1.1
The lifecycle of a springtime Arctic surface-baed mixed-phase cloud
Paquita Zuidema, CIRES/NOAA/ETL, Boulder, CO
An important objective of the Surface Heat Budget of the Arctic experiment was to foster the understanding of cloud-surface-radiative feedbacks. Towards this end, the lifecycle of a long-lived surface-based mixed-phase cloud observed from May 1-9 during the Surface Heat Budget of the Arctic experiment is presented and analyzed. It is found that the liquid phase of the cloud is primarily responsible for the cloud's radiative (flux) impact, while the ice phase regulates the liquid amount. The presence of the ice phase, in turn, is highly influenced by large-scale dynamics. The overriding radiative importance of the liquid occurs despite an average cloud temperature of approximately -20 C. Upper-level ice was occasionally present, and so thin it was missed by aircraft observers, however, when present, it had an important indirect radiative impact by quickly uptaking available liquid within the lower mixed-phase cloud through sedimentation. A separate, local and cyclical ice production mechanism was also observed and is thought to reflect a preferential freezing of the larger liquid drops, followed by their fallout and then a replenishment of the large liquid drops through collision-coalescence. Synoptics controlled the presence of the upper-level ice clouds. Interestingly, the local ice formation mechanism also appeared to be correlated with the large-scale flow, as it was more active when the NCEP large-scale subsidence rate, combined with the observed inversion height, implied increased entrainment of the polluted air layer overlying the cloud. The results imply that the impact of clouds upon the Arctic surface energy budget can only be understood if both the underlying mixed-phase cloud microphysical processes, and their dependence upon large-scale dynamics, are known.
This talk is based upon observations presented within a submitted manuscript available through http://www.etl.noaa.gov/~pzuidema
.Session 1, Clouds
Monday, 9 August 2004, 9:15 AM-12:00 PM, Vermont Room
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