Virtually all weather and climate models do switch off their convection parameterization at a arbitrary resolution within the grey zone. Hence there is urgent need to systematically explore of the behaviour of these models operating in the "Grey Zone" through varying the resolution with and without parameterized convection. As a response to this need the Working Group on Numerical Experimentation (WGNE) has initiated the Grey Zone Project that aims to answer the following questions:

• What are the relative contributions of the parameterized versus the resolved contributions to the convective transport at the various resolutions in the Grey Zone ?
• How do models operate in the Grey Zone without an explicit convection parameterization?
• How do models operate in the Grey Zone with a convection parameterization?
• To what extend do scale-aware convection parameterizations improve the representation of convective transport within the Grey Zone?

Proposal
The Grey Zone Project that we propose here, is driven by a few expensive numerical experiments (controls) at a ultra-high turbulent eddy resolving resolution (Δ x= 100~200m) on a sufficiently large domain (~2000 X 2000 X 200 grid points). These runs will serve as a reference against which the degrading behaviour of convective transport and the cloud properties of lower resolution runs can be assessed.
Such resolution dependencies can be analysed in 3 ways:

• Coarse grain the output and diagnostics of the control run at resolutions of 0.5km, 1km, 2km, 4km, 8km, (a posteriori coarse graining: COARSE),
• Repeat the control run at coarser resolutions of 0.5km, 1km, 2km, 4km, 8km, without convection parameterization (a posteriori coarse graining: NOPARAMS)
• Repeat the control run at coarser resolutions of 0.5km, 1km, 2km, 4km, 8km, with convection parameterization (a posteriori coarse graining: PARAMS)

Objectives and Aims
Through a systematic exploration of resolution dependence in the gery zone this project aims to:

• Gain insight and understanding of model behaviour in the grey zone with and without convential convection parameterizations
• Provide guidance and benchmarks for the design of new scale-aware convection parameterizations that can operate in the grey zone

A First Case for the Grey Zone
If we define the Grey Zone as the range of resolutions in which both the resolved and the subgrid contributions to the convective transport give a non-negligible contribution, it is immediately clear that the range of the Grey Zone is dependent on the type of convection. The Grey Zone for convection in the clear convective boundary layer is in the rang of 100 m to a few kilometres while for deep cumulus convection this range will extend to 10's of kilometres, depending on how the convection organises. So the Grey Zone project will have to look at different forms of convection.

The first case proposed in the Grey Zone Project is a cold air outbreak such as observed during the CONSTRAIN field campaign. Cold air outbreaks are a common feature in the winter time to the north of the British Isles where cold air from the polar cap sweeps off the ice edge over open water. The convection begins as organised rolls near to the ice edge but eventually changes into open cellular convection as the boundary layer evolves. These cellular structures can be as large as 50 km and their spatial structure does influence the convective transport Cold air outbreaks have been a modelling challenge for numerical weather and climate models since many years. As this will be the first intercomparison study on a cold air outbreak it is already interesting in its own right. In addition the resolution dependence will be explored in a systematic way in the spirit of the Grey Zone Project.

Grey Zone Committee

Pier Siebesma
Royal Netherlands Meteorology Institute
De Bilt, The Netherlands
siebesma@knmi.nl

Andy Brown
UK Met Office
Exeter, United Kingdom
andy.brown@metoffice.gov.uk

Martin Miller
martin.miller@ecmwf.int

Jeanette Onvlee
Royal Netherlands Meteorology Institute
De Bilt, The Netherlands
onvlee@knmi.nl