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
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
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