Title: Impacts of Stochastic Mixing in Idealized Convection-Permitting Simulations of Squall Lines

Abstract

This study investigates impacts of altering subgrid-scale mixing in “convection-permitting” kilometer-scale horizontal-grid-spacing (Δ_h) simulations by applying either constant or stochastic multiplicative factors to the horizontal mixing coefficients within the Weather Research and Forecasting Model. In quasi-idealized 1-km Δ_h simulations of two observationally based squall-line cases, constant enhanced mixing produces larger updraft cores that are more dilute at upper levels, weakens the cold pool, rear-inflow jet, and front-to-rear flow of the squall line, and degrades the model’s effective resolution. Reducing mixing by a constant multiplicative factor has the opposite effect on all metrics. Completely turning off parameterized horizontal mixing produces bulk updraft statistics and squall-line mesoscale structure closest to an LES “benchmark” among all 1-km simulations, although the updraft cores are too undilute. The stochastic mixing scheme, which applies a multiplicative factor to the mixing coefficients that varies stochastically in time and space, is employed at 0.5-, 1-, and 2-km Δ_h. It generally reduces midlevel vertical velocities and enhances upper-level vertical velocities compared to simulations using the standard mixing scheme, with more substantial impacts at 1- and 2-km Δ_h compared to 0.5-km Δ_h. Further, the stochastic scheme also increases updraft dilution to better agree with the LES for one case, butmore » has less impact on the other case. Stochastic mixing acts to weaken the cold pool but without a significant impact on squall-line propagation. It also does not affect the model’s overall effective resolution unlike applying constant multiplicative factors to the mixing coefficients.« less

Authors:

Stanford, McKenna W. ^[1]; Morrison, Hugh ^[2]; Varble, Adam C. ^[3]

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+ Show Author Affiliations

1. Univ. of Utah, Salt Lake City, UT (United States)
2. National Center for Atmospheric Research, Boulder, CO (United States)
3. Univ. of Utah, Salt Lake City, UT (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Publication Date:

Thu Dec 10 00:00:00 EST 2020

Research Org.:

Pacific Northwest National Lab. (PNNL), Richland, WA (United States); National Center for Atmospheric Research (NCAR), Boulder, CO (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Biological and Environmental Research (BER)

OSTI Identifier:

1735951

Alternate Identifier(s):

OSTI ID: 1786978

Report Number(s):

PNNL-SA-156541
Journal ID: ISSN 0027-0644; TRN: US2205273

Grant/Contract Number:  

AC05-76RL01830; SC0016476

Resource Type:

Accepted Manuscript

Journal Name:

Monthly Weather Review

Additional Journal Information:

Journal Volume: 148; Journal Issue: 12; Journal ID: ISSN 0027-0644

Publisher:

American Meteorological Society

Country of Publication:

United States

Language:

English

Subject:

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY