Conceptual Model of:Orographic Cloudiness:

Lee cloudinessLee waves

High lee cloudiness

Cloud Structure in Satellite Image

Lee Waves

• Narrow cloud bands in the lee of the mountain ranges: width between approx. 4 - 30 Km

• Perpendicular to the wind direction

• Easily visible in VIS: white; only dark grey in IR and only visible if broad enough

High Lee Cloud

• Ci shields in the lee of the mountain range

• Bright in IR only translucent grey in VIS

• Is blown downstream with the upper level wind:

– Large extensions can occur (up to 1000s kms)

– Can be detached from mountain chain

• Life duration is several hours.

Superposition of both lee cloud types

lee

highlee

lee highlee

lee

Meteorological - Physical Background

Conditions for the development of lee cloud

• Air flowing perpendicular onto a mountain range is forced to rise

• If the air is stable:– begins to oscillate in the lee

• dependant on different conditions: – stability, wind speed, dimension of the mountains.

• Internal gravity waves:

– lower layers: short wave length: parallel cloud lines

– higher layers: longer wave length: high lee cloudiness

Some parameters describing the process • Brunt - Vaisala Frequency

– N**2 = g/T (dT/dz + g/cp)– thresholds for stability (N2>0 - stable)

• Scorer Parameter– l(z) = N(z)/U(z)– Stability combined with characteristic of wind field (U:

wind perpendicular to mountains on windward side)– Waves are formed at small values

• Critical mountain width– L > 2π U/N: lee waves can be formed

Key Parameters

Relevant numerical parameters and their typical distribution

• Wind direction must have a component perpendicular to the mountain range

• Wind speed increases with height

• Stable layer

Relevant numerical parameters and their typical distribution

• High lee cloudiness occurs preferably on the anticyclonic jet side

cyclonic

anti-cyclonic

Carpathian

anticycloniccyclonic