A New Method for the Determination of Area-Averaged Turbulent Surface Fluxes from Low-Level Flights Using Inverse Models

Abstract

The low-level flight method (LLF) has been combined with linear inverse models (IM) resulting in an LLF+IM method for the determination of area-averaged turbulent surface fluxes. With this combination, the vertical divergences of the turbulent latent and sensible heat fluxes were calculated from horizontal flights. The statistical errors of the derived turbulent surface fluxes were significantly reduced. The LLF+IM method was tested both in numerical and field experiments. Large-eddy simulations (LES) were performed to compare 'true' flux profiles with 'measurements' of simulated flights in an idealised convective boundary layer. Small differences between the 'true' and the 'measured' fluxes were found, but the vertical flux divergences were correctly calculated by the LLF+IM method. The LLF+IM method was then applied to data collected during two flights with the Helipod, a turbulence probe carried by a helicopter, and with the research aircraft Do 128 in the LITFASS-98 field campaign. The derived surface fluxes were compared with results from eddy-covariance surface stations and with large-aperture scintillometer data. The comparison showed that the LLF+IM method worked well for the sensible heat flux at 77 and 200 m flight levels, and also for the latent heat flux at the lowest level. The model quality control indicated failures for the latent heat flux at the 200 m level (and higher), which were probably due to large moisture fluctuations that could not be modelled using linear assumptions. Finally the LLF+IM method was applied to more than twenty low-level flights from the LITFASS-2003 experiment. Comparison with aggregated surface flux data revealed good agreement for the sensible heat flux but larger discrepancies and a higher statistical uncertainty for the latent heat flux.

Energy balance closure for the LITFASS-2003 experiment

“…5 can be considered as quite good for the sensible heat flux whilst larger differences were found for the latent heat flux. The statistical errors of latent heat flux measurements are larger in general, as shown by several other investigations (Bange et al 2006b;Flamant et al 1997;Linne et al 2006). …”

“…Airborne measurements with the Helipod were performed on several days of the LITFASS-2003 experiment. The Helipod is an autonomous turbulence probe attached to a 15-m-long rope and carried by a helicopter (Bange et al 2006b). It is equipped with high-precision, fast-response sensors for the measurement of the turbulent fluctuations of wind, temperature and humidity.…”

“…The well-known box method (linear extrapolation using flux measurements at several flight levels) leads to large systematic errors (Bange et al 2006b). The lowlevel flight and inverse modelling method (see also Bange et al 2006b) could not be applied to this data set, since the horizontal and temporal dependence of air temperature and humidity cannot be detected from single flight legs in general. Thus, the flux measurements at the individual flight levels (between 80 and 100 m above the surface) were plotted in Fig.…”

Sensible and latent heat flux from radiometric surface temperatures at the regional scale: methodology and evaluation

“…Whenever a vertical profile was not available, the flux divergence correction was not applied to the data. No flux divergence correction was applied to Latent Heat Flux data, since this flux has been shown not to have a theoretical and predictable behaviour (Bange et al, 2006).…”

Area-Averaged Surface Fluxes Over the Litfass Region Based on Eddy-Covariance Measurements

“…This was well below 0.1 z i (where z i is the boundary-layer height) in most cases so that the fluxes measured along these legs can be considered to closely represent the surface fluxes. Extrapolation and inverse modelling methods were used to determine the averaged surface fluxes over the whole LITFASS area from the Helipod data (Bange et al, 2006a).…”