(EUROPE) (SWITZERLAND)

Transcription

1 3: CASE STUDY OF 3.1: SYNOPTIC 20 JUNE 2005 WEATHER PATTERN (EUROPE) The synoptic weather pattern over Europe on 20 June 2005 was dominated at mid-to-upper tropospheric levels by a strong ridge of high pressure stretching northwards over western Europe, bringing warm and stable conditions (see Figure 3.1). The highest geopotential heights at the 500 hpa pressure level lay over Spain and France, with Switzerland lying in a gently northerly flow of air on the eastern flank of the ridge. In the absence of frontal boundaries, this location is usually considered as an unlikely place for the formation of severe or widespread thunderstorms, and is often associated with negative relative vorticity. Air temperature at 500 hpa was relatively high, ranging from -10 to -12 C across almost the whole of western Europe. Surface air pressure was also relatively high, but pressure gradients were slack over central Europe, with a just weak south-easterly flow prevailing over Switzerland and southern Germany (see Figure 3.2). There was a marked north-south surface moisture gradient across the Alps, with a dewpoint of only around 10 C over Germany, but around C in northern Italy. Incursion of moist air into the southern Alps may therefore have been possible due to prevailing southerly onshore winds in northern Italy and mountain venting on the south side of the Alps. 3.2: MESOSCALE WEATHER PATTERN (SWITZERLAND) Because of the flow of warm and dry air at mid-to-upper levels, combined with anticyclonic subsidence associated with the high pressure, clear sky were predominant across almost the whole of Europe, including Switzerland, for most of the day (see Figure 3.3). As a result, surface air temperatures rose rapidly from 11 C or 12 C near dawn to over 30 C across the Swiss plateau during the late afternoon, peaking at 31.5 C at Basel-Binningen weather station. Thus, diurnal air temperature ranges were large (typically C across the Swiss plateau), indicating nearmaximum insolation, a non-hazy and dry atmosphere (allowing maximum infra-red radiation), and a shallow inversion. All MeteoSwiss ANETZ stations (as reported in MeteoSwiss' Tageswetter daily summary for 20 June 2005) recorded between 10 and 15 hours of bright sunshine during the day (but with the notable exception of Adelboden at 9.4 hours). This strong sunshine, rapidly rising temperatures, weak surface pressure gradient, and weak upper synoptic forcing (little windshear) gave rise to near-ideal conditions for the development of local or mesoscale mountain-valley / mountain-plain wind systems across Switzerland from mid-morning onwards, eventually leading to isolated convergence and near-stationary, temporary orographic convection. 1

2 3.3: THE PAYERNE SONDE AT 12 UTC The Payerne radiosonde launched at 12 UTC (see Figure 3.4) indicated the remains of a shallow inversion near 830 hpa (1800 m) capped by very dry conditions above through to the top of the troposphere - the mean relative humidity between 820 hpa and 200 hpa was only 23%, suggesting considerable descent of air by anticyclonic subsidence. The lack of available moisture in the profile to provide any additional latent heat stimulus for convection meant that there was a considerably reduced likelihood of convection. Using Normand's construction on Figure 3.3, it is estimated that a surface temperature of at least 35 C at Payerne would be necessary to saturate a surface parcel and rise it through the depth of the troposphere (assuming no mixing); such high surface temperatures were unlikely on this day. Slightly lower maximum temperatures (~33 C) would limit convection to between 2100 and 4300 metres. However, as stated already in the previous Chapter (section 2.4) of this document, it is important to note here that the Payerne radiosonde data may be representative of conditions only on the Swiss plateau and therefore may be different from a sonde profile taken in the Alps at the same time. Nevertheless, selected thermodynamic indices were calculated using the Payerne sonde data as input, and these are presented in Table : CALCULATION OF CONVECTIVE INDICES Despite the limitations of using Payerne sonde data and extrapolating it to broader scales for thunderstorm prediction in the Alps, a selection of five convective indices were calculated for 20 June 2005 using the sonde data for 12 UTC and are presented in Table 3.1. For full explanations of each of these indices, the reader is referred to Huntrieser et al. (1997), Koffi et al. (2007) or an official NOAA website. The results show that most of the indices indicate little chance of deep convection occurring, with values remaining below the threshold for severe storm development. The low values of Convective Available Potential Energy (CAPE) are essentially meaningless, presumably because not enough moisture was available in the Payerne boundary layer to saturate in upper layers, even if surface convection was powerful enough. Additionally, CIN values of less than -200 J/kg are usually not overcome by even the strongest convection. However, it is worth noting that these indices have largely been developed for the mid-west plains of Normand's construction using a tephigram: A parcel of air is assumed to rise without mixing or entrainment of surrounding air. The surface temperature at which deep convection is initiated is determined by following the coolest possible saturated adiabat line downwards from the tropopause until it intersects with line of constant water vapour mixing ratio from the surface, and then following a dry adiabat to the surface. (accessed 29 December 2008) 2

3 North America, and therefore may be partially or wholly unsuitable for the small, rugged and mountainous landscape of Switzerland, leading Huntrieser et al. (1997) to propose their own Swiss thunderstorm index. In this study, however, we are only interested in cases of marginal, localised convection, aided by convergence of mountain-valley and mountain-plain wind systems, so the use of such thermodynamic indices are only partially beneficial, at best, for our purposes. Convective Indices: 20 June 2005, 12 UTC, Payerne, Switzerland Index Value Likelihood of Convection? LI 2.6 Stable: thunderstorms not likely TI 17.4 Stable: thunderstorms not likely DCI 21.1 Below threshold CAPE 33 Weak instability, no available energy CIN -238 Below threshold, values below 200 not usually overcome Table 3.1: The convective indices of Lifted Index (LI), Totals Index (TI), Deep Convective Index (DCI), Convective Available Potential Energy (CAPE; J/kg) and Convective Inhibition Energy (CIN, J/kg). The values of LI, TI and DCI have been calculated for parcels originating from a mean surface layer of 100 hpa thickness. The CAPE and CIN values have been calculated for parcels originating at the surface. For definitions and further information on each of these indices, please see Koffi et al. (2007) or the NOAA website: (accessed 29 December 2008). 3.5: BERN EXWI AND TROWARA DATA Data from the University of Bern Exact Sciences (EXWI) building roof-top instruments of TROWARA and EXWI weather station for 20 June 2005 are presented in Figures 3.5 to 3.7. The weather data shows near-sinusoidal profiles of solar radiation curve (indicative of clear sky), air temperature and atmospheric air pressure (solar tide), with very good visibility throughout the day (e.g. deep blue skies, as seen later in Figure 3.9). A dip in water vapour pressure (hpa) can be seen near 1800 UTC in Figure 3.5, and is also evident in Figure 3.6 when expressed as a water vapour density (g/m³). This dip is interesting, and is better explained when the TROWARA data (Figure 3.6) are considered as well. The TROWARA IWV (Figure 3.6) shows a change to higher variability from about 1300 UTC onwards 3

4 (onset of convection in nearby mountains), with a sudden front-like drop at about 1500 UTC (commencement of deep convection over the Alps). The shape of this drop is reminiscent to the main author of traces of the onset of the sea-breeze (density current) in the British Isles (e.g. Simpson, 1994). Whilst there is no sea in Switzerland, like the sea-breeze, a mountain-plain air circulation system is also an air ascent-descent circulation system, caused by pressure differences. Thus, sea-breeze like features (i.e. frontal lobes, density currents) sometimes occurring in Switzerland cannot not be ruled out. Figure 3.7 shows the Beta parameter for 20 June 2005, calculated using TROWARA microwave radiometer data and the refined algorithm of Mätzler and Morland (2009). Higher beta values indicate the predominance of the water vapour signal from lower altitudes, and vice versa. There is a marked diurnal variation, probably related to the strong solar heating and rise/fall of the boundary layer over Bern during the day. Thus, higher values during the morning suggest a shallower boundary layer, rising gradually during the day due to surface heating. Note, however, the sudden peaks at around 1200, 1500 and 1900 UTC indicating sudden changes in the depth of the boundary layer over Bern and possible effects of a mountain-plain air circulation system. 3.6: DEVELOPMENT OF THUNDERSTORMS OVER JURA AND WESTERN ALPS Large towers of cumulus congestus convection began at favoured orographic points around 1200 UTC (2 pm local time) over the Swiss Jura (near Lac de Joux) and over the the western Swiss Alps (parts of cantons Vaud, Bern, Fribourg and Wallis, referred to as the Bernese Oberland henceforth) during the afternoon and early evening from 1300 UTC to 1800 UTC, quickly dying out thereafter. The thunderstorms were not exceptional by Swiss standards, but because of their isolated nature they can be easily identified as mesoscale phenomena using high resolution satellite and surface remote sensing techniques. Apart from these instances, there were no other cases of convection in the whole of Switzerland on this day, and skies remained largely clear - thus, the areas of convection remained very-well defined and small scale. The situation is captured well by the NOAA-12 visible satellite picture of 1645 UTC, just after the time of maximum convection, and is shown in Figure 3.8 (a). Also shown alongside in Figure 3.8 (b) is the MeteoSwiss MeteoSwiss 24-hour (0000 UTC to 2350 UTC) accumulated precipitation operational precipitation radar for the same day (provided by MeteoSwiss). For the reasons discussed in Germann et al. (2006), the radar accumulated values should not be taken as direct quantitative values of precipitation, but rather they should be considered as fairly reliable estimates of the location of precipitation on this day and their relative intensity. Locally extreme precipitation (greater than 50 mm) may have fallen in the Bernese mountains, but this was not confirmed by any available raingauge data. In addition, the MeteoSwiss radar beams of La Dôle and Monta Lema cannot see into the central valley of Wallis, so the true accumulated precipitation map is unknown. 4

5 Meanwhile, Figure 3.9 shows a special four-panel image of four different observational systems at approximately the same time ( UTC) on 20 June 2005, namely a Meteosat Second Generation High Resolution Visible 1km image of Switzerland, a MeteoSwiss precipitation radar image, a Global Positioning Satellite (GPS) integrated water vapour (IWV) anomaly map, and a visible photograph looking south from Fribourg, Switzerland (please note that the satellite image and cloud photographs are available for viewing as animations on the STARTWAVE website, which better show their temporal development and decay). Note the co-incidence of features between all four independent observational images, particularly the sharp precipitation boundary and IWV gradients near the source of the convective towers, but fanning more gently south-eastwards under the thunderstorm cirrus anvils. Note also the deep blue sky in the photographic image (indicating a very dry atmosphere), devoid of all smaller clouds except the thunderstorm cloud itself in the distance (indicating marginal conditions for thunderstorms, and possible descent near the observer's location). Figure 3.10 presents a six-hourly time sequence of co-incidental satellite and GPS IWV images from 1300 to 1900 UTC. The evolution of the thunderstorm over the Bernese Oberland is seen clearly in the first column of images. Alongside to the right, the GPS IWV images are presented in two different formats; (i) Middle column: corrected to an altitude of 500 metres above sea-level, according to the method described by Morland and Mätzler (2007), and (ii) Right column: as normalised anomaly, where the anomaly is divided by the mean monthly standard deviation. This latter method of portrayal largely eliminates the occurrence of visual bulls-eyes caused by the Jungfraujoch data on GPS IWV maps (e.g. compare the two GPS images for UTC), as Jungfraujoch IWV values are typically only one fifth of corresponding IWV values over the Swiss lowlands. The GPS IWV images show a remarkable concentration of water vapour features over Saanen GPS station in the Bernese Oberland (up to 50%, or ~10mm, greater than at nearby stations) from 1300 UTC onwards, with general higher IWV values south of the Alps at all times (as stated earlier). Note also the incursion of very dry air over the northern Swiss plateau later in the afternooon. Whilst the origin of this dry air was probably synoptic (spreading from the north-east) possibly leading to thunderstorms on a dry line frontal boundary over the Alps, it should not be ruled out that local enhancement of the dry air may have been aided by a mountain-plain air circulation system. 3.7: DISCUSSION OF CONVERGENCE / DIVERGENCE AND TOTAL MASS OF IWV RESULTS Figure 3.11 (top panel) presents the time series of total GPS integrated water vapour (kg) mass for the Bernese Oberland area (the area marked within the black rectangle on Figure 2.2), corrected to a level of 1000 metres (using the same method as described by Morland and Mätzler, 2007) from

6 to 2400 UTC on 13 June 2006, as described earlier in Chapter 2. The second panel of Figure 3.11 presents a time series of Divergence / Convergence (sec-1) for the same period (for the area marked within the red polygon in Figure 2.2), also as described earlier in Chapter 2. The third panel of Figure 3.11 presents the number of distant (3-30km) and near (0-3km) and Cloud-to-Ground (CG) lightning strikes recorded by the ANETZ weather stations. The fourth (bottom) panel of Figure 3.11 shows the mean precipitation rate (mm/hr) for the ANETZ weather stations located within the box for the same time period. Total IWV mass values rise from about 1.1 X 10¹¹ kg (110 megatons) during the early morning to nearly 1.3 X 10¹¹ kg during the time of maximum orographic convection in mid-afternoon. This represents a 20% increase in total IWV within the selected area, although an individual station increase of nearly 50% (~10 mm IWV) occurred at Saanen GPS station, as already stated. The maximum IWV mass values co-incide extremely well with the timing of maximum extent of the thunderstorm development and anvil (see earlier satellite pictures, Figure 3.10). Meanwhile, the convergence / divergence values in Figure 3.11 show a steady decrease (convergence) during the morning, steepening by early afternoon during the time of maximum convection (strong convergence), in agreement with results of Done et al. (2005). A sudden switch to divergence (positive) values occurs at UTC, a little earlier than might be expected, when considering the evolution of the satellite and GPS images. However, Figure 3.11 (bottom panel) shows that this sudden switch is co-incidental with a precipitation event (i.e. downdraught divergence) that occurred at Sion ANETZ station, which explains the sudden switch to divergence of Figure 3.11 (second panel). Note the occurrence of lightning (third panel) just after the time of maximum convergence and during the period of greatest water vapour concentration, indicating changes in the microphysics of the cloud. Note also the IWV total mass starts to decrease when the precipitation occurs (i.e. loss), divergence occurs (i.e. downdraughts) and the peak in lightning activity subsides (i.e. less convection keeping droplets or ice bouyant). Finally, Figure 3.12 presents the hourly values of GPS IWV at Saanen (solid black line) and Payerne (dashed blue line) GPS receiving stations for the same day. Note the strong increase in Saanen values, co-incident with decreases in the Payerne values (only 43 km to the north on the Swiss plain), during the time of maximum orographic convection in the locality. Note also the general anticorrelation of the Payerne series with the Bernese Oberland convergence values (bottom panel). These data again point towards local ascent / descent in the arms of a mountain-plain air circulation system during the day on 20 June 2005 (although synoptic forcings cannot be ruled out as well). 6

7 Figure 3.1: MeteoSwiss contour map of 500 hpa geopotential height over western Europe and north-east Atlantic at 12 UTC on 20 June 2005 (units of 10 metres = 1 decametre). T and H mark low ( Tiefdruck ) and high ( Hochdruck ) pressure centres. Also shown are the 500 hpa radiosonde reports for selected stations (e.g. Payerne, Switzerland: vector barb indicating northerly winds of 10 knots, air temperature -12 C and altitude of 582 decametres). Image taken from MeteoSwiss daily weather summary report ( Tageswetter ). 7

8 Figure 3.2: MeteoSwiss contour map of surface air pressure (interval 5 hpa) and frontal analysis over western Europe at 12 UTC on 20 June Also shown are a selection of standard meteorological station reports (e.g. Geneva, Switzerland: clear skies, weak north-easterly breeze of less than 5 knots, air temperature 26 C, dewpoint 12 C). Image taken from MeteoSwiss daily weather summary report ( Tageswetter ). 8

9 Figure 3.3: Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infra-red Imager (SEVIRI) image of Europe at 1312 UTC on 20 June Note clear skies across much of Europe (including Switzerland). There is stratus and fog near the Bay of Biscay (light grey or purple colours) and isolated deep convection (yellow colours) beginning over the Pyrenees, southern France and the French Alps (source: EUMETSAT). 9

10 Figure 3.4: A stuve thermodynamic diagram of Payerne radiosonde data for 12 UTC 20 June Stuve diagrams are similar to tephigrams; the horizontal dark blue lines are altitude, the sloping green lines are dry adiabats, the curved light blue lines are saturated adiabats and the curved purple lines are lines of constant water vapour mixing ratio. The thick solid plotted black lines are air temperature (right) and dewpoint (left). Also shown on the right are wind barbs for each given level. The diagram shows a shallow super-adiabatic surface layer, a weak shallow inversion at around 1800 m, and a very dry atmosphere above 850 hpa. Image created online using the University of Wyoming (USA) ( accessed 14 November 2008). 10 upper-air database

11 Figure 3.5: Roof-top weather station data at the University of Bern Exact Sciences (EXWI) building for 20 June 2005 at 10-minute intervals. Green = air temperature ( C); black = wind gust + 30 m/sec; dotted black line = (wind direction / 18)+20, 30 = calm; dark blue = water vapour pressure (hpa); yellow = sea-level air pressure (-990 hpa); red = solar radiation (W/m² / 20); light blue = visibility (km); hatched red and black lines = rain gauge and cumulative rain sensor (mm; no precipitation recorded). Please note that some instruments suffer from insufficient exposure. 11

12 Figure 3.6: TROWARA microwave radiometer and EXWI weather station data at the University of Bern Exact Sciences (EXWI) building roof-top site for 20 June Thick black line = air temperature ( C); Thick blue line = IWV (integrated water vapour, mm); thin blue line = water vapour density (g/m³); red = 100 X ILW (integrated liquid water, mm); thin grey line = cloud temperature C (no clouds seen). Please see Mätzler and Morland (2009) for the IWV and ILW refined algorithms. 12

13 Figure 3.7: Beta parameter for 20 June 2005, calculated using TROWARA microwave radiometer data and the refined algorithm of Mätzler and Morland (2009). Higher beta values indicate the predominance of the water vapour signal from lower altitudes, and vice versa. For example, higher values during the morning suggest a shallower boundary layer, rising gradually during the day due to surface heating. Note, however, sudden peaks at around 1200, 1500 and 1900 UTC. 13

14 (a) (b) Figure 3.8: (a) NOAA-12 combined Channels 1, 2 and 4 satellite image of western Switzerland at 1645 UTC on 20 June Note the isolated cumulus congestus towers (white colours) over the Jura and western Alps, with glaciated cumulonimbus ice anvils (purple colours) stretching downwind from the north (image provided by Bernard Burton of Wokingham Weather); (b) MeteoSwiss 24-hour (0000 UTC to 2350 UTC) accumulated precipitation for 20 June 2005, calculated from MeteoSwiss operational precipitation radar (image provided by Urs Germann of MeteoSwiss, with permission given for re-production). 14

15 Special four-panel image at UTC (a) 1527 UTC: MSG HRV visible satellite (b) MeteoSwiss precipitation radar 1520 UTC (c) GPS normalised anomaly IWV (mm), 1500 UTC (d) 1515 UTC visual photograph 15

16 Figure 3.9 (previous page): Four-panel image of four different observational systems at approximately the same time ( UTC) on 20 June 2005; (a) Meteosat Second Generation High Resolution Visible (MSG HRV) 1km image of Switzerland at 1527 UTC (image provided by EUMETSAT); (b) MeteoSwiss precipitation radar at 1520 UTC (image retrieved directly from internet ); (c) Global Positioning Satellite (GPS) average integrated water vapour (IWV) anomaly (mm) at 1500 UTC; (d) Visible photograph looking south from Fribourg, Switzerland at approximately 1515 UTC (photograph taken by Eddie Graham). 16

17 UTC MSG HRV sat-pic GPS Alt-Corr-500m GPS Norm. Anomaly

18 Figure 3.10 (previous page): Hourly time sequence of images spanning 6 hours from 1300 to 1900 UTC on 20 June 2005 (Left column): Meteosat Second Generation High Resolution Visible (MSG HRV) 1km image of Switzerland (image provided by EUMETSAT). (Middle column): Global Positioning Satellite (GPS) average integrated water vapour (IWV) corrected to an altitude of 500 metres above sea-level, according to the method described by Morland and Mätzler (2007). (Right column): the same GPS IWV data, but presented as normalised anomalies (normalised anomaly = anomaly divided by the mean monthly standard deviation). Note the co-incidence of cloud features with water vapour anomalies. 18

19 19

20 Figure 3.11 (previous page): (First [top] panel): Time series of total GPS integrated water vapour (kg) for the area marked within the black rectangle on Figure 2.2, corrected to a level of 1000 metres (using the same method as described by Morland and Mätzler, 2007) from 0000 to 2400 UTC on 13 June (Second panel): Divergence or Convergence (sec-1) for the same period for the area marked within the red polygon on Figure 2.2, as calculated using the four nearest MeteoSwiss ANETZ weather station 10-minute (red circles and thick dashed red line) and 30-minute (black asterix and thin solid black line) wind vector data. Values below zero indicate that convergence is taking place, values above zero values indicate that divergence is occurring within the designated area. (Third panel): Number of distant (3-30km) and near (0-3km) and Cloud-to-Ground (CG) lightning strikes recorded by the ANETZ weather stations within each 10-minute time period. Note the occurrence of lightning just after the time of maximum convergence and during the period of greatest water vapour concentration. (Fourth [bottom] panel): Mean precipitation rate (mm/hr) for the same ANETZ weather stations located within the box for the same time period. Note the occurrence of precipitation later in the afternoon when the IWV mass starts to decrease, divergence occurs and after the peak in lightning activity. 20

21 Figure 3.12: (Top panel): Hourly values of GPS IWV at Saanen (solid black line) and Payerne (dashed blue line) GPS receiving stations for 20 June Note the strong increase in Saanen values (by up to 50%), with coincident decrease in Payerne values. Note also the anti-correlation of the Payerne series with the bottom panel. (Bottom panel): Divergence / Convergence : same as second-panel of Figure 3.11, shown here for comparative reasons. 21