The Moist Boundary Layer with a Higher Order Turbulence Closure Model

Stephen D. Burk National Severe Storms Laboratory, NOAA, Norman, Okla. 73069

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Abstract

A one-dimensional higher order turbulence closure model is used to investigate moisture structure within the diurnally varying planetary boundary layer. The diurnal character of the moist boundary layer as a whole and a variety of micrometeorological features are examined in a series of experiments having differing lower boundary conditions on the moisture field. In one case, midafternoon surface evaporation and turbulent moisture transfer to higher levels act as competing processes in determining low-level moisture content. A double wave in low-level daily specific humidity results (specific humidity minima in early morning and midafternoon). In another experiment, a moisture inversion develops when there is a strong nocturnal moisture flux to the surface such as occurs with dew formation.

A simple, analytic method of calculating the moist layer's growth rate is compared with the numerical results. The analytic method provides good flux estimates when the shoulder in the specific humidity profiles (where the moisture lapse first sharply deviates from its mixed-layer value) is treated as being the top of the moist boundary layer.

The specified initial moisture distribution has a considerable lapse above 0.5 km. However, during the afternoon a well-mixed moist layer develops despite dry air entrainment above and surface moisture influx from below. This suggests that rapid growth into a dry environment cannot explain the coincidence of strong moisture lapses with thermally well-mixed regions.

Abstract

A one-dimensional higher order turbulence closure model is used to investigate moisture structure within the diurnally varying planetary boundary layer. The diurnal character of the moist boundary layer as a whole and a variety of micrometeorological features are examined in a series of experiments having differing lower boundary conditions on the moisture field. In one case, midafternoon surface evaporation and turbulent moisture transfer to higher levels act as competing processes in determining low-level moisture content. A double wave in low-level daily specific humidity results (specific humidity minima in early morning and midafternoon). In another experiment, a moisture inversion develops when there is a strong nocturnal moisture flux to the surface such as occurs with dew formation.

A simple, analytic method of calculating the moist layer's growth rate is compared with the numerical results. The analytic method provides good flux estimates when the shoulder in the specific humidity profiles (where the moisture lapse first sharply deviates from its mixed-layer value) is treated as being the top of the moist boundary layer.

The specified initial moisture distribution has a considerable lapse above 0.5 km. However, during the afternoon a well-mixed moist layer develops despite dry air entrainment above and surface moisture influx from below. This suggests that rapid growth into a dry environment cannot explain the coincidence of strong moisture lapses with thermally well-mixed regions.

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