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PhD Defense

Department of Atmospheric Science

Thurs., Mar. 23, 3:10 pm, EN6085

Evaluation of the Impact of Turbulence and Overturning Cells on Mixed-phase Clouds and Precipitation using Idealized WRF Large Eddy Simulations

Xia Chu

University of Wyoming

Abstract

Much research has been devoted to the question how microscale turbulence (i.e., at the scale of the spacing between particles) affects precipitation growth in warm and cold clouds, but the question how large-eddy turbulence or coherent overturning in stratiform clouds has hardly been addressed. This study uses idealized WRF large eddy simulations (LES) with a bulk microphysics scheme (the Thompson scheme) to examine the impact of planetary boundary layer (PBL) turbulence, as well as shear-induced overturning cells (Kelvin Helmholtz billows) above boundary layer, on hydrometeor growth and precipitation in stratiform mixed-phase cloud. The basic argument in favor of enhanced growth is that deep vertical eddies sufficiently alter the saturation vapor pressure of water and ice, such that depositional snow growth in updrafts is not fully compensated by snow sublimation in downdrafts. This process, proposed in some observational studies (Houze and Medina 2005; Medina and Houze 2015; Geerts et al. (2011), is offset by the enhanced detrainment of cloudy air and entrainment of dry air by large eddies, if turbulence (or coherent billows) extends to the cloud base/top.These two counter-acting processes compete in the PBL turbulence case, where mixing leads to an elevated cloud base even when the initial (stable) profile is saturated down to the ground. The net impact of PBL turbulence on snow growth and precipitation may vary depending on relative humidity and stability above cloud top, cloud base height, cloud depth, turbulence intensity, temperature, as well as cloud microphysical aspects not captured in the Thompson scheme. We then examined KH billows triggered in a shear layer in a deep stratiform cloud. Extra vapor condensation in updrafts leads to additional snow growth, but this is largely offset by snow sublimation in downdrafts. Without breaking into turbulence, these coherent overturning cells horizontally redistribute the hydrometeors but do not significantly affect hydrometeor growth and precipitation on average, even in the absence of entrainment/detrainment.

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