Conceptual Models of Cold Air Features: Comma

Cloud Structures in Satellite Images

Typical development in initial phase often „ enhanced cumuli cloud (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

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

CA unstable

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 upper level front

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