Condensate variability in ice clouds - a life cycle effect?

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Description

The analysis of ground-based cloud condensate retrievals from five ARM sites has identified a regime dependence of liquid condensate variability in warm boundary layer clouds, and has led to the development of a new parametric description for use in GCMs. Here, we further explore observations in order to extend the new parameterization to include ice clouds. Shielding by low-level clouds or precipitation is a more common problem for upper level ice clouds, so satellite-borne observations from active instruments (radar, lidar) are used to complement the ground based observations of ice condensate variability and provide a more complete picture. Consistent with previous studies, we find that ice condensate variability is greater in regions typically associated with deep convection (Tropics). However, we also identify an apparent height dependence, and hypothesize that this height dependence can be best understood as a life cycle effect with greater variability found in "young" congestus clouds barely reaching beyond the melting level, and lower variability found in "mature" clouds that have reached the tropopause and detrained into large anvils. A superposition of individual clouds at different life stages then leads to an apparent height dependence in the long term mean. The prognostic nature of the cloud scheme in the IFS provides a limited form of "memory" for the cloud's state. The relative contribution from the convection scheme to the existing cloud's ice mass is used as a proxy for the modelled cloud's life stage and tested as part of a new parameterization for ice cloud condensate variability.