| Version 5 (modified by boeske, 11 years ago) (diff) |
|---|
Page is currently under construction!!!
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 PALM this can be realized by using large scale forcing.
Horizontal large scale (LS) advection:
![\[ \left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{LS}} = - \left( u_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial x \vphantom{y}} + v_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial y} \right) \]](/trac/tracmath/d236b792c12052eabf8f46fcb065b49263e4db60.png)
with
![\[ \varphi \in \{\theta,q\}. \]](/trac/tracmath/73c0d1a61e78382e9397df50297bf6ace644aa9e.png)
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:
![\[ \left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{subs}} = - w_{\text{subs}} \dfrac{\partial \varphi_{\text{LES}}}{\partial z \vphantom{y}} = - w_{\text{LS}} \dfrac{\partial \varphi_{\text{LES}}}{\partial z \vphantom{y}}. \]](/trac/tracmath/a9251fd6891cdc94db2e625db6282a3210a9deea.png)
Alternatively, large scale subsidence tendencies, derived from larger scale models, can be directly applied to the prognostic equations:
![\[ \left. \dfrac{\partial \varphi}{\partial t} \right|_{\text{subs}} = - w_{\text{LS}} \dfrac{\partial \varphi_{\text{LS}}}{\partial z \vphantom{y}} \]](/trac/tracmath/f1e9dec7946dbe73301d9f2d8604910243707c21.png)
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
