DARGAN M. W. FRIERSON UNIVERSITY OF WASHINGTON, DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 2: 9/30/13 Fundamentals of Climate Change (PCC 587): Water VaporWater… ! Water is a remarkable molecule ¡ Water vapor ÷ Most important greenhouse gas ¡ Clouds ÷ Albedo effect & greenhouse effect ¡ Ice ÷ Albedo in polar latitudes ÷ Ice sheets affect sea level ¡ Ocean ÷ Circulation, heat capacity, etc ! Next topic: ¡ Phase changes of water (e.g., condensation, evaporation…) are equally fundamental for climate dynamics ÷ Phase change à heat released or lost (“latent heating”)Outline… ! Why water vapor content will increase with global warming ¡ Feedback on warming, not a forcing ! Effect of water vapor on current climate: ¡ Vertical temperature structure ¡ Horizontal temperature structureAlong the way… ! The controversy about whether the upper troposphere has warmed in recent decades ¡ And why global warming would be a lot worse if it didn’t warm up there… Modeled temperature trends due to greenhouse gasesAlong the way… ! Why the tropopause will rise with global warming ¡ Three separate effects cause a rise in tropopause height: result is the sum of all three! Tropopause height rise in observations versus modelsIntroduction to Moisture in the Atmosphere ! Saturation vapor pressure: ¡ Tells how much water vapor can exist in air before condensation occurs ! Exponential function of temperature ¡ Warmer air can hold much more moistureSaturation Vapor Pressure ! Winters are much drier than summers ¡ Simply because cold temperatures means small water vapor content January average surface humidity (in C) July average surface humidity (in C)Saturation Vapor Pressure ! Temperatures decrease with height in the troposphere ! This means most water vapor is confined in the lower levels in the atmosphere as well ¡ Also concentrated in the warmer tropics… Annual and zonal mean humidity content (in kg/kg) Latitude PressureRelative Humidity ! Relative humidity = vapor pressure/saturation vapor pressure ¡ Saturation at 100% RH ¡ Dry à 0% RH ¡ Generally higher in rainier regions, and lower in drier regions Zonally and annually averaged relative humidty Latitude PressureWater Cycle ! Evaporation occurs from ocean surfaces or wet land surfaces ¡ Proportional to 1-RH (drier air evaporates more) ! Saturation occurs within the atmosphere ¡ Condensation & precipitation is formed ! Evaporation and precipitation balances in steady state ¡ This also determines the relative humidity of the atmosphereRelative Humidity ! Relative humidity expected to stay roughly the same with climate changes ¡ If it didn’t, there would be more evaporation, which would humidify the atmosphere Simulated changes in relative humidity with global warming (note there are only small changes)Water Vapor Content and Global Warming ! Constant relative humidity à warmer climates have much more moisture! ¡ 7% increase per degree of warming ! More water vapor à more water vapor greenhouse effect ¡ Primary positive feedback to global warming ! Water vapor is a feedback to climate change, not a forcing of climate change ¡ Can’t change water vapor content directly: it responds to the global mean temperatureTropical Water Vapor Content ! Water vapor content has been increasing (tropics only shown here) ¡ Big peaks are El Niño events ¡ Climate models can simulate water vapor content well given sea surface temperaturesSummary so far… ! Warmer temperatures à more moisture can exist in air ! Next: the effects on energetics ¡ Phase changes are associated with heating/coolingCondensation and Latent Heating ! We’re all familiar with the idea that evaporation causes cooling ¡ Evaporation of sweat cools you off ¡ Getting out of a pool on a windy day à cold! ! Similarly, condensation -> heating of the atmosphere ¡ Condensation of water vapor is associated with a release of latent heat ¡ Huge heat source: ÷ Average tropical lower tropospheric moisture values: 45o C of heating potential!Freezing as a Latent Heat Source ! Freezing is also associated with latent heat release ¡ It’s a significantly smaller heat source though ¡ Latent heat of vaporization: 2.5 x 10^6 J/kg ¡ Latent heat of fusion: 3.3 x 10^5 J/kgMoisture Content of Atmosphere ! Observed surface water vapor content ¡ Measured in degrees (Celsius) of warming that would occur if all the moisture was condensed out at once Source: NCEP Reanalysis Latent heating is huge potential heat source! All of this moisture is condensed out in storms or large scale circulationsLatent Heating in Energy Budget ! Evaporation/condensation is primary way that energy is transferred from surface to atmosphere: Latent heat flux = 78 Sensible (thermal) heat flux = 24 Net radiative flux from surface to atmos = 350-324 = 26 Over 60% of heat is transferred off of surface by moisture!Latent Heating in Hurricanes ! Hurricanes ¡ Evaporation from ocean surface is fuel ÷ Why hurricanes dissipate over land ¡ Condensation is like the combustion of that fuel! ÷ Huge latent heat release powers winds, etc Hurricane Fran (1996)Water Vapor and Global Warming ! With global warming, atmospheric moisture content will increase ¡ Over 20% increase in humidity with 3o C global temperature increase ÷ Tropics will have 55o C of latent heating potential instead of 45o C ! What effects will the increased moisture content have on the Earth’s climate? ¡ More fuel for hurricanes & extratropical storms ÷ Don’t panic yet though… ¡ Also affects temperature gradients, vertical temperature profile ÷ We’ll show these are both negative feedbacksSummary so far… ! Warmer temperature à more moisture can exist in air ! Condensation of water vapor causes a huge amount of heating ! Next: ¡ How this impacts the vertical temperature profile of the atmosphere ÷ “Lapse rate”: rate of temperature decrease with height ¢ Primary negative feedback to global warming!Layers of the