Transcript AGU Fall Meeting 2011 - Colorado State University

Aerosol-Precipitation Responses Deduced from
Ship Tracks as Observed by CloudSat
Matthew W.
1
Christensen
and Graeme L.
A11B-0063
2
Stephens
Department of Atmospheric Sciences, Colorado State University1
Jet Propulsion Laboratory, California Institute of Technology2
Objective
Influence of Ship Plumes on Precipitation
Determine how clouds respond to aerosol plumes from ships. In
particular, how does pollution affect cloud top altitude and rainfall?
• 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.
Cloud Top Altitude Response
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.
Ship Track Database
Open
Closed
Radar Reflectivity
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.
• Reflectivity was binned vertically and normalized by the cloud top height (ztop)
over the polluted (Ship) and unpolluted (Cons) clouds.
June 2006 – December 2009
• 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.
Unpolluted Clouds (Cons)
Rain Rate
Difference (Ship – Cons)
Cloud Top Height (Differences)
• Rain rates were averaged over raining and non-raining clouds.
mean
Example of ship track analysis
• Open cells have heavier mean rainfall than closed cell clouds.
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.
Closed Cell Regime
Open Cell Regime
mean
standard
error
• Both the intensity and the spatial coverage of the rainfall (rain
cover fraction) change according to the cloud type.
• Open cells are typically associated with
lower stability, higher moisture, and
lower cloud fraction than closed cells.
Main Result:
Aerosol suppresses rainfall in closed cellular clouds (-63%).
Aerosol enhances rainfall in open cellular clouds (+88%).
Unpolluted Clouds (Cons)
• Ship plumes ingested into an unstable
atmosphere have a higher likelihood of
increasing the cloud top height.
Difference (Ship – Cons)
Rain Cover Fraction
Summary
• Rain cover fraction = # Raining Profiles / # Cloudy Profiles
MODIS: 2.1-μm
mean
standard
deviation
mean
standard
error
• Open cell clouds have a higher likelihood of raining than closed cell clouds.
• The response to aerosol primarily depends on the stratocumulus regime.
CALIPSO
Orbit
Total Attenuated Backscatter
(CALIPSO)
Total Attenuated Backscatter
Main Result:
Aerosol reduces the spatial extent of rainfall in closed cellular clouds (-55%).
Aerosol enhances rain cover fraction in open cellular clouds (+22%).
(CALIPSO)
532 nm
Radar Reflectivity
(CloudSat) grey: Ground Clutter
Radar Reflectivity
(CloudSat)
(2C-PRECIP-COLUMN) red: Non-raining
Precipitation
(2C-PRECIP-COLUMN)
• 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.
Droplet Radius
Optical Depth
• 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.
Differences (Ship – Cons)
MODIS Cloud Properties
• Droplet growth is inhibited in polluted clouds.
Precipitation
• Lower troposphere stability (LTS =
Θ700-Θsfc) is the difference in potential
temperature between 700 hpa and the
surface (ECMWF-AUX).
standard
deviation
Liquid Water Path
mean
standard
error
• 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:10.1029/2010JD014638.
Acknowledgements: This work was supported through the NASA
grants NNX07AR11G, NAS5-99237, and NNX09AK02G.