Numerical Experiments Showing the Response of Parameterized Convection to Large-Scale Forcing

Charles Cohen Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

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Abstract

In a series of experiments with a two-dimensional mesoscale numerical model that uses the Frank-Cohen cumulus parameterization, different types of large-scale forcing are held constant and the response of the model atmosphere is examined. A dynamic equilibrium between convection and the specified large-scale forcing is achieved in many of the 24 h simulations, with the convection oscillating around a mean state.

In the numerical experiments, the different physical processes that make up the large-scale forcing in the model are examined. Removing the adiabatic cooling and including only the moisture convergence effect of the modeled intertropical convergence zone ITCZ causes a substantial decrease in the precipitation. Including just the adiabatic cooling, without the moisture convergence, produces the same total rainfall over 24 h as including just the moisture convergence. The large-scale upward motion of the ITCZ, as one component of the grid-scale upward mass flux at cloud base, strengthens the parameterized convection temporarily, but over a few hours the total amount of convection is limited by the stability. Other experiments examine the influence of radiative cooling and synoptic easterly waves.

With different types of large-scale forcing, the resulting vertical profiles of changes in temperature and water vapor mixing ratio show some common features. The parameterized tropical convection causes the domain- averaged virtual potential temperature sounding to move closer to a true moist adiabat.

Abstract

In a series of experiments with a two-dimensional mesoscale numerical model that uses the Frank-Cohen cumulus parameterization, different types of large-scale forcing are held constant and the response of the model atmosphere is examined. A dynamic equilibrium between convection and the specified large-scale forcing is achieved in many of the 24 h simulations, with the convection oscillating around a mean state.

In the numerical experiments, the different physical processes that make up the large-scale forcing in the model are examined. Removing the adiabatic cooling and including only the moisture convergence effect of the modeled intertropical convergence zone ITCZ causes a substantial decrease in the precipitation. Including just the adiabatic cooling, without the moisture convergence, produces the same total rainfall over 24 h as including just the moisture convergence. The large-scale upward motion of the ITCZ, as one component of the grid-scale upward mass flux at cloud base, strengthens the parameterized convection temporarily, but over a few hours the total amount of convection is limited by the stability. Other experiments examine the influence of radiative cooling and synoptic easterly waves.

With different types of large-scale forcing, the resulting vertical profiles of changes in temperature and water vapor mixing ratio show some common features. The parameterized tropical convection causes the domain- averaged virtual potential temperature sounding to move closer to a true moist adiabat.

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