Study of near surface boundary layer characteristics during pre-monsoon seasons using micrometeorological tower observations

AbstractWithin mountainous regions, estimating the exchange of sensible heat and water vapor between the surface and the atmosphere is an important but inexact endeavor. Measurements of the turbulence characteristics of the near-surface boundary layer in complex mountain terrain are relatively scarce, leading to considerable uncertainty in the application of flux-gradient techniques for estimating the surface turbulent heat and mass fluxes. An investigation of the near-surface boundary layer within a 7-ha snow-covered forest clearing was conducted in the Kananaskis River valley, located within the Canadian Rocky Mountains. The homogeneous measurement site was characterized as being relatively calm and sheltered; the wind exhibited considerable unsteadiness, however. Frequent wind gusts were observed to transport turbulent energy into the clearing, affecting the rate of energy transfer at the snow surface. The resulting boundary layer within the clearing exhibited perturbations introduced by the surrounding topography and land surface discontinuities. The measured momentum flux did not scale with the local aerodynamic roughness and mean wind speed profile, but rather was reflective of the larger-scale topographical disturbances. The intermittent nature of the flux-generating processes was evident in the turbulence spectra and cospectra where the peak energy was shifted to lower frequencies as compared with those observed in more homogeneous flat terrain. The contribution of intermittent events was studied using quadrant analysis, which revealed that 50% of the sensible and latent heat fluxes was contributed from motions that occupied less than 6% of the time. These results highlight the need for caution while estimating the turbulent heat and mass fluxes in mountain regions.

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Measurements of Enhanced Near-Surface Turbulence under Windrows

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0265.1 ◽

2020 ◽

Vol 50 (1) ◽

pp. 197-215

Author(s):

Seth F. Zippel ◽

Ted Maksym ◽

Malcolm Scully ◽

Peter Sutherland ◽

Dany Dumont

Keyword(s):

Boundary Layer ◽

Surface Flux ◽

Breaking Waves ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface ◽

Surface Turbulence ◽

Order Of Magnitude ◽

Convergence Zones ◽

Shallow Bay

AbstractObservations of waves, winds, turbulence, and the geometry and circulation of windrows were made in a shallow bay in the winter of 2018 outside of Rimouski, Québec. Water velocities measured from a forward-looking pulse-coherent ADCP mounted on a small zodiac show spanwise (cross-windrow) convergence, streamwise (downwind) velocity enhancement, and downwelling in the windrows, consistent with the view that windrows are the result of counterrotating pairs of wind-aligned vortices. The spacing of windrows, measured with acoustic backscatter and with surface imagery, was measured to be approximately twice the water depth, which suggests an aspect ratio of 1. The magnitude and vertical distribution of turbulence measured from the ADCP are consistent with a previous scaling and observations of near-surface turbulence under breaking waves, with dissipation rates larger and decaying faster vertically than what is expected from a shear-driven boundary layer. Measurements of dissipation rate are partitioned to within, and outside of the windrow convergence zones, and measurements inside the convergence zones are found to be nearly an order of magnitude larger than those outside with similar vertical structure. A ratio of time scales suggests that turbulence likely dissipates before it can be advected horizontally into convergences, but the advection of wave energy into convergences may elevate the surface flux of TKE and could explain the elevated turbulence in the windrows. These results add to a limited number of conflicting observations of turbulence variability due to windrows, which may modify gas flux, and heat and momentum transport in the surface boundary layer.

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The Three-Dimensional Structure and Evolution of a Tornado Boundary Layer

Weather and Forecasting ◽

10.1175/waf-d-13-00070.1 ◽

2013 ◽

Vol 28 (6) ◽

pp. 1552-1561 ◽

Cited By ~ 47

Author(s):

Karen A. Kosiba ◽

Joshua Wurman

Keyword(s):

Boundary Layer ◽

Three Dimensional ◽

Flow Region ◽

Ground Level ◽

Dimensional Structure ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface ◽

Three Dimensional Structure ◽

Flow Swirl

Abstract The finescale three-dimensional structure and evolution of the near-surface boundary layer of a tornado (TBL) is mapped for the first time. The multibeam Rapid-Scan Doppler on Wheels (RSDOW) collected data at several vertical levels, as low as 4, 6, 10, 12, 14, and 17 m above ground level (AGL), contemporaneously at 7-s intervals for several minutes in a tornado near Russell, Kansas, on 25 May 2012. Additionally, a mobile mesonet anemometer measured winds at 3.5 m AGL in the core flow region. The radar, anemometer, and ground-based velocity-track display (GBVTD) analyses reveal the peak wind intensity is very near the surface at ~5 m AGL, about 15% higher than at 10 m AGL and 25% higher than at ~40 m AGL. GBVTD analyses resolve a downdraft within the radius of maximum winds (RMW), which decreased in magnitude when varying estimates for debris centrifuging are included. Much of the inflow (from −1 to −7 m s−1) is at or below 10–14 m AGL, much shallower than reported previously. Surface outflow precedes tornado dissipation. Comparisons between large-eddy simulation (LES) predictions of the corner flow swirl ratio Sc and observed tornado intensity changes are consistent.

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Baroclinic Frontal Instabilities and Turbulent Mixing in the Surface Boundary Layer. Part II: Forced Simulations

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0179.1 ◽

2017 ◽

Vol 47 (10) ◽

pp. 2429-2454 ◽

Cited By ~ 12

Author(s):

Eric D. Skyllingstad ◽

Jenessa Duncombe ◽

Roger M. Samelson

Keyword(s):

Boundary Layer ◽

Surface Wind ◽

Vertical Mixing ◽

Shear Instability ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface ◽

Large Eddy Simulation Model ◽

Geostrophic Balance ◽

Geostrophic Shear

AbstractGeneration of ocean surface boundary layer turbulence and coherent roll structures is examined in the context of wind-driven and geostrophic shear associated with horizontal density gradients using a large-eddy simulation model. Numerical experiments over a range of surface wind forcing and horizontal density gradient strengths, combined with linear stability analysis, indicate that the dominant instability mechanism supporting coherent roll development in these simulations is a mixed instability combining shear instability of the ageostrophic, wind-driven flow with symmetric instability of the frontal geostrophic shear. Disruption of geostrophic balance by vertical mixing induces an inertially rotating ageostrophic current, not forced directly by the wind, that initially strengthens the stratification, damps the instabilities, and reduces vertical mixing, but instability and mixing return when the inertial buoyancy advection reverses. The resulting rolls and instabilities are not aligned with the frontal zone, with an oblique orientation controlled by the Ekman-like instability. Mean turbulence is enhanced when the winds are destabilizing relative to the frontal orientation, but mean Ekman buoyancy advection is found to be relatively unimportant in these simulations. Instead, the mean turbulent kinetic energy balance is dominated by mechanical shear production that is enhanced when the wind-driven shear augments the geostrophic shear, while the resulting vertical mixing nearly eliminates any effective surface buoyancy flux from near-surface, cold-to-warm, Ekman buoyancy advection.

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Wind, Waves, and Fronts: Frictional Effects in a Generalized Ekman Model

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0162.1 ◽

2016 ◽

Vol 46 (2) ◽

pp. 371-394 ◽

Cited By ~ 33

Author(s):

Jacob O. Wenegrat ◽

Michael J. McPhaden

Keyword(s):

Boundary Layer ◽

Surface Waves ◽

Ocean Currents ◽

Surface Boundary ◽

Pressure Gradients ◽

Surface Boundary Layer ◽

Near Surface ◽

Thermal Wind ◽

Surface Ocean ◽

Wide Range

AbstractOcean currents in the surface boundary layer are sensitive to a variety of parameters not included in classic Ekman theory, including the vertical structure of eddy viscosity, finite boundary layer depth, baroclinic pressure gradients, and surface waves. These parameters can modify the horizontal and vertical flow in the near-surface ocean, making them of first-order significance to a wide range of phenomena of broad practical and scientific import. In this work, an approximate Green’s function solution is found for a model of the frictional ocean surface boundary layer, termed the generalized Ekman (or turbulent thermal wind) balance. The solution admits consideration of general, more physically realistic forms of parameters than previously possible, offering improved physical insight into the underlying dynamics. Closed form solutions are given for the wind-driven flow in the presence of Coriolis–Stokes shear, a result of the surface wave field, and thermal wind shear, arising from a baroclinic pressure gradient, revealing the common underlying physical mechanisms through which they modify currents in the ocean boundary layer. These dynamics are further illustrated by a case study of an idealized two-dimensional front. The solutions, and estimates of the global distribution of the relative influence of surface waves and baroclinic pressure gradients on near-surface ocean currents, emphasize the broad importance of considering ocean sources of shear and physically realistic parameters in the Ekman problem.

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A Simple Analytical Model of the Diurnal Ekman Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0031.1 ◽

2016 ◽

Vol 46 (9) ◽

pp. 2877-2894 ◽

Cited By ~ 7

Author(s):

Jacob O. Wenegrat ◽

Michael J. McPhaden

Keyword(s):

Boundary Layer ◽

Turbulent Viscosity ◽

Low Frequency ◽

Time Variability ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface ◽

Surface Ocean ◽

Boundary Layer Turbulence ◽

Frequency Rectification

AbstractThe effects of time-varying turbulent viscosity on horizontal currents in the ocean surface boundary layer are considered using a simple, theoretical model that can be solved analytically. This model reproduces major aspects of the near-surface ocean diurnal cycle in velocity and shear, while retaining direct parallels to the steady-state Ekman solution. The parameter dependence of the solution is explored, and quantitative measures of the low-frequency rectification of velocity and shear are derived. Results demonstrate that time variability in eddy viscosity leads to significant changes to the time-averaged velocity and shear fields, with important implications for the interpretation of observations and modeling of the near-surface ocean. These findings mirror those of more complete numerical modeling studies, suggesting that some of the rectification mechanisms active in those studies may be independent of the details of the boundary layer turbulence.

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Lagrangian Investigation of Wave-Driven Turbulence in the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0081.1 ◽

2019 ◽

Vol 49 (2) ◽

pp. 409-429 ◽

Cited By ~ 4

Author(s):

Tobias Kukulka ◽

Fabrice Veron

Keyword(s):

Boundary Layer ◽

Turbulent Transport ◽

Ocean Surface ◽

Inertial Subrange ◽

Breaking Wave ◽

Surface Boundary ◽

Lagrangian Analysis ◽

Surface Boundary Layer ◽

Near Surface ◽

Eddy Simulation

AbstractTurbulent processes in the ocean surface boundary layer (OSBL) play a key role in weather and climate systems. This study explores a Lagrangian analysis of wave-driven OSBL turbulence, based on a large-eddy simulation (LES) model coupled to a Lagrangian stochastic model (LSM). Langmuir turbulence (LT) is captured by Craik–Leibovich wave forcing that generates LT through the Craik–Leibovich type 2 (CL2) mechanism. Breaking wave (BW) effects are modeled by a surface turbulent kinetic energy flux that is constrained by wind energy input to surface waves. Unresolved LES subgrid-scale (SGS) motions are simulated with the LSM to be energetically consistent with the SGS model of the LES. With LT, Lagrangian autocorrelations of velocities reveal three distinct turbulent time scales: an integral, a dispersive mixing, and a coherent structure time. Coherent structures due to LT result in relatively narrow peaks of Lagrangian frequency velocity spectra. With and without waves, the high-frequency spectral tail is consistent with expectations for the inertial subrange, but BWs substantially increase spectral levels at high frequencies. Consistently, over short times, particle-pair dispersion results agree with the Richardson–Obukhov law, and near-surface dispersion is significantly enhanced because of BWs. Over longer times, our dispersion results are consistent with Taylor dispersion. In this case, turbulent diffusivities are substantially larger with LT in the crosswind direction, but reduced in the along-wind direction because of enhanced turbulent transport by LT that reduces mean Eulerian shear. Our results indicate that the Lagrangian analysis framework is effective and physically intuitive to characterize OSBL turbulence.

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A Study of Near-Surface Boundary Layer Characteristics During the 2015 Chennai Flood in the Context of Urban-Induced Land Use Changes

Pure and Applied Geophysics ◽

10.1007/s00024-018-2069-5 ◽

2018 ◽

Vol 176 (6) ◽

pp. 2607-2629

Author(s):

Sudhansu S. Rath ◽

Jagabandhu Panda

Keyword(s):

Boundary Layer ◽

Land Use ◽

Land Use Changes ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface

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REFRACTT 2006

Bulletin of the American Meteorological Society ◽

10.1175/2008bams2412.1 ◽

2008 ◽

Vol 89 (10) ◽

pp. 1535-1548 ◽

Cited By ~ 32

Author(s):

Rita D. Roberts ◽

Frédéric Fabry ◽

Patrick C. Kennedy ◽

Eric Nelson ◽

James W. Wilson ◽

...

Keyword(s):

Boundary Layer ◽

Real Time ◽

Index Of Refraction ◽

Surface Boundary ◽

Short Term ◽

Surface Boundary Layer ◽

Near Surface ◽

Large Domain ◽

Short Term Forecasting ◽

Moisture Variability

The Refractivity Experiment for H2O Research and Collaborative Operational Technology Transfer (REFRACTT), conducted in northeast Colorado during the summer of 2006, provided a unique opportunity to obtain high-resolution gridded moisture fields from the operational Denver Next Generation Weather Radar (NEXRAD) and three research radars using a radar-based index of refraction (refractivity) technique. Until now, it has not been possible to observe and monitor moisture variability in the near-surface boundary layer to such high spatial (4-km horizontal gridpoint spacing) and temporal (4–10-min update rates) resolutions using operational NEXRAD and provide these moisture fields to researchers and the National Weather Service (NWS) forecasters in real time. The overarching goals of REFRACTT were to 1) access and mosaic the refractivity data from the operational NEXRAD and research radars together over a large domain for use by NWS forecasters in real time for short-term forecasting, 2) improve our understanding of near-surface water vapor variability and the role it plays in the initiation of convection and thunderstorms, and 3) improve the accuracy of quantitative precipitation forecasts (QPF) through improved observations and assimilation of low-level moisture fields. This paper presents examples of refractivity-derived moisture fields from REFRACTT in 2006 and the moisture variability observed in the near-surface boundary layer, in association with thunderstorm initiation, and with a cold frontal passage.

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Estimation of nocturnal CO<sub>2</sub> and N<sub>2</sub>O soil emissions from changes in surface boundary layer mass storage

Atmospheric Measurement Techniques ◽

10.5194/amt-11-2119-2018 ◽

2018 ◽

Vol 11 (4) ◽

pp. 2119-2133 ◽

Cited By ~ 3

Author(s):

Richard H. Grant ◽

Rex A. Omonode

Keyword(s):

Boundary Layer ◽

Control Volume ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Near Surface ◽

Soil Emissions ◽

N2o Fluxes ◽

Diffusive Fluxes ◽

Accumulation Method ◽

Mass Accumulation

Abstract. Annual budgets of greenhouse and other trace gases require knowledge of the emissions throughout the year. Unfortunately, emissions into the surface boundary layer during stable, calm nocturnal periods are not measurable using most micrometeorological methods due to non-stationarity and uncoupled flow. However, during nocturnal periods with very light winds, carbon dioxide (CO2) and nitrous oxide (N2O) frequently accumulate near the surface and this mass accumulation can be used to determine emissions. Gas concentrations were measured at four heights (one within and three above canopy) and turbulence was measured at three heights above a mature 2.5 m maize canopy from 23 July to 10 September 2015. Nocturnal CO2 and N2O fluxes from the canopy were determined using the accumulation of mass within a 6.3 m control volume and out the top of the control volume within the nocturnal surface boundary layer. Diffusive fluxes were estimated by flux gradient method. The total accumulative and diffusive fluxes during near-calm nights (friction velocities < 0.05 ms−1) averaged 1.16 µmol m−2 s−1 CO2 and 0.53 nmol m−2 s−1 N2O. Fluxes were also measured using chambers. Daily mean CO2 fluxes determined by the accumulation method were 90 to 130 % of those determined using soil chambers. Daily mean N2O fluxes determined by the accumulation method were 60 to 80 % of that determined using soil chambers. The better signal-to-noise ratios of the chamber method for CO2 over N2O, non-stationary flow, assumed Schmidt numbers, and anemometer tilt were likely contributing reasons for the differences in chambers versus accumulated nocturnal mass flux estimates. Near-surface N2O accumulative flux measurements in more homogeneous regions and with greater depth are needed to confirm the conclusion that mass accumulation can be effectively used to estimate soil emissions during nearly calm nights.

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