SPEAKER: Dr. Daniel Kirshbaum - McGill University
DATE: October 12, 2021
Time: 3:30 - 4:30 pm MST
WEBCAST: https://operations.ucar.edu/live-eol
QUESTIONS: Slido will be used to answer your questions during the seminar.
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ABSTRACT
Large-eddy simulations are conducted to quantify and physically interpret the impacts of heterogeneous, low terrain on tropical convection initiation (CI). The simulations are based on a case of shallow-to-deep convective transition over the Amazon River basin, and use idealized terrains with varying levels of ruggedness. The terrain is designed by specifying its power-spectral shape in wavenumber space, inverting to physical space with random phases for all wave modes, and rescaling the terrain to have a peak relief of 200 m. For the case in question, these modest terrain fields expedite CI by up to 2-3~h, largely due to the impacts of the terrain on the size of, and subcloud support for, incipient cumuli. Terrain-induced circulations enhance subcloud kinetic energy on the mesoscale, which is realized in the form of wider and longer-lived subcloud circulations. When the updraft branches of these circulations breach the level of free convection, they initiate wider and more persistent cumuli that subsequently undergo less entrainment-induced cloud dilution and detrainment-induced mass loss. As a result, the clouds become more vigorous and penetrate deeper into the troposphere. Smoother terrains are found to be more effective than more rugged terrains in promoting CI due to their larger spatiotemporal forcing scales, which favor wider and more plume-like clouds supported by more persistent subcloud updrafts.
