Increase In Aerosol Concentration Research Articles

Variations of cloud condensation nuclei (CCN) and aerosol activity during fog–haze episode: a case study from Shanghai

Abstract. Measurements of cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were performed simultaneously at an urban site in Shanghai from 6 to 9 November 2010. The variations of CCN number concentration (NCCN) and aerosol activity (activated aerosol fraction, NCCN/NCN) were examined during a fog–haze co-occurring event. Anthropogenic pollutants emitted from vehicles and unfavorable meteorological conditions such as low planetary boundary layer (PBL) height exerted a great influence on PM2.5 and black carbon (BC) loadings. NCCN at 0.2% supersaturation (SS) mostly fell in the range of 994 to 6268 cm−3, and the corresponding NCCN/NCN varied between 0.09 and 0.57. NCCN and NCCN/NCN usually were usually higher in the hazy case due to increased aerosol concentration in the accumulation mode (100–500 nm), and lower in the foggy–hazy and clear cases. The BC mass concentration posed a strong positive effect on NCCN in the foggy–hazy and hazy cases, whereas it poorly correlated with NCCN in the clear case. NCCN/NCN was weakly related with BC in both foggy–hazy and hazy cases. By using a simplified particle hygroscopicity (κ), the calculated critical dry size (CDS) of activated aerosol did not exceed 130 nm at 0.2% SS in spite of diverse aerosol chemical compositions. The predicted NCCN at 0.2% SS was very successful compared with the observed NCCN in clear case (R2=0.96) and foggy–hazy/hazy cases (R2=0.91). In addition, their corresponding ratios of predicted to observed NCCNwere on average 0.95 and 0.92, respectively. More organic matter is possibly responsible for this closure difference between foggy–hazy/hazy and clear cases. These results reveal that the particulate pollutant burden exerts a significant impact on NCCN, especially NCCN/NCN promotes effectively during the polluted periods.

Aerosol indirect effects on continental low-level clouds over Sweden and Finland

Abstract. Aerosol effects on low-level clouds over the Nordic Countries are investigated by combining in situ ground-based aerosol measurements with remote sensing data of clouds and precipitation. Ten years of number size distribution data from two aerosol measurement stations (Vavihill, Sweden and Hyytiälä, Finland) provide aerosol number concentrations in the atmospheric boundary layer. This is combined with cloud satellite data from the Moderate Resolution Imaging Spectroradiometer and weather radar data from the Baltic Sea Experiment. Also, how the meteorological conditions affect the clouds is investigated using reanalysis data from the European Centre for Medium-Range Weather Forecasts. The cloud droplet effective radius is found to decrease when the aerosol number concentration increases, while the cloud optical thickness does not vary with boundary layer aerosol number concentrations. Furthermore, the aerosol–cloud interaction parameter (ACI), a measure of how the effective radius is influenced by the number concentration of cloud active particles, is found to be somewhere between 0.10 and 0.18 and the magnitude of the ACI is greatest when the number concentration of particles with a diameter larger than 130 nm is used. Lower precipitation intensity in the weather radar images is associated with higher aerosol number concentrations. In addition, at Hyytiälä the particle number concentrations is generally higher for non-precipitating cases than for precipitating cases. The apparent absence of the first indirect effect of aerosols on low-level clouds over land raises questions regarding the magnitude of the indirect aerosol radiative forcing.

Observation analysis on characteristics of formation, evolution and transition of a long-lasting severe fog and haze episode in North China

An unusual fog and haze event lasted for one week took place during 1–7 December, 2011 over North China. To investigate the characteristics and mechanism of formation, evolution, and transition of the fog and haze event, we studied the microphysical properties such as aerosol, cloud condensation nuclei (CCN), fog droplet spectrum and liquid water content (LWC), as well as horizontal visibility and boundary layer properties, using the data collected in the Project of Low-Visibility Weather Monitoring and Forecasting in the Beijing-Tianjin region. The results indicate that the long-lasting fog and haze event occurred in a high pressure weather system and calm wind condition. The stable boundary-layer structure resulted from temperature inversions that were built by warm advection and radiation cooling provided a favorable condition for the accumulation of polluted aerosols and the formation and development of the fog and haze event. In particular, the continuous southerly wet flow advection made the process a persistent and long-lasting event. The horizontal visibility was almost below 2 km in the whole process, and the lowest visibility was only 56 m. The average LWC was about 10−3 g m−3, and the maximum LWC reached 0.16 g m−3. The aerosol number concentration was more than 10000 cm−3, and its mass concentration ranged from 50 to 160 μg m−3. The further study shows that the fog and haze event experienced three main processes in different intensities during the whole period, each process could be divided into three main stages: aerosol accumulation, transition and mixture of aerosol and fog, and dissipation. Each stage had different physical features: the aerosol accumulation stage was characterized by the increase of aerosol number concentration in Aitken nuclei and accumulation mode sequentially. In the transition and mixing stage of fog and haze, the latent heating produced by fog droplet condensation process and high aerosol number concentration condition intensified the Brownian coagulation process, which induced the small size of aerosols to become larger ones and enhanced the CCN activation process, thereby promoting the explosive development of the fog event. The ratio of aerosol activated to CCN reached 17%, and the ratio of CCN converted to fog droplet exceeded 100%, showing an explosively broadening of fog droplet spectrum. The decrease and dissipation of the fog was caused by an increased solar radiation heating or the passage of cold frontal system.

Links between satellite-retrieved aerosol and precipitation

Abstract. Many theories have been proposed detailing how aerosols might impact precipitation, predicting both increases and decreases depending on the prevailing meteorological conditions and aerosol type. In convective clouds, increased aerosol concentrations have been speculated to invigorate convective activity. Previous studies have shown large increases in precipitation with increasing aerosol optical depth, concluding an aerosol effect on precipitation. Our analysis reveals that these studies may have been influenced by cloud effects on the retrieved aerosol, as well as by meteorological covariations. We use a regime-based approach to separate out different cloud regimes, allowing for the study of aerosol–cloud interactions in individual cloud regimes. We account for the influence of cloud properties on the aerosol retrieval and make use of the diurnal sampling of the TRMM satellite and the TRMM merged precipitation product to investigate the precipitation development. We find that whilst there is little effect on precipitation at the time of the aerosol retrieval, in the 6 h after the aerosol retrieval, there is an increase in precipitation from cloud in high-aerosol environments, consistent with the invigoration hypothesis. Increases in lightning flash count with increased aerosol are also observed in this period. The invigoration effect appears to be dependent on the cloud-top temperature, with clouds with tops colder than 0 °C showing increases in precipitation at times after the retrieval, as well as increases in wet scavenging. Warm clouds show little change in precipitation development with increasing aerosol, suggesting ice processes are important for the invigoration of precipitation.

Spatial variability of the trends in climatic variables across China during 1961–2010

Distribution of meteorological stations is not uniform in many regions of the world, especially in developing countries like China. To eliminate the effect of uneven stations, this study produced a data set of areal average precipitation, air temperature, solar radiation, and wind speed from 736 meteorological station observations during 1961–2010 using an inverse-distance weighted technique. Based on the data set, this study detected the trends in climatic variables. Precipitation has a slight but no significant (p = 0.78) trend for the whole of China and has a significant increase trend in northwest China. Surface air temperature has a significant (p < 0.001) accelerating warming trend of 0.032 °C/a for the whole of China, and spatially larger in northern China than that in southern China. Solar radiation has a significant (p < 0.001) dimming trend of −0.14 W/(m2 · a) for the whole of China, and the largest dimming trend appears in eastern China, the possible cause for which is a high-aerosol concentration. Surface wind speed has a significant (p < 0.001) stilling trend of −0.012 m/(s·a) for the whole of China, the causes for which were speculated the changes in atmospheric circulation and surface roughness, as well as increases in aerosol concentration and the decrease in the south-north temperature gradient in China. In addition, three large-scale instrument replacements increase uncertainties of the trend analysis.

Shallow Cumulus Mixing and Subcloud-Layer Responses to Variations in Aerosol Loading

Abstract The sensitivity of lightly precipitating trade wind shallow cumulus to both aerosol concentration and domain size is investigated using large-eddy simulations (LESs). The mean states of liquid water potential temperature, total water, and velocity field exhibit negligible change between all LES runs, offering the perfect opportunity to investigate microphysical–dynamical interactions solely due to variations in aerosol concentration and not changes in meteorology. As aerosol concentration increases, two cloud population responses are found: 1) cloud and cloud-core widths decrease while their strength increases and 2) cloud and core numbers increase. The narrowing of the polluted clouds is caused by enhanced evaporation rates surrounding the cloud cores, which in turn shrinks the diameter of the cumulus toroidal circulation. The more narrow toroidal circulation in polluted clouds has a faster rise rate and imparts weaker dynamical entrainment on the cloud cores, resulting in stronger clouds as aerosol concentration increases. The reduction in cloud number for more pristine conditions occurs from greater cold pool coverage, reducing the likelihood of subcloud-layer thermals reaching their lifting condensation level. The increase in cloud number as aerosol concentration increases is compensated by narrower and stronger clouds, resulting in a cumulus-core mass flux that appears to be unaffected by aerosol concentration variability. For the weakly precipitating case studied here, the trends in the response of the cumulus clouds to aerosol concentration are found to be insensitive to domain size.

Variations in atmospheric aerosol concentration of various sizes during the total solar eclipse of 22 July 2009 over a semi urban tropical site of Northern India

The effect of total solar eclipse on aerosol size and concentration has been studied. Continuous measurements of aerosol particle concentration having various sizes (0.3–0.5, 0.5–1.0, 1.0–2.0 and 2.0–5.0 μm) along with some meteorological parameters in conjunction with total solar eclipse of 22 July 2009 have been made from 21 to 23 July 2009 at Roorkee (29°N, 77°E, 275 m above sea level). It has been observed at Roorkee 90 % maximum obscurity of eclipse since it is close to the path of totality. The event started from 05:28 h LST and ended at 07:42 h LST with total eclipse at 06:30 h LST. Results of the measurements show an increase in aerosol number concentration in sub-micron and micron size ranges during eclipse with respect to those made on any other day.

Aerosol Number Concentrations and Visibility during Dense Fog over a Subtropical Urban Site

There is now enough evidence of greater frequencies and extent of fog formation in urban areas. These could easily be linked to rapid increase in aerosol number concentration (ANC) peculiar to polluted urban environments. It is therefore pertinent to study ANC and visibility alongside the meteorological parameters in order to investigate the relationships which may possibly exist between these parameters especially during foggy conditions. This study based on field measurements of ANC for aerosol spectrum varying from 0.3 µm to 20 µm attempts to investigates whether a threshold ANC could be associated with a given visibility range during low visibility conditions including dense fog episodes. Thus, the present work explores relationship between ANC size spectrum and visibility (100 m–4500 m) in a polluted urban environment in India with specific reference to episodes of dense fog during winter period. The study depicts a threshold minimum value of ANC during foggy conditions. A power relationship between ANC and visibility is obtained. Further, aerosol number distribution and size distribution function are also studied and empirical relation is compared with previous studies. Further work is suggested to strengthen the findings presented here.

Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999–2008 derived from CARIBIC flights

This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.08–2 µm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase.

Impact of the North Atlantic Oscillation on European aerosol ground levels through local processes: a seasonal model-based assessment using fixed anthropogenic emissions

Abstract. The North Atlantic Oscillation (NAO) controls a large amount of the European climate variability with asymmetric impacts in both time and space. Here we investigate how the NAO impact on the local atmospheric processes (disregarding the NAO impact on the large inter-continental transport mechanisms) influences the levels of various aerosol species using simulated data under constant emissions, which are fixed to the 2005 levels in order to avoid anthropogenic-induced signals. In particular, we analyze interannual variations at the seasonal timescale and focus on the ground-level. The results show that positive NAO phases favor increased aerosol concentrations in southern (northern) regions during winter (summer), while negative NAO phases enhance them in northern (southern) regions during winter (summer). The underlying processes are clearly related to the NAO impact on precipitation and wind, as they act to clean the atmosphere through removal and dispersion processes, and to the NAO impact on the radiation balance (i.e., cloudiness) as it affects the biogenic emitting activity and on the oxidative capacity of the atmosphere. Differences for all the species studied (natural inert, secondary inorganic and organic aerosols) are up to 5 μg m−3, reaching 10 and 20 μg m−3 for PM10 and PM2.5 respectively, which represents variations of about 20–40% in their mean levels between opposite NAO phases.

Structural evolution of monsoon clouds in the Indian CTCZ

AbstractStructural evolution of monsoon clouds in the core monsoon region of India has been examined using multisensor data. Invigoration of warm clouds above 4.5 km (occurring in only 15.4% days of the last 11 monsoon seasons) is associated with a transition from negative to positive normalized rainfall anomaly. Cloud top pressure reduces with an increase in aerosol optical depth at a higher rate of invigoration in drier condition (characterized by large fraction of absorbing aerosols) than wet condition. Cloud effective radius for warm clouds does not show any significant change with an increase in aerosol concentration in the presence of high liquid water path, probably due to strong buffering role of meteorology. The structural evolution of monsoon clouds is influenced by both dynamic and microphysical processes that prolong the cloud lifetime, resulting in infrequent rainfall. Our results call for improved representation of aerosol and cloud vertical structures in the climate models to resolve this issue.

Measurements of aerosol particles in the Škocjan Caves, Slovenia

For the first time, continuous aerosol measurements were performed in the Škocjan Caves, one of the most important cave systems in the world, and listed by UNESCO as a natural and cultural world heritage site since 1986. Measurements of PM10 were performed during three different periods: (1) in December 2011, the average background concentration was found to be about 4 μg m(-3); (2) in June 2012, a higher concentration was measured (8 μg m(-3)); and (3) from 8 to 20 August 2012, the highest concentration of 15.3 μg m(-3) was measured. Based on the PM10 measurement results, and as compared to similar measurements outside the cave, it can be hypothesized that the increase in the cave's aerosol concentration during the summer was connected to both the higher number of visitors and the polluted atmospheric air entering the cave upon entering of the cave system. Additional measurement of nanoparticles with scanning mobility particle sizer spectrometer (size between 14.1 and 710.5 nm) confirmed these findings; during the summer period, a severe raise in the total aerosol concentration of 30-50 times was found when groups of visitors entered the cave. Our results on nanoparticles demonstrated that we were able to detect very small changes and variations in aerosol concentration inside the cave. To our knowledge, these are the first results on nanoaerosol measurements in a cave, and we believe that such measurements may lead to the implementation of better protection of delicate cave systems.

Observed Impact of Atmospheric Aerosols on the Surface Energy Budget

Abstract Atmospheric aerosols scatter and potentially absorb incoming solar radiation, thereby reducing the total amount of radiation reaching the surface and increasing the fraction that is diffuse. The partitioning of incoming energy at the surface into sensible heat flux and latent heat flux is postulated to change with increasing aerosol concentrations, as an increase in diffuse light can reach greater portions of vegetated canopies. This can increase photosynthesis and transpiration rates in the lower canopy and potentially decrease the ratio of sensible to latent heat for the entire canopy. Here, half-hourly and hourly surface fluxes from six Flux Network (FLUXNET) sites in the coterminous United States are evaluated over the past decade (2000–08) in conjunction with satellite-derived aerosol optical depth (AOD) to determine if atmospheric aerosols systematically influence sensible and latent heat fluxes. Satellite-derived AOD is used to classify days as high or low AOD and establish the relationship between aerosol concentrations and the surface energy fluxes. High AOD reduces midday net radiation by 6%–65% coupled with a 9%–30% decrease in sensible and latent heat fluxes, although not all sites exhibit statistically significant changes. The partitioning between sensible and latent heat varies between ecosystems, with two sites showing a greater decrease in latent heat than sensible heat (Duke Forest and Walker Branch), two sites showing equivalent reductions (Harvard Forest and Bondville), and one site showing a greater decrease in sensible heat than latent heat (Morgan–Monroe). These results suggest that aerosols trigger an ecosystem-dependent response to surface flux partitioning, yet the environmental drivers for this response require further exploration.

Aerosol effects on the cloud-field properties of tropical convective clouds

Abstract. Aerosol effects on condensed water and precipitation in a tropical cloud system driven by deep convective clouds are investigated for two-dimensional simulations of 2-day duration. Although an assumed 10-fold increase in aerosol concentration results in a similar temporal evolution of mean precipitation and a small (9%) difference in cumulative precipitation between the high- and low-aerosol cases, the characteristics of the convection are much more sensitive to aerosol. The convective mass flux, and temporal evolution and frequency distribution of the condensed water path WP (sum of liquid- and ice-water paths) differ significantly between unperturbed and aerosol-perturbed simulations. There are concomitant differences in the relative importance of individual microphysical processes and the frequency distribution of the precipitation rate (P). With increasing aerosol (i) the convective mass flux increases, leading to increases in condensation, cloud liquid, and accretion of cloud liquid by precipitation; (ii) autoconversion of cloud water to rain water decreases; (iii) the WP spatial distribution becomes more homogeneous; and (iv) there is an increase in the frequencies of high and low WP and P, and a decrease in these frequencies at the mid-range of WP and P. Thus, while aerosol perturbations have a small influence on total precipitation amount, for the case considered, they do have substantial influence on the spatiotemporal distribution of convection and precipitation.

Initiation of a convective dust storm over North India on 21 April 2010 inferred using satellite data

Dust storms commonly occur during the pre-monsoon (summer) season in north and northwest parts of India. Characteristics of dust events of the pressure gradient type are well understood. However, comprehensive studies on mechanism of convective dust storms in India are few. A convective dust storm which occurred on 21 April 2010 in association with a western disturbance over North India was hence studied. In the absence of in situ data, we used available satellite data to study the event. Dust storm that occurred on 20 April 2010 on the surface of the Thar Desert transported dust to northern and northwestern parts of India (Rajasthan, Haryana, Delhi and some parts of Uttar Pradesh). This formed a background of aerosols that affected the thunderstorm formed in association with western disturbance and the strong updraft in the thunderstorm carried the dust lingering in the atmosphere to higher altitudes. Large amount of aerosols carried to higher altitude suppressed the chance of precipitation by affecting the cloud top microphysics. Enhancement in evaporation due to an increase in aerosol concentration and strong downdrafts during dissipation of the thunderstorm resulted in emission of dust particles which led to the convective dust event of 21 April 2010.

Sensitivity of Warm-Frontal Processes to Cloud-Nucleating Aerosol Concentrations

Abstract An extratropical cyclone that crossed the United States on 9–11 April 2009 was successfully simulated at high resolution (3-km horizontal grid spacing) using the Colorado State University Regional Atmospheric Modeling System. The sensitivity of the associated warm front to increasing pollution levels was then explored by conducting the same experiment with three different background profiles of cloud-nucleating aerosol concentration. To the authors’ knowledge, no study has examined the indirect effects of aerosols on warm fronts. The budgets of ice, cloud water, and rain in the simulation with the lowest aerosol concentrations were examined. The ice mass was found to be produced in equal amounts through vapor deposition and riming, and the melting of ice produced approximately 75% of the total rain. Conversion of cloud water to rain accounted for the other 25%. When cloud-nucleating aerosol concentrations were increased, significant changes were seen in the budget terms, but total precipitation remained relatively constant. Vapor deposition onto ice increased, but riming of cloud water decreased such that there was only a small change in the total ice production and hence there was no significant change in melting. These responses can be understood in terms of a buffering effect in which smaller cloud droplets in the mixed-phase region lead to both an enhanced vapor deposition and decreased riming efficiency with increasing aerosol concentrations. Overall, while large changes were seen in the microphysical structure of the frontal cloud, cloud-nucleating aerosols had little impact on the precipitation production of the warm front.

Warming-induced increase in aerosol number concentration likely to moderate climate change

Atmospheric aerosol particles influence the climate system directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. Apart from black carbon aerosol, aerosols cause a negative radiative forcing at the top of the atmosphere and substantially mitigate the warming caused by greenhouse gases. In the future, tightening of controls on anthropogenic aerosol and precursor vapour emissions to achieve higher air quality may weaken this beneficial effect. Natural aerosols, too, might affect future warming. Here we analyse long-term observations of concentrations and compositions of aerosol particles and their biogenic precursor vapours in continental mid- and high-latitude environments. We use measurements of particle number size distribution together with boundary layer heights derived from reanalysis data to show that the boundary layer burden of cloud condensation nuclei increases exponentially with temperature. Our results confirm a negative feedback mechanism between the continental biosphere, aerosols and climate: aerosol cooling effects are strengthened by rising biogenic organic vapour emissions in response to warming, which in turn enhance condensation on particles and their growth to the size of cloud condensation nuclei. This natural growth mechanism produces roughly 50% of particles at the size of cloud condensation nuclei across Europe. We conclude that biosphere-atmosphere interactions are crucial for aerosol climate effects and can significantly influence the effects of anthropogenic aerosol emission controls, both on climate and air quality. © 2013 Macmillan Publishers Limited. All rights reserved.

Cloud-resolving modelling of aerosol indirect effects in idealised radiative-convective equilibrium with interactive and fixed sea surface temperature

Abstract. The study attempts to evaluate the aerosol indirect effects over tropical oceans in regions of deep convection applying a three-dimensional cloud-resolving model run over a doubly-periodic domain. The Tropics are modelled using a radiative-convective equilibrium idealisation when the radiation, turbulence, cloud microphysics and surface fluxes are explicitly represented while the effects of large-scale circulation are ignored. The aerosol effects are modelled by varying the number concentration of cloud condensation nuclei (CCN) at 1% supersaturation, which serves as a proxy for the aerosol amount in the environment, over a wide range, from pristine maritime (50 cm−3) to polluted (1000 cm−3) conditions. No direct effects of aerosol on radiation are included. Two sets of simulations have been run: fixed (non-interactive) sea surface temperature (SST) and interactive SST as predicted by a simple slab-ocean model responding to the surface radiative fluxes and surface enthalpy flux. Both sets of experiments agree on the tendency of increased aerosol concentrations to make the shortwave cloud forcing more negative and reduce the longwave cloud forcing in response to increasing CCN concentration. These, in turn, tend to cool the SST in interactive-SST case. It is interesting that the absolute change of the SST and most other bulk quantities depends only on relative change of CCN concentration; that is, same SST change can be the result of doubling CCN concentration regardless of clean or polluted conditions. It is found that the 10-fold increase of CCN concentration can cool the SST by as much as 1.5 K. This is quite comparable to 2.1–2.3 K SST warming obtained in a simulation for clean maritime conditions, but doubled CO2 concentration. Assuming the aerosol concentration has increased from preindustrial time by 30%, the radiative forcing due to indirect aerosol effects is estimated to be −0.3 W m−2. It is found that the indirect aerosol effect is dominated by the first (Twomey) effect. Qualitative differences between the interactive and fixed SST cases have been found in sensitivity of the hydrological cycle to the increase in CCN concentration; namely, the precipitation rate shows some tendency to increase in fixed SST case, but robust tendency to decrease in interactive SST case.

A model analysis of the interactions between East Asian anthropogenic aerosols and North Pacific atmospheric transients in boreal winter

Perpetual winter simulations are conducted with the Community Earth System Model under conditions of normal and zero emission of East Asian anthropogenic aerosols. The inclusion of aerosol emission and subsequently the increase of aerosol concentrations over the North Pacific due to the downstream transport induce statistically robust changes to the structure of the time‐mean atmospheric circulation across the basin. Specifically, the activity of atmospheric transient eddies significantly weakens in a zone extending from the east of Japan to the Bering Sea, Alaska, and slightly strengthens over the central North Pacific. Further partitioning in the frequency domain reveals that low‐frequency eddies with a 10–30 days time scale dictates the overall transient eddy response to aerosols. The amplitude of synoptic‐scale, high‐frequency eddies (2–6 days), on the other hand, increases from the central North Pacific to the Gulf of Alaska and decreases at the entrance of the North Pacific storm track and near the west coast of North America. The changes in the synoptic eddy field leave a distinct and consistent signal in surface precipitation. An analysis of the local energy budget of transient eddies indicates that changes in baroclinic conversion, more specifically transient eddy heat flux, largely determines the simulated differences in the activity of atmospheric transients between conditions of normal and zero emission of East Asian anthropogenic aerosols.

Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations

Abstract. In this study, we provide a comprehensive analysis of aerosol interaction with warm boundary layer clouds over the South-East Atlantic. We use aerosol and cloud parameters derived from MODIS observations, together with co-located CALIPSO estimates of the layer altitudes, to derive statistical relationships between aerosol concentration and cloud properties. The CALIPSO products are used to differentiate between cases of mixed cloud-aerosol layers from cases where the aerosol is located well-above the cloud top. This technique allows us to obtain more reliable estimates of the aerosol indirect effect than from simple relationships based on vertically integrated measurements of aerosol and cloud properties. Indeed, it permits us to somewhat distinguish the effects of aerosol and meteorology on the clouds, although it is not possible to fully ascertain the relative contribution of each on the derived statistics. Consistently with the results from previous studies, our statistics clearly show that aerosol affects cloud microphysics, decreasing the Cloud Droplet Radius (CDR). The same data indicate a concomitant strong decrease in cloud Liquid Water Path (LWP), which is inconsistent with the hypothesis of aerosol inhibition of precipitation (Albrecht, 1989). We hypothesise that the observed reduction in LWP is the consequence of dry air entrainment at cloud top. The combined effect of CDR decrease and LWP decrease leads to rather small sensitivity of the Cloud Optical Thickness (COT) to an increase in aerosol concentration. The analysis of MODIS-CALIPSO coincidences also evidences an aerosol enhancement of low cloud cover. Surprisingly, the Cloud Fraction (CLF) response to aerosol invigoration is much stronger when (absorbing) particles are located above cloud top than in cases of physical interaction. This result suggests a relevant aerosol radiative effect on low cloud occurrence: absorbing particles above the cloud top may heat the corresponding atmosphere layer, decrease the vertical temperature gradient, increase the low tropospheric stability and provide favourable conditions for low cloud formation. We also analyse the impact of anthropogenic aerosols on precipitation, through the statistical analysis of CDR-COT co-variations. A COT value of 10 is found to be the threshold beyond which precipitation is mostly formed, in both clean and polluted environments. For larger COT, polluted clouds show evidence of precipitation suppression. Results suggest the presence of two competing mechanisms governing LWP response to aerosol invigoration: a drying effect due to aerosol enhanced entrainment of dry air at cloud top (predominant for optically thin clouds) and a moistening effect due to aerosol inhibition of precipitation (predominant for optically thick clouds).