Why does convection growth upscale faster into lines in 1-km simulations compared to 3-km: An examination using CM1

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2023-08
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Luthi, Samuel T
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Gallus Jr., William A.
Franz, Kristie J.
Williams, Ian N.
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Geological and Atmospheric Sciences
Abstract
It is known from previous research that differences exist in model simulations of convective mode as horizontal grid spacing is refined from 3-km to 1-km. It is of particular interest as to why 3-km model simulations depicting clusters of cells tend to adopt a more linear structure in 1-km simulations. One theory is that this increase in linear structures at finer horizontal grid spacings is due simply to the resolving of stronger vertical motion in a 1-km run compared to a 3-km run along the leading edge of the MCS, while other theories hold that stronger cold pools create stronger lift to more rapidly create a linear morphology. CM1 was used to simulate an array of MCSs with varying wind profiles (RKW u-wind, Weisman-Klemp Multicell u-wind) and a constant thermodynamic profile (Weisman-Klemp analytic sounding). A line of seven randomly-spaced warm bubbles was used to initiate convection. Similar trends were found in rainwater and graupel mixing ratios, as well as in cold pool intensities when compared with Squitieri and Gallus (2022a). The change in Δ? from 3-km to 1-km is more impactful on the number, cumulative area, and intensity of updrafts than the change in the number of vertical levels. In 1-km Δ? simulations, gravity waves appear to play the most dominant role in the increase of vertical motion that initiates new convection for growth into lines. 3-km Δ? simulations appear to have their upscale growth into lines being driven by ascent caused by the collision of convective cold pools.
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