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Colloquium

Department of Atmospheric Science

Tues., Oct. 25, 3:10 pm, EN6085

Aerosol Impacts on Orographic Precipitation: From Accumulation Mode to Giant Sea-salt Aerosols

Dr. Alison Nugent

NCAR/EOL

Abstract

Mountains around the globe control precipitation patterns and water resources.  Here the focus of the first half of the presentation is on understanding thermally driven orographic precipitation in the tropics over a small island.  An aircraft dataset from the Dominica Experiment (DOMEX) which took place in the eastern Caribbean is utilized. The UWKA aircraft measured upstream and downstream airflow properties as well as the convective clouds and precipitation over the island. When the convection is thermally driven, DOMEX observations show clear evidence of aerosol-cloud-precipitation interactions and the aerosol-aware Thompson microphysics scheme in WRF is used to investigate.  Accumulation mode aerosols lofted from the island surface impact the precipitation development in the orographic clouds, leading to reduced precipitation.
In the second part of the presentation, the growth of giant sea-salt aerosols will be explored.  Condensational growth of cloud droplets is often assumed to be a well-understood process using the droplet growth equation.  After cloud condensation nuclei are activated into cloud droplets at cloud base, the growth rate of these droplets is assumed to be inversely proportional to their radius, and consequently the droplet spectrum is assumed to narrow with altitude.  However, this effect ignores the solute mass contained within the cloud droplet and when considering the growth on cloud droplets grown on giant sea-salt aerosol particles (GCCN, dry radius > 0.5 μm), this process is very different. These droplets typically remain concentrated salt solutions, and their condensational growth is so rapid that they attain precipitation drop sizes through condensation only.  This has strong application to orographic precipitation production over coastal mountain ranges where oceanic air containing GCCN is rapidly lifted by terrain.  Potential implications include accelerated initiation of warm rain, and features such as non-bright band precipitation.

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