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1 hPa for 1,000 km, which is in accordance with the usual values from the
climatology point of view in the Inter-tropical Convergence Zone (ITCZ). The
wind field overlaid on the sea level pressures (see Figure 1) confirms the
absence of synoptic dynamism for the whole zone between 0 and 10 degrees
North inclusive.
Figure 1: analysis of the wind at 10m and of the pressure
at sea level on 01/06/2009 at 0 h 00 (ARPEGE model)
However, even in the presence of relatively low winds, the ITCZ, which
corresponds to the "meteorological equator", is the area of a convergence,
that occurs in the first 1,000 metres of the atmosphere, between the northern
hemisphere trade winds blowing from the north-east and the trade winds
of the southern hemisphere blowing from the south east. This convergence
generates updraughts that, in an unstable atmosphere, favour the development
of powerful cumulonimbus fed with water vapour by exchanges with the
ocean surface.
The global models do not make it possible to directly perceive these stormy
phenomena and the associated small scale structures, whose life cycle lasts a
few hours.
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77
1.2 Wind field of lower and mid-altitude layers
Analysis of the wind fi elds on the ARPEGE model (see Figure 2) up to the surface
isobar 500 hPa (corresponding to around FL180) shows that the winds encountered
in the zone were weak – less than 20 knots - and from east to south-east sector.
Figure 2: analyses of the wind fi eld
at 1 000, 850, 700 and 500 hPa on 01/06/2009 at 0 h 00 (ARPEGE model)
1.3 Wind field at altitude
Above FL180, the winds were from north sector and remained weak, less
than 20 knots between 10° south and 10° north (see the charts below for
FL300, 340 and 390).
Figure 3: analysis of the wind fi eld
at 300 hPa (FL300) on 01/06/2009 (ARPEGE model)
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Figure 4 : analysis of the wind fi eld
at 250 hPa (FL340) and 200 hPa (FL390) on 01/06/2009 (ARPEGE model)
1.4 Evaluation of the tropopause
The temperature and height fields (expressed as flight level) of the
tropopause analysed by the ARPEGE model show that, in the zone west
of the equatorial Atlantic, the temperature of the tropopause was of
the order of -80 °C and that its altitude was close to FL520 (figure 5).
Figure 5: analysis of the temperature fi eld and height (expressed as fl ight level) of the
tropopause on 1st June 2009 at 0 h 00 (ARPEGE model)
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2. Description of the inter-tropical convergence zone
on 1st June 2009
2.1 Position of the inter-tropical convergence zone
Figure 6 shows the superimposition of the infrared Météosat 9 image dated
1st June 2009 at 2 h 15 and the climatological positions of the ITCZ. The
coloured lines correspond to the average positions of the ITCZ in winter and
in boreal summer, the latter extending each side of these lines, in a more or
less regular manner.
This figure shows that the part of the ITCZ between Brazil and the western
Atlantic corresponded to an inter-season position.
Figure 6: Infrared Météosat 9 image dated 01/06/2009 at 2 h 15
and average climatology positions of the Inter-tropical Convergence Zone
2.2 Storm activity in the ITCZ characterised by Meteosat 9 infrared
imagery
2.2.1 Phenomenology: cumulonimbus and associated phenomena, storm
cluster
The vertical development of the cumulonimbus is generally limited by the
tropopause, whose altitude is between 15 and 18 km in the ITCZ. When the
top of a cumulonimbus reaches the altitude of the tropopause, in its phase of
maturity, the upper part of the cloud extends horizontally at the level of the
tropopause to form "anvils" that then overhang the "tower" of the cloud.
The air that feeds a cumulonimbus spreads and cools when climbing, and at a
certain altitude level, when the top of the cloud approaches of the tropopause,
it becomes colder than its environment, and is subject to downwards
buoyancy that slows then stops its vertical development. Through inertia, the
powerful clouds penetrate temporarily beyond the tropopause and their tops
are then much colder than their environment: this phenomenon is known
as "overshoot", visible on the infrared images, and it makes it possible to
characterise most clouds.
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The strongest vertical movements are observed in the "tower" of the
cumulonimbus in its phase of rapid growth, that is to say before the top
reaches the tropopause and the anvil is formed. The upward speeds can then
reach 110 km/h and the downward speeds 50 km/h. The vertical speed can
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