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Aerosol-Precipitation Responses Deduced from Ship Tracks as Observed by CloudSat Matthew W. Christensen 1 and Graeme L. Stephens 2 Department of Atmospheric.

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Presentation on theme: "Aerosol-Precipitation Responses Deduced from Ship Tracks as Observed by CloudSat Matthew W. Christensen 1 and Graeme L. Stephens 2 Department of Atmospheric."— Presentation transcript:

1 Aerosol-Precipitation Responses Deduced from Ship Tracks as Observed by CloudSat Matthew W. Christensen 1 and Graeme L. Stephens 2 Department of Atmospheric Sciences, Colorado State University 1 Jet Propulsion Laboratory, California Institute of Technology 2 Cloud Top Altitude Response Influence of Ship Plumes on Precipitation Objective Determine how clouds respond to aerosol plumes from ships. In particular, how does pollution affect cloud top altitude and rainfall? Each track was logged by hand using MODIS 2.1 and 3.7-μm imagery. Four regions were selected as hunting grounds for ship tracks. Example of ship track analysis 1)Identify ship tracks in MODIS 2.1-μm imagery. 2)Determine cloud type subjectively as closed or open cell. 3)Obtain polluted and unpolluted pixel locations using an automated detection method based on the contrast in the MODIS 2.1-μm radiances. 4)Collocate CALIPSO and CloudSat observations to the ship track domain. Ship Track Database Rain Rate Rain rates were averaged over raining and non-raining clouds. Open cells have heavier mean rainfall than closed cell clouds. Both the intensity and the spatial coverage of the rainfall (rain cover fraction) change according to the cloud type. Main Result: Aerosol suppresses rainfall in closed cellular clouds (-63%). Aerosol enhances rainfall in open cellular clouds (+88%). Rain Cover Fraction Rain cover fraction = # Raining Profiles / # Cloudy Profiles Open cell clouds have a higher likelihood of raining than closed cell clouds. The response to aerosol primarily depends on the stratocumulus regime. Main Result: Aerosol reduces the spatial extent of rainfall in closed cellular clouds (-55%). Aerosol enhances rain cover fraction in open cellular clouds (+22%). Radar Reflectivity Reflectivity was binned vertically and normalized by the cloud top height (ztop) over the polluted (Ship) and unpolluted (Cons) clouds. Larger reflectivities were observed in open cell clouds compared to closed cells. Aerosol plumes reduced mean reflectivity throughout the cloud layer by ~3dBZ in the closed cell regime resulting in a 50% reduction of the received power. Differences in reflectivity were insignificant in the open cell regime. MODIS cloud properties averaged over 20-pixel long segments positioned at the CALIPSO/CloudSat ship track overpass. Segments had to have at least: twenty MODIS 1-km cloudy pixels (MYD06), five CALIOP cloud top height observations (CALIPSO version 3), and four CloudSat profiles (raining/non-raining  2C-PRECIP-COLUMN) identified as polluted and as unpolluted from either side of ship tracks. Segments analyzed: 367 closed cell and 109 open cell cases. Closed Cell Regime Precipitation (2C-PRECIP-COLUMN) red: Non-raining Open Cell Regime Closed Open Summary Acknowledgements: This work was supported through the NASA grants NNX07AR11G, NAS , and NNX09AK02G. Data from MODIS, CALIPSO, and CloudSat provide evidence that aerosol plumes from ships modify the microphysical and dynamical properties of marine boundary layer clouds. The extent of the dynamical response depends primarily on the mesoscale structure of the stratocumulus cloud field. For regions of closed cells, aerosol plumes decrease liquid water amounts, drizzle, and have no affect on cloud top height. Departures in rainfall were largely due reductions in rain cover fraction Ship plumes ingested into open cells result in deeper and brighter clouds with higher liquid water amounts and rain rates. The local aerosol indirect forcing was more than five times larger for ship tracks observed in the open cell regime (-59 W m -2 ) compared to those identified in the closed cell regime (-12 W m -2 ). Details describing this research can be found here: Christensen, M. W., and G. L. Stephens (2011), Microphysical and macrophysical responses of marine stratocumulus polluted by underlying ships: Evidence of cloud deepening, J. Geophys. Res., 116, D03201, doi: /2010JD Droplet RadiusOptical Depth Liquid Water Path Evidence of Cloud Deepening Dynamical Effect Not Observed Ship tracks are elevated by ~15% in regions of open cells. Changes in cloud top height are negligible in regions of closed cells. June 2006 – December 2009 A11B-0063 Lower troposphere stability (LTS = Θ 700 -Θ sfc ) is the difference in potential temperature between 700 hpa and the surface (ECMWF-AUX). Open cells are typically associated with lower stability, higher moisture, and lower cloud fraction than closed cells. Ship plumes ingested into an unstable atmosphere have a higher likelihood of increasing the cloud top height. Cloud Top Height (Differences) Difference (Ship – Cons) mean standard deviation mean standard error Unpolluted Clouds (Cons) Difference (Ship – Cons) mean standard deviation mean standard error Unpolluted Clouds (Cons) MODIS Cloud Properties Droplet growth is inhibited in polluted clouds. Closed cell regime: ship tracks lose liquid water  Enhanced entrainment of sufficiently dry air brought about by smaller droplets leads to the drying of the polluted clouds as suggested by the results of a large eddy simulation (LES) model (Ackerman et al., Nature, 432, 2004). Open cell regime: ship tracks thicken and gain liquid water  Enhanced entrainment in polluted clouds residing under a diffuse temperature inversion with sufficient moisture enables the ship track to deepen, accumulate liquid water, and produce heavier drizzle over time compared to the unpolluted clouds. Differences (Ship – Cons) MODIS: 2.1-μm CALIPSO Orbit Total Attenuated Backscatter (CALIPSO) Radar Reflectivity (CloudSat) grey: Ground Clutter Precipitation (2C-PRECIP-COLUMN) Total Attenuated Backscatter (CALIPSO) Radar Reflectivity (CloudSat) 532 nm mean standard error


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