Potential Vorticity Streamer Research Articles

The Role of Atmospheric Processes Associated with Hurricane Olga in the December 2001 Floods in Israel

Abstract Over the period from 0000 UTC 3 December to 0000 UTC 5 December 2001, heavy rains fell in northern Israel. Intensity of the rainfall in some areas exceeded 250 mm (24 h)−1. Results of an investigation of the case including back-trajectory evaluations, numerical simulation experiments, and a potential vorticity (PV) analysis are presented. It is demonstrated that the unusual eastern Mediterranean process has been initiated by the formation of a tropical storm that later became Hurricane Olga from 25 to 29 November. The consequent synoptic processes were associated with the development of a large-scale anticyclone to the NE of the tropical storm. Large-scale subsidence in the anticyclone played a central role in the process by leading to a convergence of the moist air masses in a narrow band on the outskirts of the system. The air masses from the area were later transported into the midtroposphere over western Europe. The interaction of these relatively warm and wet air masses with the cold and dry upper-tropospheric air over Europe has led to an intensification of the upper-tropospheric northerly airflow, extrusion of stratospheric air into the upper troposphere, and formation of a PV streamer system over southern Europe. Finally, the positioning and intensity of the PV streamer, as well as the large amounts of air moisture over the Mediterranean region, contributed to the intensity of the 3–5 December 2001 torrential rains over Israel.

The Influence of the Vertical Coordinate on Simulations of a PV Streamer Crossing the Alps

In this note the impact of the vertical coordinate system on upper-tropospheric and lower-stratospheric structures is studied using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Two different simulations are compared. One uses the original sigma coordinate, and the second one uses a modified coordinate system having smoother coordinate surfaces in the free atmosphere. Fields of atmospheric variables, such as wind and temperature, show only a weak signal related to the vertical coordinate system. However, diagnostic quantities involving horizontal and vertical derivatives react very sensitively to the vertical coordinate. The results indicate that, in the presence of steep topography, a meaningful computation of the potential vorticity (PV) field in the tropopause region is possible with the modified coordinate system only. This is mainly because disturbances in the horizontal wind field that appear at first sight unimportant induce large errors in the relative vorticity field with the original coordinate system. In addition, the simulation with the original coordinate indicates a spurious moisture transport across the tropopause above the orography. On the other hand, the impact of the coordinate system on the structure and the amplitude of orographic gravity waves turns out to be quite small.

Influence of Upstream Diabatic Heating upon an Alpine Event of Heavy Precipitation

Narrow extended troughs associated with elongated streamers of potential vorticity (PV) are a frequent feature of upper-tropospheric flow and they can instigate or modify surface weather systems. In this study consideration is given to the generation of one particular PV streamer that was itself a dynamical precursor of a heavy precipitation event in the Alpine region. It is shown that the streamer's parturition over the eastern North Atlantic is linked to a prior event of cyclo- and frontogenesis upstream over the western North Atlantic. Diagnostic trajectory analysis and heuristic simulations with a limited-area NWP model suggest that the streamer's origin is influenced crucially by the following chain of physical processes: cloud-diabatic heating associated with the upstream cyclogenesis enhances a downstream negative PV anomaly in the upper troposphere, and the flow associated with this anomaly subsequently helps transform a farther-downstream preexisting broad positive PV anomaly into an elongated streamer. Hence the case study extends back in time the causal chain that heralds the occurrence of heavy precipitation events in the Alpine region and moreover the existence of the chain has further implications for the classification of extratropical cyclones.

Cut-off low monitoring by the French VHF-ST-radar network during the ESTIME campaign

In order to investigate mesoscale strato–tropospheric exchanges, the field campaign ‘Echanges Stratosphère–Troposphère: Investigations à Moyenne Echelle’ was conducted in France from late 1993 to mid 1995 and focused on cut-off low events. It involved the French research network of five VHF (Very High Frequency) ST (Strato–Tropospheric) radars deployed in southern France. Observations corresponding to three Intensive Observing Periods are reported here. The radar data analyzed and discussed are time–height diagrams of the aspect ratio (AR), that is, the vertical to oblique beam-returned power ratio, used for monitoring three cut-off low events. In order to discuss the validity of the method, 506 h of radar AR data were compared with time–height diagrams of the static stability and the humidity obtained from synoptic European Center for Medium-range Weather Forecasts model analyses. A dataset corresponding to 297 h of observations is analyzed and discussed here. It is concluded that the AR is a good tracer to document cut-off low events, including tropopause folding identification and the detection of tropospheric air masses of enhanced stability, in dry or weakly humid cases. On the other hand, although the effects of the specific humidity and its gradients on VHF radar echo power could not be extensively investigated, our results suggest that the same parameter cannot be used at mid- and lower-tropospheric levels when the effects of specific humidity significantly reinforce the moist static stability. It is important to take into account these insights in the context of future observing campaigns in which a network of VHF-ST-radars will be involved, and where their role will be to observe and to document the evolution of upper-level features or potential vorticity streamers, or more generally stratospheric–tropospheric exchanges.

Low stratospheric water vapor measured by an airborne DIAL

Water vapor measurements are presented which were performed with a newly developed airborne water vapor differential absorption lidar (DIAL). The transmitter of this DIAL system consists of an injection‐seeded optical parametric oscillator at a signal wavelength of ≈940 nm capable of measuring very low values of water vapor (10 ppmv). The instrument was tested in an experiment where the aircraft crossed a potential vorticity streamer at about 12 km altitude. The downward looking DIAL can scan vertical cross sections in the atmosphere up to a range of 6 km, with a resolution of about 30 km in the horizontal and 250 m in the vertical. The measured data show significant water vapor structures of 5–100 ppmv within the stratospheric air of the streamer. The measured cross section is compared with fields simulated by a mesoscale model and analyzed from operational data. The comparison of the three data fields shows good agreement in water vapor structures and magnitudes. In addition, the streamer exhibits a clear structure in the aerosol backscatter lidar scan.

Low-Level Potential Vorticity and Cyclogenesis to the Lee of the Alps

Abstract High-resolution numerical model simulations over the Alpine region are presented that reveal the presence of low-level elongated bands of potential vorticity (PV) downstream of high topography. These PV streamers (or PV banners) occur when the synoptic-scale wind turns into a direction across the Alps. Individual pairs of banners with anomalously positive and negative values of PV can be attributed to flow splitting, either on the scale of the whole of the Alps (primary banners), or on that of individual massifs and peaks of the model topography (secondary banners). The PV bands have amplitudes of up to −2.5 and +5 pvu, a width of 50–150 km, can attain a length of up to 1500 km, and extend in the vertical from the surface up to the 500-hPa level on occasions. The PV banners are associated with zones of enhanced horizontal wind shear. The analysis of daily output from the operational NWP model run of the Swiss Meteorological Institute also demonstrates that such PV streamers are a frequent feature...

Tropopause studies to aid long-range weather forecasting : Heating and Ventilating, Vol. 39 No. 6

In order to investigate mesoscale strato–tropospheric exchanges, the field campaign ‘Echanges Stratosphère–Troposphère: Investigations à Moyenne Echelle’ was conducted in France from late 1993 to mid 1995 and focused on cut-off low events. It involved the French research network of five VHF (Very High Frequency) ST (Strato–Tropospheric) radars deployed in southern France. Observations corresponding to three Intensive Observing Periods are reported here. The radar data analyzed and discussed are time–height diagrams of the aspect ratio (AR), that is, the vertical to oblique beam-returned power ratio, used for monitoring three cut-off low events. In order to discuss the validity of the method, 506 h of radar AR data were compared with time–height diagrams of the static stability and the humidity obtained from synoptic European Center for Medium-range Weather Forecasts model analyses. A dataset corresponding to 297 h of observations is analyzed and discussed here. It is concluded that the AR is a good tracer to document cut-off low events, including tropopause folding identification and the detection of tropospheric air masses of enhanced stability, in dry or weakly humid cases. On the other hand, although the effects of the specific humidity and its gradients on VHF radar echo power could not be extensively investigated, our results suggest that the same parameter cannot be used at mid- and lower-tropospheric levels when the effects of specific humidity significantly reinforce the moist static stability. It is important to take into account these insights in the context of future observing campaigns in which a network of VHF-ST-radars will be involved, and where their role will be to observe and to document the evolution of upper-level features or potential vorticity streamers, or more generally stratospheric–tropospheric exchanges.