Authors
Meteorological and Hydrological Service of Croatia - DHMZ
Dunja Placko-Vrsnak
Natasa Strelec Mahovic
Dunja Drvar
Introduction
The case study will deal with the severe convective development within a cold front over Turkey.
On the 25th of May convective
development occurred ahead of the front in the warm sector, induced by orography. During
the 26th of May a distinct line of prefrontal convergence can be followed, as
well as the
convection over Eastern Turkey, far ahead of the front. The case is affirmed
by thunderstorm and hail reports (data is not available here).
The whole course of development and the onset of the convective events is discussed in detail with
help of the satellite and radar images as well as the relevant NWP
parameters.
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Meteosat 8 HRV : 26 May 2006, 12 UTC Click to enlarge.
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Lightning reports during 26 May 2006 Click to enlarge.
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The left image shows convective clouds over Western and Central Turkey - the biggest cells
correspond
to convective activity within the cold front. The line of smaller cells
belongs to prefrontal convergence. Convective clouds over Eastern Turkey are connected
to the upper level low. Reports of thunderstorms can be
seen in the right image - lightning occurred during the whole day on the 26th
of May. The size of the dots indicates the frequency of lighting
during 30 minutes period (the bigger the dot - more frequent the strikes during
the time period).
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Meteosat 8 RGB 01-03-09i - 26 May 2006 : 10 UTC Convective cells within cold front over Central Turkey.
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Combinations and differences of satellite channels enable a deeper
insight
into
the synoptic situation, structure of cloudiness and development
over some area, but also help us to see in more detail things that are interesting and unusual for
a particular
location.
In the image above a red spot in
mid-
Turkey
can be noticed. I
n this RGB combination yellow colour represents
low clouds or fog, bluish colour
displays
high thin cold clouds and
pinkish colour is connected to thick convective clouds. Red colour i
n the Meteosat 8 RGB 01-03-09i is usually used to detect snow cover
over mountains. However the red spot in this image represents salt lake Tuz
located at an altitude of about 900 meter. The lake is very shallow and during
the hot part of the year it evaporates. The reflectivity of the salt
crystals is similar to the reflectivity of the ice crystals, hence in this RGB
it is presented by the dark red colour.
The aim of this case study is to:
- Show the onset of convective event using various Meteosat 8 channels and artificially coloured combinations
- Show the onset of convective event using radar images (reflectivity)
- Study satellite imagery in combination with NWP fields
- Investigate the dynamical background of the processes by considering the basic and derived numerical model parameters
The origin of the convective events discussed in this case
study is very different. Severe convection within the cold front is a result of
instability in the lower parts of the troposphere and the potential instability.
The prefrontal convection that is also mentioned in this case study is
influenced by an approaching frontal system as well as the orography.
During the same day numerous cells developed over the eastern Turkey as well.
This development differs from the western one because instead of the frontal
interaction, it is influenced by an upper level low.
This case study will
show that the analysed convective episodes have similarities and differences.
To be able to follow the case study from the beginning it is preferable to study the chapter dealing with convective weather features from
the “Manual of Synoptic Satellite Meteorology”.