Confusion about PBL

Topics related to current and future physics in the WRF as well as any problems you may have.

Confusion about PBL

Postby Jennylucky » Mon Jun 13, 2016 2:58 am

Hello everyone,
I'm confused with the surface layer physics and planetary boundary physics in WRF model recently.
First, what's the lower bound of PBL in model ? the top of surface layer or ground ?
Second, what exactly the difference or relation between the surface layer and PBL scheme ? Does they transport the flux with different way ? For example, sfclay scheme computed the flux based on Monin-Obukhov similarity , but PBL shcme based on K-theory or TKE closure method and so on. I don't know whether this understanding is right , any help would be greatly appreciated.
Thanks very much !
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Re: Confusion about PBL

Postby alainaketh » Mon Jun 11, 2018 5:10 pm


Just as a brief introduction, the planetary boundary layer is the part of the troposhere where mixing of heat and moisture occurs (both crucial processes for rain cloud formation and development) [1]. It is limited by a stable layer (capping inversion) at the top [2]. The surface layer is is 10% of the height of the boundary layer.

Vertical mixing occurs at different length scales in each of the layers. In the surface layer, all eddies have length scale similar to the height of the layer [3] and turbulent fluxes vary less than 10% of their magnitude [4]. In the PBL, atmospheric variables such as wind speed, temperature, water vapour content and humidity have significant variation and vertical mixing is strong, having a response to surface forcing of usually one hour [5].

Lower bound of the PBL is the upper part of the surface layer because of its different properties.

There is a direct relationship between the PBL and the surface layer schemes in the WRF because water and energy fluxes between from the land surface go to surface layer, then to PBL (although they can also go from land surface directly to PBL) [1]. In the WRF manual chapter 5, check the table with the "&physics" parameterisations, specifically those for "bl_pbl_physics (max_dom)" and you'll see that for every PBL shceme, there is a recommended surface layer scheme.

Hope this helps.

[1] ... 15JD023927
[2] ... att94a.pdf
[3] ... dary-layer, see the "Boundary layer (atmospheric) and air pollution | Surface layer" section.
[4] ... 11-00056.1
[5] ... 01s05.html
Hydrology Researcher, University of Bristol
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