The Weekend Effect Does 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?
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)
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
CCN @ 0.3% 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: -0.36 Ireland: -0.52 Israel: -1.02 Antarctica: -0.28 (all Wm-2per year between1955-1995)
Summary • Indirect effect upon DTR likely to be reduced for more absorbing aerosols • Explanation for weekly cycle seen in the DTR?
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 DTR Weekend 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