Boundary-layer turbulent processes and mesoscale variability represented by Numerical Weather Prediction models during the BLLAST campaign
2016
Abstract. This study evaluates the ability of three operational models, with resolution varying from 2.5 to 16 km, to predict the
boundary-layerturbulent processes and mesoscale variability observed during the
Boundary LayerLate-Afternoon and
SunsetTurbulence (BLLAST) field campaign. We analyse the representation of the vertical profiles of temperature and humidity and the time evolution of near-surface atmospheric variables and the radiative and turbulent fluxes over a total of 12 intensive observing periods (IOPs), each lasting 24 h. Special attention is paid to the evolution of the
turbulent kinetic energy(TKE), which was sampled by a combination of independent instruments. For the first time, this variable, a central one in the turbulence scheme used in AROME and ARPEGE, is evaluated with observations. In general, the 24 h forecasts succeed in reproducing the variability from one day to another in terms of
cloud cover, temperature and
boundary-layerdepth. However, they exhibit some systematic biases, in particular a cold bias within the
daytime
boundary layerfor all models. An overestimation of the
sensible heatflux is noted for two points in ARPEGE and is found to be partly related to an inaccurate simplification of surface characteristics. AROME shows a moist bias within the
daytime
boundary layer, which is consistent with overestimated latent heat fluxes. ECMWF presents a dry bias at 2 m above the surface and also overestimates the
sensible heatflux. The high-resolution model AROME resolves the vertical structures better, in particular the strong
daytimeinversion and the thin evening stable
boundary layer. This model is also able to capture some specific observed features, such as the orographically driven subsidence and a well-defined maximum that arises during the evening of the water vapour mixing ratio in the upper part of the residual layer due to fine-scale advection. The model reproduces the order of magnitude of spatial variability observed at mesoscale (a few tens of kilometres). AROME provides a good simulation of the diurnal variability of the
turbulent kinetic energy, while ARPEGE shows the right order of magnitude.
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