A Mediterranean nocturnal heavy rainfall and tornadic

Transcription

ATMOS-02354; No of Pages 17
Atmospheric Research xxx (2011) xxx–xxx
Contents lists available at ScienceDirect
Atmospheric Research
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a t m o s
A Mediterranean nocturnal heavy rainfall and tornadic event. Part I:
Overview, damage survey and radar analysis
Joan Bech a,⁎, Nicolau Pineda a, Tomeu Rigo a, Montserrat Aran a, Jéssica Amaro a, Miquel Gayà b,
Joan Arús c, Joan Montanyà d, Oscar van der Velde d
a
b
c
d
Meteorological Service of Catalonia, Berlín 38, Barcelona E-08029, Spain
AEMET, DT Balears, Palma de Mallorca, Spain
AEMET, DT Catalunya, Barcelona, Spain
Electrical Engineering Dep., Universitat Politècnica de Catalunya, Terrassa, Spain
a r t i c l e
i n f o
Article history:
Received 16 February 2010
Received in revised form 5 November 2010
Accepted 14 December 2010
Available online xxxx
Keywords:
Tornado
Downburst
Catalonia
Heavy rainfall
Doppler radar
a b s t r a c t
This study presents an analysis of a severe weather case that took place during the early
morning of the 2nd of November 2008, when intense convective activity associated with a
rapidly evolving low pressure system affected the southern coast of Catalonia (NE Spain). The
synoptic framework was dominated by an upper level trough and an associated cold front
extending from Gibraltar along the Mediterranean coast of the Iberian Peninsula to SE France,
which moved north-eastward. South easterly winds in the north of the Balearic Islands and the
coast of Catalonia favoured high values of 0–3 km storm relative helicity which combined with
moderate MLCAPE values and high shear favoured the conditions for organized convection. A
number of multicell storms and others exhibiting supercell features, as indicated by Doppler
radar observations, clustered later in a mesoscale convective system, and moved northeastwards across Catalonia. They produced ground-level strong damaging wind gusts, an F2
tornado, hail and heavy rainfall. Total lightning activity (intra-cloud and cloud to ground
flashes) was also relevant, exhibiting several classical features such as a sudden increased rate
before ground level severe damage, as discussed in a companion study. Remarkable surface
observations of this event include 24 h precipitation accumulations exceeding 100 mm in four
different observatories and 30 minute rainfall amounts up to 40 mm which caused local flash
floods. As the convective system evolved northward later that day it also affected SE France
causing large hail, ground level damaging wind gusts and heavy rainfall.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The particular coastline shape and orographic configuration surrounding the Mediterranean basin favours the
concentration of cyclone activity —often associated with
hazardous weather— on some specific areas, such as the Gulf
of Genoa in the Western Mediterranean northern coast
(Alpert et al., 1990; Trigo et al., 1999). Jansà et al. (2001)
and Campins et al. (2007) studied the relationship between
Mediterranean cyclones and heavy rainfall and strong wind.
⁎ Corresponding author. Tel.: + 34 93 5676090.
E-mail address: [email protected] (J. Bech).
They demonstrated a clear link between cyclone centres
located between the Balearic Islands, and the coast of
Catalonia (NE Iberian Peninsula), and concurrent high-impact
weather effects over Catalonia. Strong maritime easterly
winds associated with deep low-pressure systems affecting
Catalonia were already examined during the first decades of
the twentieth century by the pioneer work of Fontserè (1929).
On the other hand, other specific studies evaluated the
conditions associated with local severe storms, including
tornadic thunderstorms, covering the Western Mediterranean
area (Romero et al., 2007; Tudurí and Ramis, 1997). According
to the tornado and waterspout climatologies in Spain reported
by Gayà (2005, 2009), the Balearic Islands and, particularly,
0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2010.12.024
Please cite this article as: Bech, J., et al., A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview, damage
survey and radar analysis, Atmos. Res. (2011), doi:10.1016/j.atmosres.2010.12.024
2
J. Bech et al. / Atmospheric Research xxx (2011) xxx–xxx
Catalonia are the areas where these phenomena occur more
often. Based on a 15-year average over a 30,000 km2 area,
typically one tornado per year not exceeding F2 damage in the
Fujita scale (Fujita, 1981), is likely to occur in central or
southern coast of Catalonia. This is in reasonable agreement
with previous general observations and estimates such as
those reported in Dotzek (2003). A few tornadic case studies
in Catalonia have been reported by Ramis et al. (1997), Bech et
al. (2007), Aran et al. (2009), and Mateo et al. (2009).
The objective of this work is to describe, document and
discuss the event that took place on the first hours of the 2nd
of November 2008 when two severe storms associated with
an intense low-pressure system affected the southern coast
of Catalonia. The thunderstorms produced local flash floods
and damaging surface winds, with maximum damage
estimated up to F2 in the Fujita scale, in a general context
of intense regional wind caused by the synoptic conditions.
The study is broken into two companion papers. This is the
first one, which gives an overview, a description of the
damage, and radar analysis of the event. In the second
part (Pineda et al., this issue) a detailed analysis of total
lightning observations along the thunderstorms life cycle is
performed.
The organization of this paper is as follows. In Section 2 a
description of the synoptic and mesoscale framework of the
event is given, using Numerical Weather Prediction (NWP)
analysis and forecasts, radiosonde, Meteosat Second Generation (MSG) satellite and radar and lightning observations.
Section 3 describes the damage survey of the affected areas
and details the thunderstorm analysis with Doppler radar
observations. Section 4 provides a general discussion and
Section 5 presents a summary and concluding remarks.
2. Synoptic and mesoscale framework
2.1. General setting
This description is based on 0.5° resolution Global Forecast
System (GFS) model output provided by the US NOAA
Environmental Modeling Center (EMC, 2003). From these
data a suite of specific maps to support storm convective
forecasts over Europe are routinely produced in the framework of the European Storm Forecast Experiment (ESTOFEX,
http://estofex.org). Daily updated maps are publicly available
at http://www.lightningwizard.com/maps; see van der Velde
(2007) for a detailed description. A customized version of
those maps is used here.
The synoptic framework on SW Europe on the 2nd of
November 2008 was dominated by a highly baroclinic situation,
with an extended upper level trough and an associated surface
frontal system extending from Southern Spain along the
Mediterranean coast of the Iberian Peninsula to SE France,
which moved north-eastward. A low pressure area in the
eastern coast of the Iberian Peninsula intensified from 00 to 06
UTC. Some instability in the form of Mixed Layer Convective
Available Energy (MLCAPE) of 200–500 J kg–1 was present over
the sea after 00Z (Fig. 1). The mixed layer considered in
the MLCAPE field of Fig. 1 is the lowest 1000 m above ground
level.
Since the winds were rather strong and from easterly
directions at the surface and southerly directions in mid
Fig. 1. GFS analysis and forecasts valid at 2 November 2008 00, 03, and 06
UTC (a, b, and c respectively) showing mean sea level pressure contours
(black lines, hPa), 500 hPa geopotential heights (coloured contours, gpdam),
and mixed-layer convective available energy, MLCAPE (see shaded colours
legend, in J kg–1. The position of Catalonia (NE Spain) is highlighted with a
dashed-line red rectangle in panel b. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article.)
levels, due to strong warm air advection, large vertical wind
shear was present over the coast of Catalonia. Storm-relative
environmental helicity (SREH) in the layer 0–3 km, considering storm motion given by the internal dynamics vector
method described in Bunkers et al. (2000), was around
200 m2 s− 2 at 00Z (Fig. 2), increasing to 250–300 m2 s− 2 at
03Z over north-eastern Spain. These values are supportive of
generation of rotation in storm updrafts (i.e., potential for
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supercells) when instability is present (Davies-Jones, 1984;
Davies-Jones et al., 1990). The maps also display the supercell
composite parameter (SCP, Thompson et al., 2004) which
shows where MLCAPE, deep-layer (bulk Richardson) shear,
and SREH are present together. In particular, SCP equals to the
product of those three magnitudes divided by the following
reference values: 1000, 40, and 150, for MLCAPE, 0–6 km bulk
Richardson shear, and 0–3 km SREH, respectively. Fig. 2b
Fig. 2. GFS model sequence valid at 00, 03, and 06 UTC 2nd November 2008. Left column (panels a, b and c) shows 0–3 km storm-relative helicity (shaded colours,
legend in m2 s− 2), supercell composite parameters (Thompson et al., 2004) contour lines with values 0.1, 0.25, 0.5, 1 and 2, and internal dynamics storm motion
vectors (Bunkers et al., 2000). Right column (panels d, e and f) shows 0–1 km wet-bulb potential temperature (contour lines, in Celsius) and 10 m wind
streamlines and convergence (shown as streamline colours, with different colours for values − 7 − 6 − 5 − 4 − 3 − 2 − 1 1 2 3 4 5 6 7, in 10− 5 s− 1; red colours
indicate values above 6 × 10− 5 s− 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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shows the SCP values of 2, which are remarkable given the
low CAPE.
Over land (Fig. 3a, b, and c), 0–1 km shear vector magnitude
increased to more than 12 m s− 1 which together with the
instability and low lifting condensation level (LCL) heights
(around 500 m) can enhance potential for tornadoes (Rasmussen
and Blanchard, 1998; Thompson et al., 2003). LCL and LFC are
calculated from 0 to 1000 m mixed layer average parcel so LCL
cannot be lower than half this layer (500 m).
Deep layer shear vector length (defined here between 0
and 6 km) reached values of 20–25 m s− 1 (black contours),
which is a range commonly regarded to support supercell
Fig. 3. GFS model sequence at 00, 03, and 06 UTC 2nd November 2008. First row (panels a, b, and c) shows 0–6 km shear (black contour lines, in m s− 1), and 0–
1 km shear (shaded colours, m s− 1). Second row (panels d, e, and f) shows lifting condensation level (shaded colours, in m) and vectors qualitatively indicating the
difference between LFC and LCL: smaller means less capping according to LFC–LCL values: short arrows (b 500 m), thick arrows (N 1500 m), and red arrows
(b−300 m). Third row (panels g, h, and i) shows 0–2 km mean wind deep convergence (red contours, plotted at 2, 6, 12, 20, 20, 30 and 42, in 10− 5 s− 1), and Parcel
Layer Depth (shaded colours, m). Bottom row (panels j, k, and l) shows 0–2 km moisture transport stream lines (humidity times wind, in g kg–1 m s− 1, red colour
indicates high values, above 160 g kg–1 · m s− 1), 1000–600 hPa minimum relative humidity (shaded colours, %) and moisture upslope flux (in g kg–1 m s− 1,
contours ranging from light orange to dark pink are plotted at 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.4 1.7). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
5
formation (Rasmussen and Blanchard, 1998), and thus, one of
several indicators of an enhanced risk of significant severe
weather. Lifting condensation level was relatively low
(around 600 m) and differences between LCL and level of
free convection (LFC) indicated weak capping of most
unstable parcels (Fig. 3d, e, and f), therefore, due to the
limited capping, the shear layer was an effective shear layer.
The Parcel Layer Depth field (Fig. 3g, h, and i) is an
experimental parameter (van der Velde, 2007) which
indicates the summed depth of layers, not necessarily
continuous, up to 600 hPa from which a parcel has CAPE
N50 J kg–1 and CIN b50 J kg–1. In other words, this parameter
indicates, where over deeper layer parcels, are most easily
released, if lifted to their level of free convection due to
buoyancy (i.e. spontaneous ascent), not for other reasons
such as mechanical forcing. The Parcel Layer Depth field was
rather shallow in this case, less than 1000 m, locally 1500 m,
but it is a clear signal of the possibility of convection,
especially since it was combined with deep convergence of
the mean wind in the 0–2 km AGL layer (shown in red
contours in Fig. 3g, h, and i). As discussed later, the deep
convergence signal matched very well with the location and
timing of storm development.
The forecast indicated potential for large precipitation
amounts, given the precipitable water content values (around
30 mm) and saturated profiles, low LCL, large moisture
transport, and in particular the lifting of the moist and
unstable Mediterranean air-mass as it reached higher terrain.
This latter effect is visualized as orange contours in an
experimental field, the upslope moisture flux (Alpert, 1986)
(Fig. 3j, k, and l), computed according to Lin et al. (2001). It
indicates the upward flux of moisture by horizontal flow
forced over the terrain (at the GFS grid scale). It is shown to
increase during the period 00Z–06Z and highlighted also the
region in southern France where rain accumulation became
large during the day.
2.2. MSG, radiosonde, radar and surface observations
MSG satellite images from the Meteorological Service of
Catalonia (SMC) show an area of vigorous convective cloud
development in the eastern coast of Spain, favoured by the
surface low pressure development, the upper level trough
and the presence of associated series of jets: J1, J2, and J3 (as
depicted in Fig. 4). J1 was about 120 kts, and J2 about 105 kts
while J3 was 60 kts. J2 triggered convection via potential
instability at mid and high levels and was related to the
cyclogenesis shown in Fig. 1. The analysis of satellite loops
indicates the presence of several vorticity centres (marked as
red encircled Xs in Fig. 4). One of them moved along J2 over
the coast of Catalonia, where the severe weather took place.
Fig. 4 also shows the 500 hPa −25 °C isotherm indicating the
presence of a cold core which corresponds to the upper level
low shown in Fig. 1 (note that the 552 gpdam geopotential
curve in Fig. 1 matches approximately the −25 °C isotherm in
Fig. 4).
Radiosonde data from Barcelona and Palma de Mallorca
are shown in Fig. 5. The cold core at 500 hPa was about
−25 °C at 00 UTC (− 17 °C over Barcelona, according to the
SMC radiosonde data). CAPE values were null (assuming
parcel ascent from the surface) and low if calculated from
Fig. 4. a) 2nd November 2008 00 UTC MSG satellite water vapour image
(6.2 μm) showing the convective development area in the eastern part of the
Iberian Peninsula. Features indicated are: vorticity centres (encircled red
crosses), 300 hPa jets (blue arrows, named J1, J2 and J3), 500 hPa–25 °C area
(red dashed line closed contour), 500 hPa trough axis (red dashed
segments), and 500 hPa ridge axis (swirling red line). Radiosonde stations
of Barcelona and Palma de Mallorca are also shown with black crosses. b) 2nd
November 2008 03 UTC MSG satellite infra-red (HRIT) 10.8 μm image with
false colour brightness temperatures. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article.)
mean surface-layer or most-unstable layer (33 and 80 J kg–1
respectively). Accordingly, most instability indices related to
buoyancy did not indicate strong thunderstorm potential.
However, as seen in Fig. 5, the wind shear was remarkable.
Radiosonde-derived storm relative helicity (SREH) values
over Barcelona were 284 m2 s− 2 and 145 m2 s− 2 for 0–3 km
and 0–1 km SREH, respectively. Two previous tornadic cases
took place with similar radiosonde profiles, particularly at
low levels, and SREH 0–1 km values about 150 m2 s− 2 (Aran
et al., 2009; Mateo et al., 2009).
6
Fig. 5. Radiosonde data from Palma de Mallorca (red) and Barcelona (blue) at 00 UTC 2nd November 2008. The hodograph shown is from the Barcelona radiosonde
data. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Between 00 and 06 UTC sea level pressure dropped about
7 hPa in the southern part of Catalonia and even more in the
west of Catalonia, reaching 11.7 hPa (Raimat station), which
is a value associated with rapid cyclogenesis development
according to Carlson (1991). Moreover, other typical characteristics of these systems present in this case are: a) the
appearance of the low pressure system at the polar sector of a
jet (in this case over Morocco pointing towards the Balearic
Islands at 00 UTC); and b) a strong thermal boundary, which
was clearly appreciated in the analysis performed using the
SMC LAPS system (Albers et al., 1996) over Catalonia (not
shown here).
Fig. 6 gives an overview of precipitation distribution over
Catalonia showing 1-km CAPPI radar reflectivity factor
composites from the SMC Doppler radar network (Bech
et al., 2004). The sequence covers from 1:30Z to 4:00Z, when
the most intense convective activity, in terms of heavy
rainfall, lightning and surface wind gusts, took place. Black
arrows at 3:00Z and 3:30Z panels, respectively, point
approximately to the two main areas of damage at the time
of maximum surface wind damage. During that period
thunderstorms, mostly organized as single or multicell
storms, moved NW, coming from the sea. Afterwards (not
shown in Fig. 6), approximately at 4:30Z, they merged into a
larger linear MCS system, moving to the NE. As the system
evolved northward later that day it also affected SE France
causing large hail (diameter exceeding 2 cm), ground level
damaging wind gusts and heavy rainfall.
Total rainfall amounts were remarkable during this event,
exceeding 100 mm in 24 h in 4 stations of the SMC network, 2
of them located above 2500 m, in the Western Pyrenees of
Catalonia. Fig. 7a shows a 24 h precipitation analysis and
several areas exceeding 90 mm, one of them where the strong
damaging surface winds took place. Four stations on that area
recorded 30′ amounts exceeding 20 mm, and one reached
40 mm; as a consequence, several towns of that area suffered
local flash floods. Comparison of precipitation amounts with
radar reflectivity loops indicates that high rainfall rates in
relatively short time intervals were associated with the
individual thunderstorms that caused damaging winds;
however the largest rainfall quantities were due either to
the succession of different intense storms over the same area
(along the convergence area shown in Fig. 7b) or by mostly
multicell systems in high terrain areas in the Western
Pyrenees of Catalonia (Fig. 7a).
The radar hail diagnostic product operational at SMC
(Aran et al., 2007), based on comparing the freezing level
with height of radar reflectivity cores exceeding 45 dBZ
(Waldvogel et al., 1979), indicated moderate to high
probability of hail in the thunderstorms responsible for
damaging winds. Hail with diameter b2 cm was confirmed
on two stations in the vicinity of the damaged areas.
Fig. 7b shows the 10-m hourly averaged surface winds at 3
UTC. A clear convergence line (plotted as a yellow dashed
line) is approximately oriented along one of the maximum
precipitation areas. As discussed later, the most active
Fig. 6. Radar reflectivity factor 1-km CAPPI composite from the SMC radar network on 2nd November 2008. Black arrows at 3:00Z and 3:30Z point approximately
to the two main areas of damage at the time of maximum surface wind gusts.
7
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event a damage survey was performed by trained personnel
over these two zones to collect damage data and witness
reports. Because of the time of the day the event took place no
pictures or direct visual observation of the thunderstorm
(funnel, etc.) was possible.
3.1. Zone 1: Salou–Reus
According to local reports, the coastal city of Salou was
affected by strong winds about 03 UTC, only for a few
minutes; somewhat later Reus was also affected with less
intensity. Structural damage, rated as F0 to F1 in the Fujita
scale, was observed in several buildings, with many windows,
balconies and shades broken (see Fig. 9a). Some small areas
were also affected by flash floods. A camping area near Salou
was particularly damaged with many blown tents and a
tossed caravan (Fig. 9). A local sports centre was partially
destroyed and many trees fell, some of them on a number of
cars. In total, 5 people were injured in Zone 1. Only in Salou
there were 137 insurance claims (5 caused by flooding and
132 by wind) reaching 2 million euros. The power supply was
cut and railway transport was stopped for about 4 h, mainly
because of fallen trees. The analysis of the damage patterns
seems to indicate they were originated either by straight line
winds or by one or several microbursts (in this case would be
wet microbursts) — no evidence of tornadic origin was found.
3.2. Zone 2: Miralcamp
Fig. 7. Analysis of SMC surface observations: a) 2nd November 2008 24 h
precipitation analysis performed with 165 stations (values expressed in
mm); the green dotted rectangle indicates the wind damaged area containing Zone 1 and Zone 2 (see Fig. 8 for details). b) 2nd November 2008 3 UTC
10-m hourly averaged wind observations; the dashed line indicates
approximately the position of a convergence area. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
thunderstorms of the event followed and evolved along that
line. On the other hand, during the event strong winds were
recorded, particularly on the coastal area and in the Pyrenees
mountains. Three stations measured wind gusts exceeding
100 km h–1.
3. Damage survey
Two distinct zones, within the main area depicted in Fig. 8
suffered structural damage. The first one (Zone 1), mainly
urban and periurban, is highly populated and includes the city
of Salou and Reus, near Tarragona. The second one (Zone 2),
mostly rural, includes el Pla de Santa Maria, Cabra del Camp
and Sarral — between these two the small village of
Miralcamp is located (not shown in Fig. 8). Miralcamp was
particularly affected by surface damaging winds. After the
The damage on Zone 2 was focused mainly in the small
village of Miralcamp, and to a lesser extent, in surrounding
areas. Several high voltage power line towers were knocked
down (Fig. 10a) and many houses were badly damaged
(Fig. 10b), approximately at 3:30Z. The survey indicated
widespread F1 and also localised F2 damage, particularly in
forest areas (Fig. 10c). A specific analysis over a 1 km2 of
Miralcamp was performed to examine the damage path and
swath patterns. Location recorded with GPS, orientation of
fallen trees measured with compass, and damage intensity in
terms of the Fujita scale from the survey were plotted in a
georeferenced map (Fig. 11). More damage surrounding this
area was found to the S and N/NE, indicating a damage
path following approximately a S to N/NE direction. The
Miralcamp plot indicates predominant convergence patterns
along the path, more intense damage (up to F2) to the right of
the main track (consistent with a cyclonic rotating vortex
moving northwards), and also clear damage gradients, as
shown in Fig. 10c. These elements are similar to those found
in previous damage surveys of tornadic events performed by
some of the authors of this study as reported in Ramis et al.
(1997) and Bech et al. (2007, 2009). Though the number of
limited data in this case did not allow comparing in detail
observed damage with simulated path swaths on forests as in
Bech et al. (2009) or Beck and Dotzek (2010), the survey
analysis concluded that the most likely cause of the damage
was tornadic. The damage path was about 8 km long, with
maximum width about 400 m and maximum F2 damage.
These characteristics are consistent with statistical studies
relating tornado damage path, length and intensity (Brooks,
2004).
9
Fig. 8. Map showing the two zones affected by surface damaging winds: Zone 1 (Salou–Reus, Z1), and Zone 2 (Miralcamp, Sarral, Z2). This map corresponds
approximately to the green dotted rectangle shown in Fig. 7a. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
4. Thunderstorm analyses
In this section a radar analysis of the two thunderstorms
that caused the damage in Zone 1 and Zone 2 is presented
using data from the SMC. Pineda et al. (this issue) complement
this study with a second part with further lightning data and
processing. Radar and lightning data from the SMC were
combined in a thunderstorm tracking system to allow
characterization of precipitating convective structures (Rigo
et al., 2008, 2010). Within this system, radar data from
volumetric scans, such as 12 dBZ echo-tops, vertically integrated liquid or maximum radar reflectivity factor observed,
and total lightning (cloud-to-ground and intra-cloud flashes)
rates are processed. Single radar PPI observations from the
LMI radar, located at 0.86 °E 41.09 °N and 910 m asl are also
shown and discussed in this section.
A first analysis of the two thunderstorms that caused the
damage indicates that both were originated from a very
intense parent-thunderstorm (hereafter denoted as Cell 1;
subsequent systems are Cell 2 —affecting Salou— and Cell 3 —
affecting Miralcamp). Thunderstorm splitting is a rather
frequent mechanism of supercell formation in this region of
the Mediterranean Sea in autumn according to a 4 year
analysis of supercell thunderstorms in Spain by Quirantes
Calvo (2007), and could be responsible for the generation of
Cell 2 and Cell 3. Fig. 12 shows the evolution of selected
parameters of those cells and also the timing of the maximum
ground damaging winds observed as well as (bottom left) a
summary of the thunderstorms motion. Between 00:30 and
1:30 UTC, Cell 1 and 2 directions differed approximately by
45°. Then apparently Cell 1 died and Cell 3 appeared following
a nearly parallel direction to Cell 2, finally converging. This
10
Fig. 9. Damage observed in Zone 1. Pictures correspond to Salou, including a) damaged buildings; b) camping with tossed caravan, c) sports centre destroyed, and
d) cars affected by fallen trees.
Fig. 10. Damage observed in Zone 2 (Miralcamp area), including a) power line towers knocked down, b) houses badly damaged and, c) wide forest areas with
downed trees; the dashed line indicates a sharp gradient of damage.
11
Fig. 11. Depiction of the damage survey performed over the Miralcamp village, affected by F2 damage. Four towers of high voltage power lines (NW of the picture) —
see Fig. 10a— were knocked down during the event.
predominant direction (SE to NW) was followed by most
storms (not only the two severe thunderstorms) and is the
one indicated by the Bunkers et al. (2000) storm motion
vector (rightmover) in Fig. 2. A possible explanation of the
limited difference between the severe thunderstorms and the
other storm directions was that the background wind was
already high so deviations might have been relatively small.
A remarkable feature was the increase in the total
lightning rate before the onset of ground damage. Cell 1
presented high values of maximum radar reflectivity
(N50 dBZ), with a maximum of about 1 UTC (Fig. 12). At
that time the echo morphology of Cell 1 suggested the
presence of an inflow notch radar signature, while the
infrared cloud top image exhibited a clearly expanding anvil
(Fig. 13), indicating a strong updraft as the colder cloud tops
extend downwind from the thunderstorm core. Note that fine
detail of the radar echo morphology was hampered by the
distance to the LMI radar (about 100 km).
Additional radar observations corresponding to the time of
maximum ground damage caused by Cell 2 were further
examined with the aid of reflectivity and radial velocity PPI
scans from the LMI radar displaying base level (0.6°) (Fig. 14)
and 3.0° scans (Fig. 15). Note that LMI was only about 20 km
from Salou — though the radar site was on a mountain at
910 m asl. A strong reflectivity core (about 48 dBZ) crossed over
Salou and the Doppler pattern showed clear signs of strong
radial shear (see black arrow in Fig. 14). Over a horizontal plane,
the shear pattern would correspond to a strong convergence
signature. This could be compatible with the mid-level
convergence and low-level divergence (typically below 1 km)
associated with microbursts as described by Wilson et al.
(1984); the low-level divergence would not be seen by a radar
located on a 900 m mountain as in this case. An alternative
explanation of that shear pattern could be associated with a
mesocyclone rear flank downdraft. The convergence was
located nearest to the concave-shaped sharp reflectivity
gradient of the core in a way that may be typical of a convergent
base to a mesocyclone, or at least a deep convergent signature
along the interface of a steep gust front (e.g., Lemon and Parker,
1996). The presence of an echo overhang in Fig. 18, and its
persistent nature over 15 min suggests that this convergent
signature might have extended upward from the surface and
that it may have been a mesocyclone base.
Figs. 16 and 17 show a similar sequence covering the
3:30Z maximum ground damage observed at Miralcamp. In
this case the thunderstorm of interest (Cell 3) is about 50 km
12
Fig. 12. Evolution of selected radar and lightning-derived parameters of (thunderstorms) Cells 1, 2 and 3. See inset, bottom left, to locate the Cells (C#). The arrows
in Cell 2 and Cell 3 indicate the ground damage. Magnitudes shown in the time-series are: maximum heights of 12 dBZ radar reflectivity echoes (TOP-12, in km),
vertically integrated liquid (VIL, in mm), total lightning flash rate (TL/min, in min− 1) and maximum radar reflectivity (Zmax, in dBZ).
from the radar. An increase in reflectivity is observed, in
agreement with the increase in maximum reflectivity shown
in the Cell evolution earlier. In this thunderstorm the Doppler
radial velocity patterns are not as clear as before, though
strong shear is also present. At 3:18Z Cell 2 is shielding the
vision of Cell 3 (same line of sight from the radar) and given
that LMI operates on C-band, attenuation could be an issue —
note that this also would contribute to increased uncertainty
in radial velocity measurements. Cell 2 still presents a well
defined shear pattern; in fact for several higher elevation
scans (including the 3° scan shown) the pattern is slightly
rotated with regard to lower levels and looks much more as a
typical cyclonic velocity couplet (i.e. with azimuthal shear,
typical of mesocyclone circulations).
In Fig. 18 cross sections of Cells 2 and 3 are presented at
the time of maximum surface wind speed at Zones 1 and 2,
respectively. The tilting of the intense cores, and morphology
of cells resembles to some classical supercell features such as
the weak echo region or the forward overhang (Browning,
1964; Doswell and Burgess, 1993), suggesting the presence of
intense updrafts, in addition to the shear mentioned above
indicating organized rotation at midlevels.
13
Fig. 13. Left: Zmax LMI radar reflectivity at 1:00 UTC; the arrow points to Cell 1. Right: infrared MSG satellite image. Cell 1 cloud top exhibits an expanding anvil,
indicating strong updraft.
5. Summary and final remarks
This study has presented the first part of an analysis of the
2nd November 2008 severe weather event that took place on
the southern coastal part of Catalonia, including surface
damaging winds, hail, and also heavy rainfall and subsequent
flash floods. The second part of the study, devoted specifically
to total lightning analysis is presented in a companion paper
(Pineda et al., this issue). The synoptic and mesoscale
environment of the event was characterized by a relatively
low CAPE but moderately high shear environment, as some
previous cases in Catalonia and also typical of other severe
Fig. 14. Base level (0.6°) PPI scans from the LMI radar showing radar reflectivity (dBZ) (top row) and radial velocity (m s− 1) (bottom row) at the time of maximum
ground damage over Zone 1 (black arrow). Range circles are plotted at 20 km intervals. The 2:54Z radar reflectivity panel indicates the direction and range of the
corresponding cross section shown in Fig. 18.
14
Fig. 15. As Fig. 14 but for 3.0° PPI scans (note that there is approximately a 2 minute lag between base level and 3.0° scans).
weather events described in the cold-season in the UK
(Clarks, 2009); additional insight has been obtained from
examining a number of derived fields from the GFS NWP
model, including supercell composite parameter or upslope
moisture flux which provided good guidance about the
location and timing of severe weather and heavy rainfall
Fig. 16. Base level (0.6°) PPI scans from the LMI radar showing radar reflectivity (dBZ) (top row) and radial velocity (m s− 1) (bottom row) at the time of maximum
ground damage over Zone 2 (black arrow). Range circles are plotted at 20 km intervals. The 3:24Z radar reflectivity panel indicates the direction and range of the
corresponding cross section shown in Fig. 18.
15
Fig. 17. As Fig. 16 but for 3.0° PPI scans (note that there is approximately a 2 minute lag between base level and 3.0° scans).
Fig. 18. Radar cross-sections of reflectivity (dBZ) for Cell 2 (top panel, azimuth 90°, 40 km range) and Cell 3 (bottom panel, azimuth 56°, range 72 km) at the time
of maximum ground damage at Zones 1 and 2, respectively. Directions of each cross section over the horizontal plane are indicated in Figs. 16 and 14. Horizontal
and vertical axis units are in km.
16
respectively. The damage survey indicated two distinct areas
with maximum F1 and F2 damage in the Fujita scale,
attributed either to straight line winds or microburst in the
first case and to a tornado in the second one. The analysis of
the thunderstorms associated with the convectively induced
surface damaging winds indicated several supercell features,
including persistent weak echo regions and strong shear
suggesting the presence of a mesocyclone, as in the
downburst case study described by Dotzek et al. (2007).
Further analysis could be achieved by improving the
treatment of the Doppler data (dealising) or by automatic
processing of velocity fields to match predefined patterns, as
shown in Stumpf et al. (1998) or Elizaga et al. (2007). The
documentation and analysis of this type of severe weather
events in a specific region is important to contribute to the
identification of common features, such as a surface convergence line associated with the development and movement
of the thunderstorms, to increase our understanding about
the mechanisms of formation of these phenomena.
Acknowledgements
The authors are grateful to Salvador Castán for providing
insurance data, Eduard Marimon and Jordi Bruno for useful
comments and surface observations, to Joan Gené and other
individuals for their report on the event during the damage
surveys, and to J.A. Quirantes Calvo (AEMET) for his
comments about supercell statistics in the area of study.
Thanks are also due to the Spanish power line manager and to
several web-based mass media for providing information on
the event. Finally, we are also indebted to two anonymous
reviewers who contributed to improve the final form and
content of this paper with their constructive criticisms and
suggestions.
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A Mediterranean nocturnal heavy rainfall and tornadic

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