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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
thus vary very rapidly inside of the cumulonimbus while crossing its "tower":
variations of more than 70 km/h in the space of 2 km have sometimes been
observed. This intense turbulence can occur at the flight level of airliners and
constitute a danger for them.
The conditions that are the most favourable to icing (presence of super-cooled
water) are generally located in the lower central part of the cumulonimbus
"tower", in an altitude range where the temperatures are between 0 and
-25 °C. However, the icing conditions can persist down to -40 °C or less, that’s
to say up to around flight level FL350, but ice crystals are encountered at this
altitude.
Electrical activity can be strong, with the possibility of the appearance of
lightning in the phases of growth or maturity of a cumulonimbus, at any
altitude. Lightning can appear between the cloud and the ground, within a
cloud or between two clouds, but observation by scientific satellites shows
that it is less frequent over the sea than on land.
Cumulonimbus can group together to form storm clusters or convective
clusters. The diagrams hereafter illustrate this type of situation.
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Figure 7: conceptual diagram of the formation of storm clusters (convective)
In the ITCZ, the cruise flight level of airliners is located below the altitude of
the "anvils" of the cumulonimbus.
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2.2.2 Advantages and limitations of geostationary infrared imagery
The infrared images taken by geostationary satellites make up, over the
tropical Atlantic, the main source of information to appreciate the evolution
of the storm systems in time and space.
In fact, the images taken every fifteen minutes by Météosat 9 in the infrared
ray spectrum, at 10.8 μm, with a resolution of the order of 3 km over the
zone, allow an evaluation(1) of the temperature of the top of the
clouds from which can be deduced the altitude of the top. Since
the more powerful a cumulonimbus is, the higher the altitude of its
top is, and the colder its temperature at the top is, infrared imagery
makes it possible to a certain degree to appreciate the strength of
the cumulonimbus, and to characterise its special extension, the
structure and the evolution of the cluster that they form. Imagery
can also allow identification of the most notable cumulonimbus,
through the thermal signature of the "overshoot" phenomena that
are associated with them.
However, infrared imagery has great limitations: the satellite only observes
a dense cloud mass from above, which can result in the juxtaposition of the
"anvils" of several cumulonimbus organised in a cluster. Consequently, these
images do not allow direct observation of the meteorological conditions
at the level of flight located under the cumulonimbus anvils, in the ITCZ.
Specifically, the "towers" of cumulonimbus in a rapid growth phase that can
produce the most dangerous turbulence can be masked by the anvils of older
clouds. However, after their phase of rapid growth, the tops of the strongest
cumulonimbus reach the altitude of the tropopause and exceed it temporarily,
so that they can generally be identified, later, by infrared imagery thanks to
the local very cold signature of their tops.
The complete Météosat 9 images are timed every quarter of an hour at h, h+15,
h+30 and h+45. The precise timing of a point of each of the images must
take into account the time required for the satellite (12.5 minutes) to scan
the terrestrial disk from the South to the North and thus produce a complete
image. Thus, the equatorial part of the image is observed in the middle of the
scan sequence, or around 8 minutes earlier than the timing recorded for the
complete image, h+7, h+22 and h+37, h+52.
2.2.3 Analysis of the storm activity in the ITCZ over four days (from 30 May to
3 June 2009)
At the level of the equatorial Atlantic, the fully developed tops of the
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Interim report on the accident on 1st June 2009 to the Airbu(30)
