Graeme Stephens - Profile on Academia.edu (original) (raw)

Papers by Graeme Stephens

The Earth's weather and climate is driven by the exchange of energy between the sun, atmosphere, ... more The Earth's weather and climate is driven by the exchange of energy between the sun, atmosphere, surface, and space and energy transport required to establish a global balance. Clouds and precipitation play an integral role in this exchange, enhancing reflection of solar radiation to space, trapping thermal emission from the surface, and providing a mechanism for the direct transfer of energy to the atmosphere through the release of latent heat in precipitation. As a result, there is an intimate coupling between the climate, energy budget, and global hydrologic cycle. The problem of establishing observational evidence for these connections and climate change in general, poses a significant challenge to the observational community. This dissertation seeks to address the components of this problem related to observing the hydrologic cycle and its role in modulating the tropical energy budget, from space-based measurements. This work reports on a new technique which makes use of cloud and precipitation information from the Tropical Rainfall Measuring Mission to estimate the principal components of the tropical energy budget and to examine the mechanisms by which clouds and precipitation modify it. First, three distinct retrieval algorithms are employed to determine the three-dimensional structure of cloud and precipitation in the tropical atmosphere. The first retrieves cloud and precipitation profiles from passive microwave observations from the TRMM Microwave Imager while the second applies a different technique to the same observations in an effort to derive estimates of non-precipitating liquid cloud. Finally, the third algorithm makes use of infrared radiances from the Visible and Infrared Scanner to infer-ice cloud optical properties in non-precipitating regions. The reSUlting representation of the three-dimensional strucure of cloud and precipitation in the tropical atmosphere is then used as input to a broadband radiative transfer model to derive profiles of short-and longwave fluxes. These flux profiles are composited to present a TRMM-based estimate of the short-term tropical energy budget for oceanic regions over the month of February 1998. On average, over this period, the tropical atmosphere absorbs 51 Wm-2 or 13 % of the 393 Wm-2 of solar radiation it receives. A further 112 Wm-2 is reflected by atmospheric particles, clouds, and the surface, leaving 230 Wm-2 to be absorbed by the ocean. At thermal wavelengths, it is found that the ocean emits 436 Wm-2 of energy to the atmosphere while the atmosphere emits a total of 639 Wm-2 units, 407 Wm-2 downward toward the surface and 231 Wm-2 to space. Accounting for latent heat release which amounts to an exchange of 82 Wm-2 of energy between the surface and atmosphere, the results imply a deficit of 70 Wm-2 of energy in the atmosphere and a surplus of 121 Wm-2 at the Earth's surface. The implied net gain of 51 Wm-2 in the Earth-atmosphere system is consistent iii IV

Geophysical Research Letters, 2009

While cumulus congestus as an important mode of tropical convection has been established, many of... more While cumulus congestus as an important mode of tropical convection has been established, many of the previous studies that rely on radar observations usually capture them as snapshots. A logical question to consider is: are the statistics gathered from snapshot observations of cumulus congestus really reflective of this mode of convection that ceases its growth at these intermediate levels (terminal cumulus congestus), or will the convection being observed continue to ascend to greater altitudes to become deep convection at a later time (transient cumulus congestus)? This short article strives to answer this question by analyzing simultaneous, independent measurements of CTH and CTT from CloudSat and MODIS, together with CloudSat radar profile and collocated ECMWF analysis. It is found, based on analysis of one year of data, that a significant fraction ($30-40%) of the tropical cumulus congestus observed by the snapshot views are in transient mode that will ascend to greater altitude at a later time. The analysis concept used in this study, which gives ''static'' snapshot observations some ''dynamic'' context, can be applied to analyze convection of all types.

IEEE Transactions on Geoscience and Remote Sensing, 2019

Journal of Climate, 2016

In all outputs of the 1% yr−1 increase in CO2 climate model experiments archived under the World ... more In all outputs of the 1% yr−1 increase in CO2 climate model experiments archived under the World Climate Research Programme’s (WCRP) phase 5 of the Coupled Model Intercomparison Project (CMIP5), regions exist in the low latitudes where both the clear-sky and all-sky OLR decrease with surface warming. These are identified as regions of positive longwave feedback and are regions of a super greenhouse effect (SGE). These SGE regions are identified from feedback analysis of the 4 × CO2 abrupt experiments of CMIP5, and despite their existence, there is little agreement across models as to the magnitude of the effect. The general effects of clouds on the SGE are to amplify the clear-sky SGE, but there is also poor agreement on the magnitude of the amplification that varies by an order of magnitude across models. Sensitivity analyses indicate that localized SGE regions are spatially aligned with a large moistening of the upper troposphere. The reduction in clear-sky OLR arises from a reduc...

Improved ice content, radiation, precipitation and low-level circulation over the tropical pacific from ECMWF ERA-interim to ERA5

Environmental Research Communications

This study evaluates changes in simulated Pacific climate between two ECMWF re-analyses; the ERA ... more This study evaluates changes in simulated Pacific climate between two ECMWF re-analyses; the ERA Interim (ERAI) and the newest ERA5. Changes in the Integrated Forecasting System (IFS) and possibly sea surface temperature result in greatly reduced discrepancies in ERA5’s ice water path (IWP), radiative fluxes and precipitation relative to satellite-based observational products. IWP shows the largest percentage change, increasing by over 300% from ERAI to ERA5, due to inclusion of falling ice (snow) that impacts radiative calculation. ERAI to ERA5 changes in high-cloud fraction are generally anticorrelated as expected with outgoing longwave radiation, with ERA5 having smaller longwave discrepancies versus CERES observations compared with ERAI. Reflected shortwave discrepancies are similarly reduced from ERAI to ERA5, which appears to be due to changes in both cloud fraction and optical depth. Finally, ERA5 also reduces a longstanding precipitation excess relative to the GPCP observati...

On the combination of active and passive measurements in the study of clouds and precipitation

IGARSS 2001. Scanning the Present and Resolving the Future. Proceedings. IEEE 2001 International Geoscience and Remote Sensing Symposium (Cat. No.01CH37217)

Presents an approach that combines millimetric radar measurements and radiometer measurements for... more Presents an approach that combines millimetric radar measurements and radiometer measurements for deriving the water contents of clouds and precipitation and does so within the context of the CloudSat satellite mission

A number of climate intervention concepts, referred to as “geoengineering,” are being considered ... more A number of climate intervention concepts, referred to as “geoengineering,” are being considered as a potential additional approach (beyond mitigation of greenhouse gas emissions) to manage climate change. However, before governments go down the path of attempting deliberate climate intervention including precursor field-experiments, it is essential that the scientific community take the necessary steps to validate our understanding that underpins any of the proposed intervention concepts in order to understand all likely consequences and put in place the necessary strategies for monitoring the expected and unintended consequences of such intervention. The Keck Institute for Space Studies (KISS) has sponsored a project to identify specific priorities for improved scientific understanding and focused efforts to address selected priorities. This project does not advocate the deployment of geoengineering, outdoor geoengineering experiments, or monitoring systems for such proposed geoen...

Howard, a London pharmacist and amateur meteorologist, aired his ideas about the classification o... more Howard, a London pharmacist and amateur meteorologist, aired his ideas about the classification of clouds. These ideas were presented to a small gathering of young science-minded intellectuals who called themselves The Askesian Society. Howard's lecture on that evening was titled " On the Modification of Clouds " and opened as follows: My talk this evening concerns itself with what may strike some as an uncharacteristically impractical subject: it is concerned with the modification of clouds. Since the increased attention which has been given to meteorology, the studies of various appearances of water suspended in the atmosphere has become an interesting and even necessary branch of that pursuit. If clouds were the mere result of condensation of vapour in the masses of the atmosphere which they occupy, if their variations were produced by the movements of the atmosphere alone, then indeed might the study be deemed a useless pursuit of shadows.… This was a historic lect...

What is the Role of GEWEX in R2O and O2R? Current Challenges in Evapotranspiration Determination ... more What is the Role of GEWEX in R2O and O2R? Current Challenges in Evapotranspiration Determination Land Surface Interactions with the Atmosphere over the Semi-Arid Environment (LIAISE) 2 Quarter 1 2019 Graeme Stephens Co-Chair, GEWEX Scientific Steering Group The International Geophysical Year (IGY) in 1957-58 was a watershed moment in Earth sciences, bringing together many disciplines and marking a major change in the study of Earth. What evolved out of the IGY was an appreciation that Earth was a dynamic system exemplified by the revolutionary new, emerging model of Earth plate tectonics. Today, we fully understand and embrace the concept of an Earth system that is complex with interactions occurring between its many components. The concept of Earth system science is now deeply rooted in our thinking, as expressed in the Bretherton report of the 1980's. This system realization, however, represents challenges in organizing a science enterprise like that of the World Climate Research Programme (WCRP). It also is a major challenge for designing Earth observing systems, a point expressed the recent National Academies Decadal Survey report. Hints at a more integrative approach to address this challenge from an observing system perspective are expressed by the following most important priority identified by the hydrology panel organized for the decadal study: H-1a Interaction of Water and Energy Cycles-develop and evaluate an integrated Earth system analysis. The rationale of this objective recognizes that information of the future will ultimately come through some advanced system of models and observations fused together. The challenge is to develop such an advanced system and we are not there today, although elements of it are evolving. When it comes to climate science, there is a similar need to have an integrated system approach to couple the disciplinary parts together and we rely on Earth system models to achieve this synthesis. In both cases, the foundation for these integrative tools ultimately lies with adequate, quantitative representation of the physics and chemistry and of the basic processes of the system. This sentiment was recognized in the review of WCRP by the World Meteorological Organization (WMO), International Science Council (ICS), and the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization (IOC-UNESCO) (https://council.science/cms/2018/08/ WCRP_Report_full_screen_16112018.pdf), which recommended that a critical capability of WCRP must be to sustain activities on processes across all time and space scales. GEWEX has from the outset been more process-centric in its approach rather than phenomenologically-centric, a point underscored in this newsletter with the report on the new Global Atmospheric System Studies (GASS) Panel initiative on the Impact of Initialized Land Temperature and Snowpack on Sub-seasonal to Seasonal Prediction (LS4P) and the meeting summary of the Upper Tropospheric Clouds and Convection Process Evaluation Study (UTCC PROES). The challenge for WCRP and GEWEX is to maintain a deep discipline expertise such as resides in the three Panels, while also providing a more integrative vision of the Earth system. There is no clear recipe for building such integration. The same challenges exists for WCRP, which must unify all aspects of Panel research while avoiding the arbitrary partitioning of the science that can create gaps in key areas.

The cooling of light rains in a warming world

Nature Climate Change

Recent changes to how clouds are represented in global models, especially over the Southern Ocean... more Recent changes to how clouds are represented in global models, especially over the Southern Ocean, resulted in increased climate warming. Correcting rain processes in a model shows improved cloud representation but leads to a greatly enhanced negative feedback, offsetting documented increases in model climate sensitivity.

Several inverse problems exist in the atmospheric sciences that are computationally costly when u... more Several inverse problems exist in the atmospheric sciences that are computationally costly when using traditional gradient based methods. Unfortunately, many standard evolutionary algorithms do not perform well on these problems. This paper investigates why the temperature inversion problem is so difficult for heuristic search. We show that algorithms imposing smoothness constraints find more competitive solutions. Additionally, a new algorithm is presented that addresses the difficulties of this problem and finds approximate solutions fast.

Earth and Space Science

Most of the global climate models (GCMs) in the Coupled Model Intercomparison Project, phase 5 do... more Most of the global climate models (GCMs) in the Coupled Model Intercomparison Project, phase 5 do not include precipitating ice (aka falling snow) in their radiation calculations. We examine the importance of the radiative effects of precipitating ice on simulated surface wind stress and sea surface temperatures (SSTs) in terms of seasonal variation and in the evolution of central Pacific El Niño (CP-El Niño) events. Using controlled simulations with the CESM1 model, we show that the exclusion of precipitating ice radiative effects generates a persistent excessive upper-level radiative cooling and an increasingly unstable atmosphere over convective regions such as the western Pacific and tropical convergence zones. The invigorated convection leads to persistent anomalous low-level outflows which weaken the easterly trade winds, reducing upper-ocean mixing and leading to a positive SST bias in the model mean state. In CP-El Niño events, this means that outflow from the modeled convection in the central Pacific reduces winds to the east, allowing unrealistic eastward propagation of warm SST anomalies following the peak in CP-El Niño activity. Including the radiative effects of precipitating ice reduces these model biases and improves the simulated life cycle of the CP-El Niño. Improved simulations of present-day tropical seasonal variations and CP-El Niño events would increase the confidence in simulating their future behavior. Plain Language Summary Despite considerable progress, coupled GCMs are still far from accurately representing tropical Pacific El Niño-Southern Oscillation events, mainly due to the involvement of complex air-sea interactions and sophisticated atmospheric and oceanic processes. In this study, we have identified the exclusion of atmospheric precipitating large-particle ice (also known as falling snow) radiative effects, as in most global climate models do not represent this effect, as an important factor. Our results suggest that the inclusion of the contribution of the precipitating ice and its radiative effects in models is important to improve the simulation of the Pacific mean state, seasonal cycle, and CP-El Niño.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

Progress towards achieving a quantitative understanding of the exchanges of water between Earth&#... more Progress towards achieving a quantitative understanding of the exchanges of water between Earth's main water reservoirs is reviewed with emphasis on advances accrued from the latest advances in Earth Observation from space. These exchanges of water between the reservoirs are a result of processes that are at the core of important physical Earth-system feedbacks, which fundamentally control the response of Earth's climate to the greenhouse gas forcing it is now experiencing, and are therefore vital to understanding the future evolution of Earth's climate. The changing nature of global mean sea level (GMSL) is the context for discussion of these exchanges. Different sources of satellite observations that are used to quantify ice mass loss and water storage over continents, how water can be tracked to its source using water isotope information and how the waters in different reservoirs influence the fluxes of water between reservoirs are described. The profound influence of...

Revisiting the Entrainment Relationship of Convective Plumes: A Perspective From Global Observations

Geophysical Research Letters

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Apr 1, 2020

Bulletin of the American Meteorological Society

A technology revolution in Earth observation sensor design is occurring. This revolution in part ... more A technology revolution in Earth observation sensor design is occurring. This revolution in part is associated with the emergence of CubeSat platforms that have forced a de facto standardization on the volume and power into which sensors have to fit. The extent that small sensors can indeed provide similar or replacement capabilities compared to larger and more expensive counterparts has barely been demonstrated and any loss of capability of smaller systems weighed against the gains in costs and new potential capabilities offered by implementing them with a more distributed observing strategy also has not yet been embraced. This paper provides four examples of observations made with prototype miniaturized observing systems, including from CubeSats, that offer a glimpse of this emerging sensor revolution and a hint at future observing system design.

IEEE Transactions on Geoscience and Remote Sensing

The recent successful space-borne demonstration of a miniaturized CubeSat precipitation radar is ... more The recent successful space-borne demonstration of a miniaturized CubeSat precipitation radar is highlighted. The low cost of such a radar, together with the availability of small satellite, platforms to carry it, now make it feasible to consider employing a more distributed approach to observe important atmospheric processes that relate to precipitation. An approach to quantify the transport of water and air by deep convection is described based on a clustering of small radar satellites providing measurements seconds apart. This strategy now adds time as a new dimension for observing such processes. A mission concept, referred to as D-train, comprised of a train of three satellites 30, 90, and 120 s apart is described, and the expected performance of it for providing measures of convective transport is examined based on a large ensemble of simulations of convection with an advanced cloud-resolving model.

Remote Sensing

In this paper, we present a state-of-the-art precipitation estimation framework which leverages a... more In this paper, we present a state-of-the-art precipitation estimation framework which leverages advances in satellite remote sensing as well as Deep Learning (DL). The framework takes advantage of the improvements in spatial, spectral and temporal resolutions of the Advanced Baseline Imager (ABI) onboard the GOES-16 platform along with elevation information to improve the precipitation estimates. The procedure begins by first deriving a Rain/No Rain (R/NR) binary mask through classification of the pixels and then applying regression to estimate the amount of rainfall for rainy pixels. A Fully Convolutional Network is used as a regressor to predict precipitation estimates. The network is trained using the non-saturating conditional Generative Adversarial Network (cGAN) and Mean Squared Error (MSE) loss terms to generate results that better learn the complex distribution of precipitation in the observed data. Common verification metrics such as Probability Of Detection (POD), False Al...

The research carried out under this award dealt with issues involving (1) deterministic radiative... more The research carried out under this award dealt with issues involving (1) deterministic radiative transfer, (2) remote sensing, (3) Stochastic radiative transfer, and (4) parameterization of cloud optical properties. A number of different forms of radiative transfer models in one, two, and three dimensions were developed in an attempt to build an understanding of the radiative transfer in clouds with realistic spatial structure and to determine the key geometrical parameter that influence this transfer. The research conducted also seeks to assess the relative importance of these geometrical effects in contrast to microphysical effects of clouds. The main conclusion of the work is that geometry has a profound influence on ali aspects of radiative transfer and the interpretation of this transfer. We demonstrate how this geometry can influence estimate of particle effective radius to the 30-50% level and also how geometry can significantly bias the remote sensing of cloud optical depth. We haven't yet identified the main way of parameterizing these geometric effects but we have developed a stochastic form of radiative transfer and have explored b the possibility of incorporating gross statistics of clouds in simple radiative transfer models. We have also developed a very useful parameterization of cloud optical properties which will also ultimately be incorporated into this simple model. We propose that this model will serve as a framework for a new cloud-radiation parameterization for global models. MASTER

Australian Journal of Physics

This paper examines the role of water vapour as a greenhouse gas and discusses its role in the ev... more This paper examines the role of water vapour as a greenhouse gas and discusses its role in the evolution of the atmospheres of Venus, Earth and Mars. The paper focuses on how the greenhouse effect operates on Earth and describes the feedback between temperature and water vapour that is thqught to play a key role in global warming induced by increasing concentrations of carbon dioxide. A method for analysing the contribution of water vapour to the greenhouse effect using satellite observations is discussed. It is shown how this contribution varies in a directly proportional way with the amount of water vapour vertically integrated through the column of the atmosphere. Based on the results obtained from the analyses of satellite data, it is established that the sensitivity of the greenhouse effect to changing sea surface temperature is not uniform over the globe and is significantly greater over warmer oceans. The relevance of the results to the water vapour feedback is discussed.