Stereo photogrammetry reveals substantial drag on cloud thermals

Stereo photogrammetry, which uses two synchronized cameras to measure three‐dimensional positions, is applied here to ascertain whether drag plays a role in the ascent of cloud thermals. In particular, stereo cameras are used to measure the sizes and speeds of cloud thermals in Florida. Using the ve...

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Bibliographic Details
Published in:Geophysical research letters 2015-06, Vol.42 (12), p.5051-5057
Main Authors: Romps, David M., Öktem, Rusen
Format: Article
Language:English
Subjects:
Ascent
Buoyancy
Cameras
cloud thermals
Clouds
Coefficients
Dimensional measurement
Drag
Drag coefficient
Drag coefficients
Form drag
In situ measurement
Mathematical analysis
Mathematical models
Momentum
Momentum equation
Oceanic eddies
Photogrammetry
Simulation
stereo photogrammetry
Thermals
Vertical momentum
Vortices
Wave drag
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Summary:Stereo photogrammetry, which uses two synchronized cameras to measure three‐dimensional positions, is applied here to ascertain whether drag plays a role in the ascent of cloud thermals. In particular, stereo cameras are used to measure the sizes and speeds of cloud thermals in Florida. Using the vertical momentum equation, it is found that a substantial amount of drag (a drag coefficient on the order of 1) is needed to match both the stereo‐photogrammetric data and the known buoyancy of clouds from previous in situ measurements and large‐eddy simulations. Empirical data on form drag and theoretical calculations of wave drag reveal that, for the observed Froude numbers of cloud thermals, a drag coefficient of about one is to be expected. Key Points Stereo photogrammetry can track dimensions and velocities of cloud thermals Stereo data can only be reconciled with known buoyancies if drag is substantial Wave drag, which is usually neglected, can generate the required drag
ISSN:0094-8276
1944-8007
DOI:10.1002/2015GL064009