertial term effects, is developed to investigate the diurnal wind variation in the planetary boundary layer (PBL). An
approximate analytical solution of this model is derived by using the WKB method, which extends the Tan and Farahani (1998)’s
solution by including the vertical variable eddy viscosity.
The features of the diurnal wind variation in the PBL mainly depend on three factors: the latitude, horizontal momentum advection
and eddy viscosity. The vertical variable eddy viscosity has little influence on diurnal wind variation in the PBL at the
low latitude, however its effect may be exacerbated in the mid- and high latitudes. In comparing with the constant eddy viscosity
case, the decreasing (increasing) with height eddy viscosity produces a large (small) maximum wind speed (MWS) in the PBL,
however, the eddy viscosity that has a mid-layer peak in the vertical gives rise to a higher height of occurrence of MWS.
For the boundary-layer wind structure, there is a singular point when the modified SG inertial oscillation frequency η equals
the forcing frequency ω. The isotachs of boundary-layer wind speed have almost no tilt to left or right relative to time evolution
and the occurrence time of the MWS is the earliest at the singular point. The feature will be enhanced in the decreasing with
height eddy viscosity case and weakened in the eddy viscosity initially increasing with height case.
Received April 6, 2001; accepted December 27, 2001 |
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