Understanding Climate Change Feedbacks (2003) / Chapter Skim
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2 Cloud, Water Vapor, and Lapse Rate Feedbacks
2 Cloud, Water Vapor, and Lapse Rate Feedbacks
Pages 21-40
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From page 21... ...
An accelerated, focused effort to test the simulation of cloud, water vapor, and lapse rate feedbacks in climate models, and their role in climate sensitivity should be initiated. Existing and planned observations should be used in this new emphasis to test the simulation of clouds, water vapor, and lapse rate in climate models and the response of these variables to known forcings.
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From page 22... ...
If the relative humidity distribution remains approximately constant as temperature and specific humidity increase, then water vapor greenhouse feedback nearly doubles the sensitivity of climate above what it would be in the absence of water vapor feedback. On the largest spatial scales, existing data and current climate models are basically consistent with the assumption that on interannual time scales, relative humidity is more or less constant (Soden et al., 2002; Wentz and Schabel, 2000~.
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From page 23... ...
An integrated water vapor observing system should be developed, which has sufficient accuracy to measure decadal trends in the water vapor distribution and sufficient spatial resolution to test mechanisms by which that distribution is maintained. It should include a network of in situ sounding systems capable of measuring water vapor throughout the troposphere and lower stratosphere, complemented by ground-based remote sensors (such as have already been deployed at Atmospheric Radiation Measurement Project Cloud and Radiation Testbed (ARM CART)
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From page 24... ...
The moist adiabatic lapse rate decreases with increasing surface temperature, so by itself lapse rate feedback is expected to be negative in the tropics (Hansen et al., 1984; Wetherald and Manabe, 1986~. If the assumption of fixed relative humidity is a good approximation, then the water vapor feedback is partially cancelled by the lapse rate feedback (Cess, 1975~.
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From page 25... ...
Climate models generally reproduce the observed lapse rate in the tropics and elsewhere through the incorporation of large-scale dynamics and parameterized convection and radiation. Some observations suggest relationships between surface temperature trends and temperature trends in the free troposphere that seem inconsistent with the behavior of current climate models (NRC, 2000a; Santer et al., 2000~.
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From page 26... ...
A striking example of the contribution of cloud feedbacks to uncertainty in climate sensitivity is exhibited by comparison of the current climate models at the Geophysical Fluid Dynamics Laboratory (GFDL) and the National Center for Atmospheric Research (NCAR)
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From page 27... ...
Vertical transport of water in both vapor and ice form by convection in the tropics is an important source for upper tropospheric water vapor (Pierrehumbert and Roca, 1998; Salathe and Hartmann, 1997; Udelhofen and Hartmann, 1995~. The broad role of water vapor feedback in climate change and the specific importance of upper tropospheric water vapor cannot be divorced from the associated role of clouds and cloud feedbacks.
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From page 28... ...
A complex coupling thus exists between cloud feedbacks and ice-albedo feedback processes (see Chapter 4~. Clouds and Soil Moisture The feedbacks involving soil moisture and evaporation are intimately tied to the hydrological cycle over land (see Chapter 6~.
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From page 29... ...
radiation balance and confirm our understanding of the effect of different cloud types on this budget. Although data collected from these satellite experiments provide an important source of information for testing models, they do not sufficiently constrain critical assumptions about the treatment of cloud processes in climate models.
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From page 30... ...
Thus, model parameterizations of the internal heating of the climate system cannot be tightly constrained by observations. This shortcoming is a significant source of uncertainty in understanding cloud feedbacks.
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From page 31... ...
. Cloud feedbacks are currently diagnosed primarily by using coarse resolution climate models and even simpler one-dimensional equilibrium models.
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From page 32... ...
WHY HAS PROGRESS ON CLOUD, WATER VAPOR, AND LAPSE RATE FEEDBACKS BEEN SO ELUSIVE? The 1979 NRC report on carbon dioxide and climate contains the following statement in reference to cloud effects on climate change: Trustworthy answers can be obtained only through comprehensive numerical modeling of the general circulations of the atmosphere and oceans together with validation by comparison of the observed with the model-produced cloud types and amounts.
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From page 33... ...
New cloud data are becoming available from instruments on satellites that use polarization of reflected sunlight and active scanning with cloud radars and lidars to probe the vertical structure and particle size of clouds. These data will provide an important new source of data on the global distribution of clouds that should be used to further constrain and test the simulation of clouds in climate models.
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From page 34... ...
This artificial separation creates problems when attempting to use models to advance understanding on cloud and water vapor feedbacks. Additional problems with cloud schemes in climate models include .
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From page 35... ...
That these datasets are underutilized is in part a reflection of the attention that has been paid to model intercomparison, but too little attention has been paid to testing models against data. The utility of model-to-model intercomparison exercises for characterizing and reducing uncertainty in climate change feedbacks is limited.
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From page 36... ...
Studies such as these highlight the utility of being able to run climate models in an NWP mode to perform diagnostic analyses of processes that operate on short time scales but that are critical to producing realistic
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From page 37... ...
, which is focused on developing improved parameterizations for a wide variety of cloud types, is expected to contribute substantially to this effort. The approach generally combines observations, cloud-resolving models and global climate models.
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From page 38... ...
Progress in understanding cloud, water vapor, and lapse rate feedbacks requires that an integrated effort with additional resources be developed that cross-cuts the interests of individual agencies. We propose that this effort be developed with the following elements:
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From page 39... ...
A rigorous effort to test climate models against observational metrics must be initiated and coordinated among groups performing climate modeling, climate observation, and climate analysis. Metrics should include comparison of observed and simulated response of clouds, water vapor, and lapse rate to every well-observed forcing mechanism and time scale, including the diurnal and seasonal response, the response to ENSO and the response to volcanic eruptions.
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From page 40... ...
For example, research in collection and analysis of the global datasets of cloud and water vapor information, especially those derived from space-borne observations, are supported in large part by NASA, the development of NWP models by the National Oceanic and Atmospheric Administration (NOAA) , and high-end climate modeling efforts by yet other agencies, each of which have their own objectives.
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