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Clouds and Climate: Cloud Response to Climate Change

Clouds and Climate: Cloud Response to Climate Change. SOEEI3410 Ken Carslaw. Lecture 5 of a series of 5 on clouds and climate Properties and distribution of clouds Cloud microphysics and precipitation Clouds and radiation Clouds and climate: forced changes to clouds

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Clouds and Climate: Cloud Response to Climate Change

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  1. Clouds and Climate: Cloud Response to Climate Change SOEEI3410 Ken Carslaw Lecture 5 of a series of 5 on clouds and climate • Properties and distribution of clouds • Cloud microphysics and precipitation • Clouds and radiation • Clouds and climate: forced changes to clouds • Clouds and climate: cloud response to climate change

  2. Content of this Lecture • The importance of cloud feedbacks: Climate sensitivity • Cloud radiative forcing • Factors affecting clouds • Cloud feedback in climate models

  3. Reading • Section 7.2.2 Cloud Processes and Feedbacksof IPCC 2001 • http://www.grida.no/climate/ipcc_tar/wg1/271.htm

  4. Climate Sensitivity • Climate sensitivity determines the global temperature when a radiative forcing is applied

  5. Climate Sensitivity • DT = change in global mean temperature • Q = radiative forcing (W m-2) • l = climate sensitivity (W m-2 K-1)

  6. Summer 2002 GFDL 2xCO2 Sensitivity (K) NCAR Sensitivity of Climate Models • Sensitivity to doubled CO2 (~4 Wm-2)

  7. Cloud Changes and Climate Sensitivity 1/l=4.2 K Wm-2 % Change in low cloud amount for 2xCO2 1/l=1.8 K Wm-2

  8. Change in Cloud Radiative Forcing • Today’s Earth is cooler because of clouds (net -20 Wm-2 forcing (= 4*CO2 doubling effect) • All models agree on sign of CRF • Cloud feedback is about how CRF changes as greenhouse gases increase • Models disagree greatly on this • Some clouds warm, some cool. DT depends on which clouds change

  9. Humidity and Temperature Overall increase in atmospheric water vapour and temperature • Increased T • Increased water vapour in atmosphere • Increased cloudiness? • NO • Relative humidity is the relevant quantity Overall increase in atmospheric water vapour 100% RH

  10. Cloud Radiative Forcing (CRF) • Factors that determine CRF • Location (solar intensity) • Depth/thickness • Coverage • Drop/ice concentrations Very similar SW forcing Very different LW forcing

  11. Cloud Radiative Forcing 40 40 Winter 5o N Winter 65o N high 20 20 cloud height med low 0 0 DTs (K) DTs (K) high -20 -20 med low -40 -40 0 50 100 150 0 50 100 150 liquid water path (g m-2) liquid water path (g m-2) Equilibrium surface temperature due to presence of different clouds

  12. Reasons for Cloud Changes • Large-scale dynamics/circulation • Global circulation changes in response to changes in ocean circulation, changes in ocean-atmosphere T contrast, etc • Thermodynamic/cloud-scale changes • Changes to: • vertical T profile, • atmospheric stability, • turbulence structure of boundary layer, • water substance transport • Very difficult to separate in observations

  13. Thermodynamic Changes • Influence on water vapour feedback • water vapour is much more effective GHG in the upper troposphere than near the surface • Deep Cb clouds transport water vertically high feedback low feedback

  14. Tropical Cirrus – A Proposed “Adaptive Infrared Iris Effect” Richard Lindzen, MIT • Japan’s Geostationary Meteorological Satellite • 11 and 12 mm wavelength radiometer • 130oE-170oW, 30oS-30oN (Pacific) • 260 K brightness temperature product is a measure of “high thin cloud” – cirrus • Cirrus cover decreases with increasing SST 0.2 slope = 10-20% change per 1 K SST 0.15 observations 0.1 Cloud Amount 0.05 0 25 26 27 28 29 30 sea surface temperature (K)

  15. The Adaptive Infrared Iris as a Climate Change Regulator more IR to space less cirrus less water vapour less water transport more rain cold ocean warm ocean

  16. Problems With the Infrared Iris Idea • Observations of cloud IR radiance are not directly related to cirrus coverage • Other observations from TRMM (Tropical Rainfall Measuring Mission) show that warm clouds rain more, but they also transport more water vertically • See http://www.gsfc.nasa.gov/topstory/20020915iristheory.html

  17. Circulation/Dynamical Changes • Cloud fields are determined by large-scale circulation • Non-local response • El Nino Hadley/Walker circulation Tropical convection Tradewind cumulus Sub-tropical St/Sc Equator 30oN

  18. Observed Clouds With Temperature • Observations from the International Satellite Cloud Climatology Project (see lecture 7) • Clouds become optically thinner (less reflective) at higher temperatures • +ve or –ve feedback? Ocean low clouds 0.1 0 d ln(optical depth)/dT 0.05 -0.1 -0.15 -60 -40 -20 0 20 40 60 latitude

  19. Net Cloud Feedbacks in GCMs Doubled CO2 experiments 3 WARMING 2 SW 1 Change in CRF (W m-2) 0 LW -1 net -2 COOLING -3 Different models

  20. Difficulties • Different types of clouds have different effects and may change in different ways – many separate problems • Some aspects of clouds (thickness, ice content) are difficult to observe • Sub-grid scale problems • Effects of temperature and circulation can be confused • Changes observed on short time scales (e.g., El Niño) may not always be good indicators of climate change-induced changes

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