Search for: All records

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. 

This flux-tower observational campaign occurred in Utqiagvik, AK. A 12-m tower was installed in February 2022 to collect turbulence data at a total of five heights (0.5 m, 1.5 m, 2.5 m, 3.5 m, and 7.5 m). At each height, a Campbell Scientific CSAT3B sonic anemometer was operated to measure three velocity components and virtual temperature at 50 Hz, and an R. M. Young temperature and relative sensor was operated to measure air temperature and relative humidity at 1 Hz. The effective data collection was during March--April 2022, until the tower was taken down in April 2022. This was the first dataset of Arctic turbulence collected at 50 Hz, a frequency substantially higher than previous measurements at 10 Hz and 20 Hz. Given the strongly stable conditions in the Arctic, increasing the sampling frequency to 50 Hz was critical to resolve near-surface turbulence within or at least close to the inertial subrange. 

Large-eddy simulation (LES) runs are performed to understand the influence of a one-dimensional (1D) surface heating heterogeneity on organized vertical motions within and above the atmospheric boundary layer (ABL). Two knowledge gaps are of particular interest: i) how do updrafts develop in the low free troposphere, and ii) what parameters control the updraft location and strength within the ABL? LES runs are performed for a shear-influenced, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a prescribed 1D surface-heat-flux heterogeneity. Quasi-steady-state LES results are phase-averaged over time and the horizontal dimension perpendicular to the surface-heat-flux gradient to quantify secondary circulations. Regarding the first knowledge gap, results show that organized vertical motions in the low free troposphere can be modeled as two-dimensional (2D) stationary gravity waves, whose amplitudes depend on ABL updraft strength and instability development within the free troposphere. Regarding the second knowledge gap, results show that organized updrafts within the ABL may form either above relatively warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions to secondary circulations from phase-averaged horizontal and vertical velocity fluctuations and the relative importance of horizontal advection and turbulent transport in the phase-averaged internal energy fluctuation equation. The main balances associated with each updraft location are used to propose empirical models of updraft strength, and it is shown that the presence of sufficiently strong organized vertical motions can cause a non-negligible reduction in near-surface eddy viscosity.