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Water Vapor and Lapse Rate Feedbacks

Water Vapor and Lapse Rate Feedbacks. Neil Gordon ESP Seminar April 14, 2006. Climate Change – Feedback Mechanisms. Feedback : A sequence of interactions that determines the response of a system to an initial perturbation source: AMS Glossary

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Water Vapor and Lapse Rate Feedbacks

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  1. Water Vapor and Lapse Rate Feedbacks Neil Gordon ESP Seminar April 14, 2006

  2. Climate Change – Feedback Mechanisms • Feedback: A sequence of interactions that determines the response of a system to an initial perturbation • source: AMS Glossary • Some major climate feedbacks relating to climate: • Ice-Albedo (positive feedback) • Higher surface temperatures  Less ice and snow cover • Lower albedo  More surface absorption  higher surface temperatures • Water Vapor (positive feedback) • More GHGs  Higher surface temperatures  More evaporation •  More water vapor = More GHGs • Clouds (net negative feedback) • Higher surface temperatures  More evaporation  Higher specific humidity •  More clouds  Higher albedo  Lower surface temperatures •  More longwave absorption  Higher surface temperatures

  3. Structure of the Troposphere • Temperature decays with height at a rate of ~6-7 K/km over the troposphere (lowest 10-12 km of the atmosphere) • Water vapor is mostly concentrated in the lowest 1.5 km of the troposphere

  4. Temperature Profile from http://www.atmosphere.mpg.de/media/archive/

  5. More Detail on GHG Forcing • We can think of the surface atmosphere system radiating as a whole to space • The top-of-atmopshere (TOA) incoming radiation is primarily constant but how the Earth balances that is not • As GHG concentrations increase in the atmosphere, the effective height of emission increases, thus reducing the outgoing longwave radiation (OLR) • So, more radiation is entering the climate system and the surface and atmosphere have to warm to compensate

  6. Climate Energy Balance

  7. from Soden and Held (2000)

  8. Lapse Rate Feedback • The rate of temperature decay with height, or the slope of the temperature profile (lapse rate), is controlled by radiation, large-scale dynamics and convection • If the lapse rate were to decrease, then the temperature of the effective level of emission would warm (negative feedback) • This is a proposed negative feedback in the tropics, but it is thought to be relatively small (Zhang et al, 1994)

  9. Water Vapor Feedback • As atmospheric temperature increases, the ability of that air to hold more water vapor increases • So, if relative humidity is held constant as temperature increases, the total moisture in the air increases • Water vapor is opaque to IR radiation, making it a greenhouse gas

  10. Relative Humidity (350-500mb) from Lindzen et al. (2001)

  11. Water Vapor Feedback • The increase in total moisture in the lower troposphere as temperature increases is well-observed (Wentz and Schabel, 2000) • In order for water vapor to change the radiation balance, it must increase in the free troposphere • So, to change the free tropospheric water vapor, there must be a mehcanism to transport the water aloft • Held and Soden (2000) found that for fixed RH, the total water vapour feedback contributed by increases in WV below 850mb was only 10% of the total response

  12. Mt. Pinatubo Experiment • Soden et al. (2002) used the global cooling (and drying) resulting from the eruption of Mt. Pinatubo to test the water vapor feedback hypothesis • Global climate models were only able to reproduce the observed cooling if the water vapor feedback was included

  13. Adaptive Iris Hypothesis • Lindzen et al. (2001) propose a mechanism whereby increased surface temperature leads to more vigorous tropical convection • The enhanced convection increases the ability of the cloud to precipitate • The total water then transported to the upper atmosphere is reduced

  14. Tropical Convection from http://www.divinewindbook.com/figures/images/

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