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The boundary-layer grey zone
The atmospheric boundary layer is the region adjacent to the surface in which there are significant turbulent fluxes of heat, moisture and momentum. It has a significant diurnal cycle over land and is strongly coupled to other important atmospheric processes such as large-scale weather systems and convective storms. The boundary layer diurnal cycle thus plays a key role in many aspects of high-impact weather such as extreme surface temperatures, the dispersion of pollutants and chemical species, low level cloud and fog, and the onset of thunderstorms.
During the diurnal cycle, the typical length scale of boundary layer eddies varies between of order 1 km by day to much smaller values at night. Until recently, numerical weather prediction (NWP) models used a horizontal grid length much larger than the size of the boundary-layer eddies. This scale separation meant that column-based parametrizations were formally justified. However, with current supercomputer power, and in order to provide more skilful regional forecasts, operational weather centres now run limited area models at horizontal grid lengths as small as a few kilometres. For example, the 1.5 km grid length configuration of the Met Office Unified Model (MetUM) provides high-resolution forecasts of severe convective precipitation and fog for domains covering the UK. The MetUM is also being run experimentally at horizontal grid lengths as small as 100m. The ratio of the horizontal grid length to the boundary layer eddy size is now of order one for certain phases of the diurnal cycle; such a regime is known as the "terra incognita" or the "grey zone".
The grey zone is rapidly becoming a pressing practical issue for NWP of the atmospheric boundary layer. Whilst many weather centres run models at grey-zone resolutions, there is currently little theoretical and numerical modelling basis for how to represent the boundary layer.
Some links for this work:
Papers:
1. A paper about the misrepresentation of thermals with a Smagorinsky model at coarse resolution.2. A paper about the use of stability functions in dynamic Smagorinsky modelling.
3. A review of modelling in the boundary layer grey zone.
4. A paper providing a proof of concept for a dynamic blending approach.
5. A paper on dynamic Smagorinsky modelling of the Wangara test case.
Talks:
1. An invited talk on dynamic sub-grid modelling of the evolving convective boundary layer, (presented by George Efstathiou), at the ECMWF grey zone workshop. A video recording of the talk is available here.2. An invited talk on large-eddy simulation of the convective boundary layer, (presented by Mary-Jane Bopape), at 10th CHPC national meeting in East London, South Africa.
3. An extended abstract on large-eddy simulation of the convective boundary layer, (presented by Mary-Jane Bopape), at 32nd Annual Conference of the South African Society for Atmospheric Sciences in Cape Town, South Africa.
4. A talk on testing dynamic models in the convective boundary layer, (presented by Omduth Coceal), at AMS symposium on boundary layers and turbulence in Salt Lake City.
5. A talk on a priori testing on a tensor eddy diffusivity model, (presented by Omduth Coceal), at AMS symposium on boundary layers and turbulence in Leeds.
6. A talk describing the GREYBLs project, (presented by Bob Beare), at a UM high-resolution group meeting.
7. A talk discussing the boundary layer grey zone, presented at an NCAS workshop.
Posters:
1. A poster on blending with a dynamic Smagorinksy model, at the EGU in Vienna.2. A poster on testing dynamic models in the convective boundary layer, (presented by Omduth Coceal), at the RMetS NCAS conference in Manchester.
2. A poster on a priori testing on a tensor eddy diffusivity model, (presented by Omduth Coceal), at the NCAS annual meeting in Birmingham.