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by: Aurore MacGyver Sr.


Aurore MacGyver Sr.
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This 7 page Class Notes was uploaded by Aurore MacGyver Sr. on Wednesday September 9, 2015. The Class Notes belongs to OCEAN 200 at University of Washington taught by Staff in Fall. Since its upload, it has received 12 views. For similar materials see /class/192142/ocean-200-university-of-washington in Oceanography at University of Washington.




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Date Created: 09/09/15
ATMS 571 Hartmann Tropical Feedback Notes A 2000 The Tropical Atmosphere and Climate Change Dennis L Hartmann Department of Atmospheric Sciences University of Washington Seattle Washington Overview Observations Simplified Models Feedback and Coupling Mechanisms within the Tropics Water Vapor Clouds Circulation Tropical Energy Balance On the next page is a block diagram of the tropical energy balance where the tropics are defined as the region from 30S30N About 50 Wm39 2 are taken in as the net radiation flux at the top of the atmosphere The same amount is exported to the extratropics in about equal measures from atmospheric and oceanic transport About twothirds of the absorbed solar radiation is absorbed by the ocean another 100 Watts is absorbed in the atmosphere The largest term moving heat out of the ocean is evaporation at about 115W followed by longwave at 50W and sensible cooling of the surface of about 10W Since the surface is mostly wet and warm sensible cooling is small in comparison to latent cooling of the surface So if we want to investigate the sensitivity of the tropical SST for example we should look at the sensitivity of the larger terms in the surface energy balance ATMS 571 Hartmann Tropical FeedbackNotes A 2000 Top of Atmosphere W JLW 2 5 3911 M5 W11 V Surface gt 2 5 Figure Block diagram of tropical energy uxes Units are Watts per square meter of surface area Surface Longwave Radiation 50 Wm 2 Longwave cooling of surface decreases with surface temperature in the tropics because of the water vapor greenhouse feedback In figure 98 in Global Physical Climatology we see that the actual longwave cooling of the surface decreases with increasing surface temperature quite rapidly at tropical temperatures of around 300K This is because of the rapid closing of the water vapor window between 8 and 12 microns by continuum absorption by water vapor Although the upward emission from the surface increases at about 6 Wm2 per degree K of surface temperature increase the downward longwave emission from the atmosphere INCREASES at a faster rate of about 9 Wm2 So the surface loses its capacity to remove heat by infrared emission This is a positive feedback at the surface ATMS 571 Hartmann Tropical Feedback Notes A 2000 BLW z 3 Wm2K1 3T when 4 a TS z 6 Wm 2K 1 TS 300K We have the makings of a runaway greenhouse effect but something stops the train One thing that is working in the opposite direction is evaporation Surface Evaporative Cooling 115 Wm 2 Begin by considering the aerodynamic formula for evaporative cooling of the surface which depends on the wind speed U and the contrast in specific humidity between the surface and the air LEEpaLCDUqs qa LEzpaLCDUqs l RHRH 2 TS Ta RVTS To produce a 20 increase in LE U 5 ms391 to 6 ms391 lms391 20 RH 80 to 76 4 5 TS 300K to 303K 3K 1 Fix RH and U get large negative feedback BLE a z 7 Wm 2K 1 TS 300K ATMS 571 Hartmann Tropical Feedback Notes A 2000 Surface Solar 200 Wm 2 Clouds can greatly decrease the solar energy reaching the surface If we contrast the cloudy regions which tend to occur over the warmest SST with the adjacent clear regions we can get a gigantic value for the estimate of the sensitivity of surface insolation to SST This is basically what Ramanathan and Collins1991 did to emphasize the potential importance of blocking of solar radiation by clouds in the tropics high albedo Iow albedo Warmer SST Warm SST Qsolwarm QSOIcool z 25 Wm2K1 Tswarm scool This surely represents a gross overestimate of the magnitude of the role of the radiative effect of convective cloud on tropical SST and the sign may be wrong To get a good estimate of climate sensitivity one needs to average over the area of the largescale circulation that connects the most active convection to regions of subsidence Hartmann and Michelsen 1993 Top of Atmosphere Clouds Shortwave Cloud Forcing SWCF Clouds reduce solar energy absorbed by planet Thermostat Hypothesis Ramanathan and Collins 91 ATMS 571 Hartmann Tropical Feedback Notes A 2000 Convective cloud albedos increase rapidly with SST in the tropics and this limits SST Problems 1 Strong SWCF by convective clouds is more dependent on largescale circulation than on local SST Cloud optical properties respond much more strongly to largescale forcing than to absolute value of SST Cloudresolving model experiments show this 2 SWCF is negated by LWCF in convective regions of the tropics so that the main effect is a vertical redistribution of energy in a region where vertical redistribution by convection is very efficient Strong compensation between solar heating and evaporation affects hydrological cycle but SST not sensitive to this in twobox model Top of Atmosphere Water Vapor Water vapor is 1 Greenhouse Gas CW amp Null Hypthesis Relative humidity will remain about constant and this positive feedback through saturation vapor pressure dependence on temperature gives a doubling of climate sensitivity Greenhouse Effect 2 6TS4 OLR Water vapor is injected into troposphere by convection and removed by subsidence Subsidence is driven by radiative cooling in clear air largely from water vapor Energy and Moisture Budget Consideration In convective region Precipitation 3 myr 240 Wm392 Evaporation 14 myr 115 Wm392 P E LS Transport 16 myr 125 Wm392 ATMS 571 Hartmann Tropical Feedback Notes A 2000 A Very Interesting Question Why is ARnet zero for tropical convective clouds Why is the net radiation in regions of intense tropical convection the same as the net radiation in adjoining regions Kiehl 1994 suggested that this is a fortuitous effect of tropopause height given that forcing comes from high thick clouds Too facile an explanation 1 ARnet varies from day to day 2 ARnet varies with cloud type AR 0 results from cloud distribution Hartmann et al 1992 net Alternate Hypothesis Active feedback control for ARnet approximately same Rnet as adjacent tropical regions or a circulation 0 Convective Clouds must produce anomaly will result that will adjust cloud optical properties to AR 0 netN condition Feedback to Maintain ARnet 0 that Invokes LargeScale Dynamics Temp I Ciro Convection AR lt0 Gradlent Strength IntenSIty net t I Neg Feedback Negative Feedback Some Conclusions ATMS 571 Hartmann Tropical Feedback Notes A 2000 The convectivesubsiding dichotomy paradigm of tropical climate reveals a number of interesting feedbacks in the tropics that may be important in climate change eg 1 Water vapor in the free troposphere of the subsiding region interacts strongly with the largescale circulation and the atmospheric mixed layer to produce a feedback that reduces climate response to upper tropospheric humidity in the subsiding zone Increased humidity above the mixed layer increases subsidence and thins and dries the mixed layer 2 The tendency of convective clouds to produce the same top of atmosphere net radiation as adjacent subsiding regions may be a result of an active feedback between largescale circulations and convective cloud optical properties References Hartmann D L and M L Michelsen 1993 Largescale effects on the regulation of tropical sea surface temperature J Clim 6 2049 2062 Hartmann D L BM E Ockert and M L Michelsen 1992 The effect of cloud type on Earth s energy balance global analysis J Climate 5 b1281304 Kiehl J T 1994 On the observed near cancellation between longwave and shortwave cloud forcing in tropical regions J Climate 7 559 65 Ramanathan V and W Collins 1991 Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El Nino Nature 351 2732


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