Author:
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Darbieu, Clara; Lohou, Fabienne; Lothon, Marie; Vilà Guerau de Arellano, Jordi; Couvreux, Fleur; Durand, Pierre; Pino González, David; Patton, Edward; Nilsson, Erik; Blay-Carreras, Estel; Gioli, Beniamino
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Abstract:
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We investigate the decay of planetary boundary
layer (PBL) turbulence in the afternoon, from the time the
surface buoyancy flux starts to decrease until sunset. Dense
observations of mean and turbulent parameters were acquired
during the Boundary Layer Late Afternoon and Sunset Tur-
bulence (BLLAST) field experiment by several meteorolog-
ical surface stations, sounding balloons, radars, lidars and
two aircraft during the afternoon transition. We analysed a
case study based on some of these observations and large-
eddy simulation (LES) data focusing on the turbulent vertical
structure throughout the afternoon transition.
The decay of turbulence is quantified through the temporal
and vertical evolution of (1) the turbulence kinetic energy
(TKE), (2) the characteristic length scales of turbulence and
(3) the shape of the turbulence spectra. A spectral analysis of
LES data, airborne and surface measurements is performed
in order to characterize the variation in the turbulent decay
with height and study the distribution of turbulence over eddy
size.
This study highlights the LES ability to reproduce the tur-
bulence evolution throughout the afternoon. LESs and obser-
vations agree that the afternoon transition can be divided in
two phases: (1) a first phase during which the TKE decays
at a low rate, with no significant change in turbulence char-
acteristics, and (2) a second phase characterized by a larger
TKE decay rate and a change in spectral shape, implying an
evolution of eddy size distribution and energy cascade from
low to high wave number.
The changes observed either in TKE decay (during the first
phase) or in the vertical wind spectra shape (during the sec-
ond phase of the afternoon transition) occur first in the upper
region of the PBL. The higher within the PBL, the stronger
the spectra shape changes |