LES of Shear-Influenced Unstable Atmospheric Boundary Layers with Constant Geostrophic Wind and Decreasing Surface Heat Flux

Wide-mode horizontal convective rolls (HCRs) have been observed using operational radar scans over central Oklahoma. The formation mechanisms remain unclear for these HCRs, whose cross-roll wavelengths are more than five times the atmospheric boundary layer (ABL) depth. Neither have these HCRs been replicated using turbulence-resolving large-eddy simulations (LES). In this work, we investigate whether temporally evolving environmental conditions, a key difference between realistic ABLs and previous LES configurations, are responsible for the formation of wide-mode HCRs. A wide-mode HCR event observed during the evening transition on 26 July 2014 is selected as the case study for LES to replicate. ARM-SGP measurements, including ECOR, EBBR, radar, sounding, and ceilometer, are combined with operational radar and Mesonet observations to setup LES configuration parameters, such as geostrophic wind, initial ABL depth, and time-varying surface heat fluxes. LES results of the evening-transition period are analyzed using various statistical methods of quantifying the presence, orientation, and cross-roll wavelength of HCRs. Results show that temporally evolving surface heating alone is insufficient to generate robust wide-mode HCRs in LES of horizontally homogeneous ABL flows. In combination with previous LES runs involving heterogeneous surface heating, these results suggest that the urban-to-rural transition near Oklahoma City is potentially more responsible for the formation of wide-mode HCRs, compared to the evening transition.