clouds: the wild card

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Clouds: The Wild Card. Bruno Tremblay McGill University. Effect of Clouds Night Time. Facts about Clouds. Clear-sky day are warm Cloudy-sky day day are cold Clear-sky night are cold Clear-sky day are warm. Greenhouse effect of Clouds. - PowerPoint PPT Presentation


  • Clouds:The Wild CardBruno TremblayMcGill

  • Effect of CloudsNight Time

  • Facts about CloudsClear-sky day are warmCloudy-sky day day are cold

    Clear-sky night are coldClear-sky day are warm

  • Greenhouse effect of CloudsClouds absorb LW radiation from the surface and then re-emits upwards at their own temperatureSince clouds are colder than the surface temperature decreases with heights the earth-atmosphere system emits less radiation to space

  • Low level cloudsReflects SWemits at high tempCools the earthHigh Level CloudsTransparent to SWEmits at low tempWarms the earthThick CloudsReflect SWEmits at low tempNeutral effect

  • Radiative Forcing of CloudsLow Cloud Forcing

    High Cloud Forcing

  • Different clouds

  • Global Annual MeanCooling Effect of Clouds = 48 W/m2Warming Effect of Clouds = 31 W/m2Net Effect of Clouds = 17 W/m2 (Cooling)

    Clouds in todays climate act to cool the climate

  • Comparison with CO2Anthropogenic increase in CO2: 280 490ppm (40% increase)Reduction in thermal outgoing radiation: 1.6 W/m2

    Equvalent to placing a 60W light buld every 6m by 6m surface area over the entire earth surface.

  • Conparison with CO2CO2 increase in the atmosphere is equivalent to:1.6/48 * 100 = 3% change in reflection of solar radiation by clouds1.6/31 * 100 = 5% change in the reduction of thermal radiation by clouds1.6/17 * 100 = 9% change in net effect of clouds on radiationAre clouds sensitive to changes in earth temperature?

  • Cloud feedbacks on ClimateWarmingGH EffectCloud reflectionWeakerStrongerWeakerStrongerExacerbatemitigate

  • Positive feedbackMore high level of cloudsFewer low level cloudsIncrease in height of cloud topDecrease in cloud thickness

  • Positive feedbackFewer high level of cloudsMore low level cloudsDecrease in height of cloud topIncrease in cloud thickness

  • What are the cloud net effect on climate?We dont know But we can try!

    Use Climate ModelUse data from the last 50 years

  • GCM - Uncertainties

  • Satellite DataTwo problems:Changes in clouds may not be linked with temperature changesSatellite were made for weather not climate monitoringSatellite data show decrease of high level clouds over ocean and land in accord with land obsSatellite data show a decrease in low level clouds over ocean and land in disagreement with land obs

    Low Cumulus Clouds: Close to the ground emit at high temperature low greenhouse effect . Reflects little SW radiation.Stratocumulus: - high above ground emits at low temperature large greenhouse effect. Reflects lots of SW radiation.*


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