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?

Aerosols Types NaturalAnthropogenic Primary Mineral DustIndustrial Soot Sea SaltMineral Dust Volcanic DustBiomass Burning Organic Aerosols Secondary DMSSO2 Volcanic SO2NOx 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)

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: Ireland: Israel: Antarctica: (all Wm-2per year between ) 0.3%

Summary  Indirect effect upon DTR likely to be reduced for more absorbing aerosols  Explanation for weekly cycle seen in the DTR? EffectTypedDTR DirectScattering-ve Absorbing+ve IndirectScattering-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 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