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Atmosphere 2018, 9(8), 289; https://doi.org/10.3390/atmos9080289

Lagrangian Cloud Tracking and the Precipitation-Column Humidity Relationship

Department of Land, Air and Water Resources, University of California Davis, Davis, CA 95616, USA
Received: 5 June 2018 / Revised: 11 July 2018 / Accepted: 19 July 2018 / Published: 26 July 2018
(This article belongs to the Special Issue Precipitation: Measurement and Modeling)
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Abstract

The tropical, oceanic mean relationship between column relative humidity and precipitation is highly non-linear. Mean precipitation remains weak until it rapidly picks up and grows at high column humidity. To investigate the origin of this relationship, a Lagrangian cloud tracking code, RAMStracks, is developed, which can follow the evolution of clouds. RAMStracks can record the morphological properties of convective clouds, the meteorological environment of clouds, and their effects. RAMStracks is applied to a large-domain radiative convective equilibrium simulation, which produces a complex population of convective clouds. RAMStracks records the lifecycle of 501 clouds through growth, splits, mergers, and decay. The mean evolution of all these clouds is examined. It is shown that the column humidity evolves non-monotonically, but that lower-level and upper-level contributions to total moisture do evolve monotonically. The precipitation efficiency of tropical storms tends to increase with cloud age. This is confirmed using a prototype testing method. The same method reveals that different tracked clouds with similar initial conditions evolve in very different ways. This makes drawing general conclusions from individual storms difficult. Finally, the causality of the precipitation-column humidity relationship is examined. A Granger Causality test, as well as regressions, suggest that moisture and precipitation are causally linked, but that the direction of causality is ambiguous. Much of this link appears to come from the lower-level moisture’s contribution to column humidity. View Full-Text
Keywords: precipitation; tropical clouds; object tracking precipitation; tropical clouds; object tracking
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Igel, M.R. Lagrangian Cloud Tracking and the Precipitation-Column Humidity Relationship. Atmosphere 2018, 9, 289.

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