Effects of Solar Heating on the Indirect Effect of Aerosols as Deduced from Observations of Ship Tracks

Matthew W. Christensen

College of Oceanic and Atmospheric SciencesOregon State University

GOAL: Use ship tracks to compare indirect effects of aerosols on marine stratus with particular attention focused on the effects of solar heating on the response of the clouds.

Figure 2.10

Aerosol Effects in the Atmosphere

Cloud Albedo Effect

Lifetime EffectDirect Effect

Semi-direct Effect

More CCN = Higher Reflectivities

More CCN = More Cloudsalso

Figure 2.10 IPCC 4th Assessment Report

Following Ship Tracks

Ship Track Formation• Low clouds in cold upwelling ocean regions.• Neutral stability.• Moderate winds in a shallow boundary layer.• Strong capping inversion.

Ship Track Evolution• Ship track length increases.• Ship track width increases (dispersion)• Ship tracks drift due to wind advection.

Tools• MODIS on Satellites Aqua and Terra• NCEP reanalysis wind vectors.• Mean cloud top height ~1km 925 mb pressure-level winds

July 28th 2001 1950 UTC (TERRA)

Visible Near Infrared

Ship Track IdentificationAfternoonMorning

• Track pairs were identified through visual inspection

July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Ship Track Identification

• Track pairs are identified through visual inspection.

• Automated pixel identification as ship and controls.

AfternoonMorning July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Ship Track Identification

• Track pairs are identified through visual inspection.

• Automated pixel identification as ship and controls.

• NCEP wind field is used to predict the position of the Terra ship track at the time of the Aqua pass.

AfternoonMorning July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Ship Track Identification

• Track pairs are identified through visual inspection.

• Automated pixel identification as ship and controls.

• NCEP wind field is used to predict the position of the Terra ship track at the time of the Aqua pass.

• 2nd order line fit to Aqua and predicted Terra positions

AfternoonMorning July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Ship Track Identification

• Track pairs are identified through visual inspection.

• Automated pixel identification as ship and controls.

• NCEP wind field is used to predict the position of the Terra ship track at the time of the Aqua pass.

• 2nd order line fit to Aqua and predicted Terra positions

• Segment length ~30 km.

AfternoonMorning July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Ship Track Identification

• Track pairs are identified through visual inspection.

• Automated pixel identification as ship and controls.

• NCEP wind field is used to predict the position of the Terra ship track at the time of the Aqua pass.

• 2nd order line fit to Aqua and predicted Terra positions

• Segment length ~30 km.

• Ship and control pixels showing retrieved droplet radius.

AfternoonMorning July 19th 2002 1840 UTC (Terra) July 19th 2002 2150 UTC (Aqua)

Droplet RadiusMorning clouds (Terra) Afternoon clouds (Aqua)

OVERCAST CONDITIONS

• Polluted clouds have smaller droplet radii than unpolluted clouds.

• Droplets decrease in size throughout the day.

• Decrease in size is less pronounced for polluted clouds.

Unpolluted clouds could be losing large droplets through drizzle more rapidly than evaporation of small droplets through enhanced entrainment for polluted clouds.

Optical DepthMorning clouds (Terra) Afternoon clouds (Aqua)

OVERCAST CONDITIONS

• Polluted clouds have larger optical depths than nearby unpolluted clouds.

• Terra optical depths are larger than Aqua by ~11%. Morning clouds are thicker than afternoon clouds.

• Differences in optical depth between polluted and unpolluted clouds are ~ equal for morning and afternoon.

Cloud Liquid Water AmountMorning clouds (Terra) Afternoon clouds (Aqua)

OVERCAST CONDITIONS

• Terra liquid water amounts are larger than Aqua by ~15%

morning clouds are thicker than afternoon clouds.

• For overcast conditions, polluted clouds have less liquid water than nearby unpolluted clouds overlying free troposphere sufficiently dry that the increased entrainment in clouds

with smaller droplets leads to the drying of polluted clouds as suggested by results of LES model results reported by Ackerman et al. (2004).

• Unpolluted clouds lose more liquid water than polluted clouds throughout the day.

approximately two thirds of this loss could be due to entrainment and solar heating with one third due to drizzle

Daytime Changes Droplet Radius (3.7µm)

Ensemble Average (Segrin et al., 2007)

Cloud Tracked Average

Ship Controls Ship – Controls Ship Controls Ship - Controls

Terra 10.1 ± 2.1 12.6 ± 3.1 -2.5 ± 0.1 9.9 ± 1.8 12.7 ± 2.6 -2.9 ± 0.1

Aqua 10.2 ± 2.3 12.6 ± 3.1 -2.4 ± 0.1 9.6 ± 2.2 12.0 ± 3.0 -2.3 ± 0.1

Terra – Aqua -0.1 ± 0.1 0.0 ± 0.1 0.3 ± 0.1 0.8 ± 0.1

Optical Depth

Ship Controls Ship – Controls Ship Controls Ship - Controls

Terra 18.6 ± 7.0 16.6 ± 7.0 2.0 ± 0.1 16.7 ± 6.2 14.3 ± 6.1 2.4 ± 0.2

Aqua 16.4 ± 6.9 14.2 ± 6.3 2.2 ± 0.1 14.9 ± 5.5 12.6 ± 5.1 2.3 ± 0.2

Terra – Aqua 2.2 ± 0.1 2.4 ± 0.1 1.8 ± 0.2 1.7 ± 0.2

Liquid Water Path (gm-2)

Ship Controls Ship – Controls Ship Controls Ship - Controls

Terra 127 ± 58 139 ± 69 -12.0 ± 1.4 112 ± 52 120 ± 55 -7.8 ± 1.4

Aqua 112 ± 59 120 ± 62 -7.3 ± 1.0 98 ± 52 101 ± 50 -3.0 ± 1.4

Terra – Aqua 15 ± 1.4 19 ± 1.4 13.8 ± 1.9 18.5 ± 2.0

N = 237NTerra = 659

NAqua = 545

• Cloud susceptibility is the increase in cloud albedo resulting from the addition of 1 [cloud droplet cm-3], keeping cloud liquid water constant (Platnick and Twomey, 1994)

• Clean clouds (low concentration of larger droplets) will be more susceptible than polluted clouds (high concentration of smaller droplets).

• Change in reflectance is proportional to:

Morning clouds from this study were more susceptible to the ship pollution plumes than were the clouds taken from the ensemble averages.

Evidently the more susceptible clouds stand a better chance of showing up in the afternoon pass than the less susceptible clouds.

Cloud Susceptibility

Re (3.7-µm)

First Second Third

Ship 8.3 ± 1.3 10.0 ± 0.9 11.4 ± 1.5

Controls 9.8 ± 1.2 12.8 ± 0.8 15.6 ± 1.2

Optical Depth

First Second Third

Ship 16.1 ± 4.4 16.3 ± 7.7 17.6 ± 6.2

Controls 15.5 ± 4.7 14.2 ± 7.5 13.2 ± 3.6

0.04 0.15 0.33

Morning Observations

Satellite Viewing GeometryDoes the viewing geometry of ship tracks common to both satellites affect the outcome?

Terra: morningdescending

orbiter

Aqua: AfternoonDescending

orbiter

AquaSun Glint

TerraSun Glint

Backscattered

Sunlight

Relative Viewing Geometry

Terra and Aqua

Terra and Remapped Terra

Re (µm)

Terra 12.8 ± 0.17

Aqua 12.0± 0.19

Re-mapped Terra 12.7± 0.23

Terra – Aqua 0.8 ± 0.11

Terra – Re-mapped 0.03 ± 0.23

Remapped segments are constructed using the Terra observations nearest in satellite zenith angle to the Aqua viewing geometry.

Mean differences and standard errors of the means for Terra and Aqua are more than twice as large as the differences for the Terra and Remapped Terra. Error incurred through viewing geometry is convincingly small.

Both Terra and Aqua observations show droplet radius growing towards the coast. Terra observations are consistent with the Aqua observations

Majority of segments were viewedIn the region of backscattered sunlight

Dynamics of Marine StratocumulusMixing Mechanisms1) Cloud top radiative cooling.

2) Evaporation of droplets near cloud top.

Decoupling Mechanisms1) Daytime solar heating.

2) Evaporation of droplets below cloud base.

Morning clouds coupled to the boundary layer tend to have larger entrainment rates than decoupled afternoon clouds.

Polluted vs. Unpolluted Clouds1) Polluted clouds have stronger

evaporative cooling. Larger entrainment rates (cloud top

drying)

2) Precipitation is suppressed in polluted clouds. Stronger coupling (cloud base moistening)Adapted from Nieuwstadt and Duynkerke (1996)

SYNOPTIC ANTICYCLONIC SUBSIDENCE

INV

ER

SIO

NC

LOU

D L

AY

ER

h

SU

BC

LOU

D L

AY

ER

Surface fluxes of heat and moisture

Sea

Sol

ar r

adia

tion

heat

ing

the

body

of

the

clou

d Long wave radiation causing

cooling at cloud top and heating cloud

base

Turbulent entrainment at cloud top

Evaporative cooling at cloud top

Warm updraft from cloud base

Down and updrafts produce mixing

CLOUD TOP

CLOUD BASE

Microphysical interactions

causing precipitation

Latent heat release

Evaporative cooling below cloud base supporting stability

Summary of Findings from Ship Tracks

Effects of Solar Heating on Cloud Properties

Overcast Conditions

• Droplet radius, optical depth, and liquid water amounts decreased throughout the day.

• Daytime decrease in droplet radius is greater for unpolluted clouds.

• Daytime decrease in optical depth is approximately the same for both polluted and unpolluted clouds.

• Daytime decrease in liquid water amount is greater for unpolluted clouds.

Future Work

• Increase the data base to include more ship track pairs.

• Extend study for partly cloudy conditions.

• Use geostationary observations to track diurnal changes in ship tracks.