We analyze simulated sea ice changes in eight different Earth System Models that have conducted experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP). The simulated response of balancing abrupt quadrupling of CO 2 (abrupt4xCO2) with reduced shortwave radiation successfully moderates annually averaged Arctic temperature rise to about 1°C, with modest changes in seasonal sea ice cycle compared with the preindustrial control simulations (piControl). Changes in summer and autumn sea ice extent are spatially correlated with temperature patterns but much less in winter and spring seasons. However, there are changes of ±20% in sea ice concentration in all seasons, and these will induce changes in atmospheric circulation patterns. In summer and autumn, the models consistently simulate less sea ice relative to preindustrial simulations in the Beaufort, Chukchi, East Siberian, and Laptev Seas, and some models show increased sea ice in the Barents/Kara Seas region. Sea ice extent increases in the Greenland Sea, particularly in winter and spring and is to some extent associated with changed sea ice drift. Decreased sea ice cover in winter and spring in the Barents Sea is associated with increased cyclonic activity entering this area under G1. In comparison, the abrupt4xCO2 experiment shows almost total sea ice loss in September and strong correlation with regional temperatures in all seasons consistent with open ocean conditions. The tropospheric circulation displays a Paci fi c North America pattern-like anomaly with negative phase in G1-piControl and positive phase under abrupt4xCO2-piControl.
It is known that the Amazon region plays an important role in the global energy, hydrological cycle and carbon balance. This region has been suffering from the course of the past 40 years intense land use and land cover changes. With this in mind, this study has examined possible associations between change in spatial and temporal rainfall variability and land cover change in the Amazon, using the PRECIS regional modelling system. It has been found that the impacts of land cover change by forest removal are more intense in the so-called “Arc of deforestation” over central and southern Amazonia. However, the relative impact of the simulated rainfall changes seems to be more important in the JJA dry season. In addition, the simulations under the deforestation scenarios also show the occurrence of extreme rainfall events as well as more frequent dry periods. Therefore, the results found show to be potentially important in the modulation of regional climate variations which have several environmental and socio-economic impacts.
Using collocated measurements from geostationary and polar-orbital satellites over tropical continents, we provide a large-scale statistical assessment of the relative influence of aerosols and meteorological conditions on the lifetime of mesoscale convective systems (MCSs). Our results show that MCSs' lifetime increases by 3–24 h when ver-tical wind shear (VWS) and convective available potential energy (CAPE) are moderate to high and ambient aerosol optical depth (AOD) increases by 1 SD (1$\sigma$). However, this influence is not as strong as that of CAPE, relative humidity, and VWS, which increase MCSs' lifetime by 3–30 h, 3–27 h, and 3–30 h per 1$\sigma$ of these variables and explain up to 36%, 45%, and 34%, respectively, of the variance of the MCSs' lifetime. AOD explains up to 24% of the total variance of MCSs' lifetime during the decay phase. This result is physically con-sistent with that of the variation of the MCSs' ice water content (IWC) with aerosols, which accounts for 35% and 27% of the total variance of the IWC in convective cores and anvil, respectively, dur-ing the decay phase. The effect of aerosols on MCSs' lifetime varies between different continents. AOD appears to explain up to 20–22% of the total variance of MCSs' lifetime over equatorial South America compared with 8% over equatorial Africa. Aerosols over the Indian Ocean can explain 20% of total variance of MCSs' lifetime over South Asia because such MCSs form and develop over the ocean. These regional differences of aerosol impacts may be linked to different meteorological conditions. mesoscale convective systems | aerosols | meteorological parameters T he hypothesis that aerosols may delay precipitation and increase cloud lifetime of shallow marine clouds (1) has motivated many researchers to study the aerosol indirect effect on convective clouds; however, the influence of aerosols on enhancing cloud lifetime has remained under debate. Mesoscale convective systems (MCSs) are deep convective clouds that cover several hundred kilometers. Previous studies have shown that aerosols affect deep convection, in particular that aerosols increase the number of smaller size cloud condensation nuclei (CCN) (2), which weaken coagulation and coalescence that form rain droplets, and consequently delay warm rainfall (3, 4). These processes allow more cloud droplets to rise above the freezing level and increase latent heat released due to glaciation (5), resulting in stronger updraft speed, enhanced cloud ice content (6), larger anvil size (5), and higher cloud top height (7). The top of the troposphere warms owing to the aerosol-induced changes in convective anvils (8). Although these aerosol effects have been seen in observations from field cam-paigns (9, 10), they have been undetectable on large spatial and multiyear scales. Rosenfeld et al. (11) have attributed this lack of detectability on the large scale of aerosol invigoration of convec-tion to its variation with meteorological conditions and to the lack of knowledge of the relative humidity (RH) outside the clouds. The variation of aerosol effects on convection with meteoro-logical parameters has been studied previously (11). For example, model simulation has shown that an increase in aerosol concen-trations up to an optimal level can invigorate the MCSs under weak vertical wind shear (VWS) and higher RH but suppress the MCSs under strong VWS in a dry environment (12, 13). They found that, due to a significant enhancement in the convective available po-tential energy (CAPE), corresponding to an increase in RH from 50% to 70%, aerosol impact on ice crystal mass becomes pro-nounced, with a dramatic increase in the size of the anvils and the mass of ice crystals of the deep convection. However, such impacts are negligible when RH increases from 40% to 50%, due to little increase in CAPE (13). Moreover, consumption of CAPE for a given amount of rainfall is converted to an equal amount of kinetic energy that invigorates the convection (5, 14). These studies indicate that VWS, RH, and CAPE are important factors that can influence aerosol impacts on the MCSs. However, no quantitative assessment of the relative influence of aerosols versus these meteorological parameters on convective lifetime using satellite measurements has been established (11). The influence of aerosols on the MCSs is expected to vary in different phases of the convective life cycle. For example, Rosenfeld et al. (5) hypothesized that the impact of aerosol on deep convec-tion is stronger and more prominent during the dissipating phase. Using cloud-resolving simulations, Fan et al. (15) found out that aerosol microphysical effects intensify the deep convection during the mature and decaying phases by forming a larger number of smaller and long-lasting particles, whereas additional latent heat released due to aerosols' thermodynamic effect is responsible for invigorating the deep convections during the growing phase. Hence, the detection of aerosol impacts might not be visible until the mature phase, as no satellite can directly measure the thermo-dynamic properties.
The Asian monsoon region is the most prominent moisture center of lower stratospheric (LS) water vapor during boreal summer. Previous studies have suggested that the transport of water vapor to the Asian monsoon LS is controlled by dehydration temperatures and convection mainly over the Bay of Bengal and Southeast Asia. However, there is a clear geographic variation of convection associated with the seasonal and intra-seasonal variations of the Asian monsoon circulation, and the relative influence of such a geographic variation of convection vs. the variation of local dehydration temperatures on water vapor transport is still not clear. Using the Aura Microwave Limb Sounder (MLS) satellite observations and a domain-filling forward trajectory model, we show that almost half of the seasonal water vapor increase in the Asian monsoon LS are attributable to the geographic variations of convection and resultant variations of dehydration center, comparable to the influence of the local dehydration temperature increase. In particular, dehydration temperatures are coldest over the southeast and warmest over the northwest within the Asian monsoon region. Although convective center is located over the southeastern Asia, an anomalous increase of convection over the northwestern Asian monsoon region increases the local diabatic heating in the tropopause layer and air mass entering the LS that is dehydrated at relatively warm er temperatures. The warmer dehydration temperatures allow anomalously moist air enters the LS and then moves eastward along the northern frank of the monsoon anticyclonic flow, leading to wet anomalies in the LS over the Asian monsoon region. Likewise, when convection increases over the southeastern Asian monsoon region, dry anomalies appear in the LS. On seasonal scale, this feature is associated with the march of the monsoon circulation, convection and diabatic heating towards the northwestern Asia monsoon from June to August, leading to an increasing fraction of the air mass to be dehydrated at warmer temperatures over the nort hwestern Asian monsoon region. Work presented here confirms the dominant role of temper atures and also emphasizes that one should take the geographic variations of dehydration center into consideration when studying water vapor variations in the LS, as it is linked to changes of convection and large-scale circulation.
Solar dimming and wind stilling (slowdown) are two outstanding climate changes occurred in China over the last four decades. The wind stilling may have suppressed the dispersion of aerosols and amplified the impact of aerosol emission on solar dimming. However, there is a lack of long-term aerosol monitoring and associated study in China to confirm this hypothesis. Here, long-term meteorological data at weather stations combined with short-term aerosol data were used to assess this hypothesis. It was found that surface solar radiation (SSR) decreased considerably with wind stilling in heavily polluted regions at a daily scale, indicating that wind stilling can considerably amplify the aerosol extinction effect on SSR. A threshold value of 3.5 m/s for wind speed is required to effectively reduce aerosols concentration. From this SSR dependence on wind speed, we further derived proxies to quantify aerosol emission and wind stilling amplification effects on SSR variations at a decadal scale. The results show that aerosol emission accounted for approximately 20% of the typical solar dimming in China, which was amplified by approximately 20% by wind stilling.
This study uses the droughts of 2011 in Texas and 2012 over the central Great Plains as case studies to explore the potential of satellite-observed solar-induced chlorophyll fluorescence (SIF) for monitoring drought dynamics.We find that the spatial patterns of negative SIF anomalies from the Global Ozone Monitoring Experiment 2 (GOME-2) closely resembled drought intensity maps from the U.S. DroughtMonitor for both events. The drought-induced suppression of SIF occurred throughout 2011 but was exacerbated in summer in the Texas drought. This event was characterized by a persistent depletion of root zone soil moisture caused by yearlong below-normal precipitation. In contrast, for the central Great Plains drought, warmer temperatures and relatively normal precipitation boosted SIF in the spring of 2012; however, a sudden drop in precipitation coupled with unusually high temperatures rapidly depleted soil moisture through evapotranspiration, leading to a rapid onset of drought in early summer. Accordingly, SIF reversed from above to below normal. For both regions, the GOME-2 SIF anomalies were significantly correlated with those of root zone soil moisture, indicating that the former can potentially be used as proxy of the latter for monitoring agricultural droughts with different onset mechanisms. Further analyses indicate that the contrasting dynamics of SIF during these two extreme events were caused by changes in both fraction of absorbed photosynthetically active radiation fPAR and fluorescence yield, suggesting that satellite SIF is sensitive to both structural and physiological/biochemical variations of vegetation. We conclude that the emerging satellite SIF has excellent potential for dynamic drought monitoring.
Resolving the debate surrounding the nature and controls of seasonal variation in the structure and metabolism of Amazonian rainforests is critical to understanding their response to climate change. In situ studies have observed higher photosynthetic and evapotranspiration rates, increased litterfall and leaf flushing during the Sunlight-rich dry season. Satellite data also indicated higher greenness level, a proven surrogate of photosynthetic carbon fixation, and leaf area during the dry season relative to the wet season. Some recent reports suggest that rainforests display no seasonal variations and the previous results were satellite measurement artefacts. Therefore, here we re-examine several years of data from three sensors on two satellites under a range of sun positions and satellite measurement geometries and document robust evidence for a seasonal cycle in structure and greenness of wet equatorial Amazonian rainforests. This seasonal cycle is concordant with independent observations of solar radiation. We attribute alternative conclusions to an incomplete study of the seasonal cycle, i.e. the dry season only, and to prognostications based on a biased radiative transfer model. Consequently, evidence of dry season greening in geometry corrected satellite data was ignored and the absence of evidence for seasonal variation in lidar data due to noisy and saturated signals was misinterpreted as evidence of the absence of changes during the dry season. Our results, grounded in the physics of radiative transfer, buttress previous reports of dry season increases in leaf flushing, litterfall, photosynthesis and evapotranspiration in well-hydrated Amazonian rainforests.
We estimate the extent of upper tropospheric aerosol layers (UT ALs) surrounding mesoscale convective systems (MCSs) and explore the relationships between UT AL extent and the morphology, location, and developmental stage of collocated MCSs in the tropics. Our analysis is based on satellite data collected over equatorial Africa, South Asia, and the Amazon basin between June 2006 and June 2008. We identify substantial variations in the relationships between convective properties and aerosol transport by region and stage of convective development. The most extensive UT ALs over equatorial Africa are associated with mature MCSs, while the most extensive UT ALs over South Asia and the Amazon are associated with growing MCSs. Convective aerosol transport over the Amazon is weaker than that observed over the other two regions despite similar transport frequencies, likely due to the smaller sizes and shorter mean lifetimes of MCSs over the Amazon. Variations in UT ALs in the vicinity of tropical MCSs are primarily explained by variations in the horizontal sizes of the associated MCSs and are largely unrelated to aerosol loading in the lower troposphere. We also identify potentially important relationships with the number of convective cores, vertical wind shear, and convective fraction during the growing and mature stages of MCS development. Relationships between convective properties and aerosol transport are relatively weak during the decaying stage of convective development. Our results provide an interpretive framework for devising and evaluating numerical model experiments that examine relationships between convective properties and ALs in the upper troposphere.