Rapid cyclogenesis over Poland on 28 March 1997

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1 Meteorol. Appl. 6, (1999) Rapid cyclogenesis over Poland on 28 March 1997 Jan Waclaw Parfiniewicz, Institute of Meteorology and Water Management, ul. Podleœna 61, Warszawa, Poland A small wave on the polar front crossed the Atlantic and experienced strong explosive cyclogenesis on 28 March 1997 when passing over the North Sea. It then progressed eastwards over northern Poland causing severe damage and the death of at least 11 people. A phenomenological study of the event is presented based on a potential vorticity (PV) approach supplemented by a synoptic analysis. Results confirm previous findings about the impact of a dry stratospheric intrusion on the process of explosive cyclogenesis and fit well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. Also apparent is the role of potential vorticity advection, which is associated with the rapid change of flow pattern in the upper atmosphere. The careful analysis of potential vorticity fields every 100 m suggests that further research is required into fine mesh analysis and wind field retrieval. 1. Data overview The depression of 28 March 1997 (the Good Friday storm) passed over northern Poland causing severe damage and the death of at least 11 people. During a 10- hour period nearly the whole country was affected by strong winds reaching 75 kn. The damage and number of casualties were the greatest since the catastrophe involving Heveliusz, the Polish ferry that sunk in the Baltic on 13 December Also for the Good Friday storm there was the largest amount of damage caused by wind in Poland not associated with one particular catastrophe. The forecast of this event was not correct and the Polish weather service was subjected to severe criticism. After the Good Friday storm there were two other depressions that passed over Poland in subsequent weeks. Three months later there was the largest flood cataclysm recorded in Poland s history. The passage of the depression on 28 March 1997 was captured by the meteorological radar at Legionowo (52 24 N, E) near Warsaw. At 1200 UTC the centre of the vortex was located about 30 km north-east of the radar. At the same time, the radiosonde measurements from the relatively dense network in the area, including the sounding from Legionowo, were available. So that an analysis of the event could be performed, the following data were collected: the GRID GTS output from the UK Met. Office, surface SYNOPs from the national Polish network, and satellite images from METEOSAT and the NOAA series. Figure 1 presents the passage over the Atlantic and North Sea of a small, non-significant wave depression starting at 0000 UTC on 27 March During the next 18 hours the m.s.l. pressure of the wave depression changed from 1010 mb to 1005 mb; at 1800 UTC on 27 March the depression was close to north-east England. On passing over the North Sea, the wave received a strong cyclogenetic impulse, and after 6 hours it deepened by a further 10 mb. Figure 2 shows the track of the depression from the Jutland Peninsula, via northern Germany and northern Poland, to Belarus and Lithuania. The lowest pressure of 978 mb occurred over Poland, 30 km north-east from Warsaw, at 1200 UTC on 28 March. Then the depression backed to the north-east by about 30 and started to fill. Figure 2 also indicates the position of the jet streams at 0000 UTC and 1200 UTC on the 28th, along with the jet streaks and their velocities and heights. It was found that: At 0000 UTC the maximum wind of 150 kn was measured at Goteborg (Go), Leba (Le) and Kalinigrad (Ka) on the 277 mb, 258 and 310 mb levels (~935, 980 and 865 hm) respectively. At 1200 UTC the maximum wind of 120 kn occurred at Garmsdorf (Ga) at 287 hpa (~930 hm). The location of the main jet streams changed dramatically during that 12-hour period. The Atlantic branch of the jet was weakening while over Britain a new north-north-westerly jet developed, and the winds over Britain backed by more then 60. Over the middle of Europe the westerly branch of the jet pushed south and by 1200 UTC it was parallel to the Carpathians and Alps. The jet streak from the Baltic moved south near Garmsdorf. Most of the winds over middle Europe, especially these over Germany, veered by about 30. Between these branches of the jet lay the track of the fast-moving explosive depression. The surface depression and cold front progressed at a speed of about 43 kn based on three-hourly synoptic charts. 363

2 J W Parfiniewicz Figure 1. The passage of the pressure depression on m.s.l. charts over the Atlantic and North Sea from 0000 UTC on 27 March 1997 to 0000 UTC on 28 March Figure 2. Track of the explosive cyclone episode from 0000 UTC to 1800 UTC on 28 March Indicated are the jet streams at 0000 UTC and 1200 UTC on 28 March derived from maximum wind charts, and the jet streaks and their velocities and heights. At 0000 UTC the maximum wind was measured at Goteborg (Go) 02527, Leba (Le) and Kalinigrad (Ka) At 1200 UTC the maximum wind occurred at Garmsdorf (Ga) In Figure 3 is shown the visible image from the NOAA satellite at 1225 UTC on 28 March. It shows the mature nature of the vortex with its centre located over eastern Poland. The cloud structures in the earlier images show rotational growth, with the system starting as a typical baroclinic leaf and ending as mature depression. The whole process of cyclogenesis fits well with the archetype flat trough, diffluent flow cyclogenesis of Bader et al. (1995) (also Grant, 1997). The structure of the conveyer belts was similar to that described by 364 Browning (1997) with evidence of a cloud head and warm conveyer belts (WCBs) W1 and W2 (see Figure 8 later). Also clearly seen are the dry zone north of the cold front and the dry slot over eastern Poland. Figure 4 shows the sounding from Legionowo (12375) near Warsaw at 1200 UTC on the 28th. The humidity profile shows that there is a zone of extremely dry air from above 8 km to about 2.5 km, with a layer of saturated air below (between 1.5 and 2.5 km). The wind

Rapid cyclogenesis over Poland on 28 March 1997 Figure 3. The NOAA visible satellite imagery at 1225 UTC on 28 March 1997. It shows the mature vortex with its centre over eastern Poland.

3 Rapid cyclogenesis over Poland on 28 March 1997 Figure 3. The NOAA visible satellite imagery at 1225 UTC on 28 March It shows the mature vortex with its centre over eastern Poland. profile indicates that there are three maxima at 1.8 km, between 5 and 6 km, and at km (175 mb with a maximum wind of 75 kn). The dry zone is associated with the cyclonic subsidence due to stratospheric inflow. 2. Application of potential vorticity to understand the process The benefits of using potential vorticity (PV) to understand mid-latitude weather systems have been described by Hoskins et al. (1985) and Hoskins (1997). Also in Hoskins et al. (1985) the theoretical basis of PV (including the historical background associated with the names Rossby and Ertel) is outlined, and the physical significance of PV is explained in Hoskins (1997). Mathematically the PV is the scalar product of two vectors: the gradient of potential temperature and the absolute vorticity vector, divided by density. This quantity, often called the full potential vorticity to mark the difference from the quasi-geostrophic (QG) potential vorticity, has the unique property of being conserved during adiabatic and frictionless motion along material trajectories. Considering a thin cylinder spreading between two neighbouring isentropic surfaces, Hoskins (1997) showed that mid-tropospheric ascent and descent leads to increase and decrease of vorticity and circulation, and thus they correspond to cyclonic and anticyclonic surface development. To explain the synoptic development in terms of stretching and shrinking of vorticity it is necessary to take account of the profile of vertical motion. The concise theory of such effects in the framework of QG theory is reviewed by Pedder (1997). Discussing the case of a positive PV anomaly with the zonal wind increasing with height, which is especially relevant to this particular case, the ascent to the east of the anomaly and descent to west is pronounced. The other rule for a warm anomaly near a lower boundary with a uniform PV distribution is that the isentropic 365

4 J W Parfiniewicz Figure 4. The sounding from Legionowo (12375) near Warsaw at 1200 UTC on 28 March Left: profiles of temperature and dew point ( C). Right: profiles of relative humidity (%) and wind. separation would be enhanced with the production of cyclonic motion. The effect of diabatic heating in the free atmosphere is of special importance as the release of potential instability associated with dry stratospheric intrusion is essential for rapid cyclogenesis as described here. This would be manifested in a decrease of PV above the region of latent heat release and can lead to significant enhancement of low level cyclonic circulation, and enhancement of the upper ridge in the air moving ahead of it according to Hoskins (1997). On the basis of the PV distribution, the concept of a dynamical tropopause (DT) was introduced by Hoskins et al. (1985). Measured in PV units (PVU), where 1 PVU = 10 6 m 2 K s 1 kg 1, and looking from the subtropical jet to the pole, there is a jump in PV values typically from 1.5 to 4 PVU at the tropopause and then a rapid increase with height in the stratosphere. A surface at values like PV = 2 3 is particularly useful because this can be considered to be dynamical tropopause poleward of the subtropical jet. according to Hoskins (1997). This could be taken as the definition of a DT. The DT distribution detects significant changes in the dynamic properties of the air mass under investigation (Hoskins et al., 1985; Hoskins & Berrisford, 1987), and it has now become a standard tool for recognising upper-level disturbances associated with cyclogenesis (e.g. Dickinson et al., 1997). To provide the necessary fields for a PV analysis over Europe, the fields of the basic meteorological elements (i.e. pressure, temperature, wind and humidity) were produced on the grid of nodes with a 366 horizontal gridlength of 16 km and vertical spacing of 100 m. The GRID data from the UK Met. Office and TEMP data were aggregated and during interpolation the vertical co-ordinate was transformed from pressure to height. Then potential temperature (θ), wet bulb potential temperature (θ w ), and PV (Hoskins, 1997, equations (A6/z) and (A27/z)) fields were calculated. The vertical anelastic motions were calculated based upon the mass continuity taking into account the bottom orography. All further illustrations refer to 1200 UTC on 28 March In Figure 5(a) the DT (i.e. the height of the surface of PV = 2 PVU) at 1200 UTC is presented. The figure reveals a number of interesting details especially when compared to Figure 2. While the lowest values of just less than 5 km are located close to a jet streak, the highest values of more then 13.5 km are found on the right, southern part of the jet and ahead of the explosive depression. A strong height gradient of the DT crosses the vortex (which fits exactly within the Legionowo radar area), giving a dramatic rise in the DT (see Hoskins & Berrisford, 1987) more than a 2 km rate of growth over a 200 km horizontal distance. Figures 5(c) and 5(d) illustrate the horizontal distribution of the relative humidity at 3 km and 6 km respectively. At the lower level, which is beneath the DT, there is a homogeneous, elongated belt of moist air parallel to the polar front (and eastern branch of the jet stream). Comparison with the satellite picture in

(a) Rapid cyclogenesis over Poland on 28 March

of PV where 1 PVU = 10 6 m 2 K s 1 kg 1 ), (b)

sources area of dry air that penetrate vortex

at the 3 km level, (d) relative humidity (%) at

5 (a) Rapid cyclogenesis over Poland on 28 March 1997 (b) (c) (d) (e) Figure 5. (a) Dynamic tropopause (i.e. the height of the surface of PV = 2 units of PV where 1 PVU = 10 6 m 2 K s 1 kg 1 ), (b) PV on the 315 K isentrope showing possible sources area of dry air that penetrate vortex over Poland (in PVU), (c) relative humidity (%) at the 3 km level, (d) relative humidity (%) at the 6 km level, and (e) vertical motion (cm s 1 ) on 315 K isentrope at 1200 UTC on 28 March

6 J W Parfiniewicz Figure 3 shows that this warm moist belt must be associated with the polar-front cloud band which is diminishing in its western part over southern France, the Bay of Biscay and the Atlantic. Indeed, as follows from the vertical motion fields, all this region is dominated by subsidence. The humidity field in Figure 5(d) lies, over a significant part of the domain, above the DT. This clearly shows a dry air zone which, over western Europe, overlaps the underlying warm moist air belt. Also seen is the dry tongue of stratospheric air that penetrates the sub-alps region and northern Czech Republic, and flows through the Sudet Mountains into central Poland, where the centre of the explosive vortex was situated. In Figure 5(b) the PV on the 315 K isentrope is presented. This shows a vast homogeneous region of relatively high PV, with values above 3 PVU over Central Europe. As follows from PV conservation and the wind field distribution, Holland and Belgium are indicated as source areas of dry air that penetrates the vortex over Poland. It must be stressed that on the upper isentropes (i.e. 320 K and 330 K) the source area is displaced slightly towards the North Sea coast of Germany. This picture is consistent with the dry tongue as depicted in Figure 5(d) and the subsidence area seen in Figure 5(e). The small shift towards the north is due to the 315 K isentrope being about 1 km higher. The vertical motion on the 315 K isentrope is presented in Figure 5(e). This shows two main centres of subsidence. One of them is located close to the jet streak (near Garmsdorf, south Germany) and the second one, which lies over Poland, is just to the rear of the southwestern part of the vortex close to its centre. The location of the subsidence centre near Garmsdorf in Figure 5(e) and the elongated form of dry tongue as seen on Figure 5(d), confirm the kinematic scheme of Danielsen (as reported by Browning, 1997) in that the trajectories of the dry stratospheric air originate from a small region of intense subsidence in the vicinity of a newly developing tropopause fold. 3. The radar perspective and conceptual model The passage of the vortex is well documented by the radar reflectivity data using a grid of with a gridlength of 4 km at the 700, 1000, 1500, 2000, 3000, 4000 and 5000 m levels. Although the radar at Legionowo is not Doppler, its coverage was sufficiently large (400 km 400 km) that between 1100 UTC and 1200 UTC on the 28th the whole vortex was within its range. In Figure 6 the quasi three-dimensional picture of radar reflectivity is presented, while Figure 7 shows the geographic position of the radar domain. The main flows are drawn schematically: they can be recognised as a secondary warm conveyer belt and cold conveyer belt. The main areas of strong ascent and descent identified by the radar reflectivity are of a much smaller scale than the vertical motions presented on Figure 5(e). These areas need to be studied using fine-mesh retrieval techniques and are the subject of separate research. Nevertheless, on the basis of the conceptual model as described by Browning (1997) (see Figure 8) and supplemented by the analysis of satellite imagery, the secondary WCB, labelled W2, is clearly seen as it extends to a greater distance away from the primary WCB before rising in the cloud head. The cloud system of the part of the WCB, W2, that connects the cloud head with the primary WCB is destroyed, being overrun by dry-intrusion air over a broader front. The central and Figure 6. The quasi-three-dimensional picture of radar reflectivity at 1200 UTC on 28 March The main flows shown are the secondary warm conveyer belt and cold conveyer belt. 368

7 Rapid cyclogenesis over Poland on 28 March 1997 Figure 7. Map of Poland showing the location of the Legionowo radar domain. process might not change much from case to case, each case needs separate investigation to understand all aspects of the meteorological puzzle. It was a challenge for Polish meteorologists to develop their own workshop, which has allowed such a study. Results confirm the impact of a dry stratospheric intrusion on explosive cyclogenesis and fit very well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. This is of great help in understanding the hierarchy of processes that are manifest in the data on different scales. Also apparent is the role of potential vorticity advection which is associated with the rapid change of flow pattern in the upper atmosphere. The synergetic effect of the upper field change and rapid surface cyclogenesis was observed. Figure 8. Conceptual model showing system-relative airflow associated with the diffluent-flow type of cyclogenesis. The arrows labelled W1 and W2 are the primary and secondary warm conveyor belts. The dashed arrow labelled CCB is a cold conveyor belt. The dry intrusion is seen to overrun W2 over a broad region to produce an upper cold front at its leading edge. (After Browning, 1997). rear south-western part of the vortex are not dominated by downward motion. In these areas wind gusts of 60 kn or more were reported at 1200 UTC by a number of Polish SYNOP stations. The usage of geometric height as the vertical coordinate simplifies the interpretation of some results. The high vertical resolution allowed data to be assimilated from all radiosonde levels showing a large horizontal diversity of analysed quantities especially in the regions of significant vertical shear and high winds. The careful analysis of potential vorticity fields every 100 m indicates that further research is required into fine mesh analysis and wind field retrieval. The data collected for this study are available free of charge to all who want to further investigate this case or use the data to refine numerical models. 4. Final remarks This work is a phenomenological study of the severe weather event associated with explosive cyclogenesis. A number of such events were researched and documented. Although the general description of the Acknowledgements This work was supported by Scientific Research Council of Poland (KBN) under grant No. 4 S The author acknowledges the help of K. Browning 369

8 J W Parfiniewicz for his valuable comments and the work done by R. Riddaway, who made this paper readable. Thanks are also due to J. Borkowski, J. Pruchnicki and Z. Sorbjan for their comments on an earlier version of the manuscript. Special thanks are due to Zosia Parfiniewicz for permanent assistance and laborious editorial works. References Bader, M. J., Forbes, G. S., Grant, J. R., Lilley, R. B. E. & Waters, A. J. (1995). Images in Weather Forecasting. Cambridge University Press, 499. Browning, K. A. (1997). The dry intrusion perspective of extra-tropical cyclone development. Meteorol. Appl., 4: Dickinson, M. J., Bosart, L. F., Bracken, W. E., Hakim, G. J., Schultz D. M., Bedrick, M. A. & Tyle, K. R. (1997). The March 1993 superstorm cyclogenesis: incipient phase synoptic- and convective-scale flow interaction and model performance. Mon. Wea. Rev., 125: Grant, J. R. (1997). Use of satellite imagery in forecasting cyclogenesis. Meteorol. Appl., 4: Hoskins, B. J. (1997). A potential vorticity view of synoptic development. Meteorol. Appl., 4: Hoskins, B. J. & Berrisford, P. (1988). A potential vorticity perspective on the storm of October Weather, 43: Hoskins, B. J., McIntyre, M. E. & Robertson, A. (1985). On the use and significance of isentropic potential vorticity maps. Q. J. R. Meteorol. Soc., 99: Pedder, M. A. (1997). The omega equation: Q G interpretations of simple circulation features. Meteorol. Appl., 4:

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