Mark Zelinka | Lawrence Livermore National Lab (original) (raw)

Papers by Mark Zelinka

Geophysical Research Letters, 2013

1] This study quantifies the response of the clouds and the radiative budget of the Southern Hemi... more 1] This study quantifies the response of the clouds and the radiative budget of the Southern Hemisphere (SH) to the poleward shift in the tropospheric circulation induced by the development of the Antarctic ozone hole. Single forcing climate model integrations, in which only stratospheric ozone depletion is specified, indicate that (1) high-level and midlevel clouds closely follow the poleward shift in the SH midlatitude jet and that (2) low-level clouds decrease across most of the Southern Ocean. Similar cloud anomalies are found in satellite observations during periods when the jet is anomalously poleward. The hemispheric annual mean radiation response to the cloud anomalies is calculated to be approximately +0.25 W m À2 ,a r i s i n g largely from the reduction of the total cloud fraction at SH midlatitudes during austral summer. While these dynamically induced cloud and radiation anomalies are considerable and are supported by observational evidence, quantitative uncertainties remain from model biases in mean-state cloud-radiative processes. Citation: Grise, K. M., L. M. Polvani, G. Tselioudis, Y. Wu, and M. D. Zelinka (2013), The ozone hole indirect effect: Cloud-radiative anomalies accompanying the poleward shift of the eddy-driven jet in the Southern Hemisphere, Geophys. Res. Lett., 40,

The ability of the Earth to regulate its energy budget is strongly dependent on the concentration... more The ability of the Earth to regulate its energy budget is strongly dependent on the concentration of water vapor in the atmosphere. Increases in humidity at upper levels of the troposphere have a greater radiative impact on the climate than equivalent increases closer to the surface. In this study, I investigate the evolution of the moisture and cloud distribution in deep convective systems over the tropical Pacific using retrievals of water vapor, temperature, and cloud properties from AMSR-E, AIRS, and MODIS. Using a compositing technique centered in space and time on locations having high rain rates, I have identified systematic lags in both the fractional coverage of anvil clouds and in the moistening of the upper troposphere following peak convection. Anvil cloud fractions peak 3 hours after the strongest convection while the upper troposphere is most humid 9-15 hours following the strongest convection. Composited vertical velocity profiles from the ERA-40 Reanalysis show anomalous ascent in the upper troposphere following peak convection, which likely has a role in sustaining the moist signature aloft. Correspondingly, clear-sky outgoing longwave radiation remains significantly reduced for several hours after the convection, indicating that the sustained moist anomaly at upper levels continues to affect the local radiation budget well after the convective event.

Dynamical and thermodynamical feedbacks within the climate system play a large role in determinin... more Dynamical and thermodynamical feedbacks within the climate system play a large role in determining the regional response to global warming. In this talk two examples will be discussed. First, dynamical feedbacks between baroclinic eddies and zonal jets create natural low frequency variability. If climate change forced by natural or anthropogenic causes projects onto these modes of variability, then the spatial response can be larger and more coherent than would be expected in the absence of these modes. An example is given of the effect of the Quasi-Biennial Oscillation of the tropical stratosphere on the tropospheric circulation. Second, thermodynamic feedbacks within the climate system control not only the magnitude, but also the spatial structure of global warming. An example is given of how the latitudinal structure of cloud and water vapor feedbacks, being strongly positive near the equator and weakening with latitude, cause the required meridional heat transport by the climate system to be increased and also shifted from the ocean to the atmosphere. This has a profound effect on regional response to global warming.

Kernel functions are used to compute the sensitivity of TOA fluxes to atmospheric feedback proces... more Kernel functions are used to compute the sensitivity of TOA fluxes to atmospheric feedback processes using AR4 A2 simulations of the 21st Century climate. These TOA flux feedbacks can be used to compute the apparent causes of changes in the equator to pole flux of energy by the climate system. Near the equator the net feedback is positive, so that the climate is locally unstable. This results from enhanced water vapor and cloud longwave feedbacks there. The climate system must therefore flux more energy poleward away from the equator in warmed climates. Using surface flux sensitivities the atmospheric and oceanic flux feedbacks can be computed. The ensemble of models show an increase of atmospheric meridional energy flux and a decrease in oceanic heat flux. This is true in both hemispheres, but more strongly in the Southern Hemisphere where the upward flux of cold water supports a downward flux of sensible heat from the atmosphere and enhanced poleward flux of energy in the atmosphere. This may be related to the poleward shift of the jet in high latitudes of the Southern Hemisphere that is produced in many AR4 simulations of the 21st Century.

A number of studies have investigated the relationship between tropical deep convection and sea s... more A number of studies have investigated the relationship between tropical deep convection and sea surface temperature (SST) and the implications of these relationships for climate and climate change. Deep convection and SST are somewhat indirectly related to each other through their individual relationships with the large-scale circulation, however, because regions of high SST are also regions of large-scale low-level convergence which encourages deep convection. Thus the intrinsic sensitivity of convection to SST is somewhat less apparent. Many previous studies use outgoing longwave radiation as a proxy for deep convection, but the ubiquity of thin cirrus disconnected from active deep convection in the tropics makes interpretation of observed relationships difficult. In this study we use rainfall data from the TRMM Multisatellite Precipitation Analysis as direct measures of deep convective events, SST from AMSR-E, cloud fraction, cloud top temperature, and cloud optical thickness from MODIS, longwave and shortwave TOA fluxes from CERES and vertical velocity from NCEP/NCAR reanalysis to address a number of questions: Within a given circulation regime (as determined by the magnitude and vertical shape of the vertical motion), how does the ensemble of cloud properties vary with SST, SST gradient, and precipitation rate? How do the observed changes in cloud properties and their respective radiative forcings affect the TOA and surface energy budgets? Likewise, how does the large- scale circulation respond to a change in SST gradient, and how does this circulation change impact the cloud properties? The sensitivity of convective evolution to the underlying SST will also be assessed by using a compositing technique centered on intense rain events. Implications of these results for tropical climate will be discussed.

A positive longwave cloud feedback is a robust feature of all global climate models submitted to ... more A positive longwave cloud feedback is a robust feature of all global climate models submitted to the IPCC AR4 archive. In this study we show that this is in large part due to the fact that tropical high clouds rise vertically as the planet warms, but do so in such a way as to remain at nearly the same temperature. Thus the longwave emission from tropical high clouds remains approximately constant rather than increasing in step with the warming planet, resulting in a positive longwave cloud feedback. We demonstrate that this robust feature of the models' tropical high clouds makes sense physically due to the fundamental constraint imposed by the clear-sky radiative cooling rate, as in the fixed anvil temperature (FAT) hypothesis of Hartmann and Larson (2002). Furthermore, the slight reduction in fractional coverage of high clouds that occurs over the course of the 21st Century simulations is also well-explained by considering the clear-sky balance between radiative cooling and subsidence: The simulated increase in static stability out-paces the increase in longwave cooling, thereby causing less clear-sky upper tropospheric convergence and - by mass conservation - a reduction in high cloud cover.

Cloud feedback represents the source of largest uncertainty in projections of future warming and ... more Cloud feedback represents the source of largest uncertainty in projections of future warming and observational constraints on both the sign and magnitude of the feedback remain lacking. In this study we use observations from a suite of satellite instruments (AIRS, MODIS, CERES, CloudSat, ISCCP) and in situ data to assess the sensitivity of tropical high clouds to tropical mean surface temperature anomalies associated with interannual variability. Unlike previous studies, however, we relate cloud changes to a physical governing mechanism that has sensitivity to the vertical structure of warming, removing the ambiguity associated with simple regressions of clouds on surface temperature alone. Specifically, we demonstrate that the profile of high clouds (both the peak level and fractional coverage) and its interannual variability are well-diagnosed by the profile of upper tropospheric convergence derived from the mass and energy budget of the clear-sky atmosphere. As in the global warming case, high clouds rise and exhibit a reduction in coverage when the Tropics warm. In contrast to the global warming case, the cloud fraction reduction at lower levels exceeds the increase at higher levels, implying reduced shortwave reflection and enhanced longwave emission that dominate over reduced emission from higher tops. We demonstrate that differences in the vertical structure of interannual versus transient warming lead to differences in convergence that accurately explain these differences in cloud response, providing an explanation for why cloud feedbacks operating on interannual timescales differ from those in the global warming case. This suggests that the simple convergence metric may be a powerful evaluation tool for assessing the realism of modeled clouds and for eventually reducing the uncertainty surrounding cloud feedback.

Feedbacks in the climate system act to reduce the efficiency with which the climate system comes ... more Feedbacks in the climate system act to reduce the efficiency with which the climate system comes back into equilibrium following a radiative perturbation. In this thesis, we first investigate the implications of climate feedbacks on the change in poleward energy transport as the planet warms. We find that large positive cloud and water vapor feedbacks at low latitudes and negative cloud feedbacks of high latitudes reinforce the pre-existing net radiation gradient at the top of atmosphere, requiring that more heat be fluxed to the poles under transient warming. Because anomalous heat fluxes into the ocean also tend to preferentially cool the high latitude atmosphere, the atmosphere becomes more important than the ocean in transporting heat poleward as the planet warms. The largest uncertainty in the change in poleward transport and in climate sensitivity can be attributed to cloud feedbacks, but this large inter-model spread is primarily in the shortwave component of cloud feedback. Longwave cloud feedbacks are robustly positive across models, and we show that this is primarily caused by tropical high clouds, which rise in accordance with the requirements of radiative-convective equilibrium as the planet warms in such a way as to remain at nearly the same temperature. We also show, using cloud radiative kernels, that rising extra-tropical clouds also contribute significantly to the longwave cloud feedback, but that the effect of rising clouds is strongly opposed by reductions in cloud amount. Conversely, reductions in cloud amount largely explain the positive shortwave cloud feedback at low latitudes, but increases in cloud brightness cause the strong negative cloud feedback at high latitudes. Using a suite of satellite observations of the Tropics, we show that a simple clear-sky diagnostic tool accurately marks the location of peak high cloudiness, and realistically tracks the change in cloud profile associated with interannual variability. As the Tropics warm, cloud profiles shift upward and exhibit a reduced peak that is remarkably well-diagnosed by the profile of clear-sky diabatic convergence. Furthermore, we estimate that cloud fraction anomalies result in enhanced emission of longwave radiation and reduced reflection of SW radiation to space, with the latter dominating.

Journal of Geophysical Research, 2010

1] Longwave cloud feedback is systematically positive and nearly the same magnitude across all gl... more 1] Longwave cloud feedback is systematically positive and nearly the same magnitude across all global climate models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Here it is shown that this robust positive longwave cloud feedback is caused in large part by the tendency for tropical high clouds to rise in such a way as to remain at nearly the same temperature as the climate warms. Furthermore, it is shown that such a cloud response to a warming climate is consistent with well-known physics, specifically the requirement that, in equilibrium, tropospheric heating by convection can only be large in the altitude range where radiative cooling is efficient, following the fixed anvil temperature hypothesis of Hartmann and Larson . Longwave cloud feedback computed assuming that high-cloud temperature follows upper tropospheric convergence-weighted temperature, which we refer to as proportionately higher anvil temperature, gives an excellent prediction of the longwave cloud feedback in the AR4 models. The ensemble-mean feedback of 0.5 W m −2 K −1 is much larger than that calculated assuming clouds remain at fixed pressure, highlighting the large contribution from rising cloud tops to the robustly positive feedback. An important result of this study is that the convergence profile computed from clear-sky energy and mass balance warms slightly as the climate warms, in proportion to the increase in stability, which results in a longwave cloud feedback that is slightly smaller than that calculated assuming clouds remain at fixed temperature.

Journal of Climate, 2009

29 Deep convective systems are prominent features of the tropical atmosphere that have important ... more 29 Deep convective systems are prominent features of the tropical atmosphere that have important roles at a spectrum of space and time scales from local diurnal cycles to the planetaryscale Hadley and Walker circulations. Upward motion occurring in the ascending branches of the Hadley and Walker circulations is realized not in the form of large-scale continuous ascent but in the form of a large number of relatively small-scale discrete convective plumes occurring in an otherwise subsiding environment (Yanai et al. 1973). Because it is the ensemble of transient deep convective processes that results in the measured mean ascent in the Tropics, understanding the large spatial and long temporal scale tropical circulation requires understanding deep convective processes.

Geophysical Research Letters, 2013

1] This study quantifies the response of the clouds and the radiative budget of the Southern Hemi... more 1] This study quantifies the response of the clouds and the radiative budget of the Southern Hemisphere (SH) to the poleward shift in the tropospheric circulation induced by the development of the Antarctic ozone hole. Single forcing climate model integrations, in which only stratospheric ozone depletion is specified, indicate that (1) high-level and midlevel clouds closely follow the poleward shift in the SH midlatitude jet and that (2) low-level clouds decrease across most of the Southern Ocean. Similar cloud anomalies are found in satellite observations during periods when the jet is anomalously poleward. The hemispheric annual mean radiation response to the cloud anomalies is calculated to be approximately +0.25 W m À2 ,a r i s i n g largely from the reduction of the total cloud fraction at SH midlatitudes during austral summer. While these dynamically induced cloud and radiation anomalies are considerable and are supported by observational evidence, quantitative uncertainties remain from model biases in mean-state cloud-radiative processes. Citation: Grise, K. M., L. M. Polvani, G. Tselioudis, Y. Wu, and M. D. Zelinka (2013), The ozone hole indirect effect: Cloud-radiative anomalies accompanying the poleward shift of the eddy-driven jet in the Southern Hemisphere, Geophys. Res. Lett., 40,

The ability of the Earth to regulate its energy budget is strongly dependent on the concentration... more The ability of the Earth to regulate its energy budget is strongly dependent on the concentration of water vapor in the atmosphere. Increases in humidity at upper levels of the troposphere have a greater radiative impact on the climate than equivalent increases closer to the surface. In this study, I investigate the evolution of the moisture and cloud distribution in deep convective systems over the tropical Pacific using retrievals of water vapor, temperature, and cloud properties from AMSR-E, AIRS, and MODIS. Using a compositing technique centered in space and time on locations having high rain rates, I have identified systematic lags in both the fractional coverage of anvil clouds and in the moistening of the upper troposphere following peak convection. Anvil cloud fractions peak 3 hours after the strongest convection while the upper troposphere is most humid 9-15 hours following the strongest convection. Composited vertical velocity profiles from the ERA-40 Reanalysis show anomalous ascent in the upper troposphere following peak convection, which likely has a role in sustaining the moist signature aloft. Correspondingly, clear-sky outgoing longwave radiation remains significantly reduced for several hours after the convection, indicating that the sustained moist anomaly at upper levels continues to affect the local radiation budget well after the convective event.

Dynamical and thermodynamical feedbacks within the climate system play a large role in determinin... more Dynamical and thermodynamical feedbacks within the climate system play a large role in determining the regional response to global warming. In this talk two examples will be discussed. First, dynamical feedbacks between baroclinic eddies and zonal jets create natural low frequency variability. If climate change forced by natural or anthropogenic causes projects onto these modes of variability, then the spatial response can be larger and more coherent than would be expected in the absence of these modes. An example is given of the effect of the Quasi-Biennial Oscillation of the tropical stratosphere on the tropospheric circulation. Second, thermodynamic feedbacks within the climate system control not only the magnitude, but also the spatial structure of global warming. An example is given of how the latitudinal structure of cloud and water vapor feedbacks, being strongly positive near the equator and weakening with latitude, cause the required meridional heat transport by the climate system to be increased and also shifted from the ocean to the atmosphere. This has a profound effect on regional response to global warming.

Kernel functions are used to compute the sensitivity of TOA fluxes to atmospheric feedback proces... more Kernel functions are used to compute the sensitivity of TOA fluxes to atmospheric feedback processes using AR4 A2 simulations of the 21st Century climate. These TOA flux feedbacks can be used to compute the apparent causes of changes in the equator to pole flux of energy by the climate system. Near the equator the net feedback is positive, so that the climate is locally unstable. This results from enhanced water vapor and cloud longwave feedbacks there. The climate system must therefore flux more energy poleward away from the equator in warmed climates. Using surface flux sensitivities the atmospheric and oceanic flux feedbacks can be computed. The ensemble of models show an increase of atmospheric meridional energy flux and a decrease in oceanic heat flux. This is true in both hemispheres, but more strongly in the Southern Hemisphere where the upward flux of cold water supports a downward flux of sensible heat from the atmosphere and enhanced poleward flux of energy in the atmosphere. This may be related to the poleward shift of the jet in high latitudes of the Southern Hemisphere that is produced in many AR4 simulations of the 21st Century.

A number of studies have investigated the relationship between tropical deep convection and sea s... more A number of studies have investigated the relationship between tropical deep convection and sea surface temperature (SST) and the implications of these relationships for climate and climate change. Deep convection and SST are somewhat indirectly related to each other through their individual relationships with the large-scale circulation, however, because regions of high SST are also regions of large-scale low-level convergence which encourages deep convection. Thus the intrinsic sensitivity of convection to SST is somewhat less apparent. Many previous studies use outgoing longwave radiation as a proxy for deep convection, but the ubiquity of thin cirrus disconnected from active deep convection in the tropics makes interpretation of observed relationships difficult. In this study we use rainfall data from the TRMM Multisatellite Precipitation Analysis as direct measures of deep convective events, SST from AMSR-E, cloud fraction, cloud top temperature, and cloud optical thickness from MODIS, longwave and shortwave TOA fluxes from CERES and vertical velocity from NCEP/NCAR reanalysis to address a number of questions: Within a given circulation regime (as determined by the magnitude and vertical shape of the vertical motion), how does the ensemble of cloud properties vary with SST, SST gradient, and precipitation rate? How do the observed changes in cloud properties and their respective radiative forcings affect the TOA and surface energy budgets? Likewise, how does the large- scale circulation respond to a change in SST gradient, and how does this circulation change impact the cloud properties? The sensitivity of convective evolution to the underlying SST will also be assessed by using a compositing technique centered on intense rain events. Implications of these results for tropical climate will be discussed.

A positive longwave cloud feedback is a robust feature of all global climate models submitted to ... more A positive longwave cloud feedback is a robust feature of all global climate models submitted to the IPCC AR4 archive. In this study we show that this is in large part due to the fact that tropical high clouds rise vertically as the planet warms, but do so in such a way as to remain at nearly the same temperature. Thus the longwave emission from tropical high clouds remains approximately constant rather than increasing in step with the warming planet, resulting in a positive longwave cloud feedback. We demonstrate that this robust feature of the models' tropical high clouds makes sense physically due to the fundamental constraint imposed by the clear-sky radiative cooling rate, as in the fixed anvil temperature (FAT) hypothesis of Hartmann and Larson (2002). Furthermore, the slight reduction in fractional coverage of high clouds that occurs over the course of the 21st Century simulations is also well-explained by considering the clear-sky balance between radiative cooling and subsidence: The simulated increase in static stability out-paces the increase in longwave cooling, thereby causing less clear-sky upper tropospheric convergence and - by mass conservation - a reduction in high cloud cover.

Cloud feedback represents the source of largest uncertainty in projections of future warming and ... more Cloud feedback represents the source of largest uncertainty in projections of future warming and observational constraints on both the sign and magnitude of the feedback remain lacking. In this study we use observations from a suite of satellite instruments (AIRS, MODIS, CERES, CloudSat, ISCCP) and in situ data to assess the sensitivity of tropical high clouds to tropical mean surface temperature anomalies associated with interannual variability. Unlike previous studies, however, we relate cloud changes to a physical governing mechanism that has sensitivity to the vertical structure of warming, removing the ambiguity associated with simple regressions of clouds on surface temperature alone. Specifically, we demonstrate that the profile of high clouds (both the peak level and fractional coverage) and its interannual variability are well-diagnosed by the profile of upper tropospheric convergence derived from the mass and energy budget of the clear-sky atmosphere. As in the global warming case, high clouds rise and exhibit a reduction in coverage when the Tropics warm. In contrast to the global warming case, the cloud fraction reduction at lower levels exceeds the increase at higher levels, implying reduced shortwave reflection and enhanced longwave emission that dominate over reduced emission from higher tops. We demonstrate that differences in the vertical structure of interannual versus transient warming lead to differences in convergence that accurately explain these differences in cloud response, providing an explanation for why cloud feedbacks operating on interannual timescales differ from those in the global warming case. This suggests that the simple convergence metric may be a powerful evaluation tool for assessing the realism of modeled clouds and for eventually reducing the uncertainty surrounding cloud feedback.

Feedbacks in the climate system act to reduce the efficiency with which the climate system comes ... more Feedbacks in the climate system act to reduce the efficiency with which the climate system comes back into equilibrium following a radiative perturbation. In this thesis, we first investigate the implications of climate feedbacks on the change in poleward energy transport as the planet warms. We find that large positive cloud and water vapor feedbacks at low latitudes and negative cloud feedbacks of high latitudes reinforce the pre-existing net radiation gradient at the top of atmosphere, requiring that more heat be fluxed to the poles under transient warming. Because anomalous heat fluxes into the ocean also tend to preferentially cool the high latitude atmosphere, the atmosphere becomes more important than the ocean in transporting heat poleward as the planet warms. The largest uncertainty in the change in poleward transport and in climate sensitivity can be attributed to cloud feedbacks, but this large inter-model spread is primarily in the shortwave component of cloud feedback. Longwave cloud feedbacks are robustly positive across models, and we show that this is primarily caused by tropical high clouds, which rise in accordance with the requirements of radiative-convective equilibrium as the planet warms in such a way as to remain at nearly the same temperature. We also show, using cloud radiative kernels, that rising extra-tropical clouds also contribute significantly to the longwave cloud feedback, but that the effect of rising clouds is strongly opposed by reductions in cloud amount. Conversely, reductions in cloud amount largely explain the positive shortwave cloud feedback at low latitudes, but increases in cloud brightness cause the strong negative cloud feedback at high latitudes. Using a suite of satellite observations of the Tropics, we show that a simple clear-sky diagnostic tool accurately marks the location of peak high cloudiness, and realistically tracks the change in cloud profile associated with interannual variability. As the Tropics warm, cloud profiles shift upward and exhibit a reduced peak that is remarkably well-diagnosed by the profile of clear-sky diabatic convergence. Furthermore, we estimate that cloud fraction anomalies result in enhanced emission of longwave radiation and reduced reflection of SW radiation to space, with the latter dominating.

Journal of Geophysical Research, 2010

1] Longwave cloud feedback is systematically positive and nearly the same magnitude across all gl... more 1] Longwave cloud feedback is systematically positive and nearly the same magnitude across all global climate models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Here it is shown that this robust positive longwave cloud feedback is caused in large part by the tendency for tropical high clouds to rise in such a way as to remain at nearly the same temperature as the climate warms. Furthermore, it is shown that such a cloud response to a warming climate is consistent with well-known physics, specifically the requirement that, in equilibrium, tropospheric heating by convection can only be large in the altitude range where radiative cooling is efficient, following the fixed anvil temperature hypothesis of Hartmann and Larson . Longwave cloud feedback computed assuming that high-cloud temperature follows upper tropospheric convergence-weighted temperature, which we refer to as proportionately higher anvil temperature, gives an excellent prediction of the longwave cloud feedback in the AR4 models. The ensemble-mean feedback of 0.5 W m −2 K −1 is much larger than that calculated assuming clouds remain at fixed pressure, highlighting the large contribution from rising cloud tops to the robustly positive feedback. An important result of this study is that the convergence profile computed from clear-sky energy and mass balance warms slightly as the climate warms, in proportion to the increase in stability, which results in a longwave cloud feedback that is slightly smaller than that calculated assuming clouds remain at fixed temperature.

Journal of Climate, 2009

29 Deep convective systems are prominent features of the tropical atmosphere that have important ... more 29 Deep convective systems are prominent features of the tropical atmosphere that have important roles at a spectrum of space and time scales from local diurnal cycles to the planetaryscale Hadley and Walker circulations. Upward motion occurring in the ascending branches of the Hadley and Walker circulations is realized not in the form of large-scale continuous ascent but in the form of a large number of relatively small-scale discrete convective plumes occurring in an otherwise subsiding environment (Yanai et al. 1973). Because it is the ensemble of transient deep convective processes that results in the measured mean ascent in the Tropics, understanding the large spatial and long temporal scale tropical circulation requires understanding deep convective processes.