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Article Type:
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A Lagrangian Study of Precipitation-Driven Downdrafts

Giuseppe Torri Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

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Zhiming Kuang Department of Earth and Planetary Sciences, and School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts

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Print Publication:
01 Feb 2016
DOI:
https://doi.org/10.1175/JAS-D-15-0222.1
Page(s):
839–854
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Abstract

Precipitation-driven downdrafts are an important component of deep convective systems. They stabilize the atmosphere by injecting relatively cold and dry air into the boundary layer. They have also been invoked as responsible for balancing surface latent and sensible heat fluxes in the heat and moisture budget of tropical boundary layers. This study is focused on precipitation-driven downdrafts and basic aspects of their dynamics in a case of radiative–convective equilibrium. Using Lagrangian particle tracking, it is shown that such downdrafts have very low initial heights, with most parcels originating within 1.5 km from the surface. The tracking is also used to compute the contribution of downdrafts to the flux of moist static energy at the top of the boundary layer, and it is found that this is on the same order of magnitude as the contribution due to convective updrafts, but much smaller than that due to turbulent mixing across the boundary layer top in the environment. Furthermore, considering the mechanisms driving the downdrafts, it is shown that the work done by rain evaporation is less than half that done by condensate loading.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JAS-D-15-0222.s1.

Corresponding author address: Giuseppe Torri, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138. E-mail: torri@fas.harvard.edu

Abstract

Precipitation-driven downdrafts are an important component of deep convective systems. They stabilize the atmosphere by injecting relatively cold and dry air into the boundary layer. They have also been invoked as responsible for balancing surface latent and sensible heat fluxes in the heat and moisture budget of tropical boundary layers. This study is focused on precipitation-driven downdrafts and basic aspects of their dynamics in a case of radiative–convective equilibrium. Using Lagrangian particle tracking, it is shown that such downdrafts have very low initial heights, with most parcels originating within 1.5 km from the surface. The tracking is also used to compute the contribution of downdrafts to the flux of moist static energy at the top of the boundary layer, and it is found that this is on the same order of magnitude as the contribution due to convective updrafts, but much smaller than that due to turbulent mixing across the boundary layer top in the environment. Furthermore, considering the mechanisms driving the downdrafts, it is shown that the work done by rain evaporation is less than half that done by condensate loading.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JAS-D-15-0222.s1.

Corresponding author address: Giuseppe Torri, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138. E-mail: torri@fas.harvard.edu

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