Frictional Convergence in a Decaying Weak Vortex

Abstract Cross-isobaric flow and Ekman pumping are investigated in the frictional spindown of an initially barotropic vortex in a stratified atmosphere. Consistent with early work by Holton and others, it is found that the stratification limits the vertical penetration of the secondary circulation driven by friction, resulting in more rapid spindown than in the unstratified case. As a result, the cross-isobaric flow and Ekman pumping are weaker and shallower than classical calculations ignoring the stratification would lead one to believe. The effect of stability becomes stronger as the vortex becomes smaller for fixed boundary layer depth. For weak geostrophic vortices with horizontal scales of several hundred kilometers or less, such as tropical easterly waves, the reduction is particularly pronounced, which raises questions about the efficacy of Ekman pumping in forcing convection in such vortices. These results suggest a revised conceptual model for the role of Ekman pumping in the atmosphere. The theory as it stands is limited to weak, linear vortices in which geostrophic balance holds approximately, corresponding to small Rossby number, though extensions of the analytical theory to stronger vortices may be possible.

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Diagnosis of boundary-layer circulations

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2011.0474 ◽

2013 ◽

Vol 371 (1991) ◽

pp. 20110474 ◽

Cited By ~ 5

Author(s):

Robert J. Beare ◽

Michael J. P. Cullen

Keyword(s):

Boundary Layer ◽

Vertical Plane ◽

Climate System ◽

Momentum Balance ◽

Ekman Pumping ◽

Layer Depth ◽

Low Level ◽

Geostrophic Balance ◽

Low Level Jet

Diagnoses of circulations in the vertical plane provide valuable insights into aspects of the dynamics of the climate system. Dynamical theories based on geostrophic balance have proved useful in deriving diagnostic equations for these circulations. For example, semi-geostrophic theory gives rise to the Sawyer–Eliassen equation (SEE) that predicts, among other things, circulations around mid-latitude fronts. A limitation of the SEE is the absence of a realistic boundary layer. However, the coupling provided by the boundary layer between the atmosphere and the surface is fundamental to the climate system. Here, we use a theory based on Ekman momentum balance to derive an SEE that includes a boundary layer (SEEBL). We consider a case study of a baroclinic low-level jet. The SEEBL solution shows significant benefits over Ekman pumping, including accommodating a boundary-layer depth that varies in space and structure, which accounts for buoyancy and momentum advection. The diagnosed low-level jet is stronger than that determined by Ekman balance. This is due to the inclusion of momentum advection. Momentum advection provides an additional mechanism for enhancement of the low-level jet that is distinct from inertial oscillations.

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Fluxes and Gradients in the Convective Surface Layer and the Possible Role of Boundary-Layer Depth and Entrainment Flux

Boundary-Layer Meteorology ◽

10.1007/s10546-004-2730-7 ◽

2005 ◽

Vol 116 (2) ◽

pp. 237-252 ◽

Cited By ~ 13

Author(s):

G. J. Steeneveld ◽

A. A. M. Holtslag ◽

H. A. R. Debruin

Keyword(s):

Boundary Layer ◽

Surface Layer ◽

Layer Depth

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Submesoscale Dynamics in the Northern Gulf of Mexico. Part I: Regional and Seasonal Characterization and the Role of River Outflow

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0035.1 ◽

2017 ◽

Vol 47 (9) ◽

pp. 2325-2346 ◽

Cited By ~ 22

Author(s):

Roy Barkan ◽

James C. McWilliams ◽

Alexander F. Shchepetkin ◽

M. Jeroen Molemaker ◽

Lionel Renault ◽

...

Keyword(s):

Boundary Layer ◽

Gulf Of Mexico ◽

Root Mean Square ◽

Mean Square ◽

Layer Depth ◽

Mean Values ◽

Northern Gulf Of Mexico ◽

River Outflow ◽

Submesoscale Currents

AbstractRealistic, submesoscale-resolving numerical simulations are used to characterize the flow’s statistics and the geography of surface submesoscale currents in the northern Gulf of Mexico. This study examines the role of the Mississippi–Atchafalaya River system in driving submesoscale currents during winter and summer, on and off the shelf, by investigating two sets of statistically equilibrated solutions, with and without river forcing. In this paper, the first of three, the authors analyze vorticity ζ, horizontal divergence δ, and available potential energy to eddy kinetic energy conversion and show that river forcing has an important effect on the spatial distribution and magnitudes of submesoscale currents in both seasons. During winter, solutions without river forcing display an increase in seasonal-mean values of ζ, δ and compared to solutions with river forcing, particularly east of the Mississippi River delta and offshore. On the contrary, during summer, seasonal-mean values are larger in solutions with river forcing throughout the entire region. The river effects can be rationalized in terms of scaling arguments that relate submesoscale current magnitudes to the surface boundary layer depth and lateral buoyancy gradients. River outflow enhances submesoscale currents by increasing lateral buoyancy gradients but suppresses them by decreasing the boundary layer depth. A discussion of the submesoscale-generating mechanisms that in each season may determine whether the enhancement effect overcomes the suppression effect or vice versa is presented. Regional comparisons of horizontal velocity spectra, root-mean-square ζ, root-mean-square δ, and root‐mean‐square across different resolutions show no sign of convergence even at 150-m horizontal resolution. This demonstrates the numerical challenge of modeling the full range of submesoscale currents.

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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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A Statistical Study of Process Variables to Optimize a High Speed Curtain Coater: Part I

TAPPI Journal ◽

10.32964/tj8.1.20 ◽

2009 ◽

Vol 8 (1) ◽

pp. 20-26 ◽

Cited By ~ 1

Author(s):

PEEYUSH TRIPATHI ◽

MARGARET JOYCE ◽

PAUL D. FLEMING ◽

MASAHIRO SUGIHARA

Keyword(s):

Boundary Layer ◽

Experimental Design ◽

High Speed ◽

Statistical Study ◽

Process Variables ◽

High Speeds ◽

The Stability ◽

Air Removal ◽

Removal System

Using an experimental design approach, researchers altered process parameters and material prop-erties to stabilize the curtain of a pilot curtain coater at high speeds. Part I of this paper identifies the four significant variables that influence curtain stability. The boundary layer air removal system was critical to the stability of the curtain and base sheet roughness was found to be very important. A shear thinning coating rheology and higher curtain heights improved the curtain stability at high speeds. The sizing of the base sheet affected coverage and cur-tain stability because of its effect on base sheet wettability. The role of surfactant was inconclusive. Part II of this paper will report on further optimization of curtain stability with these four variables using a D-optimal partial-facto-rial design.

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