Potential Vorticity Distribution Research Articles

Numerical study of the October 2000 torrential precipitation event over eastern Spain: analysis of the synoptic-scale stationarity

Abstract. A torrential precipitation event affected eastern Spain during 21 to 24 October 2000. Total accumulated rainfall higher than 500 mm was registered at some locations, with values up to 300 mm in a 24-h period. The synoptic-scale charts for these days show the presence of a cold cutoff low aloft, south of the Iberian Peninsula, as a part of an W-blocking structure over Europe. At low levels, persistent easterly winds, established between a dominant anticyclone over eastern Europe and a cyclone over Morocco, are found over the western Mediterranean throughout the entire period. Satellite images show the advance and breaking away of a trough, with an associated cold front, over the Iberian Peninsula, which resulted in the cutoff low formation. Later, scattered convective cells are detected along the eastern Spanish coast during more than 3 days. Numerical simulations reveal that the convective environment was developed by the low-level advection of warm and moist air from central Mediterranean, being charged of moisture by evaporation from the sea. Sensitivity runs confirm that the synoptic-scale persistent low-level easterly flow, and specifically, the presence of a low level jet, was crucial for the subsynoptic processes leading to the long lasting torrential rainfall over the exposed terrains of eastern Spain. The stagnancy of the low-level flow is attributed to the quasi-stationary characteristics of the upper level cutoff low located south of the Iberian Peninsula. Experiments with modified upper-level potential vorticity distributions reveal that slight deviations from the observed configuration result in enhanced mobility of the low-level flow pattern, and thereby a reduction of the precipitation persistence. This suggests the major importance of a correct representation in the model initial conditions of the intensity and location of the upper level features, in order to obtain valuable numerical forecasts of these heavy rainfall events.Key words. Meteorology and atmospheric dynamics (convective processes; mesoscale meteorology; precipitation)

Use of long‐lived tracer observations to examine transport characteristics in the lower stratosphere

The Halogen Occultation Experiment (HALOE) CH4 data were used in a modified Lagrangian‐mean (MLM) analysis. The implemented MLM analysis is advantageous in that it provides a unique method for capturing tracer distributions and identifying so‐called transport barriers (i.e., regions where quasi‐horizontal transport is inhibited) and mixing regions, knowing only the mixing ratios of long‐lived tracers. This analysis can be used with any long‐lived tracer provided that adequate spatial coverage can be achieved either instantaneously or over a reasonably short period of time (i.e., characteristic time over which diabatic processes responsible for cross‐isentropic transport are small or negligible, ∼7–10 days in the lower stratosphere). Previous authors have demonstrated the use of this technique with data from limb‐sounding instruments which can provide near‐hemispheric coverage in a day. We show how this technique can be implemented with HALOE occultation data. Because the solar occultation sampling pattern of HALOE requires ∼1 month to achieve near‐hemispheric coverage, synoptic distributions of CH4 are reconstructed using the UK Meteorological Office potential vorticity (PV) distributions, exploiting the tracer's correlation with PV. In previous studies, authors have used PV for filling voids in tracer distributions. It is concluded that an MLM cross section of CH4 can be constructed from the HALOE occultation data, provided that a suitable synoptic distribution of PV can be obtained for the desired period. To illustrate the feasibility and usefulness of applying the MLM technique to HALOE reconstructed CH4 data, transport characteristics in the lower stratosphere were investigated using CH4 data for two different regimes (polar and subtropical) at times when airborne in situ data were available. In this manner we were able to compare information available with respect to transport from the MLM equivalent length analyses with that provided by aircraft tracer‐tracer correlations. The very good agreement between ER‐2 and HALOE data observed in CH4‐O3 correlations and in CH4 and O3 near‐coincident comparisons increased our confidence in using the HALOE data in the MLM equivalent length analyses. Unlike the satellite data, the ER‐2 in situ measurements cannot be used in the MLM technique. Instead, correlations between the mixing ratios of various tracer constituents have been used extensively in the past to provide insight into the relative roles of chemistry and transport in the lower stratosphere. However, separating the effects of chemistry and transport and accurately interpreting tracer‐tracer correlations is often difficult. We show that associating an aircraft tracer‐tracer correlation with a coincident HALOE MLM equivalent length analysis may provide for a less ambiguous evaluation of the correlation in the sense that it provides information on transport processes in physical space. Also, there are obvious implications for use of MLM equivalent length analyses in regions/time frames when aircraft data are not available.

Steady, nonradiating geophysical flow past a cylinder with circulation

Rotating, barotropic flow past a circular cylinder with circulation in the presence of a piecewise constant distribution of potential vorticity is considered. For a given cylinder radius, linear theory is used to show that there exists a unique value of the circulation for which the downstream wake vanishes. The cylinder experiences zero drag for such flows. A numerical method, based on contour dynamics, is then used to compute steady, nonlinear, waveless flows past the cylinder. For a given upstream velocity and different choices of the cylinder radius, waveless potential vorticity interface profiles are presented and corresponding values of the circulation calculated. Possible applications to propagating geophysical vortices are discussed, including the computation of nonlinear solutions which may be interpreted as freely propagating vortices.

Formation of Low Potential Vorticity over the Deep Pacific

Low potential vorticity extends over the deep waters of the North Pacific and, possibly, the bottom waters of the North Atlantic. Isopycnic model integrations are conducted to investigate how these potential vorticity distributions are controlled, first, for an idealized double-hemisphere and, second, for the Pacific with realistic topography. Dense water is released from a southern, high-latitude source and circulates over the domain with diapycnic mixing gradually reducing its stratification. The potential vorticity contrast is large over the Southern Hemisphere, but weak over the Northern Hemisphere where the meridional changes in planetary vorticity and layer thickness oppose each other. Including an active eddy field inhibits the grounding of dense water, which increases the potential vorticity contrast in the overlying layer. Incorporating realistic topography leads to the dense fluid spreading via deep channels with tight recirculations and jets bifurcating. The experiments suggest that extensive regions of low potential vorticity are formed whenever there is both enhanced bottom mixing and a basin is filled by a single water mass entering from across the equator.

A Study of the Stratospheric Major Warming and Subsequent Flow Recovery during the Winter of 1979 with an Isentropic Vertical Coordinate Model

The principal goal of this paper is to gain further insight into the dynamical processes during the stratospheric major warming of February and early March 1979, with a special emphasis on the recovery stage. To achieve this goal, first the entire evolution of the warming event is numerically simulated using an isentropic vertical coordinate model. Then the results from the following complementary points of view are quantitatively analyzed: wave effects on the mean flow, potential enstrophy conversion and transport, and potential vorticity redistribution on a synoptic chart. There is an indication that wavenumber 1 during the recovery stage amplifies through a mechanism within the stratosphere and propagates downward. The simulated Eliassen‐Palm flux field shows that the amplified wave 1 is responsible for the mean flow acceleration in the recovery stage. It is therefore concluded that the in situ amplification mechanism for wave 1 plays a crucial role in the dynamics of the flow recovery. In order to examine the mechanism, detailed analyses of the simulated eddy potential enstrophy budget are performed. It is found that there is an ‘‘anticascade’’ of potential enstrophy in the recovery stage through which wave 1 amplifies in the midstratosphere. This result is consistent with a synoptic description of the event in terms of potential vorticity distribution on isentropic surfaces.

Forecast and simulation of stratospheric ozone filaments: A validation of a high‐resolution potential vorticity advection model by airborne ozone lidar measurements in winter 1998/1999

In the framework of the Third European Stratospheric Experiment on Ozone project Meridional Transport of Ozone in the Lower Stratosphere (METRO) an airborne ozone lidar has been flown on the French Falcon (Mystere 20) during winter 1998/1999 to investigate polar and subtropical filaments at midlatitudes. The objective of the METRO project is to quantify the proportion of the transport of polar and subtropical air and their mixing into midlatitude air masses. The dynamical evolution of the northern winter hemisphere was simulated using a high‐resolution advection model for potential vorticity (PV): Modele Isentropique de transport Mesoechelle de l'Ozone Stratospherique par Advection (MIMOSA). To validate the model for further studies, it was first utilized to forecast the appearance of filaments inside the range of the airplane so that each flight of the airborne campaign could be planned precisely. The vertical ozone distribution measured along the flight tracks was then compared to the respective PV distribution simulated by the advection model. Correlation coefficients between 0.5 and 0.7 found over the altitude range where the filaments were observed show a good agreement between PV and ozone filaments. An improvement of the correlation up to 0.8 by horizontal shifting of the ozone profiles against the PV evolution showed that small displacements of less than 1° of the modeled PV filaments can occur. These displacements can be explained by the uncertainties of the input wind velocity data and of the lidar data. Thus we can conclude that the PV advection model MIMOSA reproduces the position, size, and structure of polar filaments and subtropical intrusions well in the range of the expected accuracy. The model is a suitable tool for further studies of the quantification of the global, long‐term transport and mixing of polar and subtropical air into mid‐latitudes.

Effects of Isopycnal and Diapycnal Mixing on the Stability of Oceanic Currents

Abstract This paper considers several influences of mixing on the stability of oceanic boundary currents, analyzed primarily in terms of the effects of mixing on the distribution of potential vorticity. This perspective relates both to the Charney–Stern criterion for the stability of currents and the Haynes–McIntyre constraints on the evolution of potential vorticity when mixing occurs. It is shown that diapycnal mixing can alter the structure of a stable current so that it becomes potentially unstable according to the Charney–Stern criterion. The effects of isopycnal diffusion and a viscous lateral boundary layer are also considered. Occurrences of instability are then illustrated with both computational solutions and experiments in a rotating tank. Finally, subsurface float trajectories are shown near the coast of Portugal that are suggestive of the instability of the boundary current downstream from the Mediterranean outflow, known to be subject to strong mixing.

Hierarchies of Balance Conditions for thef-Plane Shallow-Water Equations

Abstract For the f-plane shallow-water primitive equations (PEs), hierarchies of balance conditions relating the gravity manifold (divergence δ and ageostrophic vorticity γ = fζ − g∇2h) to the Rossby manifold (linearized potential vorticity ql = ζ − fh/H) are introduced. These hierarchies are ∂Nδ/∂tN = ∂N+1δ/∂tN+1 = 0 (δ balance), ∂Nδ/∂tN = ∂Nγ/∂tN = 0 (δ–γ balance), and ∂Nγ/∂tN = ∂N+1γ/∂tN+1 = 0 (γ balance), for N = 0, 1, … . How well these balance conditions represent the balance accessible to a given PE flow is explored. Detailed numerical experiments are carried out on an idealized potential vorticity distribution for which the domain maximum Rossby and Froude numbers are Romax ≐ 0.73 and Frmax ≐ 0.28. The numerical results reveal that all these hierarchies are asymptotic: as N increases, imbalance first decreases and then increases, as measured for instance by a linearized available energy. The minimum imbalance, over all the balance conditions considered, is attained by γ balance at N = 2. The most ...

Laboratory and numerical model studies of a negatively-buoyant jet discharged horizontally into a homogeneous rotating fluid

The results of laboratory experiments and numerical model simulations are described in which the motion of a round, negatively-buoyant, turbulent jet discharged horizontally above a slope into a rotating homogeneous fluid has been investigated. For the laboratory study, flow visualisation data are presented to show the complex three-dimensional flow fields generated by the discharge. Analysis of the experimental data indicates that the spatial and temporal developments of the flow field are controlled primarily by the lateral and vertical discharge position of the jet (with respect to the bounding surfaces of the container of width W) and the specific momentum (M 0) and buoyancy (B 0) fluxes driving the jet. The flow is seen to be characterised by the formation of (i) a primary anticyclonic eddy (PCC) close to the source, (ii) an associated secondary cyclonic eddy (SCE) and (iii) a buoyancy-driven bottom boundary current along the right side boundary wall. For the parameter ranges studied, the size L p, s and spatial location x p, s of the PCC and SCE (and the nose velocity u N of the boundary current) are shown to be only weakly-dependent upon the value of the mixed parameter M 0Ω/B 0, where Ω is the background rotation rate. Both L p and x p are shown to scale with the separation distance y*/W of the right side wall (y = 0) from the source (y = y*), both L s and x s scale satisfactorily with the length scale l M (= M 0 3/4/B 0 ½) and u N is determined by the appropriate gravity current speed [(g']0 H]½ and the separation distance y*/W. Numerical model results show good qualitative agreement with the laboratory data with regard to the generation of the PCC, SCE and boundary current as characteristic features of the flow in question. In addition, extension of the numerical model to diagnose potential vorticity and plume thickness distributions for the laboratory cases allow the differences in momentum-and buoyancy-dominated flows to be clearly delineated. Specifically, the characteristic features of the SCE are shown to be strongly dependent upon the value of M 0Ω/B 0 for the buoyant jet flow; not least, the numerical model data are able to confirm the controlling role played by the boundary walls in the laboratory experiments. Quantitative agreement between the numerical and laboratory model data is fair; most significantly, the success of the former model in simulating the dominant flow features from the latter enables the reliable extension of the numerical model to be made to cases of direct oceanic interest.

Determination of wind-driven ocean circulation inside closed characteristics

Using two-layer quasigeostrophic equations neglecting relative vorticity, it is shown that part of the lower layer flow inside closed geostrophic contours in the Sverdrup interior is determined neither by applying the integral condition nor by matching with the frictional western boundary layer. This flow is a higher approximation than might be supposed, so that the problem reduces to a perturbation one related to a local forcing-dissipation balance with flow paths being prescribed. A significant correction to this interior flow comes from the vorticity balance south of the closed region, so that to what degree the interior circulation is influenced by the western coast depends on the meridional size of the closed region. The extension of the theory to a 2(½)-layer model is immediate. Thus the present analysis strongly suggests that on shallow isopycnal surfaces the flow pattern and the associated potential vorticity distribution are not affected by the frictional western boundary current. Our asymptotic argument may partly be applied to more general cases provided that no O(1) external forcing acts on the lower layer.

Upper-Tropospheric Potential Vorticity Fluctuations and the Dynamical Relevance of the Time Mean

Analysis of frequency distribution functions of potential vorticity (PV) on middleworld isentropic surfaces that intersect the tropopause reveals that dynamics on these surfaces are dominated by the tropopause separating subtropical and polar PV pools with definitive characteristic values. The dominance of the tropopause on these surfaces suggests that the subseasonal transient variance should be a function solely of the time mean PV. Exploiting this relation, it is shown that knowledge of the annual mean PV distribution allows quite skillful deduction of interannual anomalies in transient variance. This analysis is extended to the dynamic tropopause, where it is shown that polar and subtropical jets clearly emerge in fields of the most frequent tropopause potential temperature. Implications of these results on the current understanding of how patterns of interannual variance arise are discussed.

Stationary Wave Accumulation and the Generation of Low-Frequency Variability on Zonally Varying Flows

Abstract A potent mechanism for the generation of low-frequency atmospheric variability on vortex basic states consisting of a single potential vorticity jump, or contour, separating two regions of uniform equivalent barotropic potential vorticity is described. Such basic states represent in a simple manner the potential vorticity distribution of the extratropical upper troposphere. It is shown that the group velocity for stationary waves propagating on such states can vanish for realistic zonal variations in the basic-state flow along the vortex edge, leading to local exponential disturbance growth due to the accumulation of wave action. Further, pseudo-energy stability criteria are derived that suggest that exponentially growing global disturbances are possible for sufficiently strong zonal variations in the flow along the vortex edge. These predictions are examined using linear and nonlinear initial value problem calculations. For wavenumber-1 flow variations in the basic-state zonal flow along the vor...

A similarity model for the windy jovian thermocline

A new conceptual model for the observed banded wind structure of the giant outer planets is proposed, assuming a smoothly distributed potential vorticity (PV) atop a layer of deeply seated, latitudinally variable stratification. With a vanishing horizontal entropy contrast presumably enforced at the bottom of the flow layer by the underlying convection, the thermal-wind balance of observed cloud-top motions implies a mapping of constant potential temperature surfaces, nearly vertical at upper tropospheric levels, down to a deeper, flatter, but variably sloped “thermocline”. For a generally colder-poleward distribution of isentropes consistent with the strong equatorial jets of both Jupiter and Saturn, this kind of mapping would also imply a poleward decreasing static stability. The proposed temperature-stability distribution is just the reverse of the Earth’s troposphere, where the strongest latitudinal potential temperature gradients are at the bottom instead of the top, with static stability generally increasing toward the pole. The warmer–stabler correlation for Jupiter would be consistent, however, with a nearly monotonic distribution of PV from low to high latitudes, over planetary scales for which the planetary vorticity dominates the relative vorticity of the jets. In this way efficient PV mixing for the isentropically bounded thermocline can account for the dynamical maintenance of the cyclonic flanks of the equatorial jet. Over latitudinal intervals comparable to the internal deformation scale, however, the gradient of the absolute vorticity is dominated by the flow curvature, correlated for a well-mixed PV distribution with the local latitudinal gradient of the static stability, itself proportional for the assumed thermocline structure to the local potential temperature gradient and therefore the local departures in geostrophic velocity. The resulting correlation of velocities and vorticity gradients would imply an alternation of the flow over the internal deformation radius set by the vertical entropy contrast. The requisite size of the deep stability is itself roughly consistent with the diagnostic interpretation of the apparent dispersive character and vertical wavelength of observed temperature oscillations in Jupiter’s upper troposphere and stratosphere, as well as a plausible extrapolation of the deep sub-adiabatic lapse rate detected by the Galileo probe.

Potential Vorticity Inversion on a Hemisphere

Several different kinds of accurate potential vorticity (PV) inversion operators, and the associated balanced models, are tested for the shallow water equations on a hemisphere in an attempt to approach the ultimate limitations of the balance, inversion, and slow-manifold concepts. The accuracies achieved are far higher than for standard balanced models accurate to one or two orders in Rossby number R or Froude number F (where F = |u|/c; |u| = flow speed; and c = gravity wave speed). Numerical inversions, and corresponding balanced-model integrations testing cumulative accuracy, are carried out for cases that include substantial PV anomalies in the Tropics. The balanced models in question are constructed so as to be exactly PV conserving and to have unique velocity fields (implying, incidentally, that they cannot be Hamiltonian). Mean layer depths of 1 and 2 km are tested. The results show that, in the cases studied, the dynamical information contained in PV distributions is remarkably close to being complete even though R = ∞ at the equator and even though local maximum Froude numbers, Fmax, approach unity in some cases. For example, in a 10-day integration of the balanced model corresponding to one of the most accurate inversion operators, “third-order normal mode inversion,” the mean depth was 1 km, the minimum depth less than 0.5 km, and Fmax ≃ 0.7, hardly small in comparison with unity. At the end of 10 days of integration, the cumulative rms error in the layer depth was less than 15 m, that is, less than 5% of the typical rms spatial variation of 310 m. At the end of the first day of integration the rms error was 5 m, that is, less than 2%. Here “error” refers to a comparison between the results of a balanced integration and those of a corresponding primitive equation integration initialized to have low gravity wave activity on day 0. Contour maps of the PV distributions remained almost indistinguishable by eye over the 10-day period. This remarkable cumulative accuracy, far beyond anything that could have been expected from standard scale analysis, is probably related to the weakness of the spontaneous-adjustment emission or “Lighthill radiation” studied in the companion paper by

Lagrangian Observations of the Deep Western Boundary Current in the North Atlantic Ocean.

Twenty-six RAFOS floats were deployed in the deep western boundary current (DWBC) of the North Atlantic Ocean between the Grand Banks and Cape Hatteras in 1994–95 and tracked acoustically for up to two years. Half of the floats were launched in the upper chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (∼800 m), and the other half near the deep CFC maximum that identifies the overflow water from the Nordic seas (∼3000 m). The float observations reveal the large-scale pathways of these recently ventilated water masses in the subtropics. The shallow float tracks show directly that upper Labrador Sea Water is diverted away from the western boundary and into the interior at the location where the DWBC encounters the Gulf Stream near 36°N (the “crossover region”), consistent with previous hydrographic studies. East of the crossover region, only one upper Labrador Sea Water float out of seven (∼15%) “permanently” crossed to the south side of the stream in two years, caught in a cold core ring formation event. The other shallow floats recirculated north of the Gulf Stream, apparently confined by the mean potential vorticity gradient aligned with the stream. The deep floats closely followed the topography to the crossover region, then revealed a bifurcation in fluid parcel pathways. One branch continues equatorward along the western boundary, and the other turns first eastward along the Gulf Stream path, then southward. The deep float pathways, including the bifurcation in the crossover region, can be explained in terms of the deep potential vorticity distribution. Comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth. The effective spreading rates of upper Labrador Sea Water and overflow water estimated directly from the float data, southward at 0.6 ± 0.2 cm s−1 and 1.4 ± 0.4 cm s−1, respectively, agree well with tracer-derived spreading rates. Mean velocities in the DWBC, equatorward at 2–4 cm s−1 (upper Labrador Sea Water) and 4–5 cm s−1 (overflow water), are consistent with other in situ measurements. One deep float drifted almost 4000 km along the western boundary in two years, revealing a “fast track” for the spreading of overflow water in the DWBC. These observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses, addressed in Part II of this study.

Does the Potential Vorticity Distribution Constrain the Spreading of Floats in the North Atlantic?

Float trajectories are compared with the distribution of climatological potential vorticity, Q, on approximate isentropic surfaces for intermediate waters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of floats in terms of their movement along and across Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 208‐308 in magnitude in the ocean interior. The implied Peclet number in the ocean interior ranges from 1 to 19 with a weighted-mean value of 4.4. This mean Peclet number suggests that there is significant eddy mixing in the ocean interior: tracers should only be quasi-conserved along mean streamlines over a subbasin scale, rather than over an entire basin. The mean flow also strongly crosses Q contours near the western boundary in the Tropics, where the implied Peclet number is 0.7; this value may be a lower bound as Q contours are assumed to be zonal and relative vorticity is ignored. Float clusters with a lifetime greater than 200 days show anisotropic dispersion with greater dispersion along Q contours, than across them; float clusters with shorter lifetimes are ambiguous. This anisotropic dispersion along Q contours cannot generally be distinguished from enhanced dispersion along latitude circles since Q contours are generally zonal for these cases. However, for the null case of uniform Q for the Gulf Stream at 2000 m, there is strong isotropic dispersion, rather than enhanced zonal dispersion. In summary, diagnostics suggest that floats preferentially spread along Q contours over a subbasin scale and imply that passive tracers should likewise preferentially spread along Q contours in the ocean interior.

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre‐WOCE hydrography are used to study the water‐mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.

An Alpine Rainstorm: Sensitivity to the Mesoscale Upper-Level Structure

Abstract Rainstorms that occur on the south side of the European Alps during spring and fall are usually accompanied at the tropopause level by a single elongated filament of intruded stratospheric air. This study examines the dependency of one such storm (the Piedmont event of 5–6 November 1994) upon the filament’s structure. The focus is on quantitative precipitation forecasting and particular attention is given to the timing, distribution, and amplitude of the storm’s rainfall. In a first step the storm’s synoptic evolution and the upper-level filament’s mesoscale structure are examined cursorily using data from a range of sources (ECMWF operational fields, radar estimate of accumulated precipitation, Meteosat water vapor imagery, and two limited-area pseudoforecasts of different duration to 0600 UTC 6 November). This examination helps identify distinct elements of the filament’s potential vorticity (PV) distribution. In the second step a set of atmospheric states are generated for 1800 UTC 4 November ...

Thermal versus dynamical tropopause in upper‐tropospheric balanced flow anomalies

AbstractThis paper systematically investigates differences between the thermal and the dynamical tropopause for upper‐tropospheric balanced flow anomalies. Idealized cyclonic and anticyclonic anomalies are considered, which are either axisymmetric or plane symmetric. Given a distribution of potential vorticity (PV), the inversion problem is solved numerically to obtain the corresponding balanced flow (i.e. wind and temperature). The control parameter is the aspect ratio of the PV anomaly, which governs the partitioning into a thermal and a dynamical anomaly. For PV anomalies of intermediate and tall aspect ratios, the location of the thermal tropopause differs significantly from the location of the dynamical tropopause. The thermal tropopause is rather indistinct for intermediate aspect ratios, while it is sharp and well defined for both tall and shallow anomalies. A barotropic deformation flow field superimposed on a plane symmetric anomaly induces an ageostrophic wind which modifies the static stability throughout the PV anomaly such that the thermal and dynamical tropopauses evolve differently. Recent observations concerning the correlation between the thermal and ozone tropopauses can be interpreted consistently in terms of the present results.

The signature of cyclonic PV filaments in the lower stratosphere

The perturbation temperature and wind fields induced by a given zonally invariant potential vorticity (PV) filament in an isothermal atmosphere with uniform westerly background flow is investigated using a two‐dimensional isentropic semigeostrophic model. Such PV filaments are commonplace in the lower midlatitude and polar stratosphere during winter and spring. The model is formulated in isentropic geostrophic coordinates because this formulation greatly simplifies the equations of motion describing the system. Given the potential vorticity distribution, the dynamics of the filament problem are obtained by inverting a single nonlinear elliptic equation. The model diagnoses relatively cool air below and relatively warm air above cyclonic filaments. In the case of anticyclonic filaments, air above the filament is cooler and air below the filament is warmer than the ambient air. The thermal perturbations in an isothermal atmosphere are sensitive to vertical gradients in PV, while the zonal along‐filament flow associated with PV filaments is sensitive to quasi‐horizontal PV gradients. If the PV anomaly marking the stratospheric filament is weak compared to ambient PV values, the magnitude of the thermal perturbations in the isothermal atmosphere is a linear function of the magnitude of the PV anomaly. In principle, it is possible to remotely detect the thermal signature of PV filaments using a Microwave Temperature Profiler (MTP), for example. This instrument, mounted aboard an aircraft flying in the lower stratosphere, can measure vertical profiles of the thermal structure below and above the flight track.