Comparison between measured and modelled cloudless‐sky solar irradiances at the surface and at the top of the atmosphere

Abstract

Cloudless‐sky solar fluxes calculated by the radiative transfer algorithm used in the Canadian Climate Centre's general circulation climate model are compared with measurements of upwelling radiation at the top of the atmosphere (TOA) and downwelling radiation at the surface. The 12‐layer model partitions the solar spectrum into two broad wavebands (0.25–0.68 and 0.68–4.00 μm). The comparison utilized TOA fluxes estimated from Nimbus‐ 7 measurements and measured downwelling fluxes at the surface for Kalgoorlie, West Australia, and downwelling fluxes at the surface for Woodbridge, Ontario. Model estimates and measurements agreed to within experimental error for most solar zenith angles. Estimates improved, especially at Woodbridge, when aerosol effects were included. The mean bias error was less than 4% for surface irradiance and less than 6% for upwelling TOA irradiance, which produces a TOA albedo error of about 0.01.     

Surface‐absorbed and top‐of‐atmosphere radiation fluxes for the Mackenzie River basin from satellite observations and a regional climate model and an evaluation of the model

Abstract

Both the earth‐reflected shortwave and outgoing longwave radiation (OLR) fluxes at the top of the atmosphere (TOA) as well as surface‐absorbed solar fluxes from Canadian Regional Climate Model (CRCM) simulations of the Mackenzie River Basin for the period March 2000 to September 2003 are compared with the radiation fluxes deduced from satellite observations. The differences between the model and satellite solar fluxes at the TOA and at the surface, which are used in this paper to evaluate the CRCM performance, have opposite biases under clear skies and overcast conditions, suggesting that the surface albedo is underestimated while cloud albedo is overestimated. The slightly larger differences between the model and satellite fluxes at the surface compared to those at the TOA indicate the existence of a small positive atmospheric absorption bias in the model. The persistent overestimation of TOA reflected solar fluxes and underestimation of the surface‐absorbed solar fluxes by the CRCM under all sky conditions are consistent with the overestimation of cloud fraction by the CRCM. This results in a larger shortwave cloud radiative forcing (CRF) both at the TOA and at the surface in the CRCM simulation. The OLR from the CRCM agrees well with the satellite observations except for persistent negative biases during the winter months under all sky conditions. Under clear skies, the OLR is slightly underestimated by the CRCM during the winter months and overestimated in the other months. Under overcast conditions the OLR is underestimated by the CRCM, suggesting an underestimation of cloud‐top temperature by the CRCM. There is an improvement in differences between model and satellite fluxes compared to previously reported results largely because of changes to the treatment of the surface in the model.     

Quantifying the contributions of anthropogenic and natural forcings to climate changes over arid-semiarid areas during 1946–2005

In this study, the contributions from changes in man-made greenhouse gases (GHG), anthropogenic aerosols (AA), and land use (LU), as well as natural solar and volcanic (NAT) forcing changes, to observed changes in surface air temperature (T) and precipitation (P) over global land, especially over arid-semiarid areas, during 1946–2005 are quantified using observations and climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results show that the anthropogenic (ANT) forcings dominate the ubiquitous surface warming seen in observations and lead to slight increases in precipitation over most land areas, while the NAT forcing leads to small cooling over land. GHG increases are the primary factor responsible for the anthropogenic climate change, while the AA forcing offsets a large part of the GHG-induced warming and P changes. The LU forcing generally contributes little to the T and P changes from 1946 to 2005 over most land areas. Unlike the consistent temperature changes among most model simulations, precipitation changes display a large spread among the models and are incomparable with the observations in spatial distributions and magnitude, mainly due to its large internal variability that varies among individual model runs. Using an optimal fingerprinting method, we find that the observed warming over land during 1946–2005 can be largely attributed to the ANT forcings, and the combination of the ANT and NAT forcings can explain about 85~95% of the observed warming trend over global land as well as over most arid-semiarid regions such as Northern China. However, the anthropogenic influences on precipitation over the past 60 years are generally undetectable over most land areas, including most arid-semiarid regions. This indicates that internal variability is still larger than the forced change for land precipitation.     

Future projections of the surface heat and water budgets of the Mediterranean Sea in an ensemble of coupled atmosphere–ocean regional climate models

Within the CIRCE project “Climate change and Impact Research: the Mediterranean Environment”, an ensemble of high resolution coupled atmosphere–ocean regional climate models (AORCMs) are used to simulate the Mediterranean climate for the period 1950–2050. For the first time, realistic net surface air-sea fluxes are obtained. The sea surface temperature (SST) variability is consistent with the atmospheric forcing above it and oceanic constraints. The surface fluxes respond to external forcing under a warming climate and show an equivalent trend in all models. This study focuses on the present day and on the evolution of the heat and water budget over the Mediterranean Sea under the SRES-A1B scenario. On the contrary to previous studies, the net total heat budget is negative over the present period in all AORCMs and satisfies the heat closure budget controlled by a net positive heat gain at the strait of Gibraltar in the present climate. Under climate change scenario, some models predict a warming of the Mediterranean Sea from the ocean surface (positive net heat flux) in addition to the positive flux at the strait of Gibraltar for the 2021–2050 period. The shortwave and latent flux are increasing and the longwave and sensible fluxes are decreasing compared to the 1961–1990 period due to a reduction of the cloud cover and an increase in greenhouse gases (GHGs) and SSTs over the 2021–2050 period. The AORCMs provide a good estimates of the water budget with a drying of the region during the twenty-first century. For the ensemble mean, he decrease in precipitation and runoff is about 10 and 15% respectively and the increase in evaporation is much weaker, about 2% compared to the 1961–1990 period which confirm results obtained in recent studies. Despite a clear consistency in the trends and results between the models, this study also underlines important differences in the model set-ups, methodology and choices of some physical parameters inducing some difference in the various air-sea fluxes. An evaluation of the uncertainty sources and possible improvement for future generation of AORCMs highlights the importance of the parameterisation of the ocean albedo, rivers and cloud cover.     

Are homeowners willing to adapt to and mitigate the effects of climate change?

The need to adapt to climate change impacts, whilst simultaneously limiting greenhouse gas emissions, requires that the government’s efforts are joined by public action. In England and Wales, housing contributes significantly to the emissions and many properties are at risk of flooding. This paper investigates the preparedness of homeowners in England and Wales to make changes to their homes in response to the predicted effects of climate change. A telephone survey of 961 homeowners investigated their interest in purchasing mitigation and adaptation improvements against their concern about climate change, awareness of flood risk and attribution of responsibility for action. Whilst the majority of homes had some energy-saving improvements, few were found to have property-level flood protection. The high levels of awareness about climate change and flooding were coupled with the perception of risks as low. Whilst some respondents accepted personal responsibility for action, most believed that the authorities were responsible for flood protection, and would not pay the costs required to make their home more energy-efficient and better prepared for the eventuality of floods. The results suggest that there is scope for further improvement of energy-saving measures, and that the levels of adoption of flood-protection measures are very low. Multi-faceted strategies, including more effective communication of risks and responsibilities, incentives, and material support for the poorest, will need to be developed to overcome the current reluctance by homeowners to invest in flood-protection measures and further energy conservation solutions in the future.     

“Pollution Pods”: The merging of art and psychology to engage the public in climate change

Environmental artists have risen to the challenge of communicating the urgency of public action to address environmental problems such as air pollution and climate change. Joining this challenge, the immersive artwork Pollution Pods (PPs) was created through a synthesis of knowledge from the fields of environmental psychology, empirical aesthetics, and activist art. This study summarizes the scientific process in this transdisciplinary project and reports the findings from a questionnaire study (N = 2662) evaluating the effect of the PPs on visitors. Data were collected at the first two exhibitions of the installation, one in a public park in Trondheim, Norway, and one at Somerset House, London, UK. Intentions to act were strong and slightly increased after visiting the art installation. Individual changes in intentions were positively associated with self-reported emotions of sadness, helplessness, and anger and self-reported cognitive assessment their awareness of the environmental consequences of their action, their willingness to take responsibility for their consequences, and belief in the relevance of environmental problems for daily life. Education and age were negatively associated with intentions. Despite favorable intentions, however, taking advantage of an actual behavioral opportunity to track one's climate change emissions behavior after visiting the PPs could not be detected. We conclude that environmental art can be useful for environmental communication and give recommendations for communicators on how to best make use of it. We emphasize the potential benefits of art that encourages personal responsibility and the need for valid behavior measures in environmental psychological research.     

Bimodality and regime behavior in atmosphere–ocean interactions during the recent climate change

Maximum covariance analysis (MCA) and isometric feature mapping (Isomap) are applied to investigate the spatio-temporal atmosphere–ocean interactions otherwise hidden in observational data for the period of 1979–2010. Despite an established long-term surface warming trend for the whole northern hemisphere, sea surface temperatures (SST) in the East Pacific have remained relatively constant for the period of 2001–2010. Our analysis reveals that SST anomaly probability density function of the leading two Isomap components is bimodal. We conclude that Isomap shows the existence of two distinct regimes in surface ocean temperature, resembling the break and active phases of rainfall over equatorial land areas. These regimes occurred within two separated time windows during the past three decades. Strengthening of trade winds over Pacific was coincident with the cold phase of east equatorial Pacific. This pattern was reversed during the warm phase of east equatorial Pacific. The El Niño event of 1997/1998 happened within the transition mode between these two regimes and may be a trigger for the SST changes in the Pacific. Furthermore, we suggest that Isomap, compared with MCA, provides more information about the behavior and predictability of the inter-seasonal atmosphere–ocean interactions.     

An atmosphere–ocean regional climate model for the Mediterranean area: assessment of a present climate simulation

We present an atmosphere–ocean regional climate model for the Mediterranean basin, called the PROTHEUS system, composed by the regional climate model RegCM3 as the atmospheric component and by a regional configuration of the MITgcm model as the oceanic component. The model is applied to an area encompassing the Mediterranean Sea and compared to a stand-alone version of its atmospheric component. An assessment of the model performances is done by using available observational datasets. Despite a persistent bias, the PROTHEUS system is able to capture the inter-annual variability of seasonal sea surface temperature (SST) and also the fine scale spatio-temporal evolution of observed SST anomalies, with spatial correlation as high as 0.7 during summer. The close inspection of a 10-day strong wind event during the summer of 2000 proves the capability of the PROTHEUS system to correctly describe the daily evolution of SST under strong air–sea interaction conditions. As a consequence of the model’s skill in reproducing observed SST and wind fields, we expect a reliable estimation of air–sea fluxes. The model skill in reproducing climatological land surface fields is in line with that of state of the art regional climate models.     

Correlation between climate sensitivity and aerosol forcing and its implication for the “climate trap”

Climate sensitivity and aerosol forcing are dominant uncertain properties of the global climate system. Their estimates based on the inverse approach are interdependent as historical temperature records constrain possible combinations. Nevertheless, many literature projections of future climate are based on the probability density of climate sensitivity and an independent aerosol forcing without considering the interdependency of such estimates. Here we investigate how large such parameter interdependency affects the range of future warming in two distinct settings: one following the A1B emission scenario till the year 2100 and the other assuming a shutdown of all greenhouse gas and aerosol emissions in the year 2020. We demonstrate that the range of projected warming decreases in the former case, but considerably broadens in the latter case, if the correlation between climate sensitivity and aerosol forcing is taken into account. Our conceptual study suggests that, unless the interdependency between the climate sensitivity and aerosol forcing estimates is properly considered, one could underestimate a risk involving the “climate trap”, an unpalatable situation with a high climate sensitivity in which a very drastic mitigation may counter-intuitively accelerate the warming by unmasking the hidden warming due to aerosols.     

Application of the delta‐Eddington method to the absorption of solar radiation in the atmosphere

Abstract

A model to compute rapidly the absorption of solar radiation in the atmosphere is described. The model is based partially on the parameterization of Lacis and Hansen and also makes use of the delta‐Eddington method. In addition to absorption by ozone and water vapour, and scattering by air molecules and clouds, the mode1 includes absorption and scattering by aerosols. Good agreement is found in comparison with the Lacis and Hansen parameterization in the absence of clouds and aerosol. The present model represents an improvement in the treatment of scattering by clouds. Its main advantage though, is in its flexibility in allowing for interactions with the atmospheric aerosol.     

Impact of land–surface processes on the interannual variability of tropical climate in the LMD GCM

Tropical monsoon circulations exhibit substantial interannual variability. Establishing clear links between this variability and the slowly varying boundary forcing (sea surface temperatures, SSTs, and land surface conditions) has proved difficult. For example, no clear relationships have been found between SST anomalies associated with El Nino/La Nina events and monsoon rainfall. Despite much research over the past 50 years, there are still questions regarding how different components of the land-atmosphere-ocean system contribute to tropical monsoon variability. This study examines the question of land-surface-atmosphere interactions in large-scale tropical convection and their role in rainfall interannual variability. The analysis method is based on a conceptual model of convection energetics applied every day of the simulation at the grid points within the region of interest. This allows for a distinction between the frequency and the characteristic energy and water cycle of these events. With two ensembles of five and three experiments in which different land-surface schemes are used, the relation between land-surface processes and variation of the frequency of convection is studied. It has been found in this modeling study that the formulation of land surface schemes may be important for both the simulation of mean tropical precipitation and its interannual variability by way of the frequency of convective events. Linked to this is an increased response of hydrological cycle over land to SSTAs. Numerous studies have suggested that large-scale factors, such as SST, are the dominant control. However the influence of surface processes depends on the areal extent and distance that separates the region from the ocean. The fact that differences between tropical regions decreases as convection intensifies strengthens this hypothesis. The conclusion is that it is inappropriate to separate the causes of interannual variability between SSTAs and land-surface anomalies to explain precipitation variations as land surface processes play a significant mediating role in the relationship between SSTs and monsoon strength. However there remains the possibility that a substantial portion of variability is due to dynamical processes internal to the atmosphere. Determining the relative roles of internal and lower boundary forcing processes in producing interannual variations in the tropical climate is a major objective of future research.     

Evaluating climate change at the Croatian Adriatic from observations and regional climate models’ simulations

The 2m temperature (T2m) and precipitation from five regional climate models (RCMs), which participated in the ENSEMBLES project and were integrated at a 25-km horizontal resolution, are compared with observed climatological data from 13 stations located in the Croatian coastal zone. The twentieth century climate was simulated by forcing RCMs with identical boundary conditions from the ERA-40 reanalysis and the ECHAM5/MPI-OM global climate model (GCM); climate change in the twenty-first century is based on the A1B scenario and assessed from the GCM-forced RCMs’ integrations. When forced by ERA-40, most RCMs exhibit cold bias in winter which contributes to an overestimation of the T2m annual cycle amplitude and the errors in interannual variability are in all RCMs smaller than those in the climatological mean. All models underestimate observed warming trends in the period 1951–2010. The largest precipitation biases coincide with locations/seasons with small observed amounts but large precipitation amounts near high orography are relatively well reproduced. When forced by the same GCM all RCMs exhibit a warming in the cold half-year and a cooling (or weak warming) in the warm period, implying a strong impact of GCM boundary forcing. The future eastern Adriatic climate is characterised by a warming, up to +5 °C towards the end of the twenty-first century; for precipitation, no clear signal is evident in the first half of the twenty-first century, but a reduction in precipitation during summer prevails in the second half. It is argued that land-sea contrast and complex coastal configuration of the Croatian coast, i.e. multitude of island and well indented coastline, have a major impact on small-scale variability. Orography plays important role only at small number of coastal locations. We hypothesise that the parameterisations related to land surface processes and soil hydrology have relatively stronger impact on variability than orography at those locations that include a relatively large fraction of land (most coastal stations), but affecting less strongly locations at the Adriatic islands.     

Spatiotemporal changes of freezing/thawing indices and their response to recent climate change on the Qinghai–Tibet Plateau from 1980 to 2013

Theoretical and Applied Climatology - The spatial and temporal changes of the ground surface freezing indices (GFIs), ground surface thawing indices (GTIs), air freezing indices (AFIs), and air...     

Uncertainty in the 2°C warming threshold related to climate sensitivity and climate feedback

Climate sensitivity is an important index that measures the relationship between the increase in greenhouse gases and the magnitude of global warming. Uncertainties in climate change projection and climate modeling are mostly related to the climate sensitivity. The climate sensitivities of coupled climate models determine the magnitudes of the projected global warming. In this paper, the authors thoroughly review the literature on climate sensitivity, and discuss issues related to climate feedback processes and the methods used in estimating the equilibrium climate sensitivity and transient climate response (TCR), including the TCR to cumulative CO₂ emissions. After presenting a summary of the sources that affect the uncertainty of climate sensitivity, the impact of climate sensitivity on climate change projection is discussed by addressing the uncertainties in 2°C warming. Challenges that call for further investigation in the research community, in particular the Chinese community, are discussed.