Impact of coherent eddies on airborne measurements of vertical turbulent fluxes
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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.
KeywordsAircraft measurements Heat fluxes Mesoscale model Organised structures Sahelian boundary layer
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- Goutorbe JP, Dolman AJ, Gash JHC, Kerr YH, Lebel T, Prince SD, Stricker JNM (1997) HAPEX-Sahel. Elsevier (reprinted from J Hydrol 1079 pp 188–189/1-4)Google Scholar
- Grunwald J, Kalthoff N, Fiedler F and Corsmeier U (1998). Application of different flight strategies to determine areally average turbulent fluxes. Contr Atmos Phys 71: 283–302 Google Scholar
- Halpert MS, Ropelewski CF (1992) Fourth Annual Climate Assessment. Climate Analysis Center. Dept. of Commerce, NOAA/NWS/NMC, 90 ppGoogle Scholar
- Lafore JP, Stein J, Asencio N, Bougeault P, Ducrocq V, Duron J, Fischer C, Héreil P, Mascart P, Redelsperger JL, Richard E and Vilà-Guerau de Arellano J (1998). The Meso-NH atmospheric simulation system. Part I: Adiabatic formulation and control simulations. Ann Geophys 109: 16–90 Google Scholar
- Masson V, Champeaux JL, Chauvin F, Meriguet C and Lacaze R (2003). A global database of land surface parameters at 1-km resolution in meteorological and climate models. J Climate 16: 1261–1282 Google Scholar
- Weckwerth T (1995) A study of horizontal convective rolls occurring within clear-air convective boundary layers. Ph.D. thesis, University of California, Los AngelesGoogle Scholar
- Wyngaard JC (1983). Lectures on the planetary boundary layer. In: Gal-Chen, T and Lilly, DK (eds) Mesoscale meteorology theories, observations and models., pp 781. D. Reidel, Publ. Co., Dordrecht Google Scholar