The Weekend EffectDoes it really exist in surface temperature

and if so, what is the cause?

David Ridley(Ken Carslaw and Martyn Chipperfield)

The Weekend Effect

> Why look at the weekend effect?> Aerosol & Cloud - Direct Effects - Indirect Effects - Radiative Effects

> Global Dimming - The evidence - Implications

> The Weekend Effect

Importance of the weekend effect

Definition: The weekend effect in DTR is that the average

DTR on weekdays appears to be significantly different to that of the weekend.

DTR is an indicator of global climate change Source likely to be anthropogenic in origin A way of quantifying aerosol effects?

Aerosols TypesNatural Anthropogenic

Primary

Mineral Dust Industrial Soot

Sea Salt Mineral Dust

Volcanic Dust Biomass Burning

Organic Aerosols

Secondary

DMS SO2

Volcanic SO2 NOx

VOCs VOCs

NOx

Aerosol Lifetime Lifetime of the order of days Growth processes

• Nucleation• Coagulation

Removal processes• Dry deposition• Rain out• Impact scavenging

Aerosol Direct Effect Aerosol can scatter and absorb SW

radiation

Important parameters: - Aerosol Optical Depth (AOD)

- Chemical Composition - Single Scattering Albedo (SSA)

sa

sSSA

Aerosol Direct Effect Models show reduction in solar flux reaching

surface of ~27Wm-2 for SSA=1.0 (purely scattering)

~90Wm-2 for SSA=0.8 (highly absorbing)

Effect upon surface temperature is very SSA dependent

(Yu et al. JGR 2002)

Aerosol Indirect Effects Albedo effect - Increase in aerosol burden increases cloud albedo

(Assumes constant liquid water path)

Aerosol Indirect Effects Semi-indirect effect - precipitation suppression leading to increased

lifetime

(D. Rosenfeld, Science, 2000)

Effect Of Cloud Upon Energy Balance

Reduces SW radiation reaching surface Reduces LW radiation leaving atmosphere

Net effect is to increase minimum temperature and decrease maximum temperature

Average cloud albedo increase of 5% would result in similar (but opposite) forcing to doubled CO2 scenario

Global Dimming Sunlight reaching the surface has consistently

decreased by ~0.3% per year over the past 40 years

Sunlight has decreased by over 20% in some regions

Arctic: -0.36Ireland: -0.52Israel: -1.02Antarctica: -0.28(all Wm-2per year between1955-1995)

CCN @ 0.3%

Summary

Indirect effect upon DTR likely to be reduced for more absorbing aerosols

Explanation for weekly cycle seen in the DTR?

Effect Type dDTR

Direct Scattering -ve

Absorbing +ve

Indirect Scattering -ve

Absorbing -ve

Motivation

Understanding anthropogenic effects

A method of quantifying aerosol effects

Because its interesting!

DTR Weekend Effect 50 years of DTR data (GDCN) Over 14000 stations worldwide 1400 WMO stations

Average of Wednesday, Thursday and Friday DTR gives weekday DTR

Average of Saturday, Sunday and Monday DTR gives weekend DTRWeekend effect (WEE) = weekend DTR – weekday DTR

DTR Weekend Effect ~0.2K magnitude on

average Coherent spatial

patterns Significant in China

and Japan

(Forster and Solomon, PNAS 2004)

Why would effect change sign? Why is effect not significant in Europe? Why does it not correlate with emissions, such as NOx? Can the effect be seen in some DTR-affecting parameter?

Cloud Cover Weekend Effect

18 Years of ISCCP cloud cover data 2.5x2.5deg

Global map weekly cycle plot

Results are insignificant Weekend effect may exist but <3% and

undetectable

DTR Weekend Effect

Why would weekend effect change sign?

Modelled Weekend Effect Normally-distributed random

dataset generated Weekend effects imposed upon

random data Idealised dataset What is the minimum weekend

effect that can be retrieved?

Weekend Effect

Is weekend effect a function of DTR variability?

Weekend Effect

Weekend effect appears to be a function of DTR variability

Variability of DTR in USA over 15% of that in Europe No weekend in China until recently!

Conclusions Clear weekend effect in aerosol

precursor gases and in aerosol measurements (AQI)

Unable to detect weekend effect in cloud cover

Weekend effect in DTR unclear

Further Research Future of global dimming

• Is the future bright?• Predicting aerosol forcings• Consequences for global warming