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| Titel |
A comprehensive investigation on afternoon transition of the atmospheric boundary layer over a tropical rural site |
| VerfasserIn |
A. Sandeep, T. N. Rao, S. V. B. Rao |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 13 ; Nr. 15, no. 13 (2015-07-14), S.7605-7617 |
| Datensatznummer |
250119891
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| Publikation (Nr.) |
 copernicus.org/acp-15-7605-2015.pdf |
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| Zusammenfassung |
The transitory nature
of the atmospheric boundary layer (ABL) a few hours before and after the time of
sunset has been studied comprehensively over a tropical station, Gadanki
(13.45° N, 79.18° E), using a suite of in situ and remote
sensing devices. This study addresses the following fundamental and important
issues related to the afternoon transition (AT): which state variable first
identifies the AT? Which variable best identifies the AT? Does the start time
of the AT vary with season and height? If so, which physical mechanism is
responsible for the observed height variation in the start time of the
transition?
At the surface, the transition is first seen in temperature (T) and wind
variance (σ2WS), ~ 100 min prior to the time of
local sunset, then in the vertical temperature gradient and finally in water
vapor mixing ratio variations. Aloft, both signal-to-noise ratio (SNR) and
spectral width (σ) show the AT nearly at the same time. The T at
the surface and SNR aloft are found to be the best indicators of transition.
Their distributions for the start time of the AT with reference to time of
sunset are narrow and consistent in both total and seasonal plots. The start
time of the transition shows some seasonal variation, with delayed
transitions occurring mostly in the rainy and humid season of the
northeast monsoon. Interestingly, in contrast to the general perception, the signature
of the transition is first seen in the profiler data, then in the sodar data,
and finally in the surface data. This suggests that the transition follows a
top-to-bottom evolution. It indicates that other processes, like entrainment,
could also play a role in altering the structure of the ABL
during the AT, when the sensible heat flux decreases progressively. These
mechanisms are quantified using a unique high-resolution data set to
understand their variation in light of the intriguing height dependency of
the start time of the AT. |
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