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Explicit Simulation of Cumulus Ensembles with the GATE Phase III Data: Comparison with Observations

Kuan-Man XuDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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David A. RandallDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

The macroscopic behavior of cumulus convection and its mesoscale organization during Phase III of the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) is simulated with a two-dimensional (2D) cloud ensemble model. The model includes a three-phase bulk microphysics parameterization, a third-moment turbulence closure and an interactive, radiative transfer parameterization. The observed large-scale, horizontal advective effects and large-scale vertical velocity me imposed on the model's thermodynamic equations uniformly in the horizontal. The simulated, domain-averaged horizontal wind components are nudged toward the observed winds.

A detailed comparison with available observations is made in this study. The observed time variations of the surface precipitation rate, surface evaporation rate, outgoing longwave radiation flux, and the vertical distributions of temperature, water vapor mixing ratio, and relative humidity are successfully reproduced by the model, as well as the vertical structure and time evolution of major convective systems. The most significant result is that the model is able to reproduce the negative correlation between the intensity of convection and the convective available potential energy. The simulated total cloud amount compares favorably with the whole-sky camera observations of Holle et al., but the low-level cloud amount is significantly underestimated. In spite of its success, sensitivity tests suggest that the 2D model has stronger inhibiting effects on convection and is more efficient in vertical transports than is observed when the vertical wind shear is strong. The CEM also produces smaller amplitude of the daily fluctuations in cloud amount and precipitable water than observed, due possibly to the shortcomings of the microphysics parameterization.

Abstract

The macroscopic behavior of cumulus convection and its mesoscale organization during Phase III of the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) is simulated with a two-dimensional (2D) cloud ensemble model. The model includes a three-phase bulk microphysics parameterization, a third-moment turbulence closure and an interactive, radiative transfer parameterization. The observed large-scale, horizontal advective effects and large-scale vertical velocity me imposed on the model's thermodynamic equations uniformly in the horizontal. The simulated, domain-averaged horizontal wind components are nudged toward the observed winds.

A detailed comparison with available observations is made in this study. The observed time variations of the surface precipitation rate, surface evaporation rate, outgoing longwave radiation flux, and the vertical distributions of temperature, water vapor mixing ratio, and relative humidity are successfully reproduced by the model, as well as the vertical structure and time evolution of major convective systems. The most significant result is that the model is able to reproduce the negative correlation between the intensity of convection and the convective available potential energy. The simulated total cloud amount compares favorably with the whole-sky camera observations of Holle et al., but the low-level cloud amount is significantly underestimated. In spite of its success, sensitivity tests suggest that the 2D model has stronger inhibiting effects on convection and is more efficient in vertical transports than is observed when the vertical wind shear is strong. The CEM also produces smaller amplitude of the daily fluctuations in cloud amount and precipitable water than observed, due possibly to the shortcomings of the microphysics parameterization.

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