conceptual model of: orographic cloudiness: lee cloudiness lee waves high lee cloudiness
Post on 22-Dec-2015
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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.
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
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