Effects of under-resolved convective dynamics on mesoscale convective evolution

 
Poster PDF

Authors

Adam Varble — Pacific Northwest National Laboratory
Hugh Clifton Morrison — University Corporation for Atmospheric Research

Category

Convective clouds, including aerosol interactions

Description

A MC3E squall line simulation using 750-m horizontal grid spacing produces wide convective regions with strongly upshear tilted convective updrafts and mesoscale bowing segments that are not produced in radar observations. Similar features occur in simulations for several different bulk microphysics schemes. An observed rear inflow jet remains more elevated than simulated whereas the simulated rear inflow jet descends to low levels, reinforcing the cold pool circulation that overpowers the pre-squall line low level vertical wind shear. Nesting a 250-m grid spacing domain into the simulation improves comparisons with observations with rear inflow that remains more elevated than in the 750-m run. Despite a stronger cold pool eventually developing in the 250-m run, the more elevated rear inflow counteracts the cold pool circulation to prevent convective cores from becoming overly sheared like those in the 750-m run. The different structure in the 750-m run produces excessive mid-level front-to-rear detrainment that widens the convective region relative to the 250-m run and observations while continuing the cycle of excessive latent cooling and rear inflow descent at the rear of the stratiform region in a positive feedback. Differences between the two resolution simulations can be traced back to greater downdraft mass flux in initial deep convective cells. Convective downdraft condensate, latent cooling, and downward motion all increase as downdraft width increases with very similar relationships in both resolution simulations. However, on a common 750-m grid, the 750-m run has a greater number of wide downdrafts and a lesser number of thin downdrafts than the 250-m run that more efficiently transport mid-upper level positive zonal momentum downward to low levels. Unsurprisingly, wider convective updrafts are also found in the 750-m run, but more interestingly, the number and width of convective updrafts are correlated in time with the number and width of convective downdrafts such that the two are clearly linked. These results imply that under-resolved convective drafts in simulations may transport horizontal momentum too efficiently and too far vertically, which has the ability to bias mesoscale convective system evolution.