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Abstract 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 interest: (i) how updrafts develop in the low free troposphere and (ii) what parameters control updraft location and strength within the ABL? LES runs are performed for a sheared, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a 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, the 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. For the second gap, the results show that organized updrafts within the ABL may form above warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions of horizontal and vertical velocities to the phase-averaged vorticity fluctuations tied to secondary circulations, 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 potentially change parameters used by atmospheric models that do not resolve ABL turbulence. Significance StatementThe purpose of this study is to better understand how heterogeneous surface heating affects updraft location and strength in the lowest kilometers of the atmosphere. We focus on horizontal heterogeneity scales comparable to the most frequently observed cloud size, a necessary step toward the parameterization of cloud shadow effects in weather and climate models. The results show that persistent updrafts may form above either warm or cool surfaces, with their location depending on the relative importance of terms in certain budget equations. Near-surface updrafts become stronger as the background mean wind becomes more perpendicular to the surface-heat-flux gradient, but their potential to influence clouds peaks when the background mean wind is neither parallel nor perpendicular to the surface-heat-flux gradient. 

Abstract Understanding the interactions between turbulent and nonturbulent motions has been a persistent challenge faced by the community studying stably stratified turbulent flows. For flows with high Reynolds number, high Rossby number, and stable stratifications, nonturbulent motions involve physical mechanisms acting against instability development. Because turbulent motions are generated through an energy cascade via instability development, the presence of nonturbulent motions is expected to modify the energy distribution across scales compared to that of solely turbulent motions. The objective of this work is to identify in field data statistical signals of nonturbulent motions caused by stable stratification. The need to resolve energy-containing motions in both space and time requires high-frequency time series of velocity fluctuations collected using arrays of sonic anemometers. The analysis is performed using data from the Canopy Horizontal Array Turbulence Study (CHATS), during which a total of 31 sonic anemometers were deployed on a horizontal array and on a 30-m tower. Compared to other field campaigns which were also equipped with arrays of sonic anemometers, CHATS took an important advantage of already published nighttime canopy-scale waves derived from aerosol backscatter lidar images. After precluding complexities caused by nonstationarity and horizontal heterogeneity, signals of nonturbulent motions caused by stable stratification are identified from spatial autocorrelations of time-block-averaged velocity fluctuations. These signals are interpreted using existing understanding of turbulent canopy flows and two-dimensional Kelvin–Helmholtz instability development. The associated estimates of critical wavelengths and buoyancy periods agree well with the overall properties of nighttime canopy-scale waves derived from lidar images. Significance StatementThis work investigates statistical signals of nonturbulent motions caused by stable stratification in sonic anemometer measurements of near-surface atmospheric flows. The detected signals of nonturbulent motions agree with theoretical predictions of the impacts of stable stratification on turbulent canopy flows. This agreement suggests potential advantages for understanding stably stratified near-surface flows using canopy-resolving simulations. The automatic, objective, statistical detection procedures, as well as the intermediate products of the periods of statistically stationary, horizontally homogeneous, approximately two-dimensional mean flows, are useful for improving the understanding of canopy flows for various stability conditions. 

For atmospheric turbulence, multiplying an estimate of the convection velocity with the integral time scale is useful for estimating the integral length scale. Velocity scales that have been used to estimate the convection velocity include the local mean velocity, the ratio of $$e$$-folding length and time scales, and the ratio of a prescribed spatial separation and the time lag at which the space-time autocorrelation peaks. A knowledge gap is the lack of evaluation of these velocity scales directly against the convection velocity, especially for canopy flows where previous studies have reported somewhat inconsistent results. The objective of this work is to assess the ability of each candidate velocity scale to estimate the convection velocity in canopy flows. Firstly, large-eddy simulation (LES) results of neutral canopy flows are used to compare these velocity scales to directly quantified convection velocity. When the direction of interest roughly aligns with the mean pressure gradient force (specifically, for an angle of $$7.5^\circ$$ or smaller), all candidate velocity scales other than the local mean wind component approximate the convection velocity fairly well. When the direction of interest departs from the mean pressure gradient force for more than $$15^\circ$$, the ability of each velocity scale to approximate the convection velocity changes substantially. Secondly, data collected during the Canopy Horizontal Array Turbulence Study (CHATS) are used as an example of interpreting estimates of the convection velocity in the field with the guidance from LES findings. Because observational periods are never perfectly neutral, the guidance does not involve direct comparison between observed and simulated velocity scales, but focuses on uncertainties of velocity scale estimates and potential caution needed when using these estimates. 

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. 

Understanding the interactions between turbulent and non-turbulent motions has been a persistent challenge faced by the community studying stably stratified turbulent flows. For flows with high Reynolds number, high Rossby number, and stable stratifications, non-turbulent motions share a common characteristic to involve physical mechanisms acting against instability development. Because turbulence is generated through energy cascade via instability development, the presence of non-turbulent motions is expected to modify the energy distribution across scales compared to that of solely turbulent motions. The objective of this work is to identify statistical signals of non-turbulent motions caused by stable stratification. The need to resolve energy-containing motions in both space and time requires high-frequency time series of velocity fluctuations collected using arrays of sonic anemometers. The analysis is performed using data from the Canopy Horizontal Array Turbulence Study (CHATS), during which a total of 31 sonic anemometers were deployed on a horizontal array and on a 30-m tower. Compared to other field campaigns which were also equipped with arrays of sonic anemometers, CHATS took an important advantage of already published nighttime canopy-scale waves derived from aerosol backscatter lidar images. After precluding complexities caused by nonstationarity and horizontal heterogeneity, signals of non-turbulent motions caused by stable stratification are identified from spatial autocorrelations of time-block-averaged velocity fluctuations. These signals agree with existing understanding of turbulent canopy flows and two-dimensional Kelvin-Helmholtz instability development, which predicts a critical wavelength at which motions shift from free instability growth to internal gravity waves. The estimates of critical wavelengths and buoyancy periods agree well with the overall properties of nighttime canopy-scale waves derived from lidar images.