Download Monitoring the environment of a tornado producing storm by using

Mediterranean Storms
(Proceedings of the 4th EGS Plinius Conference held at Mallorca, Spain, October 2002)
 2003 by Universitat de les Illes Balears (Spain)
MONITORING THE ENVIRONMENT OF A TORNADO PRODUCING STORM BY USING DATA FROM
SATELLITE WATER VAPOUR CHANNEL AND SURFACE OBSERVATIONS
C. G. Georgiev
National Institute of Meteorology and Hydrology, Tsarigradsko shausee 66, 1784 Sofia, Bulgaria
e-mail: [email protected]
ABSTRACT
Meteosat water vapour (WV) imagery and data from synoptic surface observations are used to infer the potential
instability in a case study of a severe storm close to the Mediterranean coast. The use of the Potential Instability WV
Index (IWV) introduced in a previous study is discussed. The IWV is calculated using a combination of two different kinds
of data: synoptic station observations of surface moisture/temperature and HRI Meteosat WV data averaged in an area
of 7×7 pixels around the synoptic stations. It is shown that the WV index might serve for assessing potential instability
and providing early warnings in cases, in which the convection is initiated by lifting, which starts at the surface. It is
not useful if the lift comes from a synoptic-scale disturbance as mid- and upper-level fronts or jet maxima and the lowlevel moisture is not a significant ingredient of the storm environment.
1
INTRODUCTION
During the afternoon of 15 May 1999, a convective storm rapidly developed in a mountain region, close to the
Mediterranean coast. The case was associated with a tornado and hail over Zhaltusha village in the most southern part
of Bulgaria. The synoptic-scale mechanism for generation of the severe storm was studied in Simeonov & Georgiev
(2001). Using Meteosat water vapour (WV) channel imagery as well as data from surface observations, Georgiev
(2002) revealed some properties of the tornado environment and introduced a Potential Instability WV Index (IWV) to
serve as a measure of potential instability. There were difficulties in studying this case due to the lack of a proximity
sounding as well as because radar observations were not performed in Bulgaria on 15 May 1999.
The aim of this study is to investigate in more detail what was happening in the meso-scale and to discuss the
usefulness of IWV. Some thermodynamic features of the pre-convective boundary layer are considered in Section 2.
Surface data from synoptic observations and Meteosat WV imagery are used in Section 3 to assess the potential of the
approach in inferring the potential instability.
2
ANALYSIS OF MESO-SCALE THERMODYNAMIC LOW-LEVEL FEATURES
Figure 1 shows Meteosat WV image at about half an hour after the release of the tornado. The positions of six
synoptic stations, which are used as a source of data for studying meso-scale low-level thermodynamyc features in the
region of convection are indicated by numbers from 1 to 6. The tornado occurred over the hilly and wooded vicinity of
village Zhaltusha located between stations 2 and 3, at the white arrow in Figure 1.
Since the synoptic station altitude varies in a significant range (138−1923 m, see Georgiev, 2002), conservative
quantities are calculated from three hourly surface observations and used for thermodynamic description of boundary
layer of the pre-convective area.
− Potential temperature (θ), which is representative for the temperature of the air mass.
− Equivalent potential temperature (θe), which is used to represent the air mass moisture and temperature in a
single quantity.
A critical point in the severe convection close to station No 2 is the unusual high air temperature observed during the
night and early morning on 15 May 1999. Figure 2(a) shows potential temperature values at 0600 UTC in the period
from 13 to 18 May. The early morning θ was maximum on the day of tornadic event due to a strong
south−southwesterly flow of Mediterranean air. At the lower terrain stations (No 1, No 3 and No 6) the wind exhibited a
south component (see Georgiev, 2002) but it was much less strong in comparison with those at the highest-latitude
station No 2, while at No 4 and No 5, northern-component surface winds are observed.
Another important ingredient, which favoured the production of severe convection and tornado on 15 May 1999 was
moistening of the lower troposphere over this mountain region. This was due to supply of Mediterranean air as well as
to evaporation from the surface, which was wetted by precipitations (41 l/m2) which had fallen over station No 2 during
the afternoon and the night of 13 May. Due to its high temperature (Figure 2(a)) the Mediterranean air elevating at this
mountain area near station No 2 on 15 May had the potential to accumulate a much greater water vapour content and it
was extremely moistened. As a result, its equivalent potential temperature significantly increased from a value of 324 K
up to 338 K during the next 6 hour (0600 − 1200 UTC). As depicted in Figure 2(b) this increasing was much more
pronounced on the day of the tornadic event in comparison with the days just before and after that.
Figure 1. Meteosat image showing convective cloud systems on 15 May 1999 at 1500 UTC, about half an hour after the release of a
tornado at the white arrow between the indications 2 and 3. The numbers 1, 2, 3, 4, 5 and 6 indicate the location of the synoptic
stations (see Georgiev 2002) that are used as sources of data for the case study.
(a)
(b)
340
E q u iv a le n t P o te n tia l T e m p e ra tu re ( K )
o
o
P o te n tia l T e m p e ra tu re , u n s a tu ra te d ( K )
305
300
295
290
335
330
325
320
315
310
13
14
15
16
D ay of M ay 1999
17
18
13
14
15
16
17
18
D ay of M ay 1999
Figure 2. (a) Potential temperature θ for 0600 UTC and (b) Equivalent potential temperature θe for 0600 (dotted) and 1200 (solid)
UTC derived by using data from synoptic surface observations during the period 13–18 May 1999 at station No 2, located close to
the release point of tornado on 15 May (see Figure 1).
3
USE OF SURFACE OBSERVATIONS AND WV DATA TO INFER POTENTIAL INSTABILITY
The relation between Meteosat WV channel radiance and upper tropospheric relative humidity is well known
(Schmetz & Turpeinen, 1988). In order to help assessing potential instability that was responsible for producing
convective storm on 15 May 1999, surface data from synoptic stations (see section 2) are interpreted jointly with
Meteosat WV data. The method was presented and discussed in Georgiev (2002). This tornado-producing storm
developed as a result of formation of a potentially unstable air mass in which moist low-level air has been capped by a
deep dry layer. After proximity soundings are not available, such a kind of potential instability might be easily
identified by using the Potential Instability WV Index, which was proposed in Georgiev (2002):
IWV = (θ + θ e + TWV ) 3
(1)
where θ is the potential temperature for unsaturated moist air parcel, θe − the equivalent potential temperature, TWV –
the averaged WV brightness temperature over 7×7 pixel segments centred at the pixel of the synoptic stations at which
θ and θe are derived, all temperatures are expressed in °C. The values of IWV at the time of hourly WV images are
calculated by applying a linear interpolation of surface data between the 3 hourly periods of synoptic observations.
(a)
(b)
310
o
20
23 6
23 2
15
22 8
10
22 4
13
14
15
16
D ay of M ay 1999
17
18
o
P o te n tia l T e m p e ra tu re , u n s a tu ra te d ( K )
24 0
W V B rig h tn e s s te m p e ra tu re ( K , d o tte d )
24 4
o
W V P o te n tia l In s ta b ility In d e x ( C , s o lid )
25
305
300
295
290
1
2
3
4
5
6
S y n o tic S ta tio n
Figure 3. (a) Potential instability WV index (solid, left axis) and Meteosat WV channel brightness temperatures averaged in a 7×7
pixel segment (dotted, right axis) for 1500 UTC during the period 13–18 May 1999 at station No 2. (b) Potential temperature derived
from data of surface observations for 1500 UTC on 15 May at the synoptic stations indicated 1, 2, 3, 4, 5 and 6 in Figure 1.
The tornado occurred at the downstream (as regards to the mid-level flow) area of station No 2 on 15 May 1999.
Figure 3(b) depicts the changes of WV index IWV at this station from 13 to 18 May as well as of its third additive
component TWV in the expression (1). During this period, the index exhibits its maximum on the day of tornadic event.
This result may serve as a sign that the stratification over the region was potentially unstable since very dry air masses
lay above warm and moist air of Mediterranean origin. It is well known, however, that in cases of potential instability,
convection is only initiated when air is lifted through the negative buoyant layer, if present, to the level of free
convection (see Kerkmann, 1998).
To have an idea for the lifting mechanism, Figure 3(b) shows the surface potential temperature at the six synoptic
stations at 1500 UTC. It reveals the sharp θ maximum at station No 2 that was seen also at 0900 and 1200 UTC
(Georgiev, 2002). This means that the most favourable conditions for initiation of deep convection were present over
the adjacent to station No 2 area. Really, No 2 is the nearest station to the most severe covective cell, which produced
the tornadic event and dry hailfall, of about 5 cm hailstone size (Simeonov & Georgiev, 2001). The lifting necessary for
triggering the convection at the high altitude station No 2 came from warm advection, differential heating and
interaction of the flow with topography of the Rhodopes mountain. The extremely severe weather was associated with
the southern most convective cell (Figure 1) where the supply of moist and warm Mediterranean air was significant.
The northern convective cells developed after lifting associated with terrain produced elevating of Balkan mountain as
well as because of convergence of the surface flow along the stations No 5 and No 6 where the wind direction was
changed from north-eats to south-east (see Georgiev, 2002).
Another possible mechanism for lifting may be associated with upper-level synoptic scale disturbances (Kerkmann,
1998). Such a case is presented in Figure 4, where WV images depict the convective system marked ‘E’, which
produced the precipitation at station No 2 in the night of 13 May 1999. Being associated with a mid-to upper level
front/jet (arrowed in white on Figure 4), the convecrtive development at 13 May seems to be the case in which the
lifting responsible for convection does not start close to the surface, but initiates above the boundary layer. At 1500
UTC, the WV channel radiance at station No 2 was maximum on 13 May (Figure 3(a)), associated with the existence of
a dark zone ‘D’ of dry air aloft on the polar side of the jet maximum and over the upstream area of the developing
convective cloud system ‘E’. However, the WV index exhibits low values at 1500 UTC on 13 May, due to the low
values of θe and θ (i.e. because the low level moisture is not a significant ingredient). A similar situation of low-level
stability is considered in (Colman, 1990) that is due to the effect of the cold boundary layer, which is rising to levels
above 850 hPa and does not represent the air above the frontal inversion. Therefore, since the WV index (1) uses
thermodynamic parameters from the boundary layer, it might be a measure of potential instability only for cases in
which the convection is initiated by lifting, which starts at the surface.
(a)
(b)
Figure 4. Meteosat WV images for 13 May 1999 at (a) 1400 UTC and (b) 1800 UTC. The transition area of light to dark WV image
shades (arrowed in white) indicates an upper-level front/jet. Also marked in (a), ‘D’ a dark zone of dry air aloft on the polar side of
the jet maximum and ‘E’ the convective cloud system, which produced precipitations on 13 May 1999.
4
CONCLUSIONS
Analysis of Figures 1 and 6 shows (see also Georgiev, 2002) that convective clouds developed over the upstream
areas of darkening processes in the water vapour imagery. The convective cell appeared close to station No 2 on 15
May (Figure 1) associated with a characteristic dark feature on its south-southwestern edge and produces extremely
severe weather events. As shown in Section 3 the Potential Instability WV index (IWV) defined in Georgiev (2002) might
serve as a measure of potential instability, in which the convection is initiated by lifting, that starts at the surface.
In the case on 13 May (Figure 4) however, it seems that the lifting responsible for convection did not start close to
the surface, but initiated above the boundary layer. For that reason, the WV index exhibits low values prior to this
convective weather event. Since IWV uses thermodynamic parameters from the boundary layer, it should not be useful if
the lift comes from a synoptic-scale disturbance as mid- and upper-level fronts, jet maxima, etc.
The proposed approach of using data from satellite WV channel and surface observations can be useful as an
analytical tool for monitoring pre-convection situations and convective activity in cases of lack of proximity soundings
for severe storm events. The ability of presented technique to provide early warning in operational forecasting of severe
weather is a subject for further investigations.
Acknowledgements. The Meteosat HRI data using in this study was kindly provided to the National Institute of
Meteorology and Hydrology of Bulgaria by EUMETSAT.
REFERENCES
Colman, B. R. Thunderstorms above frontal surfaces in environment without positive CAPE. Part II: organisation and
stability mechanisms. Mon. Wea. Rev., 1990, 118, 1123–1144.
Simeonov, P. & Georgiev, C. G. A case study of tornado-producing storm south of Rodopes mountain in the Eastern
Mediterranean. Atmos. Res., 2001, 57, 187−199.
Georgiev, C.G. Use of data from Meteosat water vapour channel and surface observations for studying pre-convective
environment of a tornado-producing storm. Submitted for publication in the special issue of Atmospheric Research
on the European Conference on Severe Storms, Prague, 26 - 30 August 2002.
Kerkmann, J. Instability indices retrived from satellite data. In SAF Training Wirkshop on Nowcasting and Very Short
Range Forecasting, (Madrid, 9-11 December 1998), EUM P 25, EUMETSAT, Darmstadt, Germany, 1998, 136147.
Schmetz, J. & Turpeinen, O. M. Estimation of the upper tropospheric relative humidity field from Meteosat water
vapour image data, J. Appl. Meteor., 1988, 27, 889-899.