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Zelinka and Hartmann 2011 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D23103, doi:10.1029/2011JD016459, 2011 The observed sensitivity of high clouds to mean surface temperature anomalies in the tropics Mark D. Zelinka1,2 and Dennis L. Hartmann1 Received 23 June 2011; revised 9 September 2011; accepted 28 September 2011; published 2 December 2011. [1] Cloud feedback represents the source of largest diversity in projections of future warming. Observational constraints on both the sign and magnitude of the feedback are limited, since it is unclear how the natural variability that can be observed is related to secular climate change, and analyses have rarely been focused on testable physical theories for how clouds should respond to climate change. In this study we use observations from a suite of satellite instruments to assess the sensitivity of tropical high clouds to interannual tropical mean surface temperature anomalies. We relate cloud changes to a physical governing mechanism that is sensitive to the vertical structure of warming. Specifically, we demonstrate that the mean and interannual variability in both the altitude and fractional coverage of tropical high clouds as measured by CloudSat, the Moderate Resolution Imaging Spectroradiometer, the Atmospheric Infrared Sounder, and the International Satellite Cloud Climatology Project are well diagnosed by upper tropospheric convergence computed from the mass and energy budget of the clear‐sky atmosphere. Observed high clouds rise approximately isothermally in accordance with theory and exhibit an overall reduction in coverage when the tropics warms, similar to their behavior in global warming simulations. Such cloud changes cause absorbed solar radiation to increase more than does outgoing longwave radiation, resulting in a positive but statistically insignificant net high cloud feedback in response to El Niño–Southern Oscillation. The results suggest that the convergence metric based on simple mass and energy budget constraints may be a powerful tool for understanding observed and modeled high cloud behavior and for evaluating the realism of modeled high cloud changes in response to a variety of forcings. Citation: Zelinka, M. D., and D. L. Hartmann (2011), The observed sensitivity of high clouds to mean surface temperature anomalies in the tropics, J. Geophys. Res., 116, D23103, doi:10.1029/2011JD016459. 1. Introduction GCMs integrated under the Special Report on Emission Scenarios (SRES) A2 emissions scenario and submitted to [2] The role of cloud‐induced changes in top of atmo- the Coupled Model Intercomparison Project phase 3 (CMIP3) sphere radiative fluxes as a feedback on a warming climate multimodel database. They estimated that the tendency for is a subject of great debate and uncertainty [e.g., Bony et al., tropical high clouds to rise as the climate warms contributes 2006]. The magnitude of cloud feedback is generally posi- − − 0.5Wm2 K 1 to the global mean LW cloud feedback, tive in global climate models (GCMs), but exhibits con- making it robustly positive. Furthermore, they demonstrated siderable intermodel spread that arises primarily from the that the radiatively driven clear‐sky diabatic convergence, spread in shortwave (SW) cloud feedbacks that can be whose peak corresponds closely with the level of peak attributed to the wide range of modeled responses of sub- convective detrainment and abundant high cloudiness, pro- tropical marine boundary layer clouds [e.g., Bony and vides a useful tool for accurately diagnosing the upward shift Dufresne, 2005]. Although most of the spread in estimates in cloud fraction in all of the CMIP3 GCMs analyzed. The of climate sensitivity from GCMs can be attributed to the robust nature of the positive LW cloud feedback, therefore, inter‐model variance in SW cloud feedback, Zelinka and arises simply as a fundamental result of the approximate Hartmann [2010] (hereafter ZH10), showed that the long- radiative‐convective equilibrium that any model must wave (LW) cloud feedback is robustly positive in twelve maintain in the tropics regardless of the details of its con- vection scheme. However, unlike the isothermal upward shift of high clouds expected from the fixed anvil tempera- 1Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA. ture (FAT) hypothesis of Hartmann and Larson [2002], 2Program for Climate Model Diagnosis and Intercomparison, Lawrence ZH10 found that the peak clear‐sky radiatively driven con- Livermore National Laboratory, Livermore, California, USA. vergence and attendant high clouds warmed slightly in the A2 simulations, a feature they referred to as the propor- Copyright 2011 by the American Geophysical Union. tionately higher anvil temperature (PHAT). 0148‐0227/11/2011JD016459 D23103 1of16 D23103 ZELINKA AND HARTMANN: TEMPERATURE SENSITIVITY OF HIGH CLOUDS D23103 [3] Considering that climate models include a convective the responses it spawned have utilized the clear‐sky diag- parameterization that adjusts toward radiative‐convective nostics that operate on a gross tropicswide scale to explain equilibrium, it is perhaps not surprising that the tropical changes in high clouds in observations. Rather, all have mass and energy balance is effective at diagnosing the attempted to link cloud properties to the underlying SSTs, altitude of peak modeled high cloud coverage. Nevertheless, which we argue offers a weak constraint on high cloud Kubar et al. [2007] demonstrated the close relationship properties and thus makes it difficult to draw conclusions between clear sky convergence and high cloud fraction relevant to a warming climate. measured by the Moderate Resolution Imaging Spectro- [6] In this study we assess the degree to which the dis- radiometer (MODIS) in three regions of the Pacific Inter- tribution of tropical cloud tops as measured by a suite of tropical Convergence Zone (ITCZ), indicating that the real satellite instruments changes in a manner consistent with atmosphere is also reasonably explained by an assumption that predicted by the clear‐sky energy budget as the tropics of radiative‐convective balance. Still, it is unclear on what warms and cools. We focus primarily on the period Sep- spatial and temporal scales the constraints imposed by this tember 2002 through July 2010, for which a wealth of sat- balance are most applicable. Convection responds quickly to ellite information is available from A‐Train instruments, but variations in temperature and humidity in its near vicinity, also make use of the longer cloud record from the Interna- and by moistening the near environment convection can tional Satellite Cloud Climatology Project (ISCCP) that improve the conditions for its own existence, aggregate, and extends back to July 1983. Additionally, information about achieve higher altitudes. Furthermore, organized large‐scale the height and optical depth of clouds from MODIS and motion can cool the air adiabatically and provide for deeper ISCCP will be used in conjunction with a radiative transfer convection. For convection to continue, however, radiation model to estimate the impact of interannual cloud fluctua- must destabilize the vertical profile of temperature diabati- tions on the top‐of‐atmosphere (TOA) energy budget. cally, and this becomes an inefficient process at low tem- [7] We wish to stress that our analysis is not predicated peratures in the upper tropical troposphere, where the on the assumption that cloud fluctuations associated with saturation vapor pressure is very low and water vapor El Niño–Southern Oscillation (ENSO) are surrogates for becomes a less effective emitter [Hartmann et al., 2001a]. those accompanying global warming forced by increased [4] In contrast to the lack of sensitivity of tropical high greenhouse gas concentrations. Rather, we demonstrate that cloud top temperatures to surface temperature (Tsfc) changes a metric based on fundamental principles of saturation vapor expected from the FAT hypothesis, Chae and Sherwood pressure, radiative transfer, and mass and energy balance [2010] showed that cloud top temperatures observed by accurately diagnoses the vertical structure of tropical high the Multiangle Imaging Spectroradiometer (MISR) exhibit clouds and its fluctuations observed in nature, just as it does appreciable seasonal fluctuations (∼5 K) that are associated in global warming simulations of GCMs (ZH10) and in with lapse rate changes in the upper troposphere. This study cloud resolving model experiments of Kuang and Hartmann looked at a limited domain rather than at the cloud proper- [2007] and B. E. Harrop and D. L. Hartmann (Testing the ties of the entire tropics; thus it remains unclear whether role of radiation in determining tropical cloud top temper- tropical cloud fields exhibit compensatory changes in ature, submitted to Journal of Climate, 2011). The obser- structure that result in minimal changes when integrated vational results presented here reinforce the value of this over the entire tropics. Indeed, Xu et al. [2005, 2007] and diagnostic tool for understanding the varied response of high Eitzen et al. [2009] demonstrated that the distribution of clouds to different forcings operating across time scales and tropical high cloud top temperatures remains qualitatively for evaluating modeled tropical high cloud changes and their unchanged across significantly different SST distributions. implied feedbacks. These studies did not attempt to show consistency between high clouds and the radiatively driven clear‐sky conver- 2. Data gence,
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