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Version 12 (modified by gronemeier, 9 years ago) (diff)

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Large scale forcing

Usually, the LES model PALM is used to simulate the flow in the boundary layer which is a certain part of the atmosphere. Processes occurring on larger scales than those in the boundary layer including large scale advection of scalars, large scale pressure gradients or large scale subsidence have also to be considered in the model, especially when focusing on realistic situations observed during measurement campaigns. In limited domain models with non-cyclic boundary conditions the large scale state enters through the boundary conditions at the lateral walls, and is usually taken from larger-scale models. This method isn't possible with cyclic horizontal boundary conditions (because there are no real lateral boundaries). In case of cyclic boundary conditions PALM uses a method developed by Neggers et al. (2012), which is switched on by parameter large_scale_forcing. This method does not work with non-cyclic horizontal boundary conditions! Furthermore, it requires to use a flat topography.

Horizontal large scale (LS) advection:

Effects of horizontal large scale advection are taken into account by adding large scale tendencies

$\left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{LS}} = - \left( u_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial x \vphantom{y}} + v_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial y} \right)$ \quad with \quad $\varphi \in \{\theta,q\}$

to the prognostic equations. These tendencies are typically derived from larger scale models and should be spatially averaged over a large domain so that local-scale perturbations are avoided.


Large scale subsidence:

Effects of large scale subsidence and ascent can be considered by two different methods in PALM. The default method uses a large scale subsidence velocity w_subs which is usually taken from larger scale models. For the vertical advection of the prognostic variables an additional tendencies are computed:

$\left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{subs}} = - w_{\text{subs}} \dfrac{\partial \varphi_{\text{LES}}}{\partial z \vphantom{y}} = - w_{\text{LS}} \dfrac{\partial \varphi_{\text{LES}}}{\partial z \vphantom{y}}.$


Alternatively, large scale subsidence tendencies, derived from larger scale models, can be directly applied to the prognostic equations:

$\left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{subs}} = - w_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial z \vphantom{y}} .$

This method can be used by setting the control parameter use_subsidence_tendencies to .T..

Note that in any case large_scale_subsidence has to be set to .T. in the parameter file.


Nudging:

When performing runs with large scale forcing it is recommended to use additional nudging, so that excessive model drift in time is prevented (Neggers et al., 2012).


A complete example for a PALM run with large scale forcing and nudging is documented here.

References

  • Neggers, R. A. J., A. P. Siebesma and T. Heus, 2012: Continous single-column model evaluation at a permanent meteorological supersite. Bull. Amer. Meteor. Soc, 29, 91-115. ​doi
                                                                                                                                                                                                                                                                                                                                                                               
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