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Understanding Orographic Cloudiness: High Lee Waves and Cloud Structure Dynamics

This study focuses on the phenomenon of orographic cloudiness, particularly high lee cloudiness and lee waves. It describes the formation of narrow cloud bands in the lee of mountain ranges, visible primarily in visible light (VIS) as white and in infrared (IR) as dark grey. The influence of air stability, wind speed, and mountain dimensions is examined in detail. Key parameters, such as the Brunt-Vaisala Frequency and Scorer Parameter, highlight the conditions conducive to the development of lee waves. This research contributes to the understanding of atmospheric dynamics and cloud formation in mountainous regions.

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Understanding Orographic Cloudiness: High Lee Waves and Cloud Structure Dynamics

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Presentation Transcript

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

  2. Cloud Structure in Satellite Image

  3. 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

  4. 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.

  5. Superposition of both lee cloud types

  6. lee

  7. high lee

  8. lee lee high lee

  9. Meteorological - Physical Background

  10. 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

  11. 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

  12. Key Parameters

  13. 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

  14. Relevant numerical parameters and their typical distribution • High lee cloudiness occurs preferably on the anticyclonic jet side cyclonic anti- cyclonic

  15. cyclonic anticyclonic Carpathian

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