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James C. McWilliams
and
M. Jeroen Molemaker

the lower atmosphere or upper ocean. The most familiar atmospheric occurrences are evident in the rainfall patterns in wintertime storms, but fronts are also common in oceanic surface temperature images from satellites ( Castellao et al. 2006 ; Ullman et al. 2007 ). Spall (1997) summarizes in situ observations of oceanic surface fronts. Frontogenesis occurs for favorably aligned buoyancy gradients in the presence of a larger-scale horizontal deformation flow ( Bergeron 1928 ). Okubo (1970

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Callum J. Shakespeare

such, many fronts are curved, with a radius of curvature comparable to the radius of the eddy. In some cases the eddy will remain in a stable, near-balanced state, while in other cases the curved front may be shed from the eddy and further strained and deformed by the surrounding flow field. The process by which an eddy and associated front attains a stable steady state is called adjustment, and the process by which a front is strained and sharpened is called frontogenesis. Adjustment and

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Callum J. Shakespeare
and
Leif N. Thomas

). Thomas and Shakespeare (2015) described a mode water formation mechanism involving the mixing of cross-front temperature–salinity contrasts, cabbeling, and frontogenesis that has the potential to select the density, temperature, and salinity of a particular mode water. The fronts that border mode waters are characterized by density-compensated temperature and salinity contrasts that decrease in magnitude with depth. Along-isopycnal mixing of the disparate water masses across the fronts leads to an

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Audric G. Collignon
and
Mark T. Stacey

terms of Eq. (10) during the ebb. The Coriolis and lateral advection terms are smaller but also contribute at leading order. c. Implications for convergence fronts Various frontogenesis mechanisms at the edge of a channel during ebb tides have been identified and analyzed in previous studies. The effect of Earth’s rotation have been identified as a potential frontogenesis mechanism by Mied et al. (2000) and studied further by Handler et al. (2001) , Mied et al. (2002) , and Handler et al

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Sara A. Ganetis
and
Brian A. Colle

-polarized radar site (KBOX); Albany, NY, upper-air site (KALB); and Albany, NY, dual-polarized radar site (KENX). Over the Northeast United States there are mesoscale snowbands within the comma head of ~85% of extratropical cyclones during the winter months ( Novak et al. 2004 ). Well-defined primary bands have been identified as forming north of the surface cyclone center in the region of enhanced midlevel (~700 hPa) frontogenesis and reduced stability ( Novak et al. 2004 , 2010 ). For single band formation

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Alan F. Srock
and
Lance F. Bosart

/ocean airmass contrasts on the precipitation distribution of TCs was suggested as a way to improve understanding and forecast skill by the Fifth Prospectus Development Team of the U.S. Weather Research Program (PDT-5; Marks et al. 1998 ). Nearshore surface features can modify the final TC precipitation distribution through mesoscale effects such as orographically forced ascent (upslope), coastal frontogenesis, and cold-air damming (CAD). This paper will examine the formation and enhancement of coastal

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Alberto C. Naveira Garabato
,
Xiaolong Yu
,
Jörn Callies
,
Roy Barkan
,
Kurt L. Polzin
,
Eleanor E. Frajka-Williams
,
Christian E. Buckingham
, and
Stephen M. Griffies

submesoscale motions Π ω (with positive values indicating a downscale KE transfer), (b) submesoscale lateral buoyancy gradient magnitude |∇ b |, and (c) mesoscale frontogenesis function F S (with positive and negative values respectively indicating frontogenesis and frontolysis). Red arrows on the upper axis of (a) indicate the main events of downscale KE transfer. The mixed layer depth as determined from glider measurements is shown by the black contour in (a)–(c). (d) Histogram synthesis of the

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Ryusuke Masunaga
,
Hisashi Nakamura
,
Bunmei Taguchi
, and
Takafumi Miyasaka

://www.ecmwf.int/node/18765 . Hewson , T. D. , 1998 : Objective fronts . Meteor. Appl. , 5 , 37 – 65 , https://doi.org/10.1017/S1350482798000553 . 10.1017/S1350482798000553 Holton , J. R. , 2004 : An Introduction to Dynamic Meteorology . 4th ed. Academic Press, 535 pp. Hoskins , B. J. , 1982 : The mathematical theory of frontogenesis . Annu. Rev. Fluid Mech. , 14 , 131 – 151 , https://doi.org/10.1146/annurev.fl.14.010182.001023 . 10.1146/annurev.fl.14.010182.001023 Hoskins , B. J. , and P. J. Valdes

Open access
Raphaël Rousseau-Rizzi
and
Kerry Emanuel

Abstract

Potential intensity (PI) is an analytical bound on steady, inviscid, axisymmetric hurricane wind speed. Studies have shown that simulated hurricane azimuthal wind speed can greatly exceed a PI bound on the maximum gradient wind. This disparity is called superintensity (SI) and has been attributed to the contribution of the unbalanced flow to the azimuthal wind. The goals of this study are 1) to introduce a new surface wind PI (PI s ), based on a differential Carnot cycle and bounding the magnitude of the surface winds; 2) to evaluate SI in numerical simulations with respect to diagnostic PI bounds on gradient wind (PI g ), azimuthal wind (PI a ), and surface wind (PI s ); and 3) to evaluate the validity of each PI bound based on the SI computations. Here, we define superintensity as the normalized amount by which each version of PI is exceeded by the quantity it bounds. Axisymmetric tropical cyclone simulations are performed while varying the parameterized turbulent mixing as a way of estimating SI in the inviscid limit. As the mixing length decreases, all three bounded wind speeds increase similarly from a sub-PI state to a marginally superintense state. This shows that all three forms of PI evaluated here are good approximations to their respective metrics in numerical simulations.

Open access
Aviv Solodoch
,
Jeroen M. Molemaker
,
Kaushik Srinivasan
,
Maristella Berta
,
Louis Marie
, and
Arjun Jagannathan

Abstract

We present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.

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