Abstracts – Session D3
IMPACT OF AEROSOL EMISSIONS ON CLOUDS AND PRECIPITATION
D301 - 0183: Aircraft measured vertical profiles of cloud microstructure and precipitation initiation under contrasting aerosol conditions over
Daniel Rosenfeld1, Ramon Braga2
1The Hebrew University of Jerusalem, Jerusalem, Israel 2Centro de Previsão de Tempo e Estudos Climáticos, Instituto Nacional de Pesquisas Espaciais, Cachoeira Paulista, Brazil
We have investigated how pollution aerosols affect the height above cloud base of rain and ice hydrometeor initiation and the subsequent vertical evolution of cloud droplet size and number concentrations in growing convective cumulus. For this purpose we used in-situ data of hydrometeor size distributions measured with instruments mounted on HALO (High Altitude and Long Range Research Aircraft) during the ACRIDICON-CHUVA campaign over the Amazon during September 2014. The results show that the height of rain initiation by collision and coalescence processes (Dr, in units of meters above cloud base) is linearly correlated with the number concentration of droplets (Nd in cm-3) nucleated at cloud base (Dr ≈ 5·Nd). When Nd exceeded values of about 1000 cm-3, Dr became greater than 5000 m, and the first observed precipitation particles were ice hydrometeors. Therefore, no liquid water raindrops were observed within growing convective cumulus during polluted conditions. Furthermore, also the formation of ice particles took place at higher altitudes in the clouds in polluted conditions, because the resulting smaller cloud droplets froze at colder temperatures compared to the larger drops in the unpolluted cases. The measured vertical profiles of droplet effective radius (re) were close to those estimated by assuming adiabatic conditions (rea), supporting the hypothesis that the entrainment and mixing of air into convective clouds is nearly inhomogeneous. Secondary activation of droplets on aerosol particles from biomass burning and air pollution reduced re below rea, which further inhibited the formation of raindrops and ice particles and resulted in even higher altitudes for rain and ice initiation.
D302 - ORAL-0123: Evidence of a reduction in cloud condensation nuclei activity of submicron water-soluble aerosols caused by biogenic
emissions in a cool-temperate forest
Astrid Müller1, Yuzo Miyazaki2, Eri Tachibana2, Tsutom Hiura3
1Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan 2Institute of Low Temperature Science, Hokkaido University, Saporo, Japan 3Field Science Center for Northern Biosphere, Hokkaido University, Tomakomai, Japan
Biogenic secondary organic aerosols (SOA) can significantly alter the hygroscopicity and cloud condensation nuclei (CCN) properties, which can subsequently impact climate change. Large uncertainties exist in how the difference in the types of terrestrial biogenic sources and the abundance of organics relative to sulfate affect CCN properties. We show that the hygroscopicity parameter κCCN (0.44 ± 0.07) exhibited a distinct seasonal trend with a minimum in autumn (κCCN = 0.32–0.37) for submicron water-soluble aerosols collected for two years in a cool-temperate forest in northern Japan. The temporal variability of κCCN was controlled by the water-soluble organic matter (WSOM)-to-sulfate ratio (R2 > 0.60), where the significant reduction of κCCN in autumn was linked to the increased WSOM/sulfate ratio. Positive matrix factorization analysis indicates that α-pinene-derived SOA and primary biological aerosol particles (PBAP) associated with fungi/bacteria substantially contributed to the WSOM mass (~75%) in autumn, the majority of which was attributable to emissions from litter/soil microbial activity near the forest floor. These findings suggest that WSOM originated from the forest floor can significantly suppress the cloud forming potential of particles in cool-temperate forests, which have implications for predicting climate effects by changes in biogenic emissions in future.
D303 - 0389: Evaluating Arctic aerosol particle characteristics and cloud changes in the general circulation model NorESM
Tanja Dallafior1, Srinath Krishnan2, Eyal Freud1, Annica Ekman2, Ilona Riipinen1, Hans-Christen Hansson1
1ACES Department of Environmental Science and Analytical Chemistry, Stockholm, Sweden 2MISU Department of Meteorology, Stockholm, Sweden
Previous studies using the general circulation model NorESM have shown that changes in aerosol particle emissions in various regions in the northern mid-latitudes result in significant temperature responses in the Arctic.
The present work aims at understanding the processes that lead to such temperature response remote from the location where the forcing, in this case aerosol particle and precursor emission changes, occurs. Our research efforts focus on two chains of effects in particular: First, transport of anthropogenic aerosol particles from the mid to high-latitudes and subsequent direct and indirect aerosol effects altering Arctic energy fluxes and triggering feedback processes. Second, altered atmospheric and ocean meridional heat transport to the Arctic initiated by the changes in energy fluxes in the mid-latitudes. The degree of contribution of each of these effects in causing the Arctic temperature response is still open to debate.
To investigate the first chain of effects, we evaluate the modeled aerosol particle population at high latitudes against new observational datasets of aerosol particle size and number concentrations from five ground-based sites within the Arctic Circle. Next, efforts are aimed at assessing how NorESM captures aerosol transport from the mid- to high-latitudes. This will be achieved through comparisons with backward trajectory calculations from ground-based measurements and with remote observations and datasets such as the MACC aerosol climatology. Finally, ground-based and remote (e.g. CALIPSO) observations of Arctic precipitation and cloud properties such as cloud droplet number concentration, cloud fraction, and liquid water path will be used to assess high-latitude cloud properties.
The goal of this research is to gain a process-based understanding of local and remote aerosol effects on high-latitude climate on timescales from weeks to years and the feedback processes they trigger.
D304 - 0390: Future reductions of anthropogenic aerosol emissions in the mid latitudes can affect the temperature in the Arctic and the
precipitation in the Tropics
Juan C Acosta Navarro1, Annica Ekman1, Francesco S. R. Pausata1, Anna Lewinschal1, Vidya Varma2, Oeyvind Seland3, Michael Gauss3, Trond Iversen3, Alf Kirkevag3, Ilona Riipinen1, Hans-Christen Hansson1
1Stockholm University and Bolin Centre for Climate Research, Stockholm, Sweden 2National Institute of Water and Atmospheric Research, , Wellington, New Zealand 3Norwegian Meteorological Institute, Oslo, Norway
European sulfur emissions changes has been shown using the NorESM earth system model to have had substantial warming on the Arctic climate since the 1980ties (Acosta et al, 2016, Nature Geoscience). Using the same model experiments to evaluate how possible future emission changes may affect the global and the Arctic climate. The simulations show that by 2025–49 a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 and 0.84 K, respectively, as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM model simulations yield a nonsignificant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the representative concentration pathway 4.5 (RCP4.5) emission scenario leads to a global and Arctic warming of 0.35 and 0.94 K, respectively. The model yields a marked annual average northward shift in the intertropical convergence zone with decreasing aerosol emissions and subsequent warming of the Northern Hemisphere. The shift is most pronounced in the MFR scenario but also visible in the CLE scenario. The strong reduction in aerosol emissions in the MFR scenario also leads to a net southward cross-hemispheric energy transport anomaly both in the atmosphere and ocean, and enhanced monsoon circulation in Southeast Asia and East Asia causing an increase in precipitation over a large part of this region.