Dynamics of in-cloud turbulence and cloud-surface turbulent coupling in the Arctic Stratocumulus topped boundary layer

This thesis examined the behaviour of the cloud dynamics and mixed layer depth (MLD), the layer of turbulence mixed down into the boundary layer by in-cloud driven processes, in response to changes in the cloud’s microphysical and turbulent structure, via both observational and model analysis. The behaviour of the MLD was examined in observational data collected during the Arctic Summer Cloud Ocean Study (ASCOS) project’s field campaign in late August 2008. The dissipation rate data enabled periods of decoupling between cloud and surface to be diagnosed and the MLD calculated. It was found that of all the cloud properties examined in the observational data available the MLD was most sensitive to variations in Liquid Water Path (LWP), most notably when the LWP <40 g m-2. A LWP of 40 g m-2 being the point at which the cloud layers switch between grey- and black-body radiative states, pointing to the cause of the relationship being due to changes in the cloud layers adsorption and emission of LW radiation. Radiation modelling using the Edwards-Slingo radiation code, in combination with the profiles of the gradient Richardson number, was conducted to examine the link between turbulent and radiative structure. The top third of the cloud layer during ASCOS, the portion above the base of the temperature inversion, was found to be non-turbulent even though LW cooling was occurring within the layer. Idealised Large Eddy Model (LEM) runs were carried out based upon the ASOCS observations and the trend in LWP with MLD observed during ASCOS was seen in LEM simulations. Further LEM runs expanded upon the impact of cloud extending above the inversion on turbulence and cloud behaviour. The greater the extension of the cloud above the base of the temperature inversion, the weaker the cloud layer turbulence and shallower the MLD. LW cooling occurring within the inversion is found to primarily cause condensation rather than buoyancy driven turbulent motion, restricting LW cooling cloud driven turbulence.