Microphysical Variability of Cirrus in Tropical Storm Outflow

The response of the tropical atmosphere to changes in climate forcing has received much attention (IPCC, 2007) since a positive feedback, arising from increased supply of water vapour by convection in the tropical tropo-pause layer (TTL), may be countered by negative feedbacks due to increasing amounts of thin cirrus clouds also being formed in this region. These cloud types have been identified as a significant source of uncertainty in the earth's radiation budget (Lynch et al. 2002) with several recent studies showing that their contribution to the earth-atmosphere radiation and water budgets in the TTL can be significant. Many studies show that optically thin cirrus are prevalent in the TTL (McFarquhar et al. 2007, Massie et al. 2002 and Dessler et al. 2006). Recently it has been suggested that their frequency, and hence radiative contribution may be larger than hitherto assumed, (Lee et al. 2009). To reduce uncertainties in cloud model radiative predictions of cirrus in the TTL more intensive in-situ measurements are needed to provide better understanding of their microphysical and radiative properties as well as the spatial variability as a function of outflow fluxing to environmental air. In situ airborne microphysical measurements were conducted as part of the ACTIVE experiment, during a typical “Hector” storm, that took place in the Austral summer, on the 9th December, 2005, to the North of Darwin, Australia (12.47oS, 130.85oE). Anvil cirrus cloud penetrations were conducted by the ARA Egrett high altitude aircraft in the storm outflow region that was confined to a vertical layer between 11.5-13.2 km and with temperatures ranging from -47 to -59oC. We present an analysis of the cloud microphysical properties within the outflow region of the storm as a function of increasing distance from the storm centre, Figure 1. Key regions of aggregation, sedimentation and ice particle nucleation are identified and characterised and their evolution and interaction with the core outflow are discussed. Examples of spatial variations in observed ice particle size distributions (PSD) in the outflow region are summarized in Figures 2(a)-(c) showing latitudinal changes in IWC (g/m3), PSD slope, λ0 (/cm) and PSD intercept, no (/cm3), with increasing distance (1-4, in Figure 1) downwind of the storm. Images of the ice crystals recorded by the aircraft corresponding to each track are presented in Figures 3(a) and (b) showing the key regions influenced by aggregation, sedimentation and new ice particle nucleation and the latitudinal evolution and interaction with the core outflow. The region closest to the storm centre, displayed the highest IWC's, >2 g/m3, but with low values of no. The PSD here are dominated by large complex aggregates or chain aggregates ejected from the anvil core. Concentrations of these decline, lowering the IWC, through sedimentation, Figure 3(a) and (b). In the outermost wings of the outflow region the PSD are dominated by high concentrations of small, recently nucleated, pristine ice crystals, which present as proto bullet rosettes. The symmetry of the outflow in this case appears to have been significantly perturbed by a gravity wave generated by interaction of the Hector high level outflow with the outflow from a second storm further to the south in the Beagle Gulf. This results in asymmetry of the latitudinal structure in cloud microphysical parameters, seen most clearly in the progression of IWC profiles. The patterns are further complicated by; additional convective instability generating convective cirrus within the distant outflow; and by entrainment of environmental air. The reduction in IWC in the core following sedimentation removal of large aggregates leads to an increase in super-saturation, with two consequences - allowing ice initiation by haze freezing in the outer regions of the outflow or promoting sustained growth of the remaining smaller ice crystals within the core as suggested by the images shown in Figures 3. This would lead to the large increases in no in such regions coupled with eventual recovery in λ0. Considering the extremities of the anvil outflow there is clear and striking evidence of a transition to regions with nucleation of new ice particles. The habit of these ice crystals is very different to those in the centre of the outflow presenting as small polycrystalline or proto-bullet rosettes. The impact of these various processes on the large scale variability in cloud microphysical and optical properties will be discussed further. References Dessler et al. 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