Can we produce realistic boundary layer turbulence by coupling large-eddy simulations with mesoscale model data?

Usually, large-eddy simulation (LES) models are used to study rather idealized atmospheric boundary layer flow situations. However, more realistic turbulence fields are of great interest for example for wind engineering or environmental applications. To make a step towards less-idealized flows, processes occurring on larger scales than those covered by LES like large-scale advection, subsidence or large-scale pressure-gradients have to be considered. One way to incorporate large-scale tendencies in LES is to couple LES with mesoscale model output in terms of prescribing advective forcing and applying continuous nudging. By means of nudging the LES state is relaxed towards a mesoscale state by means of adding artificial tendency terms which are proportional to the difference between the LES and the mesosocale state.

Two LES models, PALM and UCLA-LES, are used to simulate a period observed during the observational prototype experiment of the project HD(CP)2 (High Definition Clouds and Precipitation for Advancing Climate Prediction) which took place in April and May 2013 in Germany. The simulations are initialized with profiles stemming from simulations with the mesoscale model COSMO (Consortium for Small-scale Modeling). Time-dependent advective tendencies calculated from COSMO output are prescribed and continuous nudging towards the COSMO data is used to incorporate meteorological forcing. A comparison of first and second-order statistics with the multi-sensor data set collected during the prototype experiment is presented to assess whether it is possible to produce a realistic turbulent flow. The effect of nudging is further explored by changing the magnitude and the profile of the relaxation time-scale. The implications of our findings for the generation of realistic geophysical turbulence are discussed.