Hidden climate change – urban meteorology and the scales of real weather

This paper discusses the scale at which the weather is experienced and modified by human activities in urban environment. The climates of built-up areas differ from their non-urban counterparts in many aspect: wind-flows, radiation, humidity, precipitation and air quality all change in the presence of human settlement, transforming each city into a singularity within its regional weather system. Yet this pervasive category of anthropogenic climate change has always tended to be hidden and difficult to discern. The paper first describes the sequence of discovery of the urban heat island since the early nineteenth century, and the emergence and consolidation of a scientific field devoted to the climatology of cities. This is followed by a discussion of various attempts to apply knowledge of climatic factors to the design and management of settlement. We find that real-world application of urban climatology has met with limited success. However, the conclusion suggests that global climate change gives a new visibility and practical relevance to urban-scale climate science.     

Long run surface temperature dynamics of an A-OGCM: the HadCM3 4×CO<Subscript>2</Subscript> forcing experiment revisited

The global mean surface temperature (GMST) response of HadCM3 to a 1,000 year 4×CO₂ forcing is analysed using a transfer function methodology. We identify a third order transfer function as being an appropriate characterisation of the dynamic relationship between the radiative forcing input and GMST output of this Atmosphere-Ocean General Circulation Model (A-OGCM). From this transfer function the equilibrium climate sensitivity is estimated as 4.62 (3.92–11.88) K which is significantly higher than previously estimated for HadCM3. The response is also characterised by time constants of 4.5 (3.2–6.4), 140 (78–191) and 1,476 (564–11,737) years. The fact that the longest time constant element is significantly longer than the 1,000 year simulation run makes estimation of this element of the response problematic, highlighting the need for significantly longer model runs to express A-OGCM behaviour fully. The transfer function is interpreted in relation to a three box global energy balance model. It was found that this interpretation gave rise to three fractions of ocean heat capacity with effective depths of 63.0 (46.7–85.4), 1291.7 (787.3–2,955.3) and 2,358.0 (661.3–17,283.8) meters of seawater, associated with three discrete time constants of 4.6 (3.2–6.5), 107.7 (68.9–144.3) and 537.1 (196.2–1,243.1) years. Given this accounts for approximately 94% of the ocean heat capacity in HadCM3, it appears HadCM3 could be significantly more well mixed than previously thought when viewed on the millennial timescale.     

Correction to “the vertical distribution of climate forcings and feedbacks from the surface to top of atmosphere”

We discuss here a mistake in the analysis of Previdi and Liepert (Clim Dyn, 2011). In that article, the surface albedo radiative kernels were calculated incorrectly. We present in this brief comment the corrected albedo kernels. We then use these kernels to compute the surface albedo radiative feedback in climate model simulations driven by increasing carbon dioxide, as in Previdi and Liepert (Clim Dyn, 2011). We find that the use of the corrected albedo kernels does not change the conclusions of our earlier work.     

A deep stratospheric intrusion event down to the earth’s surface of the megacity of Athens

This case study investigates a stratospheric intrusion event down to the earth’s surface (near sea-level pressure) of the greater area of Athens (23.43°E 37.58°N), which occurred on 9 October 2003 and caused a remarkable increase in surface ozone concentrations not related to photochemical production. This event is among the rare case studies investigating, on the one hand, a deep stratospheric intrusion down to the earth’s surface at near sea-level pressure and, on the other, an event affecting the near surface ozone of a megacity such as Athens. The synoptic situation is described by a deep upper lever trough at 300 and 500 hPa extending over Greece, which is related to a deep tropopause fold as revealed by vertical cross sections of potential vorticity, relative humidity, divergence and vertical velocity. The analysis of potential vorticity at several isentropic levels indicates a hook-shaped streamer of high PV values (greater than 4 pvu at the 315 K isentropic level) over southeast Europe, which coincides with a streamer of dry air as observed from satellite images of water vapor. The aforementioned structure characterizes a textbook case study of stratosphere-to-troposphere transport. The Lagrangian particle dispersion model FLEXPART was used to calculate the trajectories of air particles reaching the receptor site and the fraction of particles with stratospheric origin. It reveals an important direct stratospheric impact within 1 day related to the tropopause fold described in this study with the fraction of stratospheric particles reaching maximum values of 1.9 and 4.5% for threshold values of the dynamical tropopause 2 and 1.5 pvu, respectively. Furthermore, a larger indirect aged stratospheric contribution is also revealed 4 to 5 days prior to the release, related to stratospheric intrusion events at the western Atlantic Ocean, reaching maximum values of 2.5 and 6.9% of particles crossing the 2 and 1.5 pvu potential vorticity surfaces, respectively.     

Does sea surface temperature outside the tropical Pacific contribute to enhanced ENSO predictability?

In this paper we seek to identify inter-annual sea surface temperature anomalies (SSTA) patterns outside the tropical Pacific that may influence El Niño/Southern Oscillation (ENSO) through atmospheric teleconnections. We assume that a linear ENSO hindcast based on tropical Pacific warm water volume and Niño3.4 SSTA indices captures tropical Pacific intrinsic predictability inherent to recharge oscillator dynamics. This simple hindcast model displays statistically significant skill at the 95 % confidence level at leads of up to seven seasons ahead of the ENSO peak. Our results reveal that ENSO-independent equatorial wind stress anomalies only significantly improve the skill of that linear hindcast at the 95 % level in boreal spring and summer before the ENSO peak and in boreal fall, five seasons ahead of the ENSO peak. At those seasons, the robust large-scale SST patterns that provide a statistically significant enhancement of ENSO predictability are related to the Atlantic meridional mode and south Pacific subtropical dipole mode in spring, the Indian Ocean Dipole and the south Atlantic subtropical dipole mode in fall. While the first two regions display significant simultaneous correlations with western equatorial Pacific wind stress in three reanalyses (ERA-I, NCEP and NCEP2), the Indian Ocean Dipole and south Atlantic subtropical dipole mode correlation with Pacific winds is less robust amongst re-analyses. We discuss our results in view of other studies that suggest a remote influence of various regions on ENSO. Although modest, the sensitivity of our results to the dataset and to details of the analysis method illustrates that finding regions that influence ENSO from the statistical analysis of observations is a difficult task.     

Impact of global warming on the geobotanic zones: an experiment with a statistical�Cdynamical climate model

In this study, a zonally-averaged statistical climate model (SDM) is used to investigate the impact of global warming on the distribution of the geobotanic zones over the globe. The model includes a parameterization of the biogeophysical feedback mechanism that links the state of surface to the atmosphere (a bidirectional interaction between vegetation and climate). In the control experiment (simulation of the present-day climate) the geobotanic state is well simulated by the model, so that the distribution of the geobotanic zones over the globe shows a very good agreement with the observed ones. The impact of global warming on the distribution of the geobotanic zones is investigated considering the increase of CO₂ concentration for the B1, A2 and A1FI scenarios. The results showed that the geobotanic zones over the entire earth can be modified in future due to global warming. Expansion of subtropical desert and semi-desert zones in the Northern and Southern Hemispheres, retreat of glaciers and sea-ice, with the Arctic region being particularly affected and a reduction of the tropical rainforest and boreal forest can occur due to the increase of the greenhouse gases concentration. The effects were more pronounced in the A1FI and A2 scenarios compared with the B1 scenario. The SDM results confirm IPCC AR4 projections of future climate and are consistent with simulations of more complex GCMs, reinforcing the necessity of the mitigation of climate change associated to global warming.     

What happened to surface temperature with sunspot activity in the past 130 years?

Based on composite analysis using categories of solar inactivity and activity, the responses of surface temperature to different categorized solar activity and a plausible mechanism have been discussed. The results show the increasing solar sunspot activity during solar cycles 13 to 24 (1880–2010) and it seemed to make a positive contribution to rising global temperature. However, the sunspots were fewer in number, if we focused on the most recent 60 years (1950–2010), and the decreasing solar activity did not contribute to a cooler Earth. This result indicates that the connection between sunspots and the Earth’s climate is sensitive to the study period. An extreme analysis was performed in an attempt to gain a better understanding of solar impacts. The extreme top 10 composite analysis demonstrated that the surface temperature response to solar activity is spatially different and is highly sensitive to El Niño–Southern Oscillation (ENSO) events. The most sensitive areas in the Pacific sector revealed a significant difference between including and excluding ENSO years. During the solar inactive years, the tropical eastern Pacific was observed as a weak El Niño-like (strong La Niña-like) pattern in the composite including (excluding) ENSO events. Another interesting feature is the strong similarity in the composites which include or exclude ENSO events during the sunspot active years, but it differs from the La Niña-like pattern observed in previous studies. The bottom-up mechanism associated with the response of the surface dynamical circulation and the heat balance when compared to the total solar irradiance forcing partially explains the connection.     

North Atlantic sea surface temperatures and their relation to the North Atlantic Oscillation during the last 230?years

August Sea Surface Temperatures (aSSTs) based on fossil diatom assemblages are generated with 2?year average resolution from a 230-year-long sediment core (Rapid 21-12B), from the Reykjanes Ridge in the subpolar North Atlantic. The results indicate a warming trend of ~0.5°C of the surface waters at the Reykjanes Ridge for the last 230?years. Superimposed on this warming trend there is a multidecadal to decadal aSST variability of up to 1°C. The interval from the 1770s to the 1830s represents the coldest period, whereas ~1860?C1880 represents the warmest period during the last 230?years. The last 25?years is characterized by a warming trend showing strong decadal aSST variability with several warm years, but also the coldest years since the 1820s. The time of these cold years in the mid-1970s, -1980s and -1990s correspond with the documented great salinity anomalies (GSA) in the North Atlantic suggesting increased fluxes of cold, low-salinity waters from the Arctic during the last decades. The aSST record and the August North Atlantic Oscillation (aNAO) index show similar multidecadal-scale variability indicating a close coupling between the oceanic and atmospheric patterns. The aSST record shows a negative correlation with the aNAO indicating cold aSST during the positive aNAO trend and vice versa. Results suggest that the wind driven variation in volume fluxes of the North Atlantic surface waters could be the major mechanism behind the observed relationship.     

Greenhouse effect and altitude gradients over the Alps – by surface longwave radiation measurements and model calculated LOR

Summary The greenhouse effect has been investigated predominantly with satellite measurements, but more than 90% of the greenhouse radiative flux affecting Earths surface temperature and humidity originates from a 1000 meter layer above the surface. Here we show that substantial improvements on surface longwave radiation measurements and very good agreement with radiative transfer model calculations allow the clear-sky greenhouse effect be determined with measured surface longwave radiation and calculated longwave outgoing radiation at the top of the atmosphere. The cloud radiative forcing is determined by measured net longwave fluxes and added to the clear-sky greenhouse effect to determine the all-sky greenhouse effect. Longwave radiation measurements at different altitudes were used to determine the clear-sky and all-sky annual and seasonal greenhouse effect and altitude gradients over the Alps. Linear altitude gradients are measured for clear-sky situations, whereas the all-sky greenhouse effect is strongly influenced by varying, cloud amounts at different altitudes. Large diurnal and seasonal variations show the importance of surface heating and cooling effects and demonstrate the strong coupling of the greenhouse effect to surface temperature and humidity.     

What happens to the ocean surface gravity waves when ENSO and MJO phases combine during the extended boreal winter?

The safety of vulnerable coastal and offshore infrastructures requires an in-depth understanding of wave variability and climate drivers. We investigate the association of significant wave height (Hs) and peak wave period (Tp) with the co-occurrence of El Niño–Southern Oscillation (ENSO) and the Madden–Julian Oscillation (MJO) at the global scale. We calculate composites of daily anomalies in modelled Hs, Tp, and surface wind for periods of ENSO–MJO phase combinations. Calculations spanned November–March seasons over the period 1979–2018. Wave anomalies are widespread across the world’s oceans, with remarkable strength during several ENSO–MJO phase combinations, demonstrating strong tropic–tropic and tropic-extratropic teleconnections. Positive Hs anomalies are strongest in the Pacific Ocean during El Niño–MJO phase 8, in the Atlantic Ocean during ENSO-neutral-MJO phase 3, and in the Indian Ocean during ENSO-neutral-MJO phase 4. Positive Tp anomalies are strongest in the Pacific Ocean during La Niña–MJO phase 8, in the Atlantic Ocean during El Niño–MJO phase 1, and in the Indian Ocean during El Niño–MJO phase 8. In the Southern Ocean, the strongest Hs anomalies occur during El Niño–MJO phase 8, whereas in the Maritime Continent, they appear during ENSO-neutral-MJO phases 5–6. Despite previous studies finding low correlations of ENSO indices with wave parameters in the North Atlantic, our results suggest that ENSO-related conditions play a significant role in the area when combined with certain MJO-related conditions. This study also reveals that the wave anomalies associated with ENSO–MJO phase combinations can be twice as strong as those found in previous work, related only to the MJO. Therefore, considering multiple concurrent climate patterns in the analysis of wave anomalies is essential to developing more reliable coastal management plans.     

Framing the frontier – Tracing issues related to soybean expansion in transnational public spheres

Rapid soybean expansion in South America has been linked to numerous socio-environmental problems, including deforestation in sensitive biomes. As a major importing region of soybeans, wider public awareness has also put pressure on the European Union. Different governance initiatives involving various groups of stakeholders have sought to address these issues. However, what is identified as a relevant problem, as a region of interest or which actors are mentioned in this context are all matters of claims-making processes between different groups and mediated through various channels of communication. This study uses a text-mining approach to trace the construction of socio-ecological problems related to soybean expansion and the actors and regions linked with these issues in public discourse. The focus lies on print media from the European Union, but several additional sources are included to investigate the similarities and differences between various communication channels and regions. These include newspaper articles from producing countries and international news agencies, scientific abstracts, corporate statements, and reports from advocacy groups gathered from the mid-1990s to 2020. The results show that European mass media have shifted their focus from consumer labeling, health, and concerns over genetically modified organisms towards more distant or abstract phenomena, such as deforestation and climate change. This has been accompanied with a broader view on different stakeholders, but also with a strong regional focus on the Amazon biome. There has also been much less attention on direct concerns for communities in producing regions, such as land conflicts or disputes over intellectual property rights. We conclude that while European public spheres appear to become more receptive to issues related to impacts in sourcing regions, there remains a narrow focus on specific problems and regions, which reflects a fundamental asymmetry in different stakeholders' ability to shape transnational deliberations and resulting governance processes.     

E − ε − 〈θ 2〉 turbulence closure model for an atmospheric boundary layer including the urban canopy

A modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is presented. The model is based on tensor-invariant parameterizations for the pressure–strain and pressure–temperature correlations that are more complete than the parameterizations used in the Mellor–Yamada model of level 3.0. The turbulent momentum and heat fluxes are calculated with explicit algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation of weakly equilibrium turbulence. The turbulent transport of heat and momentum fluxes is assumed to be negligibly small in this approximation. The three-parameter $E - \varepsilon - {\left\langle {\theta ^{2} } \right\rangle }$ model of thermally stratified turbulence is employed to obtain closed-form algebraic expressions for the fluxes. A computational test of a 24-h ABL evolution is implemented for an idealized two-dimensional region. Comparison of the computed results with the available observational data and other numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in an urban canopy layer near the urbanized ABL surface and the effect of urban roughness on a global structure of the fields of wind and temperature over a city. The results of the computational test for the new model indicate that the motion of air in the urban canopy layer is strongly influenced by mechanical factors (buildings) and thermal stratification.     

Sensitivity of precipitation to sea surface temperature over the tropical summer monsoon region—and its quantification

Over the tropical oceans, higher sea surface temperatures (SST, above 26 °C) in summer are generally accompanied by increased precipitation. However, it has been argued for the last three decades that, any monotonic increase in precipitation with respect to SST is limited to an upper threshold of 28–29.5 °C, and beyond this, the relationship fails. Based on this assessment it has often been presumed that, since the mean SSTs over the Asian monsoon basins (Indian Ocean and north-west Pacific) are mostly above the threshold, SST does not play an active role on the summer monsoon variability. It also implies that increasing SSTs due to a changing climate need not result in increasing monsoon precipitation. The current study shows that the response of precipitation to SST has a time lag, that too with a spatial variability over the monsoon basins. Taking this lag into account, the results here show that enhanced convection occurs even up to the SST maxima of 31 °C averaged over these basins, challenging any claim of an upper threshold for the SST-convection variability. The study provides us with a novel method to quantify the SST-precipitation relationship. The rate of increase is similar across the basins, with precipitation increasing at ~2 mm day^?1 for an increase of 1 °C in SST. This means that even the high SSTs over the monsoon basins do play an active role on the monsoon variability, challenging previous assumptions. Since the response of precipitation to SST variability is visible in a few days, it would also imply that including realistic ocean–atmosphere coupling is crucial even for short term monsoon weather forecasts. Though recent studies suggest a weakening of the monsoon circulation over the last few decades, results here suggest an increased precipitation over the tropical monsoon regions, in a global warming environment with increased SSTs. Thus the signature of SST is found to be significant for the Asian summer monsoon, in a quantifiable manner, seamlessly through all the timescales—from short-term intraseasonal to long-term climate scales.     

A new building energy model coupled with an urban canopy parameterization for urban climate simulations—part I. formulation, verification, and sensitivity analysis of the model

The generation of heat in buildings, and the way this heat is exchanged with the exterior, plays an important role in urban climate. To analyze the impact on urban climate of a change in the urban structure, it is necessary to build and use a model capable of accounting for all the urban heat fluxes. In this contribution, a new building energy model (BEM) is developed and implemented in an urban canopy parameterization (UCP) for mesoscale models. The new model accounts for: the diffusion of heat through walls, roofs, and floors; natural ventilation; the radiation exchanged between indoor surfaces; the generation of heat due to occupants and equipments; and the consumption of energy due to air conditioning systems. The behavior of BEM is compared to other models used in the thermal analysis of buildings (CBS-MASS, BLAST, and TARP) and with another box-building model. Eventually, a sensitivity analysis of different parameters, as well as a study of the impact of BEM on the UCP is carried out. The validations indicate that BEM provides good estimates of the physical behavior of buildings and it is a step towards a modeling tool that can be an important support to urban planners.     

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.     

Getting from here to there – energy technology transformation pathways in the EMF27 scenarios

Based on a large number of energy-economic and integrated assessment models, the Energy Modeling Forum (EMF) 27 study systematically explores the implications of technology cost and availability for feasibility and macroeconomic costs of energy system transformations toward climate stabilization. At the highest level, the technology strategy articulated in all the scenarios in EMF27 includes three elements: decarbonization of energy supply, increasing the use of low-carbon energy carriers in end-use, and reduction of energy use. The way that the scenarios differ is in the degree to which these different elements of strategy are implemented, the timing of those implementations, and the associated macroeconomic costs. The study also discusses the value of individual technologies for achieving climate stabilization. A robust finding is that the unavailability of carbon capture and storage and limited availability of bioenergy have the largest impact on feasibility and macroeconomic costs for stabilizing atmospheric concentrations at low levels, mostly because of their combined ability to remove carbon from the atmosphere. Constraining options in the electric sector such as nuclear power, wind and solar energy in contrast has a much smaller impact on the cost of mitigation.     

Exploring some of the myths of land use change: Can rural to urban migration drive declines in biodiversity?

Forest transition theory describes a reversal in land-use trends for a given area, from a period of net forest area loss, to a period of net forest area gain. Some assume that such forest gain necessarily equates with biodiversity conservation. We question this assumption, based on research conducted in Oaxaca, Mexico. In Oaxaca's northern highlands, low intensity forest use and rotational (milpa) agriculture have led to pronounced spatial heterogeneity in forest structure and composition, and created a high-biodiversity forest-agriculture mosaic. In the Zapotec community of San Juan Evangelista Analco and the Chinantec community of Santiago Comaltepec, as across much of Oaxaca, fewer people are farming; farmers are cultivating less land, working closer to settlements, and growing fewer crop varieties. Widespread agricultural abandonment has initiated unprecedented changes in ecological succession, patch size, and edge effects, which we speculate will be having an impact on the biodiversity of the landscape mosaic. Our work suggests that the decline of land use activity may result in a gradual loss of the forest-agriculture mosaic, leading to localised declines in biodiversity, despite (or because of) extensive forest resurgence. These findings support the view that indigenous cultures in Oaxaca and Mexico should not be seen as an environmental constraint but rather as an agent of landscape renewal that allows for both cultural and biological diversity to flourish, a reality that national and international conservation bodies need to more fully recognise and incorporate into policy.