scholarly journals Aerosol-boundary-layer-monsoon interactions amplify semi-direct effect of biomass smoke on low cloud formation in Southeast Asia

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Ke Ding ◽  
Xin Huang ◽  
Aijun Ding ◽  
Minghuai Wang ◽  
Hang Su ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Southeast Asia ◽  
Direct Effect ◽  
Biomass Burning ◽  
Synergetic Effect ◽  
Cloud Formation ◽  
Monsoon Circulation ◽  
Total Biomass ◽  
Aerosol Cloud ◽  
Low Cloud

AbstractLow clouds play a key role in the Earth-atmosphere energy balance and influence agricultural production and solar-power generation. Smoke aloft has been found to enhance marine stratocumulus through aerosol-cloud interactions, but its role in regions with strong human activities and complex monsoon circulation remains unclear. Here we show that biomass burning aerosols aloft strongly increase the low cloud coverage over both land and ocean in subtropical southeastern Asia. The degree of this enhancement and its spatial extent are comparable to that in the Southeast Atlantic, even though the total biomass burning emissions in Southeast Asia are only one-fifth of those in Southern Africa. We find that a synergetic effect of aerosol-cloud-boundary layer interaction with the monsoon is the main reason for the strong semi-direct effect and enhanced low cloud formation in southeastern Asia.

scholarly journals Asian monsoon amplifies semi-direct effect of biomass burning aerosols on low cloud formation

2020 ◽  
Author(s):  
Ke Ding ◽  
Xin Huang ◽  
Aijun Ding ◽  
Minghuai Wang ◽  
Hang Su ◽  
...  
Keyword(s):  
Direct Effect ◽  
Biomass Burning ◽  
Asian Monsoon ◽  
Cloud Formation ◽  
Low Cloud

scholarly journals Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

2021 ◽  
Author(s):  
Jianhao Zhang ◽  
Paquita Zuidema
Keyword(s):  
Boundary Layer ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Cloud Fraction ◽  
Small Scale ◽  
Aerosol Cloud ◽  
Ascension Island ◽  
Tropospheric Aerosol ◽  
Low Cloud ◽  
Aerosol Cloud Interactions

Abstract. Many studies examining shortwave-absorbing aerosol-cloud interactions over the southeast Atlantic apply a seasonal averaging. This disregards a meteorology that raises the mean altitude of the smoke layer from July to October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading at Ascension Island from July to October, based on measurements from Ascension Island (8º S, 14.5º W), satellite retrievals and reanalysis. In July and August, variability in the smoke loading predominantly occurs in the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol-cloud interactions consistent with a boundary-layer semi-direct effect can explain the cloudiness changes. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the land-based anticyclonic circulation transporting free-tropospheric aerosol to closer to the coast. Stronger boundary layer winds help deepen, dry, and cool the boundary layer near the main stratocumulus deck compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. Longwave cooling rates generated by a sharp water vapor gradient at the aerosol layer top facilitates small-scale vertical mixing, and could help to maintain a better-mixed September free troposphere. The October meteorology is more singularly dependent on the strength of the free-tropospheric winds advecting aerosol offshore. Free-tropospheric aerosol is less, and moisture variability more, compared to September. Low-level clouds increase and are more stratiform, when the smoke loadings are higher. The increased free-tropospheric moisture can help sustain the clouds through reducing evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region increasing cloud droplet number concentrations may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free-tropospheric winds at Ascension supports slightly higher cloud tops during smokier conditions. Overall the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology, raising a climate importance as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

scholarly journals Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

2020 ◽  
Vol 20 (4) ◽  
pp. 2341-2351 ◽  
Author(s):  
Sam Pennypacker ◽  
Michael Diamond ◽  
Robert Wood
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Biomass Burning ◽  
Liquid Water ◽  
Marine Boundary Layer ◽  
Liquid Water Path ◽  
Accumulation Mode ◽  
Water Path ◽  
Aerosol Cloud ◽  
Ascension Island

Abstract. We study 41 d with daily median surface accumulation mode aerosol particle concentrations below 50 cm−3 (ultra-clean conditions) observed at Ascension Island (ASI; 7.9∘ S, 14.4∘ W) between June 2016 and October 2017 as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign. Interestingly, these days occur during a period of great relevance for aerosol–cloud–radiation interactions, the southeast Atlantic (SEATL) biomass-burning season (approximately June–October). That means that these critical months can feature both the highest surface aerosol numbers, from smoke intrusion into the marine boundary layer, as well as the lowest. While carbon monoxide and refractory black carbon concentrations on ultra-clean days do not approach those on days with heavy smoke, they also frequently exceed background concentrations calculated in the non-burning season from December 2016 to April 2017. This is evidence that even what become ultra-clean boundary layers can make contact with and entrain from an overlying SEATL smoke layer before undergoing a process of rapid aerosol removal. Because many ultra-clean and polluted boundary layers observed at Ascension Island during the biomass burning season follow similar isobaric back trajectories, the variability in this entrainment is likely more closely tied to the variability in the overlying smoke rather than large-scale horizontal circulation through the boundary layer. Since exceptionally low accumulation mode aerosol numbers at ASI do not necessarily indicate the relative lack of other trace pollutants, this suggests the importance of regional variations in what constitutes an “ultra-clean” marine boundary layer. Finally, surface drizzle rates, frequencies and accumulation – as well as retrievals of liquid water path – all consistently tend toward higher values on ultra-clean days. This implicates enhanced coalescence scavenging in low clouds as the key driver of ultra-clean events in the southeast Atlantic marine boundary layer. These enhancements occur against and are likely mediated by the backdrop of a seasonal increase in daily mean cloud fraction and daily median liquid water path over ASI, peaking in September and October in both LASIC years. Therefore the seasonality in ultra-clean day occurrence seems directly linked to the seasonality in SEATL cloud properties. These results highlight the importance of two-way aerosol–cloud interactions in the region.

scholarly journals Injection heights of springtime biomass burning plumes over the Peninsular Southeast Asia and their impacts on pollutant long-range transport

2013 ◽  
Vol 13 (9) ◽  
pp. 23781-23816 ◽  
Author(s):  
Y. Jian ◽  
T.-M. Fu
Keyword(s):  
Boundary Layer ◽  
Southeast Asia ◽  
Long Range ◽  
Biomass Burning ◽  
Direct Injection ◽  
Long Range Transport ◽  
Northwestern Pacific ◽  
Free Troposphere ◽  
Smoke Plumes ◽  
Range Transport

Abstract. We analyzed observations from the Multi-angle Imaging SpectroRadiometer (MISR) to determine the injection heights of biomass burning smoke plumes over the Peninsular Southeast Asia (PSEA) in spring, with the goal of evaluating the impacts on pollutant long-range transport. We retrieved the heights of twenty-two thousand MISR smoke pixels from 607 smoke plumes over the PSEA during February to April of the years 2001–2010. Forty-five percent of the analyzed smoke pixels were above the local mean boundary layer (1 km) at MISR overpass time (10:30 a.m. local time). We used the GEOS-Chem model to simulate the transport of PSEA biomass burning pollutants in March 2001. We found that the direct injection of 40% of the PSEA biomass burning emissions had little impact on the long-range transport of CO to downwind regions, compared to a control simulation where all biomass burning emissions were released in the boundary layer. This was because CO at the surface over the PSEA was efficiently lifted into the free troposphere by deep convection associated with synoptic-scale weather systems. For pollutants with lifetimes shorter than the synoptic timescale, such as black carbon aerosol (BC), their long-range transport was much more sensitive to the initial plume injection height. The direct injection of NOx from PSEA biomass burning into the free troposphere drove increased formation and transport of PAN, which in turn led to significant increases of ozone over downwind southern China and northwestern Pacific. The Pacific subtropical high transported PSEA biomass burning pollutants to the marine boundary layer over the tropical northwestern Pacific. We compared our model results to aircraft measurements over the northwestern Pacific during the TRACE-P campaign (March 2001). The direct injection of 40% of the PSEA biomass burning pollutants in the free troposphere in the model led to a more pronounced BC peak at 3 km over the northwestern Pacific, which was in better agreement with the aircraft observations compared to the control simulation. Our analyses highlighted the point that the injection heights of smoke plumes pose large uncertainty to the interpretation of BC measurements downwind of biomass burning regions.

scholarly journals Injection heights of springtime biomass-burning plumes over peninsular Southeast Asia and their impacts on long-range pollutant transport

2014 ◽  
Vol 14 (8) ◽  
pp. 3977-3989 ◽  
Author(s):  
Y. Jian ◽  
T.-M. Fu
Keyword(s):  
Boundary Layer ◽  
Southeast Asia ◽  
Long Range ◽  
Biomass Burning ◽  
Direct Injection ◽  
Long Range Transport ◽  
Northwestern Pacific ◽  
Free Troposphere ◽  
Smoke Plumes ◽  
Range Transport

Abstract. We analyzed observations from the Multi-angle Imaging SpectroRadiometer (MISR) to determine the injection heights of biomass-burning smoke plumes over peninsular Southeast Asia (PSEA, here defined as Vietnam, Cambodia, Thailand, Laos, and Myanmar) in the spring, with the goal of evaluating the impacts on long-range pollutant transport. We retrieved the heights of 22 000 MISR smoke pixels from 607 smoke plumes over PSEA during February to April of the years 2001–2010. Forty-five percent of the analyzed smoke pixels were above the local mean boundary layer (1 km) at MISR overpass time (10:30 a.m. local time). We used the GEOS–Chem model to simulate the transport of PSEA biomass-burning pollutants in March 2001. On a monthly mean basis, we found that the direct injection of 40% of the PSEA biomass-burning emissions had little impact on the long-range transport of CO to downwind regions, compared to a control simulation where all biomass-burning emissions were released in the boundary layer. This was because CO at the surface over PSEA was efficiently lifted into the free troposphere by deep convection associated with synoptic-scale weather systems. For pollutants with lifetimes shorter than the synoptic timescale, such as black carbon aerosol (BC), their long-range transport was much more sensitive to the initial plume injection height. The direct injection of NOx from PSEA biomass burning into the free troposphere drove increased formation and transport of peroxyacetyl nitrate (PAN), which in turn led to a small increase in ozone over downwind southern China and the northwestern Pacific. The Pacific subtropical high transported BC emitted from PSEA biomass burning to the marine boundary layer over the tropical northwestern Pacific. We compared our model results to aircraft measurements over the northwestern Pacific during the TRACE-P campaign (March 2001). The direct injection of 40% of the PSEA biomass-burning pollutants into the free troposphere in the model led to a more pronounced BC peak at 3 km over the northwestern Pacific. Our analysis highlights the point that the injection heights of smoke plumes presents great uncertainty over the interpretation of BC measurements downwind of biomass-burning regions.

scholarly journals Global and regional modeling of clouds and aerosols in the marine boundary layer during VOCALS: the VOCA Intercomparison

2014 ◽  
Vol 14 (5) ◽  
pp. 6537-6587 ◽  
Author(s):  
M. C. Wyant ◽  
C. S. Bretherton ◽  
R. Wood ◽  
G. R. Carmichael ◽  
A. Clarke ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Quantitative Estimation ◽  
Cloud Droplet ◽  
South American ◽  
Aerosol Cloud ◽  
Low Cloud ◽  
Forecast Models ◽  
Low Cloud Cover ◽  
Aerosol Cloud Interactions

Abstract. A diverse collection of models are used to simulate the marine boundary layer in the Southeast Pacific region during the period of the October–November 2008 VOCALS REx field campaign. Regional models simulate the period continuously in boundary-forced free-running mode, while global forecast models and GCMs are run in forecast mode. The models are compared to extensive observations along a line at 20° S extending westward from the South American coast. Most of the models simulate cloud and aerosol characteristics and gradients across the region that are recognizably similar to observations, despite the complex interaction of processes involved in the problem, many of which are parameterized or poorly resolved. Some models simulate the regional low cloud cover well, though many models underestimate MBL depth near the coast. Most models qualitatively simulate the observed offshore gradients of SO2, sulfate aerosol, CCN concentration in the MBL, and the related gradient in cloud droplet concentrations, but there are large quantitative intermodel differences in both means and gradients of these quantities. Most models underestimate large CCN (at 0.1% supersaturation) in the MBL and free troposphere. The GCMs also have difficulty simulating coastal gradients in CCN and cloud droplet number concentration. The overall performance of the models demonstrates their potential utility in simulating aerosol-cloud interactions in the MBL, though quantitative estimation of aerosol-cloud interactions and aerosol indirect effects of MBL clouds with these models remains uncertain.

scholarly journals Modeling the influences of aerosols on pre-monsoon circulation and rainfall over Southeast Asia

2014 ◽  
Vol 14 (13) ◽  
pp. 6853-6866 ◽  
Author(s):  
D. Lee ◽  
Y. C. Sud ◽  
L. Oreopoulos ◽  
K.-M. Kim ◽  
W. K. Lau ◽  
...  
Keyword(s):  
Southeast Asia ◽  
Biomass Burning ◽  
Global Climate Model ◽  
Cloud Microphysics ◽  
Seasonal Development ◽  
Monsoon Circulation ◽  
Radiative Effect ◽  
Surface Temperatures ◽  
Aerosol Effects

Abstract. We conduct several sets of simulations with a version of NASA's Goddard Earth Observing System, version 5, (GEOS-5) Atmospheric Global Climate Model (AGCM) equipped with a two-moment cloud microphysical scheme to understand the role of biomass burning aerosol (BBA) emissions in Southeast Asia (SEA) in the pre-monsoon period of February–May. Our experiments are designed so that both direct and indirect aerosol effects can be evaluated. For climatologically prescribed monthly sea surface temperatures, we conduct sets of model integrations with and without biomass burning emissions in the area of peak burning activity, and with direct aerosol radiative effects either active or inactive. Taking appropriate differences between AGCM experiment sets, we find that BBA affects liquid clouds in statistically significantly ways, increasing cloud droplet number concentrations, decreasing droplet effective radii (i.e., a classic aerosol indirect effect), and locally suppressing precipitation due to a deceleration of the autoconversion process, with the latter effect apparently also leading to cloud condensate increases. Geographical re-arrangements of precipitation patterns, with precipitation increases downwind of aerosol sources are also seen, most likely because of advection of weakly precipitating cloud fields. Somewhat unexpectedly, the change in cloud radiative effect (cloud forcing) at surface is in the direction of lesser cooling because of decreases in cloud fraction. Overall, however, because of direct radiative effect contributions, aerosols exert a net negative forcing at both the top of the atmosphere and, perhaps most importantly, the surface, where decreased evaporation triggers feedbacks that further reduce precipitation. Invoking the approximation that direct and indirect aerosol effects are additive, we estimate that the overall precipitation reduction is about 40% due to the direct effects of absorbing aerosols, which stabilize the atmosphere and reduce surface latent heat fluxes via cooler land surface temperatures. Further refinements of our two-moment cloud microphysics scheme are needed for a more complete examination of the role of aerosol–convection interactions in the seasonal development of the SEA monsoon.

scholarly journals The composition and variability of atmospheric aerosol over Southeast Asia during 2008

2012 ◽  
Vol 12 (2) ◽  
pp. 1083-1100 ◽  
Author(s):  
W. Trivitayanurak ◽  
P. I. Palmer ◽  
M. P. Barkley ◽  
N. H. Robinson ◽  
H. Coe ◽  
...  
Keyword(s):  
Southeast Asia ◽  
Gas Phase ◽  
Biomass Burning ◽  
Organic Aerosol ◽  
Sea Salt ◽  
Dust Aerosol ◽  
Small Scale ◽  
Formation Mechanisms ◽  
Sulphate Aerosol ◽  
Phase Oxidation

Abstract. We use a nested version of the GEOS-Chem global 3-D chemistry transport model to better understand the composition and variation of aerosol over Borneo and the broader Southeast Asian region in conjunction with aircraft and satellite observations. Our focus on Southeast Asia reflects the importance of this region as a source of reactive organic gases and aerosols from natural forests, biomass burning, and food and fuel crops. We particularly focus on July 2008 when the UK BAe-146 research aircraft was deployed over northern Malaysian Borneo as part of the ACES/OP3 measurement campaign. During July 2008 we find using the model that Borneo (defined as Borneo Island and the surrounding Indonesian islands) was a net exporter of primary organic aerosol (42 kT) and black carbon aerosol (11 kT). We find only 13% of volatile organic compound oxidation products partition to secondary organic aerosol (SOA), with Borneo being a net exporter of SOA (15 kT). SOA represents approximately 19% of the total organic aerosol over the region. Sulphate is mainly from aqueous-phase oxidation (68%), with smaller contributions from gas-phase oxidation (15%) and advection into the regions (14%). We find that there is a large source of sea salt, as expected, but this largely deposits within the region; we find that dust aerosol plays only a relatively small role in the aerosol burden. In contrast to coincident surface measurements over Northern Borneo that find a pristine environment with evidence for substantial biogenic SOA formation we find that the free troposphere is influenced by biomass burning aerosol transported from the northwest of the Island and further afield. We find several transport events during July 2008 over Borneo associated with elevated aerosol concentrations, none of which coincide with the aircraft flights. We use MODIS aerosol optical depths (AOD) data and the model to put the July campaign into a longer temporal perspective. We find that Borneo is where the model has the least skill at reproducing the data, where the model has a negative bias of 76% and only captures 14% of the observed variability. This model performance reflects the small-scale island-marine environment and the mix of aerosol species, with the model showing more skill at reproducing observed AOD over larger continental regions such as China where AOD is dominated by one aerosol type. The model shows that AOD over Borneo is approximately evenly split between organic and sulphate aerosol with sea salt representing 10–20% during May–September; we find a similar breakdown over continental Southeast Asia but with less sea salt aerosol and more dust aerosol. In contrast, East China AOD is determined mainly by sulphate aerosol and a seasonal source of dust aerosol, as expected. Realistic sensitivity runs, designed to test our underlying assumptions about emissions and chemistry over Borneo, show that model AOD is most sensitive to isoprene emissions and organic gas-phase partitioning but all fail to improve significantly upon the control model calculation. This emphasises the multi-faceted dimension of the problem and the need for concurrent and coordinated development of BVOC emissions, and BVOC chemistry and organic aerosol formation mechanisms.

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