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Sugar, Gravel, Fish, and Flowers: Mesoscale cloud patterns in the Tradewinds

Abstract : An activity designed to characterise patterns of mesoscale (20 km to 2000 km) organisation of shallow clouds in the downstream trades is described. Patterns of mesoscale organisation observed from space were subjectively defined and learned by twelve trained scientists. The ability of individuals to communicate, learn and replicate the classification was evaluated. Nine‐hundred satellite images spanning the area from 48°W to 58°W 10°N to 20°N for the Boreal winter months (Dec‐Feb) over ten years (2007/2008 to 2016/2017) were classified. Each scene was independently labeled by six scientists as being dominated by one of six patterns (one of which was “no‐pattern”). Four patterns of mesoscale organisation could be labelled in a reproducible manner, and were labelled Sugar, Gravel, Fish, and Flowers. Sugar consists of small, low clouds of low reflectivity, Gravel clouds form along apparent gust fronts, Fish are skeletal network (often fishbone‐like) of clouds, while Flowers are circular clumped features defined more by their stratiform cloud elements. Both Fish and Flowers are surrounded by large areas of clear air. These four named‐patterns were identified 40% of the time, with the most common pattern being Gravel. Sugar was identified the least and suggests that unorganised and very shallow convection is unlikely to dominate large areas of the downstream Tradewinds. Some of the patterns show signs of seasonal and inter‐annual variability, and some degree of scale selectivity. Comparison of typical patterns with radar imagery suggests that even this subjective and qualitative visual inspection of imagery appears to capture several important physical differences between shallow cloud regimes, such as precipitation and radiative effects.
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Submitted on : Saturday, November 16, 2019 - 1:19:11 PM
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Bjorn Stevens, Sandrine Bony, Hélène Brogniez, Laureline Hentgen, Cathy Hohenegger, et al.. Sugar, Gravel, Fish, and Flowers: Mesoscale cloud patterns in the Tradewinds. Quarterly Journal of the Royal Meteorological Society, Wiley, 2019, (in press). ⟨10.1002/qj.3662⟩. ⟨insu-02289697⟩

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