Conceptual Models of Cold Air Features:

Comma

Cloud Structures in Satellite Images

Typical development

• in initial phase often „ enhanced cumuli cloud (EC)“ EC

EC

EC

EC

EC

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)

Comma

Comma

Comma

Comma

Meteorological Physical Background

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;

Physical Background of "Comma"

• Conveyor belt theory:

– Comma cloud within rising branch of dry intrusion

dry intrusion

Key Parameters

Relevant numerical parameters and their

typical distribution

• situated in front of (close to) trough line – at 1000 hPa

– at 500 hPa

comma

Relevant numerical parameters and their typical distribution

• within CA• unstable

CAunstable

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

Vorticity maximum:

PVA maximum:

Height of IPV=2:

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

Jet streak

PVA

Jet streak

Typical Appearance in Vertical Cross Sections

• Isentrops:– unstable layer from

surface up to 700hPa

– frontal zone only in middle levels above the unstable layer

unstable

upperlevelfront

unstable

• Divergence and vertical motion:– convergence in low

layer– divergence above– upward motion in

thick layer within comma

convergence

divergence

• Vorticity maximum– at the rear of the

comma feature– curvature dominant – shear in case of jet

streak

• PVA – above comma feature

PVA