conceptual models of cold air features: comma. cloud structures in satellite images
Post on 19-Dec-2015
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Conceptual Models of Cold Air Features:
Comma
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Cloud Structures in Satellite Images
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Typical development
• in initial phase often „ enhanced cumuli cloud (EC)“ EC
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EC
EC
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EC
EC
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Typical cloud configurations in
comma stage • meso-scale cloud spiral• usually in cold air behind CF • bright in all three channels (WV, IR,VIS) • discriminating between:
– comma head (with embedded convective cells) – comma tail (less bright and smoother structure)
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Comma
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Comma
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Comma
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Comma
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Meteorological Physical Background
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Physical Background of "Comma"
• Two types of development observed:
– development within cold air: • barotropic instability, baroclinic instability and
CISK mechanisms
– development out of occlusion spirals: • the innermost part of the occlusion spiral is torn off;
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Physical Background of "Comma"
• Conveyor belt theory:
– Comma cloud within rising branch of dry intrusion
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dry intrusion
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Key Parameters
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Relevant numerical parameters and their
typical distribution
• situated in front of (close to) trough line – at 1000 hPa
– at 500 hPa
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comma
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Relevant numerical parameters and their typical distribution
• within CA• unstable
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CAunstable
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Relevant numerical parameters and their typical distribution
• vorticity maximum – at the rear edge
• PVA maximum – directly above cloud
feature
• height of IPV is lowest at comma cloud
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Vorticity maximum:
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PVA maximum:
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Height of IPV=2:
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Relevant numerical parameters and their typical distribution
• At the left side of the jet axis,
• In several cases in the left exit region of a jet streak
• In such case spiral feature of comma weaker – shear vorticity
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Jet streak
PVA
Jet streak
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Typical Appearance in Vertical Cross Sections
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• Isentrops:– unstable layer from
surface up to 700hPa
– frontal zone only in middle levels above the unstable layer
unstable
upperlevelfront
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unstable
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• Divergence and vertical motion:– convergence in low
layer– divergence above– upward motion in
thick layer within comma
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convergence
divergence
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• Vorticity maximum– at the rear of the
comma feature– curvature dominant – shear in case of jet
streak
• PVA – above comma feature
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PVA