Planetary Boundary Layer Flow of a Stable Atmosphere Over the Globe

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Abstract
When the temperature and pressure perturbations are taken into consideration in the boundary layer equations, the stable stratification is found to inhibit the steady, Ekman-type flow when the global Richardson number is of or exceeds order unity. This inhibiting influence of the stable stratification is mitigated greatly at middle and higher latitudes when the flow is oscillatory even at low frequency, but the boundary layer flow fades away at low latitudes. When a surface temperature anomaly is present, the stratification also generates a thermally driven boundary layer flow. This flow increases toward the equator and with the global Richardson number. At the equator the surface friction tends to produce a downward velocity when the surface wind is westerly. Under a normal stratification and the slipping lower boundary condition valid for natural flow, the maximum vertical velocity just above the surface boundary layer has an absolute maximum around latitude 10° and a minimum at the equator. This distribution of the lifting velocity can be used to explain the frequent occurrences of the intertropical convergence zone in this region and the lack of convective activities at the equator.

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