Virtual flight measurements regarding the mixing state of the convective boundary layer during LITFASS-2003 – An LES study

The knowledge of the mixing state of the convective boundary layer (CBL) under a heterogeneous surface forcing is crucial for the interpretation of area averaged turbulence measurements. In order to investigate up to which height and degree the CBL is affected by such a complex heterogeneous surface, a series of large-eddy simulations (LES) initialized and driven by the observational data from the LITFASS-2003 field campaign was conducted.

With regard to the determination of the height where the influence of the surface heterogeneity vanishes, a spatially lagged correlation analysis between the turbulent heat fluxes and the prescribed surface heat fluxes was conducted. It is shown that the influence of surface heterogeneity is still present up to the top of the CBL and the inversion layer above.

Regarding this, the question arises whether these surface heterogeneity induced heat flux differences can be captured by airborne turbulent flux measurements. For this reason an ensemble of statistically independent virtual flight measurements above the prevailing major surface types farmland, forest and water is conducted during the LES. In agreement with the results of the spatially lagged correlation analysis, the resulting ensemble averaged virtually measured heat fluxes show a clear dependence on the underlying surface type up to the top of the CBL. However, a large variability between the individual virtual flux measurements can be observed. Because the length of the legs is limited by the underlying surface patches, the ratio between the length of the given legs and the boundary layer height is only about 6 to 8, leading to an insufficient sampling of largest turbulent elements contributing to the flux, which in turn causes the large variability. Moreover, the flux variability between the individual ensemble members is even larger than the differences between the ensemble averaged heat fluxes above the different surface types, which makes it difficult to make a clear statement about the mixing state from an individual flux measurement. Furthermore, it is found that the random and systematic uncertainties of the flux measurement based on the integral length scale do not sufficiently indicate the additional error caused by inadequate sampling of the largest turbulent elements. For the given averaging lengths of about 10 km, it turns out that at least ten statistically independent flight measurements are necessary to make a clear statement about the mixing state of the CBL.