The role of clouds in climate forcings and feedbacks: assessment using global modelling and satellite observations

Variability and change of the Earth\''s climate are of fundamental importance to humankind. In particular anthropogenic climate change has been considered widely as one of the most urgent concerns for the society (United Nations, 1992, 2002). It is therefore vital to improve the understanding of the Earth\''s climate system and its variability

Environmental problems and economic development in an endogenous fertility model

Population growth is often viewed as a most oppressive global problem with respect to environmental deterioration, but the relationships between population development, economic dynamics and environmental pollution are complex due to various feedback mechanisms. We analyze society’s economic decisions on birth rates, investment into human and physical capital, and polluting emissions within an optimal control model of the coupled demographic-economic-environmental system. We show that a long-run steady state is optimal that is characterized by a stable pollution stock, and by population and economic growth rates depending on the possibilities of emission abatement and technical progress due to human capital accumulation. We derive a condition on the production technologies and opportunity costs of raising children, under which the optimal birth rate is constant even during the transition to a steady state. In particular in an economy where only human capital is needed to produce output, the optimal choice of the birth rate is not affected by the states of the economy or the environment. In such a setting, the optimal birth rate is constant and policy should concentrate on intertemporal adjustment of per-capita emissions.sustainability, endogenous fertility, externalities

Satellite observations of convection and their implications for parameterizations

Parameterization development and evaluation ideally takes a two-step approach (Lohmann et al., 2007). Insight into new processes, and initial parameterization formulation should be guided by theory, process-level observations (laboratory experiments or field studies) or, if these are unavailable, by high-resolution modelling. However, once implemented into large-scale atmospheric models, a thorough testing and evaluation is required in order to assure that the parameterization works satisfactorily for all weather situations and at the scales the model is applied to. Satellite observations are probably the most valuable source of information for this purpose, since they offer a large range of parameters over comparatively long time series and with a very large, to global, coverage. However, satellites usually retrieve parameters in a rather indirect way, and some quantities (e.g., vertical wind velocities) are unavailable. It is thus essential for model evaluation 1. to assure comparability; and, 2. to develop and apply metrics that circumvent the limitations of satellite observations and help to learn about parameterizations. In terms of comparability, the implementation of so-called \"satellite simulators\" has emerged as the approach of choice, in which satellite retrievals are emulated, making use of model information about the subgrid-scale variability of clouds, and creating summary statistics (Bodas-Salcedo et al., 2011; Nam and Quaas, 2012; Nam et al., 2014). In terms of process-oriented metrics, a large range of approaches has been developed, e.g. investigating the life cycle of cirrus from convective detrainment (Gehlot and Quaas, 2012), or focusing on the details of microphysical processes (Suzuki et al., 2011). Besides such techniques focusing on individual parameterizations, the data assimilation technique might be exploited, by objectively adjusting convection parameters and learning about parameter choices and parameterizations in this way (Schirber et al., 2013).In this chapter, we will first introduce the available satellite data, consider their limitations and the approaches to account for these, and then discuss observations-based process-oriented metrics that have been developed so far

How can aerosols affect the Asian summer monsoon?: assessment during three consecutive pre-monsoon seasons from CALIPSOsatellite data

The impact of aerosols above and around the Tibetan Plateau on the Asian Summer Monsoon during premonsoon seasons March-April-May 2007, 2008, and 2009 is investigated by means of remote sensing and radiative transfer modelling. Four source regions are found to be responsible for the high aerosol loading around the Tibetan Plateau: the Taklamakan Desert, the Ganges Plains, the Indus Plains, and the Arabian Sea. CALIPSO lidar satellite data, providing vertically resolved images of aerosols, shows aerosol concentrations to be highest in the lower 5 km of the atmosphere with only little amounts reaching the Tibetan Plateau altitude. Using a radiative transfer model we find that aerosol plumes reduce shortwave radiation throughout the Monsoon region in the seasonal average by between 20 and 30 W/m2. Peak shortwave heating in the lower troposphere reaches 0.2 K/day. In higher layers this shortwave heating is partly balanced by longwave cooling. Although high-albedo surfaces, such as deserts or the Tibetan Plateau, increase the shortwave heating by around 10%, the overall effect is strongest close to the aerosol sources. A strong elevated heating which could influence large-scale monsoonal circulations as suggested by previous studies is not found

Exploring aerosol-cloud interaction in Southeast Pacific marine stratocumulus during VOCALS regional experiment

The marine stratocumulus clouds are highly sensitive to aerosol perturbations. In this study, we have explored the cloud susceptibility to aerosol using satellite observation and multi-model simulations over the Southeast Pacific Ocean (SEP). The climatology of satellite observation indicates that SEP is a relatively clean area with low aerosol optical depth (AOD). The SEP is a region of marine stratocumulus deck with cloud fraction (CF) reaching as high as 90% in many regions, with relatively low (140 cm−3) cloud droplet number concentration (CDNC) over the marine environment, and it increases as it moves towards the coast. The joint histogram analysis shows that the AOD-CDNC relation shows positive sensitivity and a non-linear CDNC-LWP (liquid water path) relationship; however, a negative sensitivity is dominant. The multimodel analysis shows that most models have a strong positive AOD-CDNC sensitivity, suggesting that the cloud albedo effect leads to net cooling. The general circulation models (GCM) reveal a negative radiative forcing (-0.28 to -1.36Wm−2) at the top of the atmosphere (TOA) when using the flux method. It supports the positive AODCDNC sensitivity and the resulting negative radiative forcing in GCMs. However, the CDNC-LWP shows a diverse relation in the models. In the GCMs, the effect of cloud microphysics is not considered while estimating the net radiative forcing. To include the effect of cloud microphysics in the radiative forcing estimates, we have proposed a statistical approach to calculate the net radiative forcing. The results show that the net radiative forcing is sensitive to the LWP change due to the aerosol perturbation.Die marinen Stratocumulus-Wolken reagieren sehr empfindlich auf Aerosol-Störungen. In dieser Studie haben wir die Anfälligkeit der Wolken für Aerosol anhand von Satellitenbeobachtungen und Multi-Modellsimulationen über dem Südostpazifik (SEP) untersucht. Die Klimatologie der Satellitenbeobachtung zeigt, dass der SEP ein relativ sauberes Gebiet mit geringer Aerosol optischer Dicke (AOD) ist. Der SEP ist eine Region mit mariner Stratocumulus-Decke mit einer Wolkbedeckungsgrad (CF), der in vielen Regionen bis zu 90% erreicht, mit einer relativ niedrigen (140 cm−3) Wolkentröpfchenanzahlkonzentration (CDNC) über der marinen Umgebung, und sie nimmt in Richtung Küste zu. Die gemeinsame Histogramm-Analyse zeigt, dass die AOD-CDNC-Beziehung eine positive Sensitivität und eine nicht-lineare CDNC-LWP-Beziehung (Flüssigwasserpfad) aufweist; allerdings ist eine negative Sensitivität vorherrschend. Die Multi-Modellanalyse zeigt, dass die meisten Modelle eine stark positive AOD-CDNC-Empfindlichkeit aufweisen, was darauf hindeutet, dass der Wolkenalbedo-Effekt eine Nettokühlung bewirkt. Die allgemeinen Zirkulationsmodelle (GCM) zeigen einen negativen Strahlungsantrieb (-0,28 bis -1,36Wm−2) am Oberrand der Atmosphäre (TOA), wenn die Flussmethode verwendet wird. Dies unterstützt die positive AOD-CDNC-Empfindlichkeit und den daraus resultierenden negativen Strahlungsantrieb in GCMs. Der CDNC-LWP zeigt jedoch unterschiedliche Abhängigkeiten in den Modellen. In den GCMs wird die Wirkung der Wolkenmikrophysik bei der Abschätzung des Netto-Strahlungsantriebs nicht berücksichtigt. Um die Auswirkungen der Wolkenmikrophysik auf den Strahlungsantrieb einzubeziehen, haben wir einen statistischen Ansatz zur Berechnung des Nettostrahlungsantriebs gewählt. Die Ergebnisse zeigen, dass der Nettostrahlungsantrieb empfindlich auf die LWP-Änderung durch die Aerosolstörung reagiert

Regional climate engineering by radiation management: prerequisites and prospects

Radiationmanagement (RM), as an option to engineer the climate, is highly controversial and suffers from a number of ethical and regulatory concerns, usually studied in the context of the objective to mitigate the global mean temperature. In this article, we discuss the idea that RM can be differentiated and scaled in several dimensions with potential objectives being to influence a certain climate parameter in a specific region. Some short-lived climate forcers (e.g., tropospheric aerosols) exhibit strong geographical and temporal variability, potentially leading to limited-area climate responses. Marine cloud brightening and thinning or dissolution of cirrus clouds could be operated at a rather local scale. It is therefore conceivable that such schemes could be applied with the objective to influence the climate at a regional scale. From a governance perspective, it is desirable to avoid any substantial climate effects of regional RM outside the target region. This, however, could prove impossible for a sustained, long-term RM. In turn, regional RM during limited time periods could prove more feasible without effects beyond the target area. It may be attractive as it potentially provides the opportunity to target the suppression of some extreme events such as heat waves. Research is needed on the traceability of regional RM, for example, using detection and attribution methods. Incentives and implications of regional RM need to be examined, and new governance options have to be conceived

The aerosol indirect effect: parameterization in large-scale models and evaluation with satellite data

Global climate change is considered to be one of the most serious concerns of humankind (United Nations, 1992; United Nations, 2002). Anthropogenic greenhouse gases and aerosols impact considerably the energy balance of the Earth system, possibly provoking adverse effects on social, ecological, and economical equilibria. This is one of the main reasons why the understanding of the Earth’s climate system is of major importance. If better predictions of the response of the climate system to anthropogenic perturbations were available, political decisions against negative impacts could be taken, and social adaptations to changed climate conditions would be possible

Evaluation of the statistical cloud scheme in the ECHAM5 model using satellite data

An evaluation of a statistical cloud scheme taking into account subgrid-scale variability for water vapour and cloud condensate in the ECHAM5 general circulation model of the atmosphere is presented. Three-dimensional modelled water vapour, cloud liquid water and cloud ice were distributed stochastically into subcolumns of each grid box and vertically integrated to total water path (TWP). Thus the lower atmosphere is emphasized in the evaluation of TWP due to its exponential profile. The edited model dataset was compared with the globally analyzed distribution of TWP measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument. The results show that the mean TWP and mean cloud cover are on average relatively well simulated. However, large deficiencies are revealed by the evaluation of both variance and skewness of the probability density function (PDF). Systematically negative deviations of variance are found for almost all regions of the globe. Skewness of theTWPis overestimated in the Tropics and underestimated at high latitudes. Moreover, sensitivity experiments were performed to reveal the deficiencies in the parametrization leading to the observed deviations of variance and skewness of TWP. It was found that the positive bias in skewness in the Tropics can be reduced by modifying the influence of convection on the PDF

Constraining the total aerosol indirect effect in the LMDZ and ECHAM4 GCMs using MODIS satellite data

Aerosol indirect effects are considered to be the most uncertain yet important anthropogenic forcing of climate change. The goal of the present study is to reduce this uncertainty by constraining two different general circulation models (LMDZ and ECHAM4) with satellite data. We build a statistical relationship between cloud droplet number concentration and the optical depth of the fine aerosol mode as a measure of the aerosol indirect effect using MODerate Resolution Imaging Spectroradiometer (MODIS) satellite data, and constrain the model parameterizations to match this relationship. We include here “empirical” formulations for the cloud albedo effect as well as parameterizations of the cloud lifetime effect. When fitting the model parameterizations to the satellite data, consistently in both models, the radiative forcing by the combined aerosol indirect effect is reduced considerably, down to −0.5 and −0.3Wm−2, for LMDZ and ECHAM4, respectively

A six year satellite-based assessment of the regional variations in aerosol indirect effects: A six year satellite-based assessment of the regional variations inaerosol indirect effects

Aerosols act as cloud condensation nuclei (CCN) for cloud water droplets, and changes in aerosol concentrations have significant microphysical impacts on the corresponding cloud properties. Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud properties are combined with NCEP Reanalysis data for six different regions around the globe between March 2000 and December 2005 to study the effects of different aerosol, cloud, and atmospheric conditions on the aerosol indirect effect (AIE). Emphasis is placed in examining the relative importance of aerosol concentration, type, and atmospheric conditions (mainly vertical motion) to AIE from region to region. Results show that in most regions, AIE has a distinct seasonal cycle, though the cycle varies in significance and period from region to region. In the Arabian Sea (AS), the sixyear mean anthropogenic + dust AIE is −0.27Wm−2 and is greatest during the summer months (<−2.0Wm−2) during which aerosol concentrations (from both dust and anthropogenic sources) are greatest. Comparing AIE as a function of thin (LWP<20 gm−2) vs. thick (LWP≥20 gm−2) clouds under conditions of large scale ascent or decent at 850 hPa showed that AIE is greatest for thick clouds during periods of upward vertical motion. In the Bay of Bengal, AIE is negligible owing to less favorable atmospheric conditions, a lower concentration of aerosols, and a non-alignment of aerosol and cloud layers. In the eastern North Atlantic, AIE is weakly positive (+0.1Wm−2) with dust aerosol concentration being much greater than the anthropogenic or sea salt components. However, elevated dust in this region exists above the maritime cloud layers and does not have a hygroscopic coating, which occurs in AS, preventing the dust from acting as CCN and limiting AIE. The Western Atlantic has a large anthropogenic aerosol concentration transported from the eastern United States producing a modest anthropogenic AIE (−0.46Wm−2). Anthropogenic AIE is also present off the West African coast corresponding to aerosols produced from seasonal biomass burning (both natural and man-made). Interestingly, atmospheric conditions are not particularly favorable for cloud formation compared to the other regions during the times where AIE is observed; however, clouds are generally thin (LWP<20 gm−2) and concentrated very near the surface. Overall, we conclude that vertical motion, aerosol type, and aerosol layer heights do make a significant contribution to AIE and that these factors are often more important than total aerosol concentration alone and that the relative importance of each differs significantly from region to region