Papers by George Tselioudis
Research Square (Research Square), Apr 26, 2022
Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagr... more Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagreement is a necessary first step in understanding their wide variation in climate sensitivities. Here we bring together two analysis techniques that illuminate complementary aspects of cloud feedback. The first quantifies feedbacks from changes in cloud amount, altitude, and optical depth, while the second separates feedbacks due to cloud property changes within specific cloud regimes from those due to regime occurrence frequency changes. We find that in the global mean, shortwave cloud feedback averaged across ten models comes solely from a positive within-regime cloud amount feedback countered slightly by a negative within-regime optical depth feedback. These within-regime feedbacks are highly uniform: In nearly all regimes, locations, and models, cloud amount decreases and cloud albedo increases with warming. In contrast, between-regime components vary widely across models but are very small on average. This component, however, is dominant in set
<p><strong>Cold-air outbreaks (CAOs) form marine boundary layer (MBL)... more <p><strong>Cold-air outbreaks (CAOs) form marine boundary layer (MBL) clouds that undergo rapid overcast-to-broken cloud regime transitions, initiated by substantial rain. CAOs are usually accompanied by dry intrusions (DIs) that subside as free-tropospheric (FT) air into the postfrontal sector of mid-latitude storms. For an exemplary cold-air outbreak in the NW Atlantic that showed faster transitions (corresponding to reduced extents of overcast clouds) closer to the low-pressure system, we posit that varying transitions are caused by an uneven meteorological pattern imposed by the prevailing DI. We compile satellite observations, reanalysis fields, and Lagrangian large-eddy simulations (LES) translating along MERRA2-based trajectories to show that postfrontal trajectories closer to the low-pressure system are uniquely favorable to rain formation (and, thus, cloud transitions) as they show (1) weaker FT subsidence rates, (2) greater FT humidity, (3) greater MBL windspeeds, and (4) a colder MBL as well as reduced lower-tropospheric stability. We present an updated conceptual view of postfrontal cloud formation that may guide future investigations.</strong></p>
<p&amp... more <p>Recent analyses of Coupled Model Intercomparison Project phase 6 (CMIP6) models have shown higher climate sensitivities than previously reported, and this increase has been preliminary attributed to the simulation of anomalous Shortwave Cloud Radiative Effect (SWCRE) over the southern midlatitude regions. In this work, we further explore how the seasonal and annual SWCRE over different regions of the globe influence the model climate sensitivities. Our study suggests a significant contribution of SWCRE on climate sensitivities in both northern and southern midlatitudes; and the relationship remains robust across the seasons. Additionally, we assess the underlying physics of the inter-model spread to diagnose model biases. The results will contribute to quantify the severity of the Equilibrium Climate Sensitivity, as simulated by the CMIP6 models.</p>
Bulletin of the American Meteorological Society, Nov 1, 2006
Bulletin of the American Meteorological Society, Aug 1, 2019
interactions owing to proximity to aircraft bases and a wide range in aerosol concentrations coup... more interactions owing to proximity to aircraft bases and a wide range in aerosol concentrations coupled to a persistent marine cloud deck, especially in the summertime when experiments are usually conducted. Extensive ship traffic in this study region served as a focal point of many experiments (e.g., Durkee et al. 2000; Russell et al. 2013) since the formation of ship tracks represents one of the clearest visual demonstrations of how aerosol perturbations impact clouds when viewed from space. Diversity of other pollutant sources, with varying characteristic physical and chemical properties, provides an additional benefit for studying this region. The lessons learned from the California studies sponsored by the Office of Naval Research (ONR) provide motivation for a five-year NASA Earth Venture Suborbital (EVS-3) investigation off the opposite coast of the United States. A dual-aircraft approach with combined in situ and remote sensing instrumentation will be coupled to an unprecedented number of f lights to maximize statistics in a region with diverse aerosol and meteorological conditions, including the continuum of warm cloud types spanning stratiform to cumulus. The Aerosol Cloud meTeorolog y Interactions oVer the western ATlantic Experiment (ACTIVATE) is described in detail, with a description of data analysis and multiscale modeling that will address the complexity of the processes being examined ranging in spatial scale from ~10-7 to 10 6 m (i.e.,
Lectures in climate change, Nov 1, 2022
From its location on the subtropics-midlatitude boundary, the Azores is influenced by both the su... more From its location on the subtropics-midlatitude boundary, the Azores is influenced by both the subtropical high pressure and the midlatitude baroclinic storm regimes, and therefore experiences a wide range of cloud structures, from fair-weather scenes to stratocumulus sheets to deep convective systems. This project combined three types of data sets to study cloud variability in the Azores: a satellite analysis of cloud regimes, a reanalysis characterization of storminess, and a 19-month field campaign that occurred on Graciosa Island. Combined analysis of the three data sets provides a detailed picture of cloud variability and the respective dynamic influences, with emphasis on low clouds that constitute a major uncertainty source in climate model simulations. The satellite cloud regime analysis shows that the Azores cloud distribution is similar to the mean global distribution and can therefore be used to evaluate cloud simulation in global models. Regime analysis of low clouds shows that stratocumulus decks occur under the influence of the Azores high-pressure system, while shallow cumulus clouds are sustained by cold-air outbreaks, as revealed by their preference for post-frontal environments and northwesterly flows. An evaluation of CMIP5 climate model cloud regimes over the Azores shows that all models severely underpredict shallow cumulus clouds,more » while most models also underpredict the occurrence of stratocumulus cloud decks. It is demonstrated that carefully selected case studies can be related through regime analysis to climatological cloud distributions, and a methodology is suggested utilizing process-resolving model simulations of individual cases to better understand cloud-dynamics interactions and attempt to explain and correct climate model cloud deficiencies.« less
Geoscientific Model Development, Jan 25, 2017
The primary objective of CFMIP is to inform future assessments of cloud feedbacks through improve... more The primary objective of CFMIP is to inform future assessments of cloud feedbacks through improved understanding of cloud-climate feedback mechanisms and better evaluation of cloud processes and cloud feedbacks in climate models. However, the CFMIP approach is also increasingly being used to understand other aspects of climate change, and so a second objective has now been introduced, to improve understanding of circulation, regional-scale precipitation, and non-linear changes. CFMIP is supporting ongoing model inter-comparison activities by coordinating a hierarchy of targeted experiments for CMIP6, along with a set of cloud-related output diagnostics. CFMIP contributes primarily to addressing the CMIP6 questions "How does the Earth system respond to forcing?" and "What are the origins and consequences of systematic model biases?" and supports the activities of the WCRP Grand Challenge on Clouds, Circulation and Climate Sensitivity. A compact set of Tier 1 experiments is proposed for CMIP6 to address this question: (1) what are the physical mechanisms underlying the range of cloud feedbacks and cloud adjustments predicted by climate models, and which models have the most credible cloud feedbacks? Additional Tier 2 experiments are proposed to address the following questions. (2) Are cloud feedbacks consistent for climate cooling and warming, and if not, why? (3) How do cloudradiative effects impact the structure, the strength and the variability of the general atmospheric circulation in present and future climates? (4) How do responses in the climate system due to changes in solar forcing differ from changes due to CO 2 , and is the response sensitive to the sign of the forcing? (5) To what extent is regional climate change per CO 2 doubling state-dependent (non-linear), and why? (6) Are climate feedbacks during the 20th century different to those acting on long-term climate change and climate sensitivity? (7) How do regional climate responses (e.g. in precipitation) and their uncertainties in coupled models arise from the combination of different aspects of CO 2 forcing and sea surface warming? Published by Copernicus Publications on behalf of the European Geosciences Union. 360 M. J. Webb et al.: The Cloud Feedback Model Intercomparison Project (CFMIP) CFMIP also proposes a number of additional model outputs in the CMIP DECK, CMIP6 Historical and CMIP6 CFMIP experiments, including COSP simulator outputs and process diagnostics to address the following questions. 1. How well do clouds and other relevant variables simulated by models agree with observations? 2. What physical processes and mechanisms are important for a credible simulation of clouds, cloud feedbacks and cloud adjustments in climate models? 3. Which models have the most credible representations of processes relevant to the simulation of clouds? 4. How do clouds and their changes interact with other elements of the climate system?
AGU Fall Meeting Abstracts, Dec 1, 2018
AGU Fall Meeting Abstracts, Dec 18, 2015
WORLD SCIENTIFIC eBooks, Mar 13, 2022
Climate Dynamics, Sep 11, 2022
Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagr... more Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagreement is a necessary first step in understanding their wide variation in climate sensitivities. Here we bring together two analysis techniques that illuminate complementary aspects of cloud feedback. The first quantifies feedbacks from changes in cloud amount, altitude, and optical depth, while the second separates feedbacks due to cloud property changes within specific cloud regimes from those due to regime occurrence frequency changes. We find that in the global mean, shortwave cloud feedback averaged across ten models comes solely from a positive within-regime cloud amount feedback countered slightly by a negative within-regime optical depth feedback. These within-regime feedbacks are highly uniform: In nearly all regimes, locations, and models, cloud amount decreases and cloud albedo increases with warming. In contrast, between-regime components vary widely across models but are very small on average. This component, however, is dominant in set
arXiv (Cornell University), Dec 8, 2022
Improved knowledge of glacial-to-interglacial global temperature change implies that fastfeedback... more Improved knowledge of glacial-to-interglacial global temperature change implies that fastfeedback equilibrium climate sensitivity (ECS) is 1.2 ± 0.3°C (2σ) per W/m 2. Consistent analysis of temperature over the full Cenozoic era-including "slow" feedbacks by ice sheets and trace gasessupports this ECS and implies that CO2 was about 300 ppm in the Pliocene and 400 ppm at transition to a nearly ice-free planet, thus exposing unrealistic lethargy of ice sheet models. Equilibrium global warming including slow feedbacks for today's human-made greenhouse gas (GHG) climate forcing (4.1 W/m 2) is 10°C, reduced to 8°C by today's aerosols. Decline of aerosol emissions since 2010 should increase the 1970-2010 global warming rate of 0.18°C per decade to a post-2010 rate of at least 0.27°C per decade. Under the current geopolitical approach to GHG emissions, global warming will likely pierce the 1.5°C ceiling in the 2020s and 2°C before 2050. Impacts on people and nature will accelerate as global warming pumps up hydrologic extremes. The enormity of consequences demands a return to Holocene-level global temperature. Required actions include: 1) a global increasing price on GHG emissions, 2) East-West cooperation in a way that accommodates developing world needs, and 3) intervention with Earth's radiation imbalance to phase down today's massive human-made "geo-transformation" of Earth's climate. These changes will not happen with the current geopolitical approach, but current political crises present an opportunity for reset, especially if young people can grasp their situation.
Springer atmospheric sciences, Sep 10, 2016
Midlatitude storm track density shifts are correlated with satellite-derived cloud properties and... more Midlatitude storm track density shifts are correlated with satellite-derived cloud properties and radiation effects. We find that high-cloud amount constitutes the primary tracer of storm track shifts, and that high clouds have been shifting poleward in the 1984–2009 time period by 0.30–0.46 degrees per decade in all four major oceanic storm track regions. Driven by the poleward shift of high clouds, the total cloud field and the cloud radiative effect have also been shifting poleward at a rate of 0.12–0.23 degrees per decade, similar to the rate of the poleward shift of the storms in three out of the four major oceanic storm tracks. This poleward total cloud amount shift produces a change in the radiative effect of storm clouds as they move to a region of lower solar insolation.
Journal of Advances in Modeling Earth Systems
This paper presents the response to anthropogenic forcing in the GISS‐E2.1 climate models for the... more This paper presents the response to anthropogenic forcing in the GISS‐E2.1 climate models for the 21st century Shared Socioeconomic Pathways emission scenarios within the Coupled Model Intercomparison Project Phase 6 (CMIP6). The experiments were performed using an updated and improved version of the NASA Goddard Institute for Space Studies (GISS) coupled general circulation model that includes two different versions for atmospheric composition: A non‐interactive version (NINT) with prescribed composition and a tuned aerosol indirect effect and the One‐Moment Aerosol model (OMA) version with fully interactive aerosols which includes a parameterized first indirect aerosol effect on clouds. The effective climate sensitivities are 3.0°C and 2.9°C for the NINT and OMA models, respectively. Each atmospheric version is coupled to two different ocean general circulation models: The GISS ocean model (E2.1‐G) and HYCOM (E2.1‐H). We describe the global mean responses for all future scenarios ...
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Papers by George Tselioudis