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The common occurrence of highly supercooled drizzle and rain near the coastal regions of the western United States
Author(s) -
Rosenfeld Daniel,
Chemke Rei,
DeMott Paul,
Sullivan Ryan C.,
Rasmussen Roy,
McDonough Frank,
Comstock Jennifer,
Schmid Beat,
Tomlinson Jason,
Jonsson Haflidi,
Suski Kaitlyn,
Cazorla Alberto,
Prather Kimberly
Publication year - 2013
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/jgrd.50529
Subject(s) - drizzle , environmental science , ice crystals , atmospheric sciences , cloud condensation nuclei , orographic lift , ice nucleus , freezing rain , icing conditions , supercooling , aerosol , icing , meteorology , climatology , snow , geography , geology , physics , precipitation , nucleation , thermodynamics
Highly supercooled rain and drizzle from cloud tops at −12 to −21°C down to the 0° isotherm was documented by aircraft observations in clouds over a wide range of meteorological situations under relatively pristine marine aerosol conditions. The Gulfstream‐1 aircraft during the CalWater campaign in February and early March 2011 measured clouds over the coastal waters of northern California, orographically triggered convective clouds over the foothills of the Sierra Nevada, and orographic layer clouds over Yosemite National Park. Supercooled drizzle in layer clouds near Juneau, Alaska, was measured by the Wyoming King Air as part of a FAA project to study aircraft icing in this region. Low concentrations of cloud condensation nuclei (CCN) were commonly observed in all of these clouds, allowing for the formation of clouds with small concentrations of mostly large drops that coalesced into supercooled drizzle and raindrops. Another common observation was the absence of ice nuclei (IN) and/or ice crystals in measurable concentrations, associated with persistent supercooled drizzle and rain. Average ice crystal concentrations were 0.007 l −1 at the top of convective clouds at −12°C and 0.03 l −1 in the case of layer clouds at −21°C. In combination, these two conditions of low concentrations of CCN and very few IN provide ideal conditions for the formation of highly supercooled drizzle and rain. These results help explain the anomalously high incidences of aircraft icing at cold temperatures in U.S. west coast clouds and highlight the need to include aerosol effects when simulating aircraft icing with cloud models.