1. Introduction
Narrow quasi-meridionally aligned troughs are a frequent feature of atmospheric flow at tropopause levels (Holopainen and Rontu 1981) and they correspond to elongated intrusions of stratospheric air extending downward and equatorward into the troposphere. On a tropopause-transecting isentropic surface they are identifiable as PV streamers (Appenzeller and Davies 1992) extruding from the stratospheric reservoir of high potential vorticity (e.g., Hoskins et al. 1985). Observations indicate that the streamers emerge in the mature stage of the synoptic development and they are hence more prevalent at the downstream end of the midlatitude storm tracks (Appenzeller and Davies 1992; Price and Vaughan 1992; Appenzeller et al. 1996).
For the Alpine region (located downstream of the Atlantic storm track) a series of recent studies indicates that the approach of a streamer is a precursor of heavy precipitation (Massacand et al. 1998), the streamer's finescale structure can influence the location and intensity of the resulting rainfall (Fehlmann et al. 2000), and the prediction of its evolution is a key forecasting task (Fehlmann and Quadri 2000). A streamer can play a dual dynamical role by both instigating and modifying the Alpine precipitation. Translating eastward, it usually has a region of ascent on its forward flank and can thereby enhance or trigger convection. In addition the streamer's location and strength influence the horizontal component of the low-level wind field and thereby the moisture flux toward the Alpine ridge.
In the present study, consideration is given to the generation of a PV streamer that subsequently constituted the precursor of an Alpine event of heavy precipitation. In effect we seek to extend back in time the causal chain that heralds the precipitation event.
2. The approach
The PV streamer under consideration developed off the European seaboard in the second half of September 1993. It both preceded and accompanied the occurrence of heavy precipitation in the Alpine region in the so-called Brig event of the Valais region of Switzerland. An overview of the streamer's development is performed using analysis fields derived from the T213/L31 version model of the European Centre for Medium-Range Weather Forecasts (ECMWF) assimilation scheme. The ECMWF fields are available at a temporal resolution of 6 h, and for the present purpose are interpolated linearly onto a regular latitude–longitude grid with a spacing of 0.75°.
On the basis of this overview a tentative hypothesis is formulated for the streamer's development that subsequently examined in a two-step process. First a Lagrangian-based diagnosis (following Wernli and Davies 1997) is undertaken for air parcels deemed to be salient to the streamer's development. This involves the computation of their backward trajectories together with the interpolation of their physical characteristics along their paths.
Second two numerical simulations of the event are performed with the limited-area hydrostatic Europa Model (EM). The latter is a version of the operational forecasting model of the German weather service. The initial data are derived from the ECMWF analysis fields, and at the lateral boundaries the model's simulated fields are relaxed toward fields constructed from the 6-hourly ECMWF analyses. The model operates with a rotated coordinate system and with 30 levels in the vertical. Its integration domain encloses most of the northeastern Atlantic, western Europe, and the Mediterranean with a horizontal resolution of 0.5° (∼55 km). It is also pertinent to note that the parameterized physical processes include a surface-layer formulation, a second-order boundary layer and turbulence formulation, and Kessler-type cloud microphysics incorporating a representation of the ice phase [further details of the model formulation and parameterization schemes are provided in Majewski (1991) and Lüthi et al. (1996)].
3. The streamer's development
Figure 1 provides an overview of the PV streamer's evolution together with conventional synoptic charts of the lower-level flow for the period from 1200 UTC 19 September to 1200 UTC 23 September 1993. The figure displays a temporal sequence, at 24-h intervals, of the PV and horizontal flow field patterns on the 320-K isentropic surface (left panels), the vertically averaged PV in the 150–500-hPa layer (cf. Massacand et al. 1998) (middle panels), and the geopotential and thermal fields at 800 hPa along with precipitation (right panels).
The left panels of Fig. 1 indicate that a broad-scale PV anomaly in the mid-Atlantic underwent mild contraction and distention as it translated eastward between 19 and 20 September. By 1200 UTC on 22 September it had evolved to form an elongated meridionally aligned filament characterized by a sharp shear line and high potential vorticity. One day later the streamer had became detached from the stratospheric reservoir to form an isolated anomaly extending in an arc from Denmark over the Bay of Biscay to the Western Mediterranean.
The pattern of the vertically averaged potential vorticity (ΣPV) in the middle panels of Fig. 1 confirms that the mature streamer was a deep and vertically coherent structure, and that from about 1200 UTC 21 September onward a negative PV anomaly was present to the west of the streamer and located primarily above the 320-K surface. By 1200 UTC 22 September the negative anomaly was compact and evident as a distinct closed region of anticyclonic flow on the 320-K surface. Thereafter the juxtaposition and in tandem movement of the negative anomaly and streamer is noteworthy.
The right panels of Fig. 1 capture two synoptic developments of import to the present study. First, an active lower-level front was linked at the beginning of the period to a comparatively deep cyclone south of Iceland. The front was aligned with, and ahead of, the maturing streamer aloft and approached the Alps early on 21 September. Ahead of the front there was sustained southwesterly flow of warm moist air toward the central Alpine region that persisted until 23 September. It was the accompanying prolonged and heavy precipitation in the southern and central Alpine regions that resulted in a major flooding episode at Brig.
Second, major cyclo- and frontogenesis occurred on the eastern seaboard of North America between 19 and 20 September, and was located beneath the southeastern fringe of an upper-level PV anomaly—upstream of the incipient streamer. The cold front itself continued to intensify to 1200 UTC 21 September as it moved out over the Atlantic and was accompanied by a sustained and intense rainband. The latter is indicative of strong cloud-diabatic effects and hence of marked diabatically induced PV variations within the ascending air (see later).
Our objective is to examine the generation of the streamer, and the premise pursued in the next section is that the negative anomaly evident in the ΣPV pattern is implicated in the process.
4. Diagnosis
The negative PV anomaly to the west of the streamer at 1200 UTC 22 September is significant both in terms of its strength and its location. Its strength is notable since, in the balanced flow limit, large amplitude negative anomalies are linked to seemingly disproportionately strong signals in the flow and thermal fields. In effect, for balanced flow, the negative anomaly limit of the PV → 0 is the counterpart of the positive anomaly limit of PV → ∞ (Thorpe and Bishop 1995). Indeed, 800-hPa vorticity fields (not shown) indicate that the negative anomaly's anticyclonic circulation extends down to lower levels.
The anomaly's location is notable since its juxtaposition with the incipient streamer establishes an east–west-aligned PV dipole. In isolation a localized dipole configuration would result in a displacement of both poles perpendicular to their axis (i.e., southward in this case), and there is an indication of such a tandem movement in Fig. 1. This process would be important for the elongation and cutting off of the streamer.
The foregoing cursory considerations prompt a more detailed examination of the negative anomaly's origin and its influence upon the streamer's generation.
a. Origin of the negative anomaly
To examine the spatial and dynamical origin of the anomaly, backward trajectories are calculated of air parcels that satisfy two criteria: (i) they are located in the anomaly's general vicinity at 1200 UTC 22 September and (ii) they possess PV values potential vorticity units (less than 0.5 PVU). The calculation is undertaken for a 84-h time span using the 6-hourly ECMWF analysis fields, and the latitudinal and longitudinal borders of the selected domain are marked out in the corresponding ΣPV panel of Fig. 1 (bold dashed lines).
Inspection of the physical characteristics of the air parcel trajectories indicates two types of trajectories. One corresponds to the predominantly adiabatic, horizontal, and longitudinal movement of air parcels with preexisting low PV values into the anomaly. The other type corresponds to strong diabatically influenced trajectories ahead of the cold front in the western Atlantic and involving both deep ascent and a major poleward excursion of the air parcels. The latter type constitutes over 30% of the selected parcels and typically experience warming of greater than 10 K. About 20% of the parcels experience a warming of greater than 20 K, and an extreme sample of the latter trajectories (warming >30 K) is shown in Fig. 2. These air parcels track northeastward some 25° latitude (see physical characteristics of the air parcels in Table 1 and compare with the synoptic features shown in the right column of Fig. 1) and ascend diabatically from ∼850 to ∼250 hPa within the cold front's rainband. They experience a monotonic decrease in their specific humidity (∼10 g kg−1), a monotonic increase in their potential temperature (∼30 K), and first an increase in their potential vorticity (0.3 → 0.7 PVU), followed by a more accute decrease.
The appearance of low-PV air at tropopause levels can be attributed to poleward advection from the subtropics and/or cloud-diabatic generation at midlatitudes. From this standpoint the “quasi-adiabatic” trajectories imply the preexistence or the prior formation of low-PV air in the extratropics. In contrast the “diabatic” trajectories (particularly those displayed in Fig. 2) are consistent with a substantial latitudinal excursion and strong diabatic effects.
The tabulated influence of the cloud-diabatic effects upon the PV of air parcels is consistent with the following heuristic interpretation (Wernli and Davies 1997). A midlevel maximum of diabatic heating induces positive (negative) PV tendencies below (above) it, and thus sustained diabatic heating can result in a compact vertically coherent but midlevel positive PV anomaly comprising air parcels transiting through the heated region, and the deposition of a shallow layer of low-PV air aloft that subsequently advects with the flow at its level. The latter diabatically induced negative anomalies can be a feature of cyclogenesis events (Wernli 1997; Rossa et al. 2000; Pomroy and Thorpe 2000) and of strong jet stream flow (Davies and Rossa 1998).
b. Influence of diabatic effects upon the streamer's development
The diabatic origin of the negative anomaly prompts an examination of the diabatic influence upon the streamer's generation. To explore this issue two 4-day simulations from 0000 UTC 20 September were conducted with the EM as set out in section 2. The first is a control run with the full model whereas the second is performed with a dry (i.e., no cloud-diabatic effects) version of the model.
An overview comparison of the two simulations is provided in Figs. 3 and 4. Figure 3 shows the analog of the ΣPV patterns (the central column of Fig. 1) for the control and dry simulations at 1200 UTC on 21, 22, and 23 September. Note that the ΣPV patterns of the control simulation replicate those derived from the ECMWF analysis fields satisfactorily, capturing the development and detachment of the streamer, and the presence of the isolated negative PV anomaly on the streamer's western flank. In the “dry” simulation the overall pattern resembles that of the control for the first 36 h (i.e., to 1200 UTC 21 Sep) although there are some differences in the neighborhood of the incipient negative anomaly. Thereafter significant differences appear. In particular the broad positive PV anomaly that is the forerunner of the streamer recedes toward the pole, and the isolated negative anomaly is markedly weaker.
Figure 4 shows the analog for the control and dry simulations of the fields in the right column of Fig. 1 along with a depiction of the velocity difference between the two runs in the European sector. The control simulation captures the synoptic developments over both the eastern and western Atlantic seaboards. In contrast, in the dry simulation, the cyclo- and frontogenesis in the western Atlantic are less rapid and less intense, and the trough in the eastern Atlantic does not extend south to Iberia. Linked to the latter difference, the spatially coherent and sustained southwesterly jet that is directed toward the Alpine chain in the control simulation is not present in the dry run.
5. Further remarks
This study considered the evolution of one particular PV streamer that was itself a dynamical precursor of an event of heavy precipitation in the Alpine region. It points to a causal chain of physical processes that contributed to the streamer's generation.
First, cloud-diabatic heating effects within a strong frontogenetic event over the western North Atlantic were shown (Fig. 3) to result in the generation of a tropopause-level negative PV anomaly downstream of the surface front. Second, this negative anomaly, located on the incipient streamer's western flank, was shown to be instrumental to its emergence [in contrast, the weaker negative anomaly located on the streamer's eastern flank (Fig. 1) is linked to both the prior existence of the streamer and quasi-adiabatic advection of the subtropical low PV air].
The inference is that, for this particular but not atypical event, the PV streamer's generation is dependent upon the preconditioning of the upstream upper-tropospheric flow by diabatic effects. It remains to examine the general validity of this inference regarding preconditioning by undertaking further case study analyses, and to investigate the nature of the underlying dynamics and the relative contributions of adiabatic and diabatic effects by performing diagnostic studies and idealized numerical model simulations.
Confirmation of the efficacy of this preconditioning process would have significant theoretical and forecasting ramifications. From a theoretical standpoint it would highlight the role of diabatic effects in two related settings: first, its possible contribution to establishing the anticyclonic asymmetry that favors the occurrence of the paradigmatic “PV streamer” type of baroclinic wave development (e.g., Davies et al. 1991; Thorncroft et al. 1993), and second, its influence upon the occurrence, nature, and interpretation of downstream development and cyclogenesis [cf. the theoretical studies of Wernli et al. (1999), Shapiro et al. (1999), and Orlanski and Chang (1993) and the empirical diagnoses of Nielsen-Gammon and Lefevre (1996) and Orlanski and Sheldon (1993)].
From a forecasting point of view it would emphasize the desirability of accurately modeling the upstream cloud-diabatic heating to predict the subsequent occurrence of severe precipitation events in the Alpine region.
Acknowledgments
REFERENCES
Appenzeller, C., and H. C. Davies, 1992: Structure of stratospheric intrusions into the troposphere. Nature, 358 , 570–572.
Appenzeller, C., H. C. Davies, and W. A. Norton, 1996: Fragmentation of stratospheric intrusions. J. Geophys. Res, 101 , 1435–1456.
Davies, H. C., and A. Rossa, 1998: PV frontogenesis and upper-tropospheric fronts. Mon. Wea. Rev, 126 , 1528–1539.
Davies, H. C., C. Schär, and H. Wernli, 1991: The palette of fronts and cyclones within a baroclinic wave development. J. Atmos. Sci, 48 , 1666–1689.
Fehlmann, R., and C. Quadri, 2000: Predictability issues of heavy Alpine south-side precipitation. Meteor. Atmos. Phys, 72 , 232–231.
Fehlmann, R., C. Quadri, and H. C. Davies, 2000: An Alpine rainstorm: Sensitivity to the mesoscale upper-level structure. Wea. Forecasting, 15 , 4–28.
Holopainen, E. O., and L. Rontu, 1981: On shearlines of the upper troposphere over Europe. Tellus, 33 , 351–359.
Hoskins, B. J., M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc, 111 , 877–946.
Lüthi, D., H. C. Davies, A. Cress, C. Frei, and C. Schär, 1996: Interannual variability and regional climate simulations. Theor. Appl. Climatol, 53 , 185–209.
Majewski, D., 1991: The Europa-Modell of the Deutscher Wetterdienst. Proc. Workshop on Numerical Methods in Atmospheric Models, Vol. 2, Reading, United Kingdom, ECMWF, 147–191.
Massacand, A. C., H. Wernli, and H. C. Davies, 1998: Heavy precipitation on the Alpine southside: An upper-level precursor. Geophys. Res. Lett, 25 , 1435–1438.
Nielson-Gammon, J. W., and R. J. Lefevre, 1996: Piecewise tendency diagnosis of dynamical processes governing the development of an upper-tropospheric mobile trough. J. Atmos. Sci, 53 , 3120–3142.
Orlanski, I., and E. K. M. Chang, 1993: Ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves. J. Atmos. Sci, 50 , 212–225.
Orlanski, I., and J. Sheldon, 1993: A case of downstream baroclinic development over western North America. Mon. Wea. Rev, 121 , 2929–2950.
Pomroy, H. R., and A. J. Thorpe, 2000: The evolution and dynamical role of reduced upper-tropospheric potential vorticity in Intensive Observing Period One of FASTEX. Mon. Wea. Rev, 128 , 1817–1834.
Price, J. D., and G. Vaughan, 1992: Statistical studies of cut-off low systems. Ann. Geophys, 10 , 96–102.
Rossa, A., H. Wernli, and H. C. Davies, 2000: Growth and decay of an extratropical cyclone's PV-tower. Meteor. Atmos. Phys, 73 , 139–156.
Shapiro, M. A., and and Coauthors, 1999: A planetary-scale to meso-scale perspective of the life cycles of extratropical cyclones: The bridge between theory and observations. The Life Cycles of Extratropical Cyclones, M. A. Shapiro and S. Grønås, Eds., Amer. Meteor. Soc., 139–185.
Thorncroft, C. D., B. J. Hoskins, and M. E. McIntyre, 1993: Two paradigms of baroclinic-wave life-cycle behaviour. Quart. J. Roy. Meteor. Soc, 119 , 17–56.
Thorpe, A. J., and C. H. Bishop, 1995: Potential vorticity and the electrostatic analogy—Ertel Rossby formulation. Quart. J. Roy. Meteor. Soc, 121 , 1477–1495.
Wernli, H., 1997: A Lagrangian-based analysis of extratropical cyclones. II. A detailed case-study. Quart. J. Roy. Meteor. Soc, 123 , 1677–1706.
Wernli, H., and H. C. Davies, 1997: A Lagrangian-based analysis of extra-tropical cyclones. I: The method and some applications. Quart. J. Roy. Meteor. Soc, 123 , 467–489.
Wernli, H., M. A. Shapiro, and J. Schmidli, 1999: Upstream development in idealized baroclinic wave experiments. Tellus, 51A , 574–587.
(left column) Distribution of isentropic PV (in PVU) plus wind vectors (arrows) on the 320-K isentropic surface. (middle column) Distribution of vertically integrated PV (ΣPV in PVU) plus wind vectors at 320 K. (right column) Distribution of geopotential heights (red contours every 50 m from 1650 to 2150 m), temperature (blue contours every 2 K) on the 800-hPa surface, and 6-h accumulated precipitation (shaded). Successive rows are for 1200 UTC on 19–23 Sep 1993 (first to fifth row, respectively)
Citation: Monthly Weather Review 129, 11; 10.1175/1520-0493(2001)129<2822:IOUDHU>2.0.CO;2
Backward trajectories (−84 h) from 1200 UTC 22 Sep 1993 for parcels from the selected domain (cf. Fig. 1 middle panel for 22 Sep) and with initial PV values less than 0.5 PVU and potential temperature difference more than 30 K over the time period
Citation: Monthly Weather Review 129, 11; 10.1175/1520-0493(2001)129<2822:IOUDHU>2.0.CO;2
Analogs of Fig. 1 middle column for the full-physics simulation (left panels) and the dry-physics simulation (right panels). Successive rows are for 1200 UTC on 21 Sep (+36 h after the start of the simulation), 22 Sep (+60 h), and 23 Sep 1993 (+84 h)
Citation: Monthly Weather Review 129, 11; 10.1175/1520-0493(2001)129<2822:IOUDHU>2.0.CO;2
Analogs of Fig. 1 right column for the full-physics simulation (left panels) and the dry-physics simulation (middle panels). In addition (right panels) the velocity difference (full minus dry physics) is given over the European sector (shaded in m s−1, direction in arrows). Successive rows are for 1200 UTC on 21 Sep (+36 h after the start of the simulation), 22 Sep (+60 h), and 23 Sep 1993 (+84 h)
Citation: Monthly Weather Review 129, 11; 10.1175/1520-0493(2001)129<2822:IOUDHU>2.0.CO;2
Mean values and variances of the position (latitude, longitude, pressure), specific humidity q, potential temperature θ, and potential vorticity PV every 12 h of the trajectory integration (cf. Fig. 2)
