LES of Classic, Narrow-Mode Horizontal Convective Rolls over a Homogeneous Surface

Horizontal convective rolls (HCRs) are elongated, counter-rotating, mixed-layer circulations in the atmospheric boundary layer (ABL). Accurately quantifying their orientation and cross-roll wavelength is essential for evaluating simulations against observations, examining theoretical models, and improving ABL parameterizations. This study evaluates and combines statistical methods for estimating HCR properties from large-eddy simulation (LES) results. Three statistical methods are considered: i) the primary mode of two-dimensional (2D) Fourier analysis, ii) the volume flux ratio (VFR), which is a simplified version of the mass flux ratio (MFR), and iii) the autocorrelation contours, with a new automated process developed. These methods are applied to two LES cases: i) updraft bands with known orientation and cross-roll wavelength enforced by heterogeneous surface heating, and ii) classic narrow-mode HCRs over a homogeneous surface. Results recommend using the VFR to obtain HCR orientation and then taking this orientation estimate as input to the new automated process of analyzing autocorrelation contours to obtain cross-roll wavelength. This combination of VFR and autocorrelation contours can be readily adopted for analyzing field observations like radar scans. If a consistent wavelength is obtained using the primary mode of 2D Fourier analysis, then the orientation suggested by the 2D Fourier analysis can be compared to that obtained using VFR for cross-validation purposes.