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Zhiwei Zhang
,
Xincheng Zhang
,
Bo Qiu
,
Wei Zhao
,
Chun Zhou
,
Xiaodong Huang
, and
Jiwei Tian

instability (MLI) and strain-induced frontogenesis are likely two most important mechanisms responsible for the submesoscale generations ( Callies et al. 2015 ; Siegelman et al. 2020 ), which are consistent with the earlier quasigeostrophic theories and idealized numerical simulations ( Lapeyre et al. 2006 ; Boccaletti et al. 2007 ). However, which mechanism (MLI or frontogenesis) is more dominant seems to be situation dependent, and it is difficult to determine based on data because the two mechanisms

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Yuehua Li
and
Trevor J. McDougall

Abstract

Double-diffusive interleaving is examined as it progresses from a linear instability toward finite amplitude. When the basic stratification is in the “finger” sense, the initial series of finger interfaces is unstable and one grows in strength at the expense of the others. At an intermediate stage of its development, the interleaving motions pass through a stage when every second interface in the vertical is stable to double diffusion. At a later time this interface turns into a “diffusive” double-diffusive interface. This study takes the fluxes of heat and salt across both the finger and diffusive interfaces to be given by the laboratory flux laws, and the authors ask whether a steady state is possible. It is found that the fluxes across the diffusive interfaces must be many times stronger relative to the corresponding fluxes across the finger interfaces than is indicated from existing flux expressions derived from laboratory experiments. The total effect of the interleaving motion on the vertical fluxes of heat and of salt are calculated for the steady-state solutions. It is found that both the fluxes of heat and salt are upgradient, corresponding to a negative vertical diffusion coefficient for all heat, salt, and density. For moderate to large Prandtl numbers, these negative effective diapycnal diffusivities of heat and salt are approximately equal so that the interleaving process acts to counteract some of the usual turbulent diapycnal diffusivity due to breaking internal waves.

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Reuben Demirdjian
,
Richard Rotunno
,
Bruce D. Cornuelle
,
Carolyn A. Reynolds
, and
James D. Doyle

⁡ ( 1 ρ ∂ ψ ∂ Z ) + ∂ ∂ X ⁡ ( ρ q g f 3 θ 0 ∂ ψ ∂ X ) = − 2 Q − g ρ f 2 θ 0 ∂ S ∂ X , where ψ is the ageostrophic streamfunction, g is the gravitational acceleration, f is the Coriolis parameter, θ 0 is the base-state potential temperature, Q is geostrophic forcing of frontogenesis

Open access
Christian E. Buckingham
,
Jonathan Gula
, and
Xavier Carton

adjustment and frontogenesis problem for curved density fronts, essentially generalizing the work of Hoskins and Bretherton (1972) . Adams et al. (2017) examined submesoscale instabilities on the edge of a mesoscale eddy in the Southern Ocean. Although Adams et al. (2017) did not examine the relevant criterion given above, the authors did examine the Ertel PV, finding that curvature did not appreciably modify their results. Last, Brannigan et al. (2017) explored these dynamics within numerical

Open access
Mirja L. Kemppi
and
Victoria A. Sinclair

different structures of the surface front at the two locations were due to an evolving large-scale flow. At both locations, the front had a larger horizontal temperature gradient aloft (~4 km) than at the surface, which Wakimoto and Bosart (2001) hypothesized may be due to turbulent fluxes of heat and momentum from the ocean surface acting to diffuse the surface front. This observation differs from the analytical model of frontogenesis presented by Hoskins and Bretherton (1972) , which predicts that

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Christian E. Buckingham
,
Jonathan Gula
, and
Xavier Carton

-D-13-0157.1 Holton , J. R. , 1992 : An Introduction to Dynamic Meteorology . 3rd ed. Academic Press, 511 pp. Hoskins , B. J. , 1974 : The role of potential vorticity in symmetric stability and instability . Quart. J. Roy. Meteor. Soc. , 100 , 480 – 482 , https://doi.org/10.1002/qj.49710042520 . 10.1002/qj.49710042520 Hoskins , B. J. , and F. P. Bretherton , 1972 : Atmospheric frontogenesis models: Mathematical formulation and solution . J. Atmos. Sci. , 29 , 11 – 37 , https

Open access
James C. McWilliams
,
Jonathan Gula
,
M. Jeroen Molemaker
,
Lionel Renault
, and
Alexander F. Shchepetkin

1. Introduction A widespread appreciation has emerged and grown in the past several years for how active the regime of submesoscale currents is within the oceanic surface layer. Examples are density fronts and filaments, their instabilities, coherent vortices, vertical material fluxes, and a forward energy cascade to an enhanced dissipation rate. Mesoscale eddies are the energy source for submesoscale flows and density gradients, and strain-induced frontogenesis is a process that can shrink the

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Marcia Cronce
,
Robert M. Rauber
,
Kevin R. Knupp
,
Brian F. Jewett
,
Justin T. Walters
, and
Dustin Phillips

1. Introduction Mesoscale precipitation bands often develop in the north and northwest quadrants of extratropical cyclones ( Novak et al. 2004 ). From many studies, it appears that the vertical motions in mesoscale bands are forced by frontogenesis, either in an environment that is stable to upright convection and characterized by small moist symmetric stability (e.g., Thorpe and Emanuel 1985 ; Sanders and Bosart 1985 ; Sanders 1986 ) or in an environment characterized by weak moist

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Lingling Xie
,
Enric Pallàs-Sanz
,
Quanan Zheng
,
Shuwen Zhang
,
Xiaolong Zong
,
Xiaofei Yi
, and
Mingming Li

Abstract

Using the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10−4) kg m−4 and vertical eddy diffusivity of O(10−5–10−4) m2 s−1. The kinematic deformation term S DEF, ageostrophic advection term S ADV, and vertical mixing forcing term S MIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive–negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches −5 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indicates that S ADV contributes most to the coastal upwelling, while term S DEF, which is dominated by the ageostrophic component S DEFa, plays a dominant role in the frontal zone. The vertical mixing forcing term S MIX, which includes the momentum and buoyancy flux terms S MOM and S BUO, is comparable to S DEF and S ADV in the upper ocean, but negligible below the thermocline. The effect of the vertical mixing on the vertical velocity is mainly concentrated at depths with relatively large eddy diffusivity and eddy diffusivity gradient in the frontal zone.

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R. D. Sharman
and
J. M. Pearson

Abstract

Current automated aviation turbulence forecast algorithms diagnose turbulence from numerical weather prediction (NWP) model output by identifying large values in computed horizontal or vertical spatial gradients of various atmospheric state variables (velocity; temperature) and thresholding these gradients empirically to indicate expected areas of “light,” “moderate,” and “severe” levels of aviation turbulence. This approach is obviously aircraft dependent and cannot accommodate the many different aircraft types that may be in the airspace. Therefore, it is proposed to provide forecasts of an atmospheric turbulence metric: the energy dissipation rate to the one-third power (EDR). A strategy is developed to statistically map automated turbulence forecast diagnostics or groups of diagnostics to EDR. The method assumes a lognormal distribution of EDR and uses climatological peak EDR data from in situ equipped aircraft in conjunction with the distribution of computed diagnostic values. These remapped values can then be combined to provide an ensemble mean EDR that is the final forecast. New mountain-wave-turbulence algorithms are presented, and the lognormal mapping is applied to them as well. The EDR forecasts are compared with aircraft in situ EDR observations and verbal pilot reports (converted to EDR) to obtain statistical performance metrics of the individual diagnostics and the ensemble mean. It is shown by one common performance metric, the area under the relative operating characteristics curve, that the ensemble mean provides better performance than forecasts from individual model diagnostics at all altitudes (low, mid-, and upper levels) and for two input NWP models.

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