Download open cell regime - Colorado State University

Aerosol-Precipitation Responses Deduced from
Ship Tracks as Observed by CloudSat
Matthew W. Christensen1 and Graeme L. Stephens2
Department of Atmospheric Sciences, Colorado State University1
Jet Propulsion Laboratory, California Institute of Technology2
Cloud Top Altitude Response
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 aligned along the ship track at the CALIPSO/CloudSat overpass.
• Segments had to have at least: twenty MODIS 1-km cloudy pixels (MYD06), two CALIOP cloud top height observations (CALIPSO version 3), and
two CPR cloud top detections (CloudSat 2B-GEOPROF) with successful rain rate retrievals (2C-PRECIP-COLUMN) identified as polluted and as
unpolluted from either side of ship tracks.
• Segments analyzed: 78 closed cell and 61 open cell cases.
Ship Track Database
Open
Closed
Aerosol Increasing Cloud Top Height
Aerosol Dynamical Effect Not Observed
Polluted clouds are elevated by
~15% in regions of open cells
Changes in cloud top height are
negligible in regions of closed cells
Radar Reflectivity
Location of ship tracks: (June 2006 – December 2009)
• Radar reflectivity binned vertically into normalized height coordinates given by the
cloud top height estimated from CloudSat. A running mean filter was applied to
both the polluted (ship) and unpolluted (con) pixels.
Each track was located and logged by hand using MODIS 2.1 and 3.7-μm
imagery. Four regions (predominately stratocumulus) were selected as hunting
grounds for ship tracks. In total, 1448 ship tracks were identified.
• Larger reflectivities are observed in open cell clouds compared to closed cells.
• Aerosol plumes reduce mean reflectivity throughout the cloud layer by ~3dBZ (one
standard deviation of the mean) in the closed cell regime resulting in a 50%
reduction of the received power.
• Reductions in reflectivity are modest in the open cell regime.
Difference (Ship – Cons)
Intensity
• Rain rates are averaged over raining and non-raining clouds detected by the
CPR (segment average for polluted and unpolluted clouds).
Closed
Open
Closed Cell
mean
mean
• Open cells have heavier rainfall than closed cell clouds.
Example of ship track analysis
1) MODIS images were constructed from the 2.1-μm radiances.
2) Cloud type was determined (subjectively) as either closed (left) or open (right) cell if the
cloud field remained uniform throughout a 100 km2 region (pink box) located over the track.
3) Colored MODIS pixels (see below) indicate the cloud droplet effective radius.
4) The automated track finding scheme (based on method from Segrin et al.,JAS,64,4330,2007)
identifies the portions of the cloud polluted by the ship and selects nearby unpolluted clouds
adjacent to the ship track to serve as controls for the analysis.
5) CALIPSO and CloudSat observations are collocated to the nearest MODIS pixels
comprising the polluted and unpolluted clouds in the ship track domain.
Closed Cell Regime
Main Result:
Aerosol suppresses rainfall in closed cellular clouds (63% decrease)
Aerosol generally enhances rainfall in open cellular clouds (88% increase)
Closed
Open
Open Cell Regime
mean
mean
MODIS: 2.1-μm
standard
deviation
standard
error
• Rainfall departures arise from changes in the intensity and spatial coverage
of the rainfall (rain cover fraction) and the response primarily depends on
the mesoscale stratocumulus cloud type.
Difference (Ship – Cons)
standard
deviation
standard
error
Rain Cover Fraction
• Rain cover fraction is the fraction of raining pixels identified from the clouds detected by
the CPR for the polluted and unpolluted portions of the ship track (#rain/#clouds pixels).
• Open cellular clouds have a higher likelihood of precipitation than closed cells.
MODIS: 2.1-μm
• The response to aerosol primarily depends on the stratocumulus regime.
CALIPSO
Orbit
Total Attenuated Backscatter (CALIPSO)
532 nm
Main Result:
Pollution significantly reduces the spatial extent of rainfall in closed cellular clouds (-55%)
Ship plumes slightly increase rain cover fraction in regions of open cellular clouds (+22%)
Ship
Control 1
Control 2
Total Attenuated Backscatter (CALIPSO)
Precipitation (2C-PRECIP-COLUMN)
Radar Reflectivity (CloudSat)
Precipitation (2C-PRECIP-COLUMN)
Open Cell
Rain rate
(mmday-1)
Height (m)
Rain rate (mmday1)
Height (m)
CON1-CON2
0.06 (0.29)
5 (3)
-0.39 (0.59)
40 (30)
SHIP-CONS
-0.75 (0.20)
5 (2)
2.23 (1.03)
131 (15)
σSHIP
0.56
17
5.75
51
σCONS
1.65
25
4.06
128
• Means and standard errors of the means for the differences in rain rate and cloud top height
between each control (CON1-CON2) and between the ship and combined controls (SHIPCONS). Also listed is the ensemble average standard deviation for the polluted (σSHIP) and
unpolluted cloud (σCONS).
• Open cells have larger variability in rainfall and cloud top height than closed cell clouds.
• Cloud top height was unchanged by the ship plume in closed cells but significantly
increased in regions of open cells.
• Departures in rainfall between the polluted and unpolluted clouds are significantly larger
than that given by the difference between controls.
 Changes in rainfall were significantly altered by the ship plume.
Summary
• Data from MODIS, CALIPSO, and CloudSat provide evidence that aerosol
plumes from ships modify the microphysical and dynamical properties of
clouds.
• The extent of the dynamical response depends primarily on the mesoscale
structure of the stratocumulus cloud field.
532 nm
MODIS Cloud Properties (Differences)
Radar Reflectivity (CloudSat)
Variability in Cloud Top Height & Rainfall
Droplet Radius
Optical Depth
Liquid Water Path
• Droplet growth is inhibited in polluted clouds.
• For regions of closed cells, aerosol plumes decrease liquid water amounts,
drizzle, and have no affect on cloud top height. Changes in rainfall were
largely due reductions in rain cover fraction.
• In the closed cell regime ship tracks lose liquid water.
 Overlying free troposphere sufficiently dry that the increased
entrainment in clouds with smaller droplets leads to the drying of polluted
clouds as suggested by the results of a large eddy simulation (LES) model
(Ackerman et al., Nature,432,1014,2004).
• Ship plumes ingested into open cells result in deeper and brighter clouds with
higher liquid water amounts and rain rates. Heavier rainfall in the nearby
clouds presumably leads to enhanced moisture convergence below the ship
track.
• In the open cell regime ship tracks thicken and gain liquid water.
 Heavy drizzle from the nearby clouds adjacent to the ship track converge
moisture through colliding cold pools below the ship track leading to
stronger updrafts and moistening the polluted clouds as suggested by the
results of a LES model (Wang and Feingold., JAS,66,3257,2009).
Further 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.
Means and standard errors of the mean are shown above for the distributions of droplet
radius, visible cloud optical depth, and liquid water path: polluted – controls for the
ensemble of closed cell clouds (red solid line) and open cell clouds (blue dashed line).
Acknowledgements: This work was supported through the NASA
grants NNX07AR11G, NAS5-99237, and NNX09AK02G.