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ANALYSIS OF A STRONG SNOWSTORM IN CATALONIA ON
DECEMBER 2001 Ramón Pascual (1), Alfons Callado
(1), Marc Berenguer (2) Instituto Nacional de
Meteorología, Barcelona, Spain (1) Grup de
Recerca Aplicada en Hidrometeorologia,
Universitat Politècnica de Catalunya, Barcelona
Spain (2)
INTRODUCTION A very complex and unusual
winter precipitation event occurred in 14-15
December 2001 in Catalonia (Fig. 1). Snowfall
accumulations of more than 20 cm were measured in
many places, within which smaller pockets of
accumulation in excess of 50-70 cm were measured
in central Catalonia (Fig. 2 Fig. 3 Fig. 4).
The event, 40 hours long, was mainly
characterized by a wide stratiform precipitation
system, associated to a deep cold low centered
west of Catalonia, and by a very persistent
northeasterly cold advection at low levels.
Synoptic pattern moved slowly, determining a
quasi-stationary precipitation pattern. The
important snowfall accumulation was consequence
of both a high/moderate snowfall intensity and
its persistence (see Doswell et al. 1996). At the
beginning of the event the interaction between
synoptic low-level northerly flow and the
east-west oriented Pyrenees range resulted in the
Tramontane-Mestral mesoscale winds system which
determined a well established Catalonian-Balearic
boundary layer Convergence-Zone (CBCZ) (Pascual
and Callado 2002). The synoptic forcing
associated to the upper-level disturbance
reinforced the upward motion associated to the
low-level convergence and rapidly transformed the
previous cloudiness field in a precipitation
field.
Fig. 1. Catalonia situation at western Europe
Fig. 4. Spatial distribution of event total
precipitation in mm (contour interval 10 mm).
INM raingages network.

• IMPACT
• Six fatalities.
• Hazardous road conditions.
• Hundreds of cars cut-off by the snow.
• Electrical supply severely affected.
• School closure.
• Villages cut-off by the snow.
• policy storm.

Fig. 2. Orographic map of Catalonia indicating
main features and Tramontane-Mestral system. Red
square Area of maximum snowfall accumulation.
Fig. 3. Collbató (388 m amsl, near Barcelona
city) covered by snow. (Top photo Jordi Gubern,
bottom photo Miquel Soro).
SYNOPTIC AND MESOSCALE ANALYSIS The synoptic
cold low and the mesoscale features reinforced
together for the development and maintenance of a
long-lived snowfall system.

• ESTRUCTURE AND EVOLUTION
• 0624 UTC-1310 UTC
• A very rapid development of a nearly circular
  stratiform and almost stationary precipitation
  area was identified (Fig. 8 Fig. 11 Fig. 12)
• 1310 UTC-2200 UTC
• Development of a southeast-northwest-oriented
  enhanced reflectivity band with a western end
  clearly defined. The band, 200 km long and wide
  varying from 50 km to 100 km, persisted during
  almost all this period, gradually changing its
  orientation to east-west but without any
  significant displacement of its center (Fig. 9
  Fig. 11).
• 2200 UTC-2354 UTC 15 th
• A continuous development of precipitation cells
  defining a broken precipitation pattern. We call
  this period the cellular stage (Fig. 10 Fig.
  11).

S Y N O P T I C S C A L E

• Retrograde cold low moving from central Europe to
  northeastern Iberian Peninsula
• T500 ? -36 ºC
• T300 ? -38 ºC (TPP sinking)

• Stationary cold low over northeastern Iberian
  Peninsula
• T500 ? from 36 ºC to 32 ºC
• T300 ? from 38ºC to 44ºC

a

• 700 hPa
• High baroclinity
• High temperature gradient (-20 ºC/ km)
• Northerly flow

• 700 hPa
• Low baroclinity
• Any temperature gradient over Catalonia
• Easterly flow

Fig. 8. Day 14/12/01. Reflectivity PPI0. 1210
UTC. Development stage.

• Low levels (850, 900 hPa and surface)
• Approaching of the secondary low to Catalonia
• North-northeasterly warm advection with
  increasing wind velocity (65 km/h)
• Moderate instability

b

• Low levels (850, 900 hPa and surface)
• Abroad low over western Mediterranean
• Development of a secondary low easter Catalonia
• North-northeasterly cold advection with
  moderate-strong winds

b
Fig. 5. MM5 forecast valid at 1800 14th.
Geopotencial height and temperature. (a) 500
hPa. (b) 850 hPa

• Moist Low Level Jet (Fig.)
• 60 km/h / Easterly (100º) / 90 humidity
• Thickness 300 m

Orographic Pyrenean low with anticyclonic vortex
M E S O S C A L E
a

• CBCZ (Fig. 2 Fig. 6)
• Northerly wind
• (Tramontane, 90 km/h Beárn Cape)
• Dry northwesterly wind (Mestral, 47 km/h
  Vandellòs)

Fig. 9. Day 14/12/01. Reflectivity PPI0. 1830
UTC. Snowband.

• Fig. 7. Satellite images.
• Meteosat IR, 1200 14th.
• NOAA 12, Channel 4, 0542 15th.

Fig. 6. MM5 analysis valid at 0900 14th. MSL
pressure wind at 10 m temperature at 900 hPa.
Fig. 13. Schematic showing the precipitation
patterns evolution. Black line stationary
precipitation area. (First stage). Blue line
Mesoscale snowband. Blue arrows Movement
direction of precipitation elements (X)
embedded in the band train effect. Red arrows
Rotation of the band (Second stage). Yellow
elements Shallow convective cells and cell
movement (third stage).
Fig. 10. Day 15/12/01. Reflectivity PPI0. 1250
UTC. Cellular stage.
Fig. 11. 2-hourly time-average reflectivity
fields, from 0804 UTC 14th to 2204 UTC 15th
(left to right and top to bottom).
WIND FIELD

• 14th Tramontane-Mestral wind system.
  Catalonian-Balearic boundary layer Convergence
  Zone. Anticyclonic vortex generating convergence
  in central Catalonia ((Fig. Fig. 3).
• 14th Easterly-southeasterly low level jet (LLJ)
  above central Catalonia (Fig. Fig.).
• 15th Reinforced north-easterly wind at low and
  middle levels (Fig.).

Fig. Hodograph and wind data for Barcelona at
1200 UTC 14th. Notice de LLJ (Red square).
Courtesy of Univer-sitat de Barcelona.
Fig. 12. Time evolution of the area covered by
radar echoes with reflectivity of more than 5 dBZ
(left), 12 dBZ (center) and 25 dBZ (right).
Notice the fast development of the precipitation
area at the beginning of the event.
Fig. 14. Persistence of reflectivity values above
5 dBZ (left), 12 dBZ (center) and 25 dBZ (right).
Contour interval 6 hours. First line depicted 6
hours.

• PERSISTENCE
• Duration of snowfall associated to
• (Fig. 13 Fig. 14)
• Quasy-stationary system (linked to a nearly
  stationary synoptic-mesoscale disturbance).
• Large precipitation area.
• Train effect (along the band and cellular).

Fig. Time-space averaged Doppler wind fields for
different time intervals (All times UTC). 06 - 12
14th (left) Convergence area in central
Catalonia, 12 14th - 00 15th (center)
Convergence area in central Catalonia and
reinforced easterly flux, 00 15th 00 16th
(right) reinforced north-easterly flux.
Fig. Time-space averaged Doppler wind fields for
different time intervals for 14th (All times
UTC). 12 - 13 (left), 13 - 14 (center), 14 - 15
(right). The south-eastern low level jet can be
observed in this 3 hours long period.

• CONCLUSIONS
• ALGUNA COSA SOBRE SITUACIÓ SINÒPTICA I MESOSCALAR
  (Alfons)
• The retrograde movement of the synoptic
  disturbance determined a quasi-stationary
  precipitation system.
• Inner kinematics and spatial structure leaded to
  a long duration precipitation system.
• Mesoscale circulation at low and middle levels
  and circulations associated to the transition
  zone supported moisture convergence and the
  upward motion.
• 3D reflectivity and radial wind fields have been
  essential in order to understand the snow event.
• ALGUNA COSA SOBRE MÈTODES DACUMULACIÓ I
  COMPARACIÓ ENTRE LES DUES IMATGES (MARC)

COMPARISON OF 2 ACCUMULATION TECHNIQUES

• b) SOFTWARE-CORRECTED RADAR SCANS
• Ground-echoes contamination identification
  substitution (Sánchez-Diezma et al., 2001).
• Mountain screening effects
• Signal instabilities (Sempere-Torres et al.,
  2003).

• a) HARDWARE-CORRECTED RADAR SCANS
• Ground-echoes contamination identification from
  Doppler spectrum vertical substitution.

Accumulation procedure simple sum of measured
precipitation fields.
Accumulation procedure displacing the
precipitation field according to an estimated
mean motion velocity and supposing linear
evolution of intensities between consecutive
radar scans.
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Holle, 2001. Multiscale structure and evolution
of an Oklahoma winter precipitation event. Mon.
Wea. Rev., 129, 486-501. ACKNOWLEDGEMENTS This
work has been done in the frame of the RD CICYT
project REN2000-17755-C01. Thanks are due to the
Spanish Instituto Nacional de Meteorología for
providing data and support.