Conceptual Model of: Orographic Cloudiness: Lee cloudiness Lee waves High lee cloudiness

Cloud Structure in Satellite Image

Lee Waves Narrow cloud bands in the lee of the mountain ranges: width between approx 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

high lee

high 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 anticyclonic cyclonic