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Precipitation and Cloud Structures of Intense Rain during the 2013 Great Colorado Flood

Katja Friedrich Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado

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Evan A. Kalina Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado

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Joshua Aikins Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, Colorado

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David Gochis National Center for Atmospheric Research,* Boulder, Colorado

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Roy Rasmussen National Center for Atmospheric Research,* Boulder, Colorado

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Print Publication:
01 Jan 2016
DOI:
https://doi.org/10.1175/JHM-D-14-0157.1
Page(s):
27–52
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Abstract

Radar and disdrometer observations collected during the 2013 Great Colorado Flood are used to diagnose the spatial and vertical structure of clouds and precipitation during episodes of intense rainfall. The analysis focuses on 30 h of intense rainfall in the vicinity of Boulder, Colorado, during 2200–0400 UTC 11–13 September. The strongest rainfall occurred along lower parts of the Colorado Front Range at >1.6 km MSL and on the northern side of the Palmer Divide. The vertical structure of clouds and horizontal distribution of rainfall are strongly linked to upslope flow and low-level forcing, which resulted in surface convergence. During times of weak forcing, shallow convection produced rain at and below the melting layer through collision–coalescence and, to a lesser extent, riming. A mesoscale circulation interacting with the local terrain produced convective rainfall with high cloud tops that favored ice crystal production. During moderate forcing with cloud tops slightly exceeding the 0°C level, both cold- and warm-phase microphysical processes dominated. Less rain with weaker rainfall rates was observed over the higher-elevation stations compared to the lower-elevation stations across the foothills.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Dr. Katja Friedrich, Dept. of Atmospheric and Oceanic Sciences, University of Colorado Boulder, UCB 311, Boulder, CO 80309. E-mail: katja.friedrich@colorado.edu

Abstract

Radar and disdrometer observations collected during the 2013 Great Colorado Flood are used to diagnose the spatial and vertical structure of clouds and precipitation during episodes of intense rainfall. The analysis focuses on 30 h of intense rainfall in the vicinity of Boulder, Colorado, during 2200–0400 UTC 11–13 September. The strongest rainfall occurred along lower parts of the Colorado Front Range at >1.6 km MSL and on the northern side of the Palmer Divide. The vertical structure of clouds and horizontal distribution of rainfall are strongly linked to upslope flow and low-level forcing, which resulted in surface convergence. During times of weak forcing, shallow convection produced rain at and below the melting layer through collision–coalescence and, to a lesser extent, riming. A mesoscale circulation interacting with the local terrain produced convective rainfall with high cloud tops that favored ice crystal production. During moderate forcing with cloud tops slightly exceeding the 0°C level, both cold- and warm-phase microphysical processes dominated. Less rain with weaker rainfall rates was observed over the higher-elevation stations compared to the lower-elevation stations across the foothills.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Dr. Katja Friedrich, Dept. of Atmospheric and Oceanic Sciences, University of Colorado Boulder, UCB 311, Boulder, CO 80309. E-mail: katja.friedrich@colorado.edu
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