Idealized model for changes in equilibrium temperature, mixed layer depth, and boundary layer cloud over land in a doubled CO2climate

This study investigates the asymmetric distribution of hurricane boundary layer height scales in a storm-motion-relative framework using global positioning system (GPS) dropsonde observations. Data from a total of 1916 dropsondes collected within four times the radius of maximum wind speed of 37 named hurricanes over the Atlantic basin from 1998 to 2015 are analyzed in the composite framework. Motion-relative quadrant mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to increase with increasing radius in all four motion-relative quadrants. It is also found that the thermodynamic mixed layer depth and height of maximum tangential wind speed are within the inflow layer in all motion-relative quadrants. The inflow layer depth and height of the maximum tangential wind are both found to be deeper in the two front quadrants, and they are largest in the right-front quadrant. The difference in the thermodynamic mixed layer depth between the front and back quadrants is smaller than that in the kinematic boundary layer height. The thermodynamic mixed layer is shallowest in the right-rear quadrant, which may be due to the cold wake phenomena. The boundary layer height derived using the critical Richardson number ( R i c ) method shows a similar front-back asymmetry as the kinematic boundary layer height.

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Buoyancy-Forced Downwelling in Boundary Currents

Journal of Physical Oceanography ◽

10.1175/2008jpo3993.1 ◽

2008 ◽

Vol 38 (12) ◽

pp. 2704-2721 ◽

Cited By ~ 11

Author(s):

Michael A. Spall

Keyword(s):

Boundary Layer ◽

Wave Propagation ◽

Heat Loss ◽

Mixed Layer ◽

General Circulation ◽

Mixed Layer Depth ◽

Kelvin Wave ◽

Surface Cooling ◽

Layer Depth ◽

Boundary Currents

Abstract The issue of downwelling resulting from surface buoyancy loss in boundary currents is addressed using a high-resolution, nonhydrostatic numerical model. It is shown that the net downwelling is determined by the change in the mixed layer density along the boundary. For configurations in which the density on the boundary increases in the direction of Kelvin wave propagation, there is a net downwelling within the domain. For cases in which the density decreases in the direction of Kelvin wave propagation, cooling results in a net upwelling within the domain. Symmetric instability within the mixed layer drives an overturning cell in the interior, but it does not contribute to the net vertical motion. The net downwelling is determined by the geostrophic flow toward the boundary and is carried downward in a very narrow boundary layer of width E1/3, where E is the Ekman number. For the calculations here, this boundary layer is O(100 m) wide. A simple model of the mixed layer temperature that balances horizontal advection with surface cooling is used to predict the net downwelling and its dependence on external parameters. This model shows that the net sinking rate within the domain depends not only on the amount of heat loss at the surface but also on the Coriolis parameter, the mixed layer depth (or underlying stratification), and the horizontal velocity. These results indicate that if one is to correctly represent the buoyancy-forced downwelling in general circulation models, then it is crucial to accurately represent the velocity and mixed layer depth very close to the boundary. These results also imply that processes that lead to weak mixing within a few kilometers of the boundary, such as ice formation or freshwater runoff, can severely limit the downwelling forced by surface cooling, even if there is strong heat loss and convection farther offshore.

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Wind-Driven Mixing below the Oceanic Mixed Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-10-05020.1 ◽

2011 ◽

Vol 41 (8) ◽

pp. 1556-1575 ◽

Cited By ~ 28

Author(s):

Alan L. M. Grant ◽

Stephen E. Belcher

Keyword(s):

Boundary Layer ◽

Shear Layer ◽

Mixed Layer ◽

Momentum Flux ◽

Layer Structure ◽

Mixed Layer Depth ◽

Layer Depth ◽

Turbulent Momentum ◽

Bulk Model ◽

Shear Production

Abstract This study describes the turbulent processes in the upper ocean boundary layer forced by a constant surface stress in the absence of the Coriolis force using large-eddy simulation. The boundary layer that develops has a two-layer structure, a well-mixed layer above a stratified shear layer. The depth of the mixed layer is approximately constant, whereas the depth of the shear layer increases with time. The turbulent momentum flux varies approximately linearly from the surface to the base of the shear layer. There is a maximum in the production of turbulence through shear at the base of the mixed layer. The magnitude of the shear production increases with time. The increase is mainly a result of the increase in the turbulent momentum flux at the base of the mixed layer due to the increase in the depth of the boundary layer. The length scale for the shear turbulence is the boundary layer depth. A simple scaling is proposed for the magnitude of the shear production that depends on the surface forcing and the average mixed layer current. The scaling can be interpreted in terms of the divergence of a mean kinetic energy flux. A simple bulk model of the boundary layer is developed to obtain equations describing the variation of the mixed layer and boundary layer depths with time. The model shows that the rate at which the boundary layer deepens does not depend on the stratification of the thermocline. The bulk model shows that the variation in the mixed layer depth is small as long as the surface buoyancy flux is small.

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Similarity methods to derive turbulence quantities and mixed-layer depth from SODAR measurements in the convective boundary layer: A review

Applied Physics B Photophysics and Laser Chemistry ◽

10.1007/bf00324095 ◽

1993 ◽

Vol 57 (1) ◽

pp. 11-17 ◽

Cited By ~ 9

Author(s):

D. Melas

Keyword(s):

Boundary Layer ◽

Mixed Layer ◽

Convective Boundary Layer ◽

Mixed Layer Depth ◽

Layer Depth ◽

Convective Boundary ◽

Sodar Measurements

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A Revised Conceptual Model of the Tropical Marine Boundary Layer. Part III: Bragg Scattering Layer Statistical Properties

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0323.1 ◽

2013 ◽

Vol 70 (10) ◽

pp. 3047-3062 ◽

Cited By ~ 10

Author(s):

Jennifer L. Davison ◽

Robert M. Rauber ◽

Larry Di Girolamo ◽

Margaret A. LeMone

Keyword(s):

Boundary Layer ◽

Conceptual Model ◽

Mixed Layer ◽

Transition Layer ◽

Marine Boundary Layer ◽

Mixed Layer Depth ◽

Bragg Scattering ◽

Layer Depth ◽

Radar Satellite ◽

Cloud Tops

Abstract This paper examines the structure and variability of the moisture field in the tropical marine boundary layer (TMBL) as defined by Bragg scattering layers (BSLs) observed with S-band radar. Typically, four to five BSLs were present in the TMBL, including the transition layer at the top of the surface-based mixed layer. The transition-layer depth (~350 m) exhibited a weak diurnal cycle because of changes in the mixed-layer depth. BSLs and the “clear” layers between them each had a median thickness of about 350 m and a lifetime over the radar of 8.4 h, with about 25% having lifetimes longer than 20 h. More (fewer) BSLs were present when surface winds had a more southerly (northerly) component. Both BSLs and clear layers increased in depth with increasing rain rates, with the rainiest days producing layers that were about 100 m thicker than those on the driest days. The analyses imply that the relative humidity (RH) field in the TMBL exhibits layering on scales observable by radar. Satellite and wind profiler measurements show that the layered RH structure is related, at least in part, to detraining cloudy air. Based on analyses in this series of papers, a revised conceptual model of the TMBL is presented that emphasizes moisture variability and incorporates multiple moist and dry layers and a higher TMBL top. The model is supported by comparing BSL tops with satellite-derived cloud tops. This comparison suggests that the layered RH structure is related, in part, to cloud detrainment at preferred altitudes within the TMBL. The potential ramifications of this change in TMBL conceptualization on modeling of the TMBL are discussed.

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Langmuir Turbulence and Surface Heating in the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0018.1 ◽

2015 ◽

Vol 45 (12) ◽

pp. 2897-2911 ◽

Cited By ~ 18

Author(s):

Brodie C. Pearson ◽

Alan L. M. Grant ◽

Jeff A. Polton ◽

Stephen E. Belcher

Keyword(s):

Boundary Layer ◽

Heat Flux ◽

Mixed Layer ◽

Surface Heat Flux ◽

Mixed Layer Depth ◽

Surface Heat ◽

Surface Boundary ◽

Langmuir Turbulence ◽

Layer Depth ◽

Surface Boundary Layer

AbstractThis study uses large-eddy simulation to investigate the structure of the ocean surface boundary layer (OSBL) in the presence of Langmuir turbulence and stabilizing surface heat fluxes. The OSBL consists of a weakly stratified layer, despite a surface heat flux, above a stratified thermocline. The weakly stratified (mixed) layer is maintained by a combination of a turbulent heat flux produced by the wave-driven Stokes drift and downgradient turbulent diffusion. The scaling of turbulence statistics, such as dissipation and vertical velocity variance, is only affected by the surface heat flux through changes in the mixed layer depth. Diagnostic models are proposed for the equilibrium boundary layer and mixed layer depths in the presence of surface heating. The models are a function of the initial mixed layer depth before heating is imposed and the Langmuir stability length. In the presence of radiative heating, the models are extended to account for the depth profile of the heating.

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Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth

Ocean Science Discussions ◽

10.5194/osd-6-277-2009 ◽

2009 ◽

Vol 6 (1) ◽

pp. 277-341 ◽

Cited By ~ 2

Author(s):

S. Elipot ◽

S. T. Gille

Keyword(s):

Boundary Layer ◽

Boundary Condition ◽

Transfer Function ◽

Southern Ocean ◽

Wind Stress ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Surface Velocity ◽

Layer Depth ◽

Second Best

Abstract. Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth.) Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m2 s−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

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Ekman layers in the Southern Ocean: spectral models and observations, vertical viscosity and boundary layer depth

Ocean Science ◽

10.5194/os-5-115-2009 ◽

2009 ◽

Vol 5 (2) ◽

pp. 115-139 ◽

Cited By ~ 21

Author(s):

S. Elipot ◽

S. T. Gille

Keyword(s):

Boundary Layer ◽

Boundary Condition ◽

Transfer Function ◽

Southern Ocean ◽

Wind Stress ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Surface Velocity ◽

Layer Depth ◽

Second Best

Abstract. Spectral characteristics of the oceanic boundary-layer response to wind stress forcing are assessed by comparing surface drifter observations from the Southern Ocean to a suite of idealized models that parameterize the vertical flux of horizontal momentum using a first-order turbulence closure scheme. The models vary in their representation of vertical viscosity and boundary conditions. Each is used to derive a theoretical transfer function for the spectral linear response of the ocean to wind stress. The transfer functions are evaluated using observational data. The ageostrophic component of near-surface velocity is computed by subtracting altimeter-derived geostrophic velocities from observed drifter velocities (nominally drogued to represent motions at 15-m depth). Then the transfer function is computed to link these ageostrophic velocities to observed wind stresses. The traditional Ekman model, with infinite depth and constant vertical viscosity is among the worst of the models considered in this study. The model that most successfully describes the variability in the drifter data has a shallow layer of depth O(30–50 m), in which the viscosity is constant and O(100–1000 m2 s−1), with a no-slip bottom boundary condition. The second best model has a vertical viscosity with a surface value O(200 m2 s−1), which increases linearly with depth at a rate O(0.1–1 cm s−1) and a no-slip boundary condition at the base of the boundary layer of depth O(103 m). The best model shows little latitudinal or seasonal variability, and there is no obvious link to wind stress or climatological mixed-layer depth. In contrast, in the second best model, the linear coefficient and the boundary layer depth seem to covary with wind stress. The depth of the boundary layer for this model is found to be unphysically large at some latitudes and seasons, possibly a consequence of the inability of Ekman models to remove energy from the system by other means than shear-induced dissipation. However, the Ekman depth scale appears to scale like the climatological mixed-layer depth.

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On the Characteristic Height Scales of the Hurricane Boundary Layer

Monthly Weather Review ◽

10.1175/mwr-d-10-05017.1 ◽

2011 ◽

Vol 139 (8) ◽

pp. 2523-2535 ◽

Cited By ~ 158

Author(s):

Jun A. Zhang ◽

Robert F. Rogers ◽

David S. Nolan ◽

Frank D. Marks

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Mixed Layer ◽

Layer Structure ◽

Mixed Layer Depth ◽

Layer Depth ◽

Layer Height ◽

Boundary Layer Structure ◽

Boundary Layer Height ◽

Hurricane Boundary Layer

AbstractIn this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.

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Surface Fluxes and Ocean Coupling in the Tropical Intraseasonal Oscillation

Journal of Climate ◽

10.1175/jcli-3212.1 ◽

2004 ◽

Vol 17 (22) ◽

pp. 4368-4386 ◽

Cited By ~ 142

Author(s):

Eric D. Maloney ◽

Adam H. Sobel

Keyword(s):

Mixed Layer ◽

Mixed Layer Depth ◽

Intraseasonal Variability ◽

Intraseasonal Oscillation ◽

Sensitivity Study ◽

Ocean Model ◽

Surface Fluxes ◽

Boreal Winter ◽

Layer Depth ◽

Idealized Model

Abstract Sensitivity of tropical intraseasonal variability to mixed layer depth is examined in the modified National Center for Atmospheric Research Community Atmosphere Model 2.0.1 (CAM), with relaxed Arakawa–Schubert convection, coupled to a slab ocean model (SOM) whose mixed layer depth is fixed and geographically uniform, but varies from one experiment to the next. Intraseasonal west Pacific precipitation variations during boreal winter are enhanced relative to a fixed-SST (infinite mixed layer depth) simulation for mixed layer depths from 5 to 50 m, with a maximum at 20 m [interestingly, near the observed value in the regions where the Madden– Julian oscillation (MJO) is active], but are strongly diminished in the 2-m depth simulation. This nonmonotonicity of intraseasonal precipitation variance with respect to mixed layer depth was predicted by Sobel and Gildor using a highly idealized model. Further experiments with the same idealized model help to interpret results derived from the modified NCAR CAM. A sensitivity study shows that the convection–surface flux feedback [wind-induced surface heat exchange (WISHE)] is important to the intraseasonal variability in the CAM. This helps to explain the behavior of the 2-m SOM simulation and the agreement with the idealized model. Although intraseasonal SST variations are stronger in the 2-m SOM simulation than in any of the other simulations, these SST variations are phased in such a way as to diminish the amplitude of equatorial latent heat flux variations. Reducing the mixed layer depth is thus nearly equivalent to eliminating WISHE, which in this model reduces intraseasonal variability. The WISHE mechanism in the model is nonlinear and occurs in a region of mean low-level westerlies. Since a very shallow mixed layer is effectively similar to wet land, it is suggested that the mechanism described here may explain the local minimum in MJO amplitude observed over the Maritime Continent region.

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