Potential Vorticity Structure Research Articles

On the Role of the Meridional Jet and Horizontal Potential Vorticity Dipole in the Iowa Derecho of 10 August 2020

Abstract On 10 August 2020, a derecho caused widespread damage across Iowa and Illinois. Des Moines station data show that the arrival of the gust front was characterized by an abrupt shift to northerly flow, exceeding 22 m s−1 for ∼20 min. To test the hypothesis that this northerly jet is associated with a horizontal potential vorticity (PV) dipole in the lower troposphere, we investigated the structure of PV in the University of Wisconsin Nonhydrostatic Modeling System (UWNMS) and of absolute vorticity in High-Resolution Rapid Refresh (HRRR) forecast analyses. This structure is described here for the first time. The negative PV member coincides with the downdraft, while the positive PV member coincides with the updraft, with a northerly jet between. The westerly inflow jet descends anticyclonically in the downdraft, joining with northerly flow from the surface anticyclone. The resulting northerly outflow jet creates the trailing comma-shaped radar echo. The speed of propagation of the derecho is similar to the westerly wind maximum in the 3–5-km layer associated with the approaching synoptic cyclone, which acts as a steering level for resonant amplification. Idealized diagrams and 3D isosurfaces illustrate the commonality of the PV dipole/northerly jet structure. Differences in this structure among the three model states are related to low-level wind shear theory. The PV dipole coincides with the pattern of diabatic stretching tendency, which shifts westward and downward relative to the updraft/downdraft with increasing tilt. The PV dipole can contribute toward dynamical stability in a derecho. Significance Statement The purpose of this work is to investigate the structure of potential vorticity (PV) in the lower troposphere in a derecho. It is found that a northerly outflow jet occurs between an east–west-oriented horizontal PV dipole, which is described here for the first time. The negative PV member coincides with the downdraft and is inertially unstable, while the positive PV member coincides with the updraft. This work contributes toward the theory of resonant structures and longevity. The 3–5-km westerly inflow layer constitutes a steering level, which controls propagation speed despite differences in structure. The degree of westward tilt with height is related to the pattern of forcing by diabatic stretching in producing the PV dipole.

The Role of Surface Potential Vorticity in the Vertical Structure of Mesoscale Eddies in Wind-Driven Ocean Circulations

Abstract The vertical structure of ocean eddies is generally surface-intensified, commonly attributed to the dominant baroclinic modes arising from the boundary conditions (BCs). Conventional BC considerations mostly focus on either flat- or rough-bottom conditions. The impact of surface buoyancy anomalies—often represented by surface potential vorticity (PV) anomalies—has not been fully explored. Here, we study the role of the surface PV in setting the vertical distribution of eddy kinetic energy (EKE) in an idealized adiabatic ocean model driven by wind stress. The simulated EKE profile in the extratropical ocean tends to peak at the surface and have an e-folding depth typically smaller than half of the ocean depth. This vertical structure can be reasonably represented by a single surface quasigeostrophic (SQG) mode at the energy-containing scale resulting from the large-scale PV structure. Due to isopycnal outcropping and interior PV homogenization, the surface meridional PV gradient is substantially stronger than the interior PV gradient, yielding surface-trapped baroclinically unstable modes with horizontal scales comparable to or smaller than the deformation radius. These surface-trapped eddies then grow in size both horizontally and vertically through an inverse energy cascade up to the energy-containing scale, which dominates the vertical distribution of EKE. As for smaller horizontal scales, the EKE distribution decays faster with depth. Guided by this interpretation, an SQG-based scale-aware parameterization of the EKE profile is proposed. Preliminary offline diagnosis of a high-resolution simulation shows the proposed scheme successfully reproducing the dependence of the vertical structure of EKE on the horizontal grid resolution.

An objective identification technique for potential vorticity structures associated with African easterly waves

Abstract. Tropical Africa and the North Atlantic Ocean are significantly influenced by African easterly waves (AEWs), which play a fundamental role in tropical rainfall and cyclogenesis in that region. The dynamics of AEWs can be described in a potential vorticity (PV) framework. The important impact of latent heat release by cloud processes is captured in this framework by the diabatic generation of PV anomalies. This paper introduces an innovative approach for the identification and tracking of PV structures within AEWs. By employing AEW tracking and computing the wave phase of each point within the AEW domain using a Hilbert transform, we are able to effectively identify and collect 3-D PV structures associated with specific AEWs. To facilitate a climatological analysis, performed here over the months of June to October from 2002 to 2022, these structures are subsequently characterized by low-dimensional descriptors, including their location, intensity, and orientation. Our climatological analysis reveals the seasonal evolution and the structural attributes of PV anomalies within AEWs over the study domain. PV feature locations closely align with the African easterly jet's latitudinal shift during the summer season. Analysis of the mean pressure level of the 3-D PV structures shows a remarkable shift during their life cycle, indicating deep moist convection characteristics over land and more shallow convection characteristics over the ocean. On average, PV features identified within AEW troughs tilt downshear over land and equatorward over the ocean. The trough-centered analysis reveals distinct differences between satellite-estimated and model-predicted rainfall. Agreement between the results of a more traditional composite analysis and our new feature analysis provides confidence in our feature approach as a novel diagnostic tool. The feature framework provides a low-dimensional representation of the PV structure of AEWs, which facilitates future statistical analyses of the relation of this structure to, for example, tropical cyclogenesis or the predictability of tropical rainfall.

Process-based classification of Mediterranean cyclones using potential vorticity

Abstract. Mediterranean cyclones (MCs) govern extreme weather events across the Euro-African Basin, affecting the lives of hundreds of millions. Despite many studies addressing MCs in the last few decades, their correct simulation and prediction remain a significant challenge to the present day, which may be attributed to the large variability among MCs. Past classifications of MCs are primarily based on geographical and/or seasonal separations; however, here we focus on cyclone genesis and deepening mechanisms. A variety of processes combine to govern MC genesis and evolution, including adiabatic and diabatic processes, topographic influences, land–sea contrasts, and local temperature anomalies. As each process bears a distinct signature on the potential vorticity (PV) field, a PV approach is used to distinguish among different “types” of MCs. Here, a combined cyclone-tracking algorithm is used to detect 3190 Mediterranean cyclone tracks in ECMWF ERA5 from 1979–2020. Cyclone-centered, upper-level isentropic PV structures in the peak time of each cyclone track are classified using a self-organizing map (SOM). The SOM analysis reveals nine classes of Mediterranean cyclones, with distinct Rossby-wave-breaking patterns, discernible in corresponding PV structures. Although classified by upper-level PV structures, each class shows different contributions of lower-tropospheric PV and flow structures down to the surface. Unique cyclone life cycle characteristics, associated hazards (precipitation, winds, and temperature anomalies), and long-term trends, as well as synoptic, thermal, dynamical, seasonal, and geographical features of each cyclone class, indicate dominant processes in their evolution. Among others, the classification reveals the importance of topographically induced Rossby wave breaking to the generation of the most extreme Mediterranean cyclones. These results enhance our understanding of MC predictability by linking the large-scale Rossby wave formations and life cycles to coherent classes of under-predicted cyclone aspects.

Interaction of microphysics and dynamics in a warm conveyor belt simulated with the ICOsahedral Nonhydrostatic (ICON) model

Abstract. Warm conveyor belts (WCBs) produce a major fraction of precipitation in extratropical cyclones and modulate the large-scale extratropical circulation. Diabatic processes, in particular associated with cloud formation, influence the cross-isentropic ascent of WCBs into the upper troposphere and additionally modify the potential vorticity (PV) distribution, which influences the larger-scale flow. In this study we investigate heating and PV rates from all diabatic processes, including microphysics, turbulence, convection, and radiation, in a case study that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign using the Icosahedral Nonhydrostatic (ICON) modeling framework. In particular, we consider all individual microphysical process rates that are implemented in ICON's two-moment microphysics scheme, which sheds light on (i) which microphysical processes dominate the diabatic heating and PV structure in the WCB and (ii) which microphysical processes are the most active during the ascent and influence cloud formation and characteristics, providing a basis for detailed sensitivity experiments. For this purpose, diabatic heating and PV rates are integrated for the first time along online trajectories across nested grids with different horizontal resolutions. The convection-permitting simulation setup also takes the reduced aerosol concentrations over the North Atlantic into account. Our results confirm that microphysical processes are the dominant diabatic heating source during ascent. Near the cloud top longwave radiation cools WCB air parcels. Radiative heating and corresponding PV modification in the upper troposphere are non-negligible due to the longevity of the WCB cloud band. In the WCB ascent region, the process rates from turbulent heating and microphysics partially counteract each other. From all microphysical processes condensational growth of cloud droplets and vapor deposition on frozen hydrometeors most strongly influence diabatic heating and PV, while below-cloud evaporation strongly cools WCB air parcels prior to their ascent and increases their PV value. PV production is the strongest near the surface with substantial contributions from condensation, melting, evaporation, and vapor deposition. In the upper troposphere, PV is reduced by diabatic heating from vapor deposition, condensation, and radiation. Activation of cloud droplets as well as homogeneous and heterogeneous freezing processes have a negligible diabatic heating contribution, but their detailed representation is important for, e.g., hydrometeor size distributions. Generally, faster-ascending WCB trajectories are heated markedly more than more slowly ascending WCB trajectories, which is linked to larger initial specific humidity content providing a thermodynamic constraint on total microphysical heating. Yet, the total diabatic heating contribution of convectively ascending trajectories is relatively small due to their small fraction in this case study. Our detailed case study documents the effect of different microphysical processes implemented in ICON's two-moment scheme for heating and PV rates in a WCB from a joint Eulerian and Lagrangian perspective. It emphasizes the predominant role of microphysical processes and provides a framework for future experiments on cloud microphysical sensitivities in WCBs.

Tropical cyclone rapid intensification and the excitation of inertia-gravity waves on the edges of an evolving potential vorticity structure

The problem of tropical cyclone rapid intensification is reduced to a potential vorticity (PV) equation and a second order, inhomogeneous, partial differential equation for the azimuthal wind. The latter equation has the form of a Klein-Gordon equation, the right-hand side of which involves the radial derivative of the evolving PV field. When the PV field evolves rapidly, inertia-gravity waves are excited at the edges of the evolving PV structure. In contrast, when the PV field evolves slowly, the second order time derivative term in the Klein-Gordon equation is negligible, inertia-gravity waves are not excited, and the equation reduces to an invertibility principle for the PV. The above concepts are presented in the context of an axisymmetric shallow water model, in both its linear and nonlinear forms. The nonlinear results show a remarkable sensitivity of vortex intensification to the percentage of mass that is diabatically removed from the region inside a given absolute angular momentum surface.

Fluid Dynamics of Polar Vortices on Earth, Mars, and Titan

Polar vortices that share many similarities are found in Earth's stratosphere and the atmospheres of Mars and Saturn's moon Titan. These vortices all occur in the winter, and are characterized by high potential vorticity (PV) in polar regions, steep meridional PV gradients and peak zonal winds in middle latitudes, and a cold pole. There are, however, differences in the daily and subseasonal variability, zonal asymmetries, and PV structure among the vortices. These differences are related to differences in the disruption of polar vortices by Rossby waves, the poleward extent of the mean meridional circulation, and condensation of major gases. There are also differences in the transport of gases and particles among the vortices. The range of polar vortex characteristics is likely much larger for terrestrial exoplanets, which include planets with, for example, a wider range of obliquities.

Application of potential vorticity tendency diagnosis method to high-resolution simulation of tropical cyclones

As the grid spacing in the numerical simulation decreases to ∼1 km, the potential vorticity (PV) structure of the simulated tropical cyclone (TC) is an annular tower of high PV with low PV in the eye and the resulting reversal of the radial PV gradient in the inner core is subject to dynamic instability, leading to complicated small-scale features in the PV field. While the PV tendency (PVT) method has been successfully used to diagnose TC motion in numerical simulations with relatively coarse resolution (∼10 km), it has been found that the PVT diagnosis method fails in the TC simulation with grid spacing of ∼1 km. This study reveals that the failure of the PVT diagnosis method in the high-resolution simulation with grid spacing of ∼1 km arises from the induced small-scale features in the PV field. The high localized PV features do not affect TC motion, but make it difficult to calculate the gradient of azimuthal mean PV when the time interval of the model output is ∼1 h. It is suggested that the PVT method can be applied to high-resolution simulations by increasing the time interval of the model output and/or smoothing the model output to reduce the influence of small-scale PV features.

Winter Weakening of Titan's Stratospheric Polar Vortices

Abstract Polar vortices are a prominent feature in Titan's stratosphere. The Cassini mission has provided a detailed view of the breakdown of the northern polar vortex and formation of the southern vortex, but the mission did not observe the full annual cycle of the evolution of the vortices. Here we use a TitanWRF general circulation model simulation of an entire Titan year to examine the full annual cycle of the polar vortices. The simulation reveals a winter weakening of the vortices, with a clear minimum in polar potential vorticity and midlatitude zonal winds between winter solstice and spring equinox. The simulation also produces the observed postfall equinox cooling followed by rapid warming in the upper stratosphere. This warming is due to strong descent and adiabatic heating, which also leads to the formation of an annular potential vorticity structure. The seasonal evolution of the polar vortices is very similar in the two hemispheres, with only small quantitative differences that are much smaller than the seasonal variations, which can be related to Titan's orbital eccentricity. This suggests that any differences between observations of the northern hemisphere vortex in late northern winter and the southern hemisphere vortex in early winter are likely due to the different observation times with respect to solstice, rather than fundamental differences in the polar vortices.

The Roles of Latent Heating and Dust in the Structure and Variability of the Northern Martian Polar Vortex

Abstract The winter polar vortices on Mars are annular in their potential vorticity (PV) structure, a phenomenon identified in observations, reanalysis, and some numerical simulations. Some recent modeling studies have proposed that condensation of atmospheric carbon dioxide at the winter pole is a contributing factor to maintaining the annulus through the release of latent heat. Dust and topographic forcing are also known to be causes of internal and interannual variability in the polar vortices. However, coupling between these factors remains uncertain, and previous studies of their impact on vortex structure and variability have been largely limited to a single Martian global climate model (MGCM). Here, by further developing a new MGCM, we decompose the relative roles of latent heat, topography, and dust as drivers for the variability and structure of the northern Martian polar vortex. Additionally, we analyze a reanalysis data set, finding that there are significant differences in vortex morphology and variability according to the spacecraft instrument used for the data assimilation. In both model and reanalysis, high atmospheric dust loading (such as that seen during a global dust storm) can disrupt the vortex, cause the destruction of PV in the low-to-mid-altitudes (>0.1 hPa), and significantly reduce spatial and temporal vortex variability. Through our simulations, we find that the combination of dust and topography primarily drives the eddy activity throughout the Martian year, and that although latent heat release can produce an annular vortex, it has a relatively minor effect on vortex variability.

Large-scale drivers of the mistral wind: link to Rossby wave life cycles and seasonal variability

Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

The Dynamical Structure of a Warm Core Ring as Inferred from Glider Observations and Along-Track Altimetry

This study investigates the vertical structure of the dynamical properties of a warm-core ring in the Gulf of Mexico (Loop Current ring) using glider observations. We introduce a new method to correct the glider’s along-track coordinate, which is, in general, biased by the unsteady relative movements of the glider and the eddy, yielding large errors on horizontal derivatives. Here, we take advantage of the synopticity of satellite along-track altimetry to apply corrections on the glider’s position by matching in situ steric height with satellite-measured sea surface height. This relocation method allows recovering the eddy’s azimuthal symmetry, precisely estimating the rotation axis position, and computing reliable horizontal derivatives. It is shown to be particularly appropriate to compute the eddy’s cyclo-geostrophic velocity, relative vorticity, and shear strain, which are otherwise out of reach when using the glider’s raw traveled distance as a horizontal coordinate. The Ertel potential vorticity (PV) structure of the warm core ring is studied in details, and we show that the PV anomaly is entirely controlled by vortex stretching. Sign reversal of the PV gradient across the water column suggests that the ring might be baroclinically unstable. The PV gradient is also largely controlled by gradients of the vortex stretching term. We also show that the ring’s total energy partition is strongly skewed, with available potential energy being 3 times larger than kinetic energy. The possible impact of this energy partition on the Loop Current rings longevity is also discussed.

GFS model fidelity in capturing the transition of low‐pressure area to monsoon depression

AbstractLow‐pressure areas and monsoon depressions are the synoptic‐scale systems that contribute largely to the Indian summer monsoon rainfall. India Meteorological Department currently uses a high‐resolution (∼12 km) Global Forecasting System (GFS) model for short‐ and medium‐range operational forecasts over the Indian region. In this study, we have evaluated the performance of the GFS model in capturing the transition of low‐pressure areas (LPAs) to monsoon depressions (L2D). GFS has a good fidelity in capturing synoptic variance over the Indian region, and can simulate the composite precipitation structure for L2D cases. The upper‐level warm core temperature structure is prominent for L2D cases, while it is at the lower level for the cases where the LPA does not intensify and remains as low (RL) pressure area. GFS can simulate the upper‐level warm core temperature structure up to 24 hr. The model has good fidelity in capturing the upper‐level potential vorticity maxima for L2D cases but amplitude is underestimated as the lead time increases. GFS fails to reproduce the potential vorticity structure and amplitude for RL cases. Positive lower‐ to middle‐level heating to the west/southwest sector of the system is seen for L2D cases but the heating is at the lower level for RL cases. Heating structures for L2D cases are slightly underestimated by GFS but it fails to simulate the structure for RL cases. The model greatly underestimates the vertical velocity for L2D cases and RL cases with increasing lead time. Positive moisture convergence is seen at the lower level near the system centre. GFS underestimates the moisture convergence for both the cases and the southward tilt with height is also not captured well. GFS has good fidelity in simulating the dynamical parameters associated with L2D cases but shows lesser fidelity for RL cases.

Comments on “On the Structure and Formation of UTLS PV Dipole/Jetlets in Tropical Cyclones by Convective Momentum Surges”

AbstractThis comment on Hitchman and Rowe first deepens their introduction by distinguishing adiabatic and diabatic tilting of vorticity. Then, it strengthens their interpretation by emphasizing that momentum must be vertically transported with reference to isentropic levels to yield the potential vorticity (PV) dipoles. Moreover, it points out a flaw in their PV budget analysis and proposes a remedy for the flaw. Their convective momentum transport paradigm and the vorticity tilting paradigm reinterpret the same physical process. However, they counted one physical process twice by associating the two paradigms with two different terms. As an attempt to remedy the flaw, this comment associates the reinterpretation of the two paradigms with a transformation of the PV equation; their paradigm corresponds to a flux form. With the proposed remedy, their paradigm can be more easily translated to advances in convective parameterization because of its horizontal locality.

The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments

Abstract. Recent studies have pointed to an important role of latent heating during cloud formation for the dynamics of anticyclonic circulation anomalies such as atmospheric blocking. However, the effect of latent heating on blocking formation and maintenance has not yet been fully elucidated. To explicitly study this cause-and-effect relationship, we perform sensitivity simulations of five selected blocking events with the Integrated Forecast System (IFS) global weather prediction model in which we artificially eliminate latent heating in clouds upstream of the blocking anticyclones. This elimination has substantial effects on the upper-tropospheric circulation in all case studies, but there is also significant case-to-case variability: some blocking systems do not develop at all without upstream latent heating, while for others the amplitude, size, and lifetime of the blocking anticyclones are merely reduced. This strong influence of latent heating on the midlatitude flow is due to the injection of air masses with low potential vorticity (PV) into the upper troposphere in strongly ascending “warm conveyor belt” airstreams and the interaction of the associated divergent outflow with the upper-level PV structure. The important influence of diabatic heating demonstrated with these experiments suggests that the accurate representation of moist processes in ascending airstreams in weather prediction and climate models is crucial for blocking dynamics.

Potential vorticity structure of Titan’s polar vortices from Cassini CIRS observations

The Cassini mission has provided the best opportunity to date to extensively study the seasonal variation in Titan’s atmosphere, with observations spanning almost half a Titan year (Ls=293−93°). An important feature in the Titan middle-atmosphere is the formation of a polar vortex. Observations have shown that an initially well-developed northern vortex enriched with trace gas species gradually breaks down after spring equinox as a new vortex emerges in southern winter. Here we use Cassini CIRS observations to derive the temperature and composition of the middle-atmosphere. We use the gradient wind equation to first estimate the mean zonal winds, and then the Potential Vorticity (PV) throughout Titan’s atmosphere over the timespan of the Cassini mission. PV is a useful diagnostic quantity for studying the dynamics of polar vortices because it is materially conserved for adiabatic and frictionless flows, and can be inverted to find all other dynamical fields. Our results show the formation of a strong zonal jet in the winter hemisphere, with wind velocities reaching 220 ms−1, which is consistent with previous studies. An annular PV structure is also observed over the winter poles, whereby a ring of PV encircles a local minima over the pole. Such distributions are often found to be unstable without a restoring force, yet they are seen here in numerous observations in both the northern and southern hemispheres. A comparison with the annular Martian vortices shows that latent heat release from condensation or subsidence-induced adiabatic heating may explain the origin and stability of the annulus. Finally, we investigate the evolution of the size of the vortices and the role of strong PV gradients as a dynamical mixing barrier for trace gas species across the vortex edge. We find that longer lived gases are less confined to the vortex than those with shorter photochemical lifetimes.

A potential vorticity perspective on cyclogenesis over centre‐eastern South America

AbstractThis study investigates the influence of upper‐level potential vorticity (PV) structures on surface cyclogenesis in central‐eastern South America using 1979–2017 ERA‐Interim reanalysis data. Surface cyclones are identified in three regions (Argentina, Uruguay and SEBrazil) and it is quantified how often PV streamers and PV cutoffs co‐occur with cyclogenesis events. There are interesting regional and seasonal differences: in Argentina PV streamers occur mainly with cyclogenesis in summer, and in Uruguay and SEBrazil in both summer and winter. In the three regions, PV streamers associated with cyclogenesis occur most frequently at 320 K, except for summer in SEBrazil, where they predominate at 340 K. PV cutoffs occur with lower frequencies than PV streamers during cyclogenesis, and they occur more often in summer than in winter in all three regions. The dynamics of cyclogenesis differs among the three regions in some important aspects. In Argentina, the environment is slightly more baroclinic in summer and genesis occurs beneath the equatorial entrance of a jet streak. In this season, the anomalous PV associated with a PV streamer is important to favour ascent in the baroclinic environment contributing to surface cyclogenesis. Similar patterns occur for cyclogenesis in summer and winter in Uruguay, and in winter in SEBrazil. In Argentina during winter, cyclogenesis is located beneath the poleward exit of a jet streak, with a weaker PV streamer upstream. In SEBrazil during summer, the jet stream is far displaced from the genesis region and cyclogenesis occurs in a more barotropic environment.

Characteristics of the potential vorticity and its budget in the surface layer over the Tibetan plateau

AbstractThe variability of interior atmospheric potential vorticity (PV) is linked with PV generation at the Earth's surface. The present paper reveals the features of the surface PV and provides a stepping stone to investigate the surface PV budget. In this study, the formats of the PV and PV budget adopting a generalized vertical coordinate were theoretically examined to facilitate the calculation of the surface PV and its budget. Results show that the formats of the PV and PV budget equations are independent of the vertical coordinate. While the vertical component of the surface PV dominates over the platform of the Tibetan Plateau, the horizontal component plays an important role over the slopes of the Tibetan Plateau, especially the southern slope owing to the strong in‐situ meridional gradient of the potential temperature. These results indicate that the employment of complete surface PV not only provides a finer PV structure but also more appropriately reveals its effect on atmospheric circulation. Diagnosis based on reanalysis and model output demonstrates that the surface PV budget equation is well balanced both in terms of the climate mean and synoptic process, and the surface PV budget in June has a prominent diurnal cycle. The diabatic heating with a minimum in the early morning and a maximum from evening to midnight contributes dominantly to this diurnal cycle. It is further indicated that positive PV generation due to diabatic heating is essential for the formation, development, and movement of the Tibetan Plateau vortex.

Forcing of the Martian polar annulus by Hadley cell transport and latent heating

AbstractA hierarchy of idealized models is used to investigate the roles of Hadley cell forcing and latent heat release from carbon dioxide condensation in determining the annular potential vorticity structure of the Martian winter polar vortex. The angular momentum conserving the Hadley cell model of Lindzen and Hou with summer hemisphere heating maximum of appropriate strength and latitude produces a strong westerly jet near 60°N, which is similar in strength to the winter polar night jet on Mars. Although the corresponding potential vorticity profile in the angular momentum conserving and thermal wind regions has no annular structure resembling the Martian one, there is an implied δ‐function at the discontinuity in zonal wind. This δ‐function is smoothed out by explicit diffusion in full axisymmetric model integrations forming a partial annular structure, though a local maximum in potential vorticity at the pole persists and is further enhanced when cooling representing the polar night is included. A distinct polar minimum and clear annular potential vorticity structure are obtained, however, when an additional representation of polar latent heating is also included. Full eddy‐permitting shallow‐water model integrations confirm the basic structure obtained by the axisymmetric model and suggest a nominal value of viscosity appropriate as a representation of the effects of eddy mixing. Instability of the polar annulus leads to vacillation‐type behaviour involving eddy growth and annulus disruption, followed by re‐formation under the influence of radiative relaxation. The degree of transience and mean eddy activity both show an increase with stronger latent heating and the resulting deeper polar potential vorticity minimum, showing that mixing in polar regions may be dependent on details of polar carbon dioxide condensation. Vacillation time‐scales are also shown to vary with radiative time‐scales, but through a modification of instability growth rate rather than as a result of direct radiative restoration.

Strength and Spatial Structure of the Perturbation Induced by a Tropical Cyclone to the Underlying Eddies

AbstractA tropical cyclone (TC) induces the oceanic geostrophic response, which perturbs the underlying ocean eddy field. This study investigates the strength and spatial structure of isopycnal undulations and potential vorticity (PV) changes linked with the geostrophic response by use of a linear, two‐layer theory and an OGCM. It is found that the strength and cross‐track length scale of the geostrophic response are well compared with those of background eddies, highlighting the ability of a TC to perturb underlying ocean eddies. More importantly, the TC‐induced PV perturbation is confined within the thermocline between 100 and 300 m depth, causing the 3D quasi‐geostrophic evolution of the perturbed eddies. The length scale of upwelling perturbation (which is ~130 km) is larger than that of PV response (~50 km). This scale disparity means that the patterns of the TC‐induced perturbations are subject to eddy‐TC separation distance. Consequently, the perturbation of a TC to an eddy is significant (negligible) when the eddy‐TC separation distance is less than 80 km (larger than 200 km). This study provides a starting point for understanding the TC‐induced 3D evolution of the perturbed ocean eddy field.