Ageostrophic Geopotential Fluxes Research Articles

Maintenance of the South Asian jet wave train: eddy kinetic energy balance

The mechanisms of the propagation and maintenance of the South Asian jet wave train are investigated in this study by examining the eddy kinetic energy (EKE) balance along the wave train. The intensity and evolution of the disturbances along the wave train can be well represented by the variation of EKE centers. The east-to-west discrepancy in the intensity of EKE centers indicates the wave train is not only maintained by the waveguide effect of the jet stream, but also fueled and damped by other physical processes along the wave train. It is found that both baroclinic and barotropic conversions are key energy sources during the lifespan of the wave train, characterized by regional differences. They evolve with the eddies and are offset mainly by energy exported by the ageostrophic geopotential fluxes after the peak of the eddies. They are strong upstream of the wave train, as extremely strong cooling/warming advection appears behind/ahead of the trough over the eastern Mediterranean Sea-Middle East where abnormal descent/ascent occurs, and potential energy is transformed into kinetic energy. As this trough tilts southwestward, kinetic energy can be converted from the mean flow to eddies on the north flank of the South Asian jet. Strong dissipation occurs over the Middle East, which might be responsible for the relatively weaker EKE centers downstream. Over China, the eddy is fueled by strong barotropic conversion again, as a result of the northward shift of the jet axis.

Downstream development during South African cut-off low pressure systems

Using 39 years of ECMWF renalysis data, an established energetics framework and simple composite analysis this study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development of a baroclinic wave. Downstream development is characterised by the evolution of two energy centres, one located upstream in the midlatitudes and another downstream in the subtropics. The upstream eddy kinetic energy, which is associated with a midlatitude jet streak, develops and reaches its maximum before the formation of the closed COL cyclonic circulation. The downstream eddy kinetic energy centre maximises at the point where the closed circulation forms. The upstream eddy kinetic energy centre grows from baroclinic conversion from eddy available potential energy to eddy kinetic energy, whilst the downstream centre grows by receiving energy by means of ageostrophic geopotential fluxes that transport eddy kinetic energy in a north-eastward direction from the upstream centre. These ageostrophic geopotential fluxes are induced, increased in magnitude and directed by processes associated with Rossby wave breaking (RWB) on the midlatitude dynamical tropopause and so the downstream energy transfer connects South African COLs to midlatitude processes. The study has further shown that the baroclinic kinetic energy configuration previously associated with wet seasons over South Africa is consistent with times when COLs forms over the country.

Wave energetics of the southern hemisphere of Mars

An assessment of the energetics of transient waves in the southern hemisphere of Mars is presented using the Mars Analysis Correction Data Assimilation (MACDA) dataset (v1.0) and the eddy kinetic energy equation. The dataset is divided into four representative periods covering the summer and winter solstices, a late fall period, and an early spring period for three Mars years. Spring eddies are the most intense, with eddies during the fall being less intense due to a marginally more stable mean-temperature profile and reduced recirculation of ageostrophic geopotential fluxes compared to the spring. Eddy kinetic energy during winter is reduced in intensity as a result of the winter solstitial pause in wave activity, and eddy kinetic energy during the summer is limited. Baroclinic energy conversion acts as a source in fall and spring but disappears during the winter as a result of a stabilized vertical temperature profile. Barotropic energy conversion acts as both a source and a sink of eddy kinetic energy, being most positive during the solstitial pause. Eddies take a northwest to southeast track across the southern highlands in the fall but have a more zonal track in the spring due to stronger eddy kinetic energy advection. Wave energetics is less intense in the southern compared to the northern hemisphere as a result of a shallower baroclinically unstable vertical profile.

Eddy kinetic energy redistribution within windstorms Klaus and Friedhelm

The formation of lower‐tropospheric wind speed maxima is analyzed during the mature stage of two distinct windstorms using ERA‐Interim reanalysis datasets and performing an eddy kinetic energy (EKE) budget. Both storms developed according to the Shapiro–Keyser conceptual model and crossed the large‐scale low‐frequency jet from its warm‐air towards its cold‐air side. The formation of strong wind regions are shown to depend on the position of the storms relative to the large‐scale jet axis, which confirm theoretical results of a companion study. As long as the storms are travelling south of the low‐frequency jet or close to the jet axis, the most intense EKE maxima as well as the total kinetic energy maxima are located in the warm sector of the surface cyclones on their southeastern side. As soon as the surface cyclones move to the north of the low‐frequency jet, EKE is cyclonically redistributed in the lower troposphere, first to the northnorthwest of the cyclone's centre, and then to the southwest along the bent‐back warm fronts. At this later stage, EKE, which is generated by baroclinic conversion in the mid‐troposphere, is redistributed downwards by the vertical ageostrophic geopotential fluxes before being further redistributed southwestwards in the lower troposphere by the ageostrophic geopotential fluxes. This EKE redistribution led to the formation of a low‐level westerly jet to the south of each cyclone centre behind the cold front. These common features between the two storms happened in spite of differences in their shape and environment.

Eddy kinetic energy study of the snowstorm over Southern China in January 2008

The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data. The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25–28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs. The magnitudes of the energy conversion terms, interaction kinetic energy conversion and baroclinic conversion, were too small to explain the development of the energy centers and the jet streaks. The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes, and then lost energy at their exit regions by the same fluxes. At the entrance regions, the fluxes converged, increasing the geopotential gradient, which generated a stronger geostrophic wind and higher kinetic energy, resulting in an ascending motion in this area. When the troughs moved to China, the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.

Eddy kinetic energy redistribution within idealized extratropical cyclones using a two‐layer quasi‐geostrophic model

Eddy kinetic energy (EKE) redistribution within midlatitude cyclones is investigated using a two‐layer quasi‐geostrophic model. The flow is composed of synoptic localized cyclones in both layers interacting with a baroclinic zonal basic flow. The effects of the planetary vorticity gradient β and the relative vorticity gradient of the westerly basic flow are to reduce the upstream ageostrophic geopotential fluxes in the lower layer and to intensify the downstream fluxes in the upper layer. Furthermore, they act to reduce the downward ageostrophic geopotential fluxes and to intensify the upward ageostrophic fluxes. When cyclones are embedded in a cyclonic shear, EKE rapidly accumulates on the southwestern flank of the lower‐layer cyclone before being cyclonically redistributed by the rearward cyclonically oriented ageostrophic fluxes and nonlinear advection. In contrast, when cyclones are embedded in an anticyclonic shear, the lower‐layer ageostrophic fluxes are mainly westward‐oriented and the pressure work combines perfectly with nonlinear advection to maintain the EKE increase at the same place upstream or downstream of the lower‐layer cyclone, depending on the value of β. Finally, a simulation showing the crossing of a westerly jet by cyclones condenses all the above effects. At the early stages, when cyclones lie on the anticyclonic side of the jet, a strong EKE increase occurs downstream of the lower‐layer cyclone. Just after the jet crossing, the EKE is rapidly rearward and cyclonically redistributed by the ageostrophic fluxes and nonlinear advection, leading to the formation of a lower‐layer jet to the south of the cyclone.

Development of an idealised downstream cyclone: Eulerian and Lagrangian perspective on the kinetic energy

ABSTRACTIn this idealised modelling study, the development of a downstream cyclone, which closely follows the life-cycle of a Shapiro-Keyser cyclone, is addressed from a quasi-geostrophic kinetic energy perspective. To this end a simulation of a dry, highly idealised, dispersive baroclinic wave, developing a primary and a downstream cyclone, is performed. Kinetic energy and processes contributing to its tendency – in particular baroclinic conversion and ageostrophic geopotential fluxes – are investigated in three dimensions both in an Eulerian and a Lagrangian framework from the genesis of the downstream cyclone as an upper-level kinetic energy centre, over frontal fracture to the fully developed cyclone showing the characteristic T-bone surface frontal structure, with a strong low-level jet along the bent-back front. Initially the downstream cyclone grows by the convergence of ageostrophic geopotential fluxes from the primary cyclone, but as vertical motions intensify this process is replaced by baroclinic conversion in the warm sector. We show that kinetic energy released in the warm sector is radiated away at all levels by ageostrophic geopotential fluxes: in the upper troposphere they are directed downstream, while in the lower troposphere they radiate kinetic energy to the rear of the cyclone. Thereby, vertical ageostrophic geopotential fluxes, their location and divergence, are identified to play a major role in the intensification of the cyclone in the lower troposphere and for the formation of the low-level jet. Low-level rearward ageostrophic geopotential fluxes converging along the bent-back front are shown to be a general characteristic of an eastward propagating baroclinic wave.

Characteristics of Target Areas Selected by the Ensemble Transform Kalman Filter for Medium-Range Forecasts of High-Impact Winter Weather

Abstract The characteristics of “target” locations of tropospheric wind and temperature identified by a modified version of the ensemble transform Kalman filter (ETKF), in order to reduce 0–7-day forecast errors over North America, are explored from the perspective of a field program planner. Twenty cases of potential high-impact weather over the continent were investigated, using a 145-member ensemble comprising perturbations from NCEP, ECMWF, and the Canadian Meteorological Centre (CMC). Multiple targets were found to exist in the midlatitude storm track. In half of the cases, distinctive targets could be traced upstream near Japan at lead times of 4–7 days. In these cases, the flow was predominantly zonal and a coherent Rossby wave packet was present over the northern Pacific Ocean. The targets at the longest lead times were often located within propagating areas of baroclinic energy conversion far upstream. As the lead time was reduced, these targets were found to diminish in importance, with downstream targets corresponding to a separate synoptic system gaining in prominence. This shift in optimal targets is sometimes consistent with the radiation of ageostrophic geopotential fluxes and transfer of eddy kinetic energy downstream, associated with downstream baroclinic development. Concurrently, multiple targets arise due to spurious long-distance correlations in the ETKF. The targets were least coherent in blocked flows, in which the ETKF is known to be least reliable. The effectiveness of targeting in the medium range requires evaluation, using data such as those collected during the winter phase of The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Field Campaign (T-PARC) in 2009.

An Energetics Study of Wintertime Northern Hemisphere Storm Tracks under 4 × CO2 Conditions in Two Ocean–Atmosphere Coupled Models

Abstract Different possible behaviors of winter Northern Hemisphere storm tracks under 4 × CO2 forcing are considered by analyzing the response of two of the ocean–atmosphere coupled models that were run for the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4), namely the Institut Pierre Simon Laplace’s global coupled model (IPSL-CM4) and the Centre National de Recherches Meteorologiques’s coupled ocean–atmosphere model (CNRM-CM3). It is interesting to compare these models due to their very different responses, especially concerning the North Atlantic storm track. A local energetics study of the synoptic variability in both models is performed, derived from the eddy energy equations, including diabatic terms. The ability of both models to simulate the present-day eddy energetics is considered, indicating no major discrepancies. Both models indicate that the primary cause for synoptic activity changes at the western end of the storm tracks is related to the baroclinic conversion process, due to mean temperature gradient changes in some localized regions of the western oceanic basins, but also resulting from changes in the eddy efficiency to convert energy from the mean flow. Farther downstream, latent heat release during the developing and mature stages of eddies becomes an important eddy energy source especially in terms of changes between 4 × CO2 and preindustrial conditions. This diabatic process amplifies the upstream synoptic (hence usually baroclinic) changes, with more and/or stronger storms implying more latent heat being released (and the converse being true for weaker synoptic activity). This amplification is asymmetrical for the models considered under the simulated 4 × CO2 conditions, due to a greater amount of water vapor contained in warmer air and hence the potential for more condensation for a given synoptic activity. The magnitude of the reduced latent heating is attenuated, whereas increased latent heating is strengthened. Ageostrophic geopotential fluxes are also important in relocating eddy kinetic energy, especially in the vertical.

Stages in the energetics of baroclinic systems

The results from several idealized and case studies are drawn together to form a comprehensive picture of “downstream baroclinic evolution” using local energetics. This new viewpoint offers a complementary alternative to the more conventional descriptions of cyclone development. These additional insights are made possible largely because the local energetics approach permits one to define an energy flux vector which accurately describes the direction of energy dispersion and quantifies the role of neighboring systems in local development. In this view, the development of a system's energetics is divided into three stages. In Stage 1, a pre-existing disturbance well upstream of an incipient trough loses energy via ageostrophic geopotential fluxes directed downstream through the intervening ridge, generating a new energy center there. In Stage 2, this new energy center grows vigorously, at first due to the convergence of these fluxes, and later by baroclinic conversion as well. As the center matures, it begins to export energy via geopotential fluxes to the eastern side of the trough, initiating yet another energy center. In Stage 3, this new energy center continues to grow while that on the western side of the trough decays due to a dwinding supply of energy via fluxes from the older upstream system and also as a consequence of its own export of energy downstream. As the eastern energy center matures, it exports energy further downstream, and the sequence begins anew. The USA “Blizzard of'93” is used as a new case study to test the limits to which this conceptual sequence might apply, as well as to augment the current limited set of case studies. It is shown that, despite the extraordinary magnitude of the event, the evolution of the trough associated with the Blizzard fits the conceptual picture of downstream baroclinic evolution quite well, with geopotential fluxes playing a critical role in three respects. First, fluxes from an old, decaying system in the Pacific were convergent over the west coast of North America, creating a kinetic energy center there and modifying the jet, resulting in a large extension of the overall kinetic energy center well into Mexico. Second, energy fluxes from this extension of the northwesterly flow were strongly convergent east of the trough, producing explosive growth of kinetic energy over the northwestern Gulf of Mexico, with baroclinic conversion following shortly thereafter. Lastly, the kinetic energy generated by the vigorous baroclinic conversion in the cold advection on the west side of the trough was very effectively transferred to the energy center on the east side of the trough via geopotential fluxes. DOI: 10.1034/j.1600-0870.1995.00108.x

On the Dynamics of a Storm Track

An idealized primitive equation model is used to determine the factors controlling the dynamics and maintenance of eddy activity in a storm track. The results show that localized regions of enhanced baroclinicity do not necessarily lead to localization of eddy activity. By studying the energetics of the storm track, it is shown that while baroclinic conversion does indeed correlate with the region of maximum baroclinicity, it is the downstream radiation of energy through the ageostrophic geopotential fluxes which acts as a trigger for the development and maintenance of eddy activity over less baroclinic regions, extending the region of eddy activity much further downstream from the region of high baroclinicity. Examples of eddy life cycles are given that show that convergence and divergence of ageostrophic fluxes can dominate baroclinic and barotropic conversion, especially in regions with weak baroclinicity. Factors that may limit the zonal extent of a storm track are discussed. Evidence of downstream development over the wintertime Pacific storm track based on analyses of ECMWF data is also shown.

Ageostrophic Geopotential Fluxes in Downstream and Upstream Development of Baroclinic Waves

Abstract With the use of a simple primitive equation model, it is demonstrated that the convergence/divergence of ageostrophic geopotential fluxes can be a major source/sink of kinetic energy for both downstream and upstream development of baroclinic waves, and can play a dominant role during the early stages of wave development. It is also shown that both surface friction and β effects lead to an asymmetry in the upstream versus downstream development, with downstream development much stronger. A total group velocity is defined based on ageostrophic fluxes, and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed.