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A theoretical problem of linear stationary disturbances of the background geostrophic flow of a stratified rotating medium (atmosphere) that are induced by inhomogeneities of the gravitational field is considered. There is a common belief that such inhomogeneities may only somewhat deform (distort) the state of hydrostatic equilibrium, but cannot affect the dynamics of the flow in the atmosphere. Generally, the problem statement is different for the processes over a solid surface and a water surface, because a water surface (the lower boundary condition for the atmosphere) is deformed by inhomogeneities of the gravitational field. The problem of disturbances over a water surface has been considered in recent papers of the authors; in this paper, the results are developed and significantly revised. The emphasis is on disturbances over a flat horizontal surface, which were not examined before. From the analytical solutions, it follows that the influence of inhomogeneities of the gravitational field on the atmospheric flows may be significant in some cases. Physical generation mechanisms of disturbances are analyzed. [ABSTRACT FROM AUTHOR]

Nocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia-gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain. Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope. [ABSTRACT FROM AUTHOR]

The proximity between the 6 yr recurrence time of the torsional Alfvén waves that have been inferred in the Earth's outer core over 1940-2010 and their 4 yr traveltime across the fluid core is nicely explained if these travelling waves are to be considered as normal modes. We discuss to what extent the emergence of free torsional modes from a stochastic forcing in the fluid core is compatible with some dissipation, specifically with an electromagnetic torque strong enough to account for the observed length of day variations of 6 yr period. In a spherical cavity enclosed by an insulating mantle, torsional normal modes consist of standing waves. In the presence of a conducting mantle, they transform into outward travelling waves very similar to the torsional waves that have been detected in the Earth's outer core. With such a resonant response a periodic forcing is not required to explain the regular recurrence of torsional waves; neither is the search for a source of motions in the vicinity of the cylindrical surface tangent to the inner core, where travelling waves seem to emerge. We discuss these results in the light of the reflection properties of torsional waves at the equator. We are able to reproduce the properties found for geophysical time-series of geostrophic flows (detection of a normal mode, almost total absorption at the equator) if the conductance of the lowermost mantle is 3 × 107 to 108 S. [ABSTRACT FROM AUTHOR]

The analysis of the growth of initial perturbations in dynamical systems is an important aspect of predictability theory because it informs on error growth. The Conditional Nonlinear Optimal Perturbation (CNOP) method is an approach where the nonlinear growth of perturbations is determined over a certain lead time. The CNOPs can be found by a nonlinear constrained optimisation problem, which is typically solved using sequential quadratic programming (SQP), a routine that requires an adjoint model. Such adjoint models are not always available and hence we here study the performance of an adjoint-free optimisation method (COBYLA), in combination with a dimension reduction technique, to determine CNOPs. The new technique is applied to a quasi-geostrophic model of the wind-driven ocean circulation. We find that COBYLA is able to find good approximations of CNOPs, albeit at a higher computational cost than conventional adjoint-based methods. [ABSTRACT FROM AUTHOR]

Numerical modeling using the Princeton Ocean Model (POM) was performed for the southeastern part of the Baltic with the objective to compare the spatial distribution of the velocity and bottom sediments data in this region. Special attention was focused on the consideration of the influence of the western and northeastern winds, which lead to the formation of the intense quasi-geostrophic currents and may cause very high velocities in the near bottom layer so that the elution of bottom sediments and transport of their fine fractions occurs. Under abrupt changing of wind velocity the effect of elution can intensify due to the generation of inertial internal waves penetrating into the bottom layer. The comparative analysis of spatial distributions of the velocity in the surface and near bottom layers with the bottom sediments data showed that the areas with the highest velocities that were formed under the western and northeastern winds in most cases coincide with the areas where the bottom sediments are presented by coarse fractions: gravel and sands. [ABSTRACT FROM AUTHOR]

The prediction of convective initiation remains a challenge to forecasters in the Great Plains, especially for elevated events at night. This study examines a subset of 287 likely elevated nocturnal convective initiation events that occurred with little or no direct influence from surface boundaries or preexisting convection over a 4-month period of May-August during the summer of 2015. Events were first classified into one of four types based on apparent formation mechanisms and location relative to any low-level jet. A climatology of each of the four types was performed focusing on general spatial tendencies over a large Great Plains domain and initiation timing trends. Simulations from five convection-allowing models available during the Plains Elevated Convection At Night (PECAN) field campaign, along with four versions of a 4-km Weather Research and Forecasting (WRF) Model, were used to examine the predictability of these types of convective initiation. A dual-peak pattern for initiation timing was revealed, with one peak near 0400 UTC and another around 0700 UTC. The times and prominence of each peak shifted depending on the region analyzed. Positive thermal advection by the geostrophic wind was present in the majority of events for three types but not for the type occurring without a low-level jet. Models were more deficient with location than timing for the five PECAN models, with the four 4-km WRF Models showing similar location errors and problems with initiating convection at a lower altitude than observed. [ABSTRACT FROM AUTHOR]

It has been suggested that upper-tropospheric Rossby wave packets propagating along the midlatitude waveguide may play a role for triggering severe weather. This motivates the search for robust methods to detect and track Rossby wave packets and to diagnose their properties. In the framework of several observed cases, this paper compares different methods that have been proposed for these tasks, with an emphasis on horizontal propagation and on a particular formulation of a wave activity flux previously suggested by Takaya and Nakamura. The utility of this flux is compromised by the semigeostrophic nature of upper-tropospheric Rossby waves, but this problem can partly be overcome by a semigeostrophic coordinate transformation. The wave activity flux allows one to obtain information from a single snapshot about the meridional propagation, in particular propagation from or into polar and subtropical latitudes, as well as about the onset of wave breaking. This helps to clarify the dynamics of individual wave packets in cases where other, more conventional methods provide ambiguous or even misleading information. In some cases, the 'true dynamics' of the Rossby wave packet turns out to be more complex than apparent from the more conventional diagnostics, and this may have important implications for the predictability of the wave packet. [ABSTRACT FROM AUTHOR]

A condition for the existence of sediment waves under turbidity currents as antidunes with the requirement of a slope gradient ≥ 3.0 × 10 − 3 is deduced. Data show no such waves on slopes < 2.5 × 10 − 3 , but some contourite mudwaves occur on slopes as low as 4.4 × 10 − 4 . However the latter also occur on slopes > 3 × 10 − 3 so no clear distinction is possible. Where turbidity current channels cross sediment drifts, or geostrophic flows traverse turbidite fans, the origin of most mudwaves will need to be determined by reference to internal features and context. A key problem is deposition of mud as antidunes from turbidity currents where even the waning flow is probably well above the critical erosion velocity for a clear flow. Deposition must occur from high concentration flows well above clear water critical depositional stresses. Once a wavy bed is set up, subsequent deposition may occur via the lee-wave mechanism proposed for contourite waves under a gradient Froude Number > 1. A steep angle (< 45°) between crest and flow axes is typical of GF waves, which may be dunes or antidunes, whereas TC waves tend to be orthogonal, but data on this discriminant are sparse. [ABSTRACT FROM AUTHOR]

A foehn wind is an important factor in the occurrence of many extreme high-temperature events in geographically complex regions. In this study, the authors verified the hypothesis that a foehnlike wind contributes to high temperatures at the end of the leeward (eastward) area using three difference approaches: field experiments, numerical experiments, and statistical analyses. According to the hypothesis, a foehnlike wind has the features of the sum of a traditional foehn effect with adiabatic heating, plus dry-diabatic heating from the ground surface along the fetch of the wind. Field experiments conducted at seven observational points on Nobi Plain, Japan, where a mesoscale westerly wind blew, revealed that the westerly wind clearly had the features of a traditional foehn effect in the western part of the Nobi Plain. In addition to field experiments, a simplified estimate using a simple mixed-layer model demonstrated that the wind was further heated by dry-diabatic heating (sensible heat supply) from the ground surface along the fetch (especially in urbanized areas in the eastern region of the Nobi Plain) of the wind. This diabatic heating effect along the fetch of the wind on the high temperature at the end of the leeward area was also supported by both additional numerical experiments and a statistical analysis. These results proved that the hypothesis is correct and indicated that ground conditions and the land use and land cover in the windward area were strongly related to air temperature at the end of the leeward area, where an extremely high temperature was observed. [ABSTRACT FROM AUTHOR]

Extensive measurements were made of the summertime Great Plains low-level jet (LLJ) in central Kansas during June and July 2015 as a component of the Plains Elevated Convection at Night (PECAN) field study. Here, the authors describe the early phase of the LLJ development on 20 June 2015. Half-hourly soundings were launched to monitor the progress of the jet. An airborne mission was also conducted using the University of Wyoming King Air research aircraft. Vertical sawtooth patterns were flown along a fixed track at 38.7°N between longitudes 98.9° and 100.3°W to document changes in the potential temperature and wind profiles. Ageostrophic winds during the LLJ formation were also assessed. In addition, a high-resolution numerical simulation of the 20 June 2015 LLJ case was conducted using the Weather Research and Forecasting Model. Observations and model results show that the early stage of development consisted of a rapid increase in wind speed in the hours just after sunset with less pronounced directional change. The LLJ evolution is similar to that expected from an inertial oscillation of the ageostrophic wind following the stabilization of the near-surface layer. [ABSTRACT FROM AUTHOR]

The meridional Ekman volume, heat, and salt transport across two trans-Atlantic sections near 14.5° N and 11° S were estimated using in situ observations, wind products, and model data. A meridional ageostrophic velocity was obtained as the difference between the directly measured total velocity and the geostrophic velocity derived from observations. Interpreting the section mean ageostrophy to be the result of an Ekman balance, the meridional Ekman transport of 6:2×2:3 Sv northward at 14.5° N and 11:7×2:1 Sv southward at 11° S is estimated. The integration uses the top of the pycnocline as an approximation for the Ekman depth, which is on average about 20m deeper than the mixed layer depth. The Ekman transport estimated based on the velocity observations agrees well with the predictions from in situ wind stress data of 6:7×3:5 Sv at 14.5° N and 13:6×3:3 Sv at 11° S. The meridional Ekman heat and salt fluxes calculated from sea surface temperature and salinity data or from highresolution temperature and salinity profile data differ only marginally. The errors in the Ekman heat and salt flux calculation were dominated by the uncertainty of the Ekman volume transport estimates. [ABSTRACT FROM AUTHOR]

When internal (inertia-)gravity waves propagate in a vertically sheared geostrophic (eddying or mean) flow, they exchange energy with the flow. A novel concept parameterizing internal wave-mean flow interaction in ocean circulation models is demonstrated, based on the description of the entire wave field by the wave-energy density in physical and wavenumber space and its prognostic computation by the radiative transfer equation. The concept enables a simplification of the radiative transfer equation with a small number of reasonable assumptions and a derivation of simple but consistent parameterizations in terms of spectrally integrated energy compartments that are used as prognostic model variables. The effect of the waves on the mean flow in this paradigm is in accordance with the nonacceleration theorem: only in the presence of dissipation do waves globally exchange energy with the mean flow in the time mean. The exchange can have either direction. These basic features of wave-mean flow interaction are theoretically derived in a Wentzel-Kramers-Brillouin (WKB) approximation of the wave dynamics and confirmed in a suite of numerical experiments with unidirectional shear flow. [ABSTRACT FROM AUTHOR]

The variability of midlatitude jets is investigated in a long-term integration of a dry three-level quasigeostrophic model on the sphere. As for most observed jets, the leading EOF of the zonal-mean wind corresponds to latitudinal shifts of the jet, and the second EOF to pulses of the jet speed. The first principal component (PC1) is also more persistent than the second one (PC2); this longer persistence arises from different eddy feedbacks both in the short term (i.e., within a few days following the peak of the PCs) and in the long term. The short-term eddy feedbacks come from two distinct mechanisms. First, a planetary waveguide effect acts as a negative feedback on both PCs. The positive phases of PC1 and PC2, which correspond to poleward-shifted and accelerated jets, respectively, are first driven then canceled by planetary waves reflecting on the equatorward flank of the jet. A similar process occurs for the negative phases when planetary waves reflect on the poleward flank of the jet. Second, synoptic waves also exert a short-term negative feedback on PC2: when the jet accelerates, the enhanced meridional wind shear increases the barotropic sink of eddy energy and depletes it very rapidly, therefore preventing synoptic eddies from maintaining the accelerated jet. Finally, at lags longer than their typical time scale, synoptic eddies drive a positive feedback on PC1 only. This feedback can be explained by a baroclinic mechanism in which the jet shift modifies the baroclinicity, causing, first, eddy heat flux anomalies and, then, momentum convergence anomalies. This feedback is absent for PC2, despite some changes in the baroclinicity. [ABSTRACT FROM AUTHOR]

Motion in planetary geostrophic equations (PGEs) is represented by the three-dimensional geostrophic wind ( ug, vg, wg) where ug and vg are the standard horizontal components while the vertical component wg can be derived, for example, from the Richardson equation. However, this vertical component appears not to have been evaluated as yet on the basis of data nor compared to the actual vertical component w. Part of this missing information is provided here by an evaluation of wg from observations and by analyzing the role of wg in linear versions of PGEs. The time mean fields [ABSTRACT FROM AUTHOR]

Recent case-study analyses produced a conceptual model suggesting that lower-stratospheric fronts, the lower-stratospheric half of a tropopause jet-front system, develop preferentially in southwesterly flow in the presence of lower-stratospheric geostrophic warm air advection and ascent within the jet core. This conceptual understanding is examined by performing climatological and case analyses of 185 objectively identified lower-stratospheric fronts that occurred over North America during the winter seasons (December, January and February) of 2004-2012 in the 1.0°×1.0°NCEP/NCAR Global Forecast System analysis. The climatological analysis shows a preference for strong southwesterly flow lower-stratospheric fronts to occur over the Intermountain West region and relatively weaker southwesterly flow lower-stratospheric fronts to occur over eastern North America. The southwesterly flow lower-stratospheric fronts were composited according to their geographical location and the analysis shows that the composite lower-stratospheric and tropospheric structures in these two locations have statistical differences. The case-study analyses of southwesterly flow lower-stratospheric fronts in these two regions highlight different, geographically dependent, frontal evolutions. The strong lower-stratospheric front identified in the Intermountain West region interacted with an orographic gravity wave and associated lower-stratospheric ascent that rearranged the local thermal field. The lower-stratospheric front in eastern North America developed parallel to a surface front and was associated with diabatically reduced static stability in the upper troposphere, geostrophic warm air advection, and enhanced ascent. [ABSTRACT FROM AUTHOR]

In developing methods for convective-scale data assimilation (DA) it is necessary to consider the full range of motions governed by the compressible Navier-Stokes equations (including non-hydrostatic and ageostrophic flow). These equations describe motion on a wide range of time-scales with non-linear coupling. For the purpose of developing new DA techniques that suit the convective-scale problem it is helpful to use so-called "toy models" that are easy to run, and contain the same types of motion as the full equation set. Such a model needs to permit hydrostatic and geostrophic balance at the large-scales, but to allow imbalance at the small-scale, and in particular, they need to exhibit intermittent convection-like behaviour. Existing "toy models" are not always sufficient for investigating these issues. A simplified system of intermediate complexity derived from the Euler equations is presented, which support dispersive gravity and acoustic modes. In this system the separation of time scales can be greatly reduced by changing the physical parameters. Unlike in existing models, this allows the acoustic modes to be treated explicitly, and hence inexpensively. In addition, the non-linear coupling induced by the equation of state is simplified. This means that the gravity and acoustic modes are less coupled than in conventional models. A vertical slice formulation is used which contains only dry dynamics. The model is shown to give physically reasonabe results, and convective behaviour is generated by localised compressible effects. This model provides an affordable and flexible framework within which some of the complex issues of convective-scale DA can later be investigated. The model is called the "ABC model" after the three tunable parameters introduced: A (the gravity wave frequency), B (the modulation of the divergent term in the continuity equation), and C (defining the compressibility). [ABSTRACT FROM AUTHOR]

For mid-latitude Rossby waves (RWs) in the atmosphere, the expression for the energy flux for use in a model diagnosis, and without relying on a Fourier analysis or a ray theory, has previously been derived using quasi-geostrophic equations and is singular at the equator. By investigating the analytical solution of both equatorial and mid-latitude waves, the authors derive an exact universal expression for the energy flux which is able to indicate the direction of the group velocity at all latitudes for linear shallow water waves. This is achieved by introducing a streamfunction as given by the inversion equation of Ertel's potential vorticity, a novel aspect for considering the energy flux. For ease of diagnosis from a model, an approximate version of the universal expression is explored and illustrated for a forced/dissipative equatorial basin mode simulated by a single-layer oceanic model that includes both mid-latitude RWs and equatorial waves. Equatorial Kelvin Waves (KWs) propagate eastward along the equator, are partially redirected poleward as coastal KWs at the eastern boundary of the basin, and then shed mid-latitude RWs that propagate westward into the basin interior. The connection of the equatorial and coastal waveguides has been successfully illustrated by the approximate expression of the group-velocity-based energy flux of the present study. This will allow for tropical-extratropical interactions in oceanic and atmospheric model outputs to be diagnosed in terms of an energy cycle in a future study. [ABSTRACT FROM AUTHOR]

An overview on the helicity of the velocity field and the role played by this concept in modern research in the field of geophysical fluid dynamics and dynamic meteorology is given. Different (both previously known in the literature and first presented) formulations of the equation of helicity balance in atmospheric motions (including those with allowance for effects of air compressibility and Earth's rotation) are brought together. Equations and relationships are given which are valid in different approximations accepted in dynamic meteorology: Boussinesq approximation, quasi-static approximation, and quasi-geostrophic approximation. Emphasis is placed on the analysis of helicity budget in large-scale quasi-geostrophic systems of motion; a formula for the helicity flux across the upper boundary of the nonlinear Ekman boundary layer is given, and this flux is shown to be exactly compensated for by the helicity destruction inside the Ekman boundary layer. [ABSTRACT FROM AUTHOR]