Impact of coherent eddies on airborne measurements of vertical turbulent fluxes |
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Authors: | Marie Lothon Fleur Couvreux Sylvie Donier Françoise Guichard Pierre Lacarrère Donald H Lenschow Joël Noilhan Frédérique Saïd |
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Institution: | (1) Centre de Recherches Atmosphériques, 8 route de Lannemezan, Campistrous, 65300, France;(2) Laboratoire d’Aérologie, UMR 5560 CNRS, Université Paul Sabatier Toulouse III, Toulouse, France;(3) Météo-France/CNRM, Toulouse, France;(4) National Center for Atmospheric Research, Boulder, CO, USA |
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Abstract: | During the Hydrological-Atmospheric Pilot Experiment (HAPEX)-Sahel, which took place in Niger in the transitional period between
the wet and dry seasons, two French aircraft probed the Sahelian boundary layer to measure sensible and latent heat fluxes.
The measurements over the Niamey area often revealed organised structures of a few km scale that were associated with both
thermals and dry intrusions. We study the impact of these coherent structures using a single day’s aircraft-measured fluxes
and a numerical simulation of that day with a mesoscale model. The numerical simulation at high horizontal resolution (250 m)
contains structures that evolve from streaks in the early morning to cells by noon. This simulation shows distribution, variance
and skewness similar to the observations. In particular, the numerical simulation shows dry intrusions that can penetrate
deeply into the atmospheric boundary layer (ABL), and even reach the surface in some cases, which is in accordance with the
observed highly negatively skewed water vapour fluctuations. Dry intrusions and thermals organised at a few km scale give
skewed flux statistics and can introduce large errors in measured fluxes. We use the numerical simulation to: (i) evaluate
the contribution of the organised structures to the total flux, and (ii) estimate the impact of the organised structures on
the systematic and random errors resulting from the 1D sampling of the aircraft as opposed to the 2D numerical simulation
estimate. We find a significant contribution by the organised structures to the total resolved fluxes. When rolls occur, and
for a leg length of about 30 times the ABL depth, the 1D sampled flux is shown to be sometimes 20% lower than the corresponding
2D flux when the 1D sampling direction is the same as the main axis of the rolls, whereas the systematic error is much lower
when the direction of the leg is transverse to the rolls. In the case of cells, an underestimate of around 10% can still be
observed with the 1D approach independent of direction, due to poor sampling of the energy-containing scales. |
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Keywords: | Aircraft measurements Heat fluxes Mesoscale model Organised structures Sahelian boundary layer |
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