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The early evening boundary layer transition
Author(s) -
Mahrt L.
Publication year - 1981
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49710745205
Subject(s) - boundary layer , geostrophic wind , daytime , atmospheric sciences , nocturnal , evening , flow (mathematics) , planetary boundary layer , diurnal temperature variation , geology , mechanics , front (military) , meteorology , environmental science , climatology , physics , astronomy
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.