Aggregated Convection and the Regulation of Tropical Climate
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Aggregated Convection and the Aggregated Convection and the Regulation of Tropical ClimateRegulation of Tropical Climate
Kerry EmanuelProgram in Atmospheres, Oceans, and Climate
MIT
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Numerical simulations of RC equilibrium show that, Numerical simulations of RC equilibrium show that, under some conditions, moist convection self-under some conditions, moist convection self-
aggregates aggregates
Day 10 Day 50
From Bretherton et al. (2005)
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Vertically integrated water vapor at 4 (a), 6 (b), 8 (c), and 10 Vertically integrated water vapor at 4 (a), 6 (b), 8 (c), and 10 (d) days (Nolan et al., QJRMS, 2007)(d) days (Nolan et al., QJRMS, 2007)
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Effect of Self-Effect of Self-Aggregation on Aggregation on
HumidityHumidity
(Bretherton et al. , 2005)
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Nolan et al., QJRMS, 2007Nolan et al., QJRMS, 2007
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Self-Aggregation is Temperature-Dependent Self-Aggregation is Temperature-Dependent (Nolan et al., 2007; Emanuel and Khairoutdinov, in preparation, 2009)
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Empirical Necessary Conditions for Self-Aggregation Empirical Necessary Conditions for Self-Aggregation (after Held et al., 1993; Bretherton et al., 2005; Nolan et al.; 2007)
• Small vertical shear of horizontal wind• Interaction of radiation with clouds and/or
water vapor• Feedback of convective downdraft surface
winds on surface fluxes• Sufficiently high surface temperature
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HypothesisHypothesis
• At high temperature, convection self-aggregates
• →Horizontally averaged humidity drops dramatically
• →Reduced greenhouse effect cools system• →Convection disaggregates• →Humidity increases, system warms• →System wants to be near phase transition to
aggregated state
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Recipe for Self-Organized CriticalityRecipe for Self-Organized Criticality(First proposed by David Neelin, but by different mechanism)(First proposed by David Neelin, but by different mechanism)
• System should reside near critical threshold for self-aggregation
• Convective cluster size should follow power law distribution
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Toy ModelToy Model
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PropertiesProperties• PBL quasi-equilibrium enforced• Bulk aerodynamic surface fluxes with convective
gustiness• Albedo and emissivity simple weighted average of clear
and cloudy regions• Water vapor-dependent clear sky emissivity• Horizontally uniform temperature but variable moist
static energy (i.e. water vapor) at mid-level• Vertical motion calculated to enforce zero horizontal
temperature gradient• PBL moist static energy adjusted to yield zero domain-
averaged vertical motion• Slow horizontal diffusion of moisture at mid-level
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ResultsResultsSelf-Aggregation Occurs for:Self-Aggregation Occurs for:
• Small or negative gross moist stability• Sufficiently large feedback between
convective gustiness and surface enthalpy fluxes
• Sufficiently high surface temperature
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Example:Example:
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Summary of Toy Model ResultsSummary of Toy Model Results• Self-aggregation driven by convective
gustiness at high temperature• No self-aggregation at low temperature• Aggregated state is much drier at mid levels• System tends towards self-organized criticality
(SOC)• Climate sensitivity of SOC state much lower
(0.04 K/Wm-2) than sensitivity of uniform convection (0.2 K/Wm-2)
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Overall SummaryOverall Summary
• Spontaneous aggregation of convection can occur in low-shear environments at sufficiently high temperature
• Aggregation results in profound drying of the troposphere (associated with increase in precipitation efficiency)
• Drying results in decrease of surface temperature• Above points suggest that low-shear tropical
environments are “attracted” to the critical point between disaggregated and aggregated states
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• Climate sensitivity of this self-organized critical state likely to be quite different from normal radiative-convective states
• Global models probably do not represent this process
• Idea currently being tested with cloud-resolving model (w/Marat Khairoutdinov)