C1. Stochastic representation of intermittent boundary-layer mixing

A new atmospheric boundary-layer (ABL) scheme based on an equilibrium turbulent kinetic energy (TKE) formulation has been developed and tested in the single column model (SCM) employing the CCCma 4th-generation atmospheric physics package which is the basis for CCCma's regional (CanRCM4) and global (CanAM4) climate models. This scheme has been coupled to a stochastic parameterization of the effects of intermittent mixing in the stable ABL. The current version of the scheme represents this effect through a simple stochastic enhancement of the vertical diffusivity for momentum in the capping inversion at the top of the ABL. It has been used successfully in a cloud-free version of the SCM to simulate observed climatological features of the diurnal cycle of the probability density function (PDF) of lower atmospheric wind speeds, in particular the characteristic long tail of the PDF toward higher wind speeds.

This boundary layer scheme will be enhanced to include coupling to the statistical cloud scheme, the effects of non-local mixing in convectively active boundary layers, and coupling to the cumulus parameterizations for deep and shallow moist convection employed in CanRCM4 and CanAM4. Further testing in the SCM using a range of case studies will be carried out to evaluate and optimize its performance in a wide range of boundary-layer regimes and explore more physically based representations of episodic mixing processes. Following this, the new fully coupled scheme will be implemented and evaluated in climate simulations using both CanRCM4 and CanAM4, and will be made available for implementation in GEM and CRCM5.

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