On the Generation of Convectively Driven Mesohighs Aloft

J. M. Fritsch Office of Weather Research and Modification, NOAA, Environmental Research Laboratories, Boulder, CO 80303

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J. M. Brown Office of Weather Research and Modification, NOAA, Environmental Research Laboratories, Boulder, CO 80303

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Abstract

In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.

Abstract

In an attempt to determine the relative contribution of the direct incorporation of cold air (detrainment from overshooting convective cloud tops) to the production of mesohighs in the vicinity of the tropopause, two numerical simulations were performed using a 20 km horizontal resolution, 20-level primitive equation model. One simulation included direct cooling and the other did not. The results showed that including the cooling increased the high-level pressure and wind perturbations by approximately 30 and 40%; respectively. The simulation results also showed that in spite of the omission of the direct cloud cooling, a high-level cold pool was still generated. The cooling was accomplished by adiabatic expansion in response to the lifting by the convectively driven mesoscale vertical circulation. Thus, it appears that the mesoscale adiabatic expansion is the dominant effect in elevated-mesohigh production and the detrainment of overshooting air is an important modifying factor.

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