scholarly journals Characteristics of the Near-Surface Boundary Layer within a Mountain Valley during Winter

2012 ◽  
Vol 51 (3) ◽  
pp. 583-597 ◽  
Author(s):  
Warren Helgason ◽  
John W. Pomeroy
Keyword(s):  
Boundary Layer ◽  
Heat Fluxes ◽  
Quadrant Analysis ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
Wind Speed Profile ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Mountainous Regions ◽  
Mass Fluxes

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.

scholarly journals Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

2021 ◽  
Vol 13 (19) ◽  
pp. 3989
Author(s):  
Hiroshi Kuroda ◽  
Takashi Setou
Keyword(s):  
Boundary Layer ◽  
Pacific Ocean ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Northwestern Pacific ◽  
Surface Boundary ◽  
Northwestern Pacific Ocean ◽  
Surface Boundary Layer ◽  
Westerly Jet ◽  
Oceanic Surface

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

scholarly journals Measurements of Enhanced Near-Surface Turbulence under Windrows

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.

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

2013 ◽  
Vol 28 (6) ◽  
pp. 1552-1561 ◽  
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.

scholarly journals Parameterizations of Sea-Spray Impact on the Air–Sea Momentum and Heat Fluxes

2011 ◽  
Vol 139 (12) ◽  
pp. 3781-3797 ◽  
Author(s):  
J.-W. Bao ◽  
C. W. Fairall ◽  
S. A. Michelson ◽  
L. Bianco
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Weather Prediction ◽  
Heat Fluxes ◽  
Sea Spray ◽  
Surface Boundary ◽  
Near Surface ◽  
The Mean ◽  
Momentum And Heat Fluxes ◽  
The Impact

Abstract This paper focuses on parameterizing the effect of sea spray at hurricane-strength winds on the momentum and heat fluxes in weather prediction models using the Monin–Obukhov similarity theory (a common framework for the parameterizations of air–sea fluxes). In this scheme, the mass-density effect of sea spray is considered as an additional modification to the stratification of the near-surface profiles of wind, temperature, and moisture in the marine surface boundary layer (MSBL). The overall impact of sea-spray droplets on the mean profiles of wind, temperature, and moisture depends on the wind speed at the level of sea-spray generation. As the wind speed increases, the mean droplet size and the mass flux of sea-spray increase, rendering an increase of stability in the MSBL and the leveling-off of the surface drag. Sea spray also tends to increase the total air–sea sensible and latent heat fluxes at high winds. Results from sensitivity testing of the scheme in a numerical weather prediction model for an idealized case of hurricane intensification are presented along with a dynamical interpretation of the impact of the parameterized sea-spray physics on the structure of the hurricane boundary layer.

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

2017 ◽  
Vol 47 (10) ◽  
pp. 2429-2454 ◽  
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.

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

2016 ◽  
Vol 46 (2) ◽  
pp. 371-394 ◽  
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.

scholarly journals Seasonal and Diurnal Cycles of Surface Boundary Layer and Energy Balance in the Central Andes of Perú, Mantaro Valley

Atmosphere ◽  
2019 ◽  
Vol 10 (12) ◽  
pp. 779 ◽  
Author(s):  
José Luis Flores-Rojas ◽  
Joan Cuxart ◽  
Manuel Piñas-Laura ◽  
Stephany Callañaupa ◽  
Luis Suárez-Salas ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Energy Balance ◽  
Bowen Ratio ◽  
Heat Fluxes ◽  
Surface Boundary ◽  
Sensible Heat ◽  
Surface Boundary Layer ◽  
Mountain Valley ◽  
Arid Conditions

The present study presents a detailed analysis of the diurnal and monthly cycles the surface boundary layer and of surface energy balance in a sparse natural vegetation canopy on Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m ASL), which is located in the central Andes of Perú (Mantaro Valley) during an entire year (May 2018–April 2019). We used a set of meteorological sensors (temperature, relative humidity, wind) installed in a gradient tower 30 m high, a set of radiative sensors to measure all irradiance components, and a set of tensiometers and heat flux plate to measure soil moisture, soil temperatures and soil heat flux. To estimate turbulent energy fluxes (sensible and latent), two flux–gradient methods: the aerodynamic method and the Bowen-ratio energy-balance method were used. The ground heat flux at surface was estimated using a molecular heat transfer equation. The results show minimum mean monthly temperatures and more stable conditions were observed in June and July before sunrise, while maximum mean monthly temperatures in October and November and more unstable conditions in February and March. From May to August inverted water vapor profiles near the surface were observed (more intense in July) at night hours, which indicate a transfer of water vapor as dewfall on the surface. The patterns of wind direction indicate well-defined mountain–valley circulation from south-east to south-west especially in fall–winter months (April–August). The maximum mean monthly sensible heat fluxes were found in June and September while minimum in February and March. Maximum mean monthly latent heat fluxes were found in February and March while minimum in June and July. The surface albedo and the Bowen ratio indicate semi-arid conditions in wet summer months and extreme arid conditions in dry winter months. The comparisons between sensible heat flux ( Q H ) and latent heat flux ( Q E ), estimated by the two methods show a good agreement (R 2 above 0.8). The comparison between available energy and the sum of Q E and Q H fluxes shows a good level of agreement (R 2 = 0.86) with important imbalance contributions after sunrise and around noon, probably by advection processes generated by heterogeneities on the surface around the Huancayo observatory and intensified by the mountain–valley circulation.

scholarly journals Inclusion of a Drag Approach in the Town Energy Balance (TEB) Scheme: Offline 1D Evaluation in a Street Canyon

2008 ◽  
Vol 47 (10) ◽  
pp. 2627-2644 ◽  
Author(s):  
R. Hamdi ◽  
V. Masson
Keyword(s):  
Boundary Layer ◽  
Energy Balance ◽  
Street Canyon ◽  
Potential Temperature ◽  
Single Layer ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
The Town ◽  
Meteorological Models

Abstract The Town Energy Balance module bridges the micro- and mesoscale and simulates local-scale urban surface energy balance for use in mesoscale meteorological models. Previous offline evaluations show that this urban module is able to simulate in good behavior road, wall, and roof temperatures and to correctly partition radiation forcing into turbulent and storage heat fluxes. However, to improve prediction of the meteorological fields inside the street canyon, a new version has been developed, following the methodology described in a companion paper by Masson and Seity. It resolves the surface boundary layer inside and above urban canopy by introducing a drag force approach to account for the vertical effects of buildings. This new version is tested offline, with one-dimensional simulation, in a street canyon using atmospheric and radiation data recorded at the top of a 30-m-high tower as the upper boundary conditions. Results are compared with simulations using the original single-layer version of the Town Energy Balance module on one hand and with measurements within and above a street canyon on the other hand. Measurements were obtained during the intensive observation period of the Basel Urban Boundary Layer Experiment. Results show that this new version produces profiles of wind speed, friction velocity, turbulent kinetic energy, turbulent heat flux, and potential temperature that are more consistent with observations than with the single-layer version. Furthermore, this new version can still be easily coupled to mesoscale meteorological models.

scholarly journals A Simple Analytical Model of the Diurnal Ekman Layer

2016 ◽  
Vol 46 (9) ◽  
pp. 2877-2894 ◽  
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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