50 Interactions between Turbulence and Ice Phase Processes in Mixed Phase Layer Clouds: Observational and Modelling Studies

Monday, 7 July 2014
Paul Alan Barrett, UK Met Office, Exeter, United Kingdom; and M. Andrejczuk, A. Blyth, and P. Brown

It is important to understand and quantify the formation of ice particles in turbulent supercooled layer clouds in order to improve model predictions of cloud systems in both mid-latitude frontal features and the Arctic boundary layer. The ice production process, which is poorly understood, is thought to proceed via the liquid phase. Traditional heterogeneous ice nucleation mechanisms do not adequately describe the formation of ice in this environment.

In order to understand the production of ice from the liquid phase it is crucial to determine the turbulent environment that controls the liquid phase. The liquid phase of an altocumulus cloud is modulated by Long Wave Radiative Cooling (LWRC) which produces negatively buoyant parcels at cloud top. The resulting turbulent circulations may then generate additional liquid water and produce the classic cellular structure that is often observed.

In-situ observations from the FAAM BAe-146 research aircraft are presented showing the turbulence structure in mid-level altocumulus clouds. The cloud-tops of these mid-level clouds are often found to slope along constant thermodynamic surfaces. This limits the utility of performing a stack of straight and level runs (SLRs) to build up a profile of the turbulence structure. Here we present a method of generating turbulence statistics in mid-level layer clouds from slant profile data. A case study comparing the two techniques, the slant profile method and a stack of SLRs, for a boundary layer stratocumulus cloud is presented.

Understanding the rate of a time dependant ice nucleation mechanism requires knowledge of the statistical distribution of in-cloud residence times of cloud drops. A modelling study using eddy-resolving models has been performed that allows this distribution to be investigated. A 3D high-resolution large eddy simulation of a mixed phase altocumulus cloud is constrained with the observed thermodynamic, dynamic and microphysical observations. Finally a 2D Lagrangian Parcel tracking model representing the liquid phase only will be used to make an assessment of the lifetimes of cloud droplets in relation to their size. Preliminary results from this modelling study will be presented.

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