Effects of Shallow Convective Mixing on Convective Organization and Heavy Precipitation Statistics

Monday, December 10, 2018 - 08:00
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This paper examines how shallow convective activity can change properties of convective organization and consequently can affect the rainfall distribution and the strength of precipitation extremes.
Previous analyses had proposed that lateral advection of moisture into convergence zones can suppress deep convection at the edges and enhance it in the cores of these zones. This statement implies that boundary layer turbulence and vertical redistribution of moisture by shallow convection may have profound effects on heavy rainfall intensities. Although shallow convective mixing is known to affect climate sensitivity, its impact on precipitation statistics has not yet been systematically quantified.

We analyze idealized simulations of radiative convective equilibrium in the System for Atmospheric Modeling run on square domains with 4km resolution. We first quantify the dependence of rainfall statistics on the degree of organization by varying the Smagorinsky coefficient in the turbulence scheme for different sea surface temperatures (SST). This also allows us to decouple the influence of SSTs from the role of aggregation on the intensification of extreme precipitation.
We then focus on organized states alone as an idealized representation of convergence zones interacting with the large-scale circulation. This setup is used to study the response of deep convection to different shallow convective mixing strengths by varying the vertical component of turbulent diffusivity by a constant factor in the dry region.

We quantify the response of mean climate quantities, including mean rainfall, components of the atmospheric energy budget, and the area of the moist patch, before characterizing changes in the entire distribution of rain. In addition to rainfall percentiles and frequencies, a normalized distribution of rain amounts is calculated as a convenient way to isolate the response in the distribution shape from changes in the domain mean rainfall. These experiments motivate the need to further quantify the role of convective aggregation, shallow convective mixing and boundary layer dynamics on the uncertainties of future rainfall projections in climate models.

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