PAOC Colloquium: Dan Cziczo (MIT)
|September 18th, 2017
||Ida Green Lounge
Title: Better Understanding Climate, Precipitation and Atmospheric Chemistry by Understanding the Formation of Mixed Phase Clouds
Abstract : Field and laboratory measurements using cloud chambers have been used to understand the atmospheric abundance of droplet and ice nucleating particles and to, in turn, construct parameterizations for mixed-phase and completely glaciated clouds in weather and climate models. This seminar investigates the measurement of droplet and cloud concentration and sizing using cloud chambers. We show here that assumptions of ideal instrument behavior are not supported by measurements. Non-ideal instrument behavior, which is likely inherent to varying degrees in all cloud chambers, is caused by exposure of particles to different humidities and/or temperatures than predicated from theory. This can result in a systematic, and variable, underestimation of reported cloud element concentrations. We further investigate the effects of undercounting on simulated cloud forcing in a global climate model. For ice clouds in particular, we find that simulated long wave ice-bearing cloud forcing in a global climate model can vary up to 0.8 W/m2 and can change sign from positive to negative within the experimentally constrained bias range. We’ll conclude with a discussion of possible instrument improvements and how these can improve our understanding of climate, precipitation and atmospheric chemistry.
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