Abstract
We evaluate the influence of a forest parametrization on the simulation of the boundary layer flow over moderate complex terrain in the context of the Perdigão 2017 field campaign. The numerical simulations are performed using the Weather research and forecasting model using its large eddy simulation mode (WRF-LES). The short-term high resolution (40 m horizontal grid spacing) and long-term (200 m horizontal grid spacing) WRF-LES are evaluated for an integration time of 12 hours and 1.5 months, respectively, with and without forest parameterization. The short-term simulations focus on low-level jet events over the valley, while the long-term simulations cover the whole intensive observation period (IOP) of the field campaign. The results are validated using lidar and meteorological tower observations. The mean diurnal cycle during the IOP shows a significant improvement of the along-valley wind speed and the wind direction when using the forest parametrization. However, the drag imposed by the parametrization results in an underestimation of the cross-valley wind speed, which can be attributed to a poor representation of the land surface characteristics. The evaluation of the high-resolution WRF-LES shows a positive influence of the forest parametrization on the simulated winds in the first 500 m above the surface.
Highlights
We evaluate the influence of a forest parametrization on the simulation of the boundary layer flow over moderate complex terrain in the context of the Perdigão 2017 field campaign
In the following, we present an evaluation of the Weather Forecast and Research model (WRF)-large eddy simulation (LES) simulations (D03 and D04) with/without the forest parametriza140 tion in the context of the Perdigão 2017 field campaign
A set of low-level jet (LLJ) events is analysed through the short-term simulations (D04) to investigate the impact of the forest parametrization on the LLJ structure over the double ridge. 3.1 Evaluation of the long-term simulations Figure 4 shows the observed and simulated mean diurnal cycle of both the along- and cross-valley wind speeds averaged over 145 the 49-day period covering the intensive observation period (IOP) for towers T20, T25 and T29
Summary
In recent years the rising computational power allowed numerical weather prediction models to run with higher spatial resolutions in real case mode. Tree clusters or urban blocks start to be in the spatial scale of numerical models, evidencing the need to improve the representation of surface characteristics (Shaw and Schumann, 1992). When the model vertical and horizontal resolution decrease to tens (or hundreds) of metres, the representation 20 of the surface obstacles becomes critical (Aumond et al, 2013). The classical representation of such obstacles is normally introduced into mesoscale models using a bulk approach such as a characteristic roughness length (Zo) in each grid cell. The effect of the ground obstacles should be taken into account through surface schemes, and within the dynamic equations of the numerical model such as in the drag force approach (Zaïdi et al, 2013)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
