Effects of Horizontal and Vertical Grid Spacing on Mixing in Simulated Squall Lines and Implications for Convective Strength and Structure

Abstract

The sensitivity of an idealized squall line to horizontal D h and vertical D y grid spacing is investigated using a new approach. Simulations are first performed at a horizontal grid spacing of 1 km until the storm reaches its mature stage. The model output is then interpolated to smaller (and larger) grid spacings, and the model is restarted using the interpolated state plus small thermodynamic perturbations to spin up small-scale motions. This framework allows an investigation of the sensitivity of the storm to changes in D h without complications from differences in storm initiation and early evolution. The restarted simulations reach a quasi steady state within approximately 1 h. Results demonstrate that there are two D h -dependent regimes with the transition between regimes occurring for D h between 250 and 500 m. Some storm characteristics, such as the mean convective core area, change substantially for D h . 250 m but show limited sensitivity as D h is decreased below 250 m, despite better resolving smaller-scale turbulent motions. This transition is found to be independent of the chosen D y . Mixing in the context of varying D h and D y is also investigated via passive tracers that are initialized 1 h after restarting the simulations (i.e., after the spin up of small-scale motions). The tracer field at the end of the simulations reveals that entrainment and detrainment are suppressed in the simulations with D h $ 500 m. For decreasing D h , entrainment and detrainment are substantially more important, limiting the flux of low-level tracer to the upper troposphere, which has important implications for modeling studies of convective transport from the boundary layer through the troposphere.

Vertical Velocity in the Gray Zone

“…This is plotted in Figure b, which shows that H has some resolution dependence, but asymptotes to roughly at fine resolution. Plugging and into equation yields for the inner edge of the gray zone, in good agreement with Figure a as well as Lebo and Morrison ().…”

“…Our focus here will be on the effects of horizontal resolution on parcel aspect ratio D / H and hence vertical acceleration, as opposed to the effects of horizontal resolution on turbulence and entrainment (e.g., Bryan et al, ; Bryan & Morrison, ; Lebo & Morrison, ; Wyngaard, ). These effects are likely related, though, as Lebo and Morrison () find that the onset of resolved turbulent mixing and the convergence of vertical motion coincide at . This gives a tentative answer to question 1 above, but the answer is purely empirical.…”

Effects of Scavenging, Entrainment, and Aqueous Chemistry on Peroxides and Formaldehyde in Deep Convective Outflow Over the Central and Southeast United States

“…With these grid spacings, entrainment may not be produced sufficiently (Bryan et al, ; Bryan & Morrison, ). Furthermore, statistical convective properties have been shown not to converge until the grid spacing was reduced to 250 m for an idealized squall line (Lebo & Morrison, ). However, computational limitations prevented us from further decreasing the horizontal grid spacing for these simulations.…”

Wet scavenging of soluble gases in DC3 deep convective storms using WRF‐Chem simulations and aircraft observations

“…Since the background CO profile is reinitialized at 21 UTC from observations, the correct WRF-Chem mean inflow and outflow values indicate that the model is representing the transport adequately. However, Lebo and Morrison [2015] determined that a horizontal grid spacing of 250 m was necessary for convergence of statistical convective properties and mixing in WRF simulations of idealized squall lines. Bryan et al [2003] and Bryan and Morrison [2012] found that entrainment in a numerical model similar to WRF was underestimated at a horizontal grid spacing of 1 km, due to the inability to properly resolve turbulent eddies.…”