The early evening boundary layer transition

✍ Scribed by L. Mahrt

• Book ID: 104575339
• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 849 KB
• Volume: 107
• Category: Article
• ISSN: 0035-9009
• DOI: 10.1002/qj.49710745205

✦ Synopsis

Abstract

Various mechanisms which lead to nocturnal accelerations and formation of the low‐level jet are examined by analysing Wangara data. the stress field is inferred from the wind field and equations of motion. the evolution of the stress divergence, during the evening transition from daytime mixed layer flow to nocturnal boundary layer flow, is found to increase the ageostrophic flow and subsequent nocturnal accelerations by roughly a factor of two. During this transition the stress divergence in the lowest few hundred metres increases due to the fact that the influence of decreasing boundary layer depth exceeds the effect of decreasing surface stress. This leads to temporary deceleration and rotation of the low‐level wind vector towards low pressure and thus increases the ageostrophic flow.

Diurnal variation of the geostrophic wind is also found to significantly strengthen the nocturnal flow. This diurnal variation is apparently due to heating and cooling over terrain which slopes gently upward, east of the location of the Wangara experiment.