Validation of the “Lokal-Modell” over heterogeneous land surfaces using aircraft-based measurements of the REEEFA experiment and comparison with micro-scale simulations |
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Authors: | B Maurer G Heinemann |
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Institution: | Meteorologisches Institut der Universit?t Bonn, Bonn, Germany
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Abstract: | Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange
processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric
model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using
the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities.
Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study
vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured
net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the
investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface
fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method,
which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL
is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed
net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible
heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation
of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes,
but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the
LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the
LM scale. |
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