Associations between pollution and the

effects of clouds on the Arctic LW and SW surface

radiation balanceTim Garrett

University of UtahCollaborations with Chuanfeng Zhao, Kyle

Tietze and Melissa Maestas at UU

Support from NSF and Clean Air Task Force

Arctic Stratus at full resolution (30 m x 300 m)

600 m

Cloud Radiative Forcing

Seasonality of Arctic Haze Winter/Spring Increase in Aerosol Nitrate and Sulfate

Sources: Diesel and gasoline engines

Coal fired power plants

0.06

0.04

0.02

0.00

1/82 1/84 1/86 1/88 1/90 1/92 1/94 1/96 1/98 1/00 1/02 1/04

0.04

0.03

0.02

0.01

0.00

1/98 1/99 1/00 1/01 1/02 1/03 1/04

0.4

0.3

0.2

0.1

0.0

1.0

0.8

0.6

0.4

0.2

0.0

Alert

Barrow

NO3-, ug N m-3

SO4=, ug S m-3

Summer - Aerosol Direct & Indirect Effects (-ΔT)

Cloud Radiative Forcing

Most Polluted

Francis and Hunter, 2006

Francis and Hunter, 2006

Winter – Enhanced Cloud Longwave Emissivity (+ΔT)

Thin, clean cloudPoor insulatorHeat escapes

Thin, polluted cloud.Better insulator. Heat istrapped and re-emitted. [Garrett and Zhao, Nature, 2006]

F(LW) = T4

Cloud emissivity depends foremost on cloud thickness

Garrett et al. (2002) JAS

DJF MAM

Blackbody

<3.5 km

JJA SON

Blackbody

<3.5 km

Cloud emissivity also depends on re and potentially also Arctic pollution

Garrett et al. (2002) JAS

Measurements

ARM

remote

sensing

Barrow Site

ERS-Gome satellite Ozone profile

NOAA

aerosol

Temperature and water vapor profiles

CO2

Strat. O3

dirty window

Looking up with FTIR at Barrow

(looking up at outer space)

Retrieval bands

Upper hazequartile

Lower haze quartile

All LowCloud

CleanPolluted

Liquid Cloud

Ice Cloud

Forcing normalized by monthly low cloud cover

Warming

Cooling

10 W/m2 ~ 2.5 K

Liquid Cloud

Microphysics

Radiation

Dynamics

Climate

Clouds

Nominally clean Nominally polluted

€

€

€

FLW FLW FLW

A-train Arctic Clouds:Winter (DJF 06)

Low clouds(z < 3km)

High clouds(z > 7 km)

Thick clouds (dz > 7 km)

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Kay et al. (in prep)

Arctic low cloud anomaly in 2007

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2006 2007Kay et al. (in prep)

Microphysics

Radiation

Dynamics

Climate

Clouds

What is the direction of this arrow?

Summary

• Seasonal pollution is associated with changes in low-level Arctic cloud properties– Higher longwave cloud emissivity– Net surface warming in winter and spring

(about 5 W/m2) – Compensating surface cooling in summer if

surface is dark

Summary

• Increases in downwelling longwave fluxes occur in late winter and early spring, at the beginning of the melting ‘push’

Summary

• The CRF increases are larger than would be expected from effective radius decreases alone

• There may be interesting interactions between pollution and cloud dynamics, associated with enhanced cloud top radiative cooling– Cloud cover– Cloud circulations– Cloud top entrainment– Cloud forcing?