Extended Abstract (44K)P4.3
Local mixing with external control in the Met Office Unified Model stable boundary layer
Alastair G. Williams, Met Office, Bracknell, Berks., United Kingdom
The current first-order closure scheme for the Met Office Unified Model (UM) stable boundary layer (SBL) calculates eddy diffusivities in terms of a Richardson number based on local gradients. Such an approach represents well the accepted localised behaviour of turbulence in stratified flows, but is limited in its ability to control the depth and vertically-integrated mixing characteristics of the SBL as a whole. This leads to problems such as unsatisfactory fog forecasts and severe surface decoupling. A new scheme for the UM is currently being developed, which attempts to retain a local character for turbulence in stable conditions, whilst at the same time imposing a strong external control upon gross SBL mixing behaviour. This is achieved by prescribing a "prototype" profile for the local Monin-Obukhov length, as a function only of surface fluxes and a diagnosed SBL depth, which describes the overall turbulence state towards which local turbulence should tend to relax at each level if external forcings remain steady. This profile is then used as input to a theoretical model for the equilibrium SBL, based upon second-order flux/variance budget relationships and solved using a simple iterative technique, which provides the required eddy diffusivities. The equilibrium model also provides a form of the turbulence kinetic energy budget that is used as the basis for the SBL depth diagnosis. Aspects of the concept and implementation of the new scheme will be discussed, along with illustrations of its performance in a single column version of the UM.
Poster Session 4, Stable BLs; Chemistry and Dispersion in the ABL
Tuesday, 16 July 2002, 2:00 PM-2:00 PM
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