Showing papers by "Michael Jensen published in 2021"
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TL;DR: In this paper, the authors present evidence of the regular and frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages, facilitated by a combination of efficient removal of existing particles by precipitation, cold air temperatures, vertical transport of reactive gases from the ocean surface, and high actinic fluxes in a broken cloud field.
Abstract: Marine low clouds play an important role in the climate system, and their properties are sensitive to cloud condensation nuclei concentrations. While new particle formation represents a major source of cloud condensation nuclei globally, the prevailing view is that new particle formation rarely occurs in remote marine boundary layer over open oceans. Here we present evidence of the regular and frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages. The new particle formation is facilitated by a combination of efficient removal of existing particles by precipitation, cold air temperatures, vertical transport of reactive gases from the ocean surface, and high actinic fluxes in a broken cloud field. The newly formed particles subsequently grow and contribute substantially to cloud condensation nuclei in the remote marine boundary layer and thereby impact marine low clouds. Globally, new particle formation represents a major source of cloud condensation nuclei. Here, the authors present evidence of frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages.
30 citations
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Washington University in St. Louis1, Brookhaven National Laboratory2, University of Washington3, Colorado State University4, Stony Brook University5, Purdue University6, Texas A&M University7, Nanjing University of Information Science and Technology8, University of Kansas9, Pacific Northwest National Laboratory10, University of North Dakota11, Missouri University of Science and Technology12, Yonsei University13, Los Alamos National Laboratory14, Université du Québec à Montréal15, University of Arizona16, Argonne National Laboratory17, Rutgers University18, Michigan Technological University19, Georgia Institute of Technology20, University of Maryland, Baltimore County21
TL;DR: The Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign as discussed by the authors was motivated by the need of comprehensive in-situ measurements for improving the understanding of marine boundary layer CCN budget, cloud and drizzle microphysics, and the impact of aerosol on marine low cloud and precipitation.
Abstract: With their extensive coverage, marine low clouds greatly impact global climate. Presently, marine low clouds are poorly represented in global climate models, and the response of marine low clouds to changes in atmospheric greenhouse gases and aerosols remains the major source of uncertainty in climate simulations. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In addition, the ENA is periodically impacted by continental aerosols, making it an excellent location to study the cloud condensation nuclei (CCN) budget in a remote marine region periodically perturbed by anthropogenic emissions, and to investigate the impacts of long-range transport of aerosols on remote marine clouds. The Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign was motivated by the need of comprehensive in-situ measurements for improving the understanding of marine boundary layer CCN budget, cloud and drizzle microphysics, and the impact of aerosol on marine low cloud and precipitation. The airborne deployments took place from June 21 to July 20, 2017 and January 15 to February 18, 2018 in the Azores. The flights were designed to maximize the synergy between in-situ airborne measurements and ongoing long-term observations at a ground site. Here we present measurements, observation strategy, meteorological conditions during the campaign, and preliminary findings. Finally, we discuss future analyses and modeling studies that improve the understanding and representation of marine boundary layer aerosols, clouds, precipitation, and the interactions among them.
17 citations
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TL;DR: In this article, the authors examined the key processes that drive the cloud condensation nuclei population in the MBL using comprehensive characterizations of aerosol and trace gas vertical profiles during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign.
Abstract: . Because of their extensive coverage, marine low clouds greatly impact the
global climate. Presently, the response of marine low clouds to the changes
in atmospheric aerosols remains a major source of uncertainty in climate
simulations. One key contribution to this large uncertainty derives from the
poor understanding of the properties and processes of marine aerosols under
natural conditions and the perturbation by anthropogenic emissions. The
eastern North Atlantic (ENA) is a region of persistent but diverse
subtropical marine boundary layer (MBL) clouds, where cloud albedo and
precipitation are highly susceptible to perturbations in aerosol properties.
Here we examine the key processes that drive the cloud condensation nuclei
(CCN) population in the MBL using comprehensive characterizations of aerosol
and trace gas vertical profiles during the Aerosol and Cloud Experiments in
the Eastern North Atlantic (ACE-ENA) field campaign. During ACE-ENA, a total
of 39 research flights were conducted in the Azores: 20 during summer 2017
and 19 during winter 2018. During summer, long-range-transported aerosol
layers were periodically observed in the lower free troposphere (FT),
leading to elevated FT CCN concentrations ( NCCN ). Both biomass burning
and pollution from North America contribute to submicron aerosol mass in
these layers, with pollution likely the dominant contributor. In contrast,
long-range transported continental emissions have a much weaker influence on
the aerosol properties in the ENA during the winter season. While the
entrainment of FT air is a major source of particle number in the MBL for
both seasons, on average it does not serve as a direct source of CCN in the
MBL because the average FT NCCN is the same or even lower than that in
the MBL. The particle number flux due to FT entrainment is dominated by
pre-CCN (particles that are too small to form cloud droplets under typical
conditions, i.e., particles with sizes below the Hoppel minimum) due to the
elevated Npre-CCN in the lower FT. Once these pre-CCN are entrained into
the MBL, they can grow and reach CCN size range through condensational
growth, representing an indirect and major source of MBL CCN in the ENA. The
impact of synoptic conditions on the aerosol properties is examined. Under
pre-front and post-front conditions, shallow convective activity often leads
to a deep and decoupled boundary layer. Coalescence scavenging and
evaporation of drizzle below clouds lead to reduced NCCN and
larger accumulation-mode particle sizes in the upper cloud-containing
decoupled layer, indicating that surface measurements overestimate the
NCCN relevant to the formation of MBL clouds under decoupled conditions.
13 citations
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30 Sep 202112 citations
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TL;DR: In this article, the authors used long-term observations from the Atmospheric Radiation Measurement (ARM) facility's Eastern North Atlantic (ENA) site on Graciosa Island, Azores, Portugal, to identify cloud cases with open- or closed-cellular organization.
Abstract: . Extensive regions of marine boundary layer cloud impact
the radiative balance through their significant shortwave albedo while
having little impact on outgoing longwave radiation. Despite this
importance, these cloud systems remain poorly represented in large-scale
models due to difficulty in representing the processes that drive their
life cycle and coverage. In particular, the mesoscale organization and
cellular structure of marine boundary clouds have important implications for
the subsequent cloud feedbacks. In this study, we use long-term
(2013–2018) observations from the Atmospheric Radiation Measurement (ARM)
Facility's Eastern North Atlantic (ENA) site on Graciosa Island, Azores,
Portugal, to identify cloud cases with open- or closed-cellular organization. More than 500 h of each organization type are identified. The ARM observations are combined with reanalysis and satellite products to quantify
the cloud, precipitation, aerosol, thermodynamic, and large-scale synoptic
characteristics associated with these cloud types. Our analysis
shows that both cloud organization populations occur during similar sea
surface temperature conditions, but the open-cell cases are distinguished by stronger cold-air advection and large-scale subsidence compared to the
closed-cell cases, consistent with their formation during cold-air
outbreaks. We also find that the open-cell cases were associated with deeper boundary layers, stronger low-level winds, and higher rain rates compared to
their closed-cell counterparts. Finally, raindrops with diameters larger
than 1 mm were routinely recorded at the surface during both
populations, with a higher number of large drops during the open-cellular
cases. The similarities and differences noted herein provide important
insights into the environmental and cloud characteristics during varying
marine boundary layer cloud mesoscale organization and will be useful for
the evaluation of model simulations for ENA marine clouds.
11 citations
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TL;DR: In this article, the authors presented an alternative presentation regarding how diurnal precipitation is modulated by convective events that developed over the central Amazon during the preceding nighttime period, and they used data collected during the Observations and Modelling of the Green Ocean Amazon(GoAmazon 2014/2015) field campaign that took place from 1-January 2014 to 30-November 2015.
Abstract: . This study offers an alternative presentation regarding
how diurnal precipitation is modulated by convective events that developed
over the central Amazon during the preceding nighttime period. We use data
collected during the Observations and Modelling of the Green Ocean Amazon
(GoAmazon 2014/2015) field campaign that took place from 1 January 2014
to 30 November 2015 in the central Amazon. Local surface-based
observations of cloud occurrence, soil temperature, surface fluxes, and
planetary boundary layer characteristics are coupled with satellite data to
identify the physical mechanisms that control the diurnal rainfall in
central Amazon during the wet and dry seasons. This is accomplished through
evaluation of the atmospheric properties during the nocturnal periods
preceding raining and non-raining events. Comparisons between these
non-raining and raining transitions are presented for the wet (January to
April) and dry (June to September) seasons. The results suggest that
wet-season diurnal precipitation is modulated by nighttime cloud coverage and
local influences such as heating induced turbulence, whereas the dry-season
rain events are controlled by large-scale circulations.
8 citations
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TL;DR: This article showed that shallow cumulus and congestus clouds play an important role in tropical climate dynamics by distributing heat and moisture vertically (Riehl et al., 1951), and in the case of congestus providing a contribution to the net diabatic heating term proportional to their precipitation.
Abstract: Convective clouds are formed from rising air parcels, to first order driven by buoyancy and modulated by environmental stability and mixing with environmental air. Both shallow cumulus and congestus cloud types play an important role in tropical climate dynamics by distributing heat and moisture vertically (Riehl et al., 1951), and in the case of congestus providing a contribution to the net diabatic heating term proportional to their precipitation (Schumacher et al., 2008; Stachnik et al., 2013). They also can act as a precursor for deeper convection (Neggers et al., 2007) by moistening and destabilizing the stable layers that inhibit them (Mechem & Oberthaler, 2013; Riehl et al., 1951; Stevens, 2007). Despite their importance, some aspects of these clouds are not fully understood, and their contribution to tropical dynamics is not correctly represented in global climate models (GCMs) (Nam et al., 2012; Williams & Tselioudis, 2007). GCMs underestimate the sensitivity of convection to the tropospheric humidity (Derbyshire et al., 2004), including over the Amazon region (Lintner et al., 2017). As a result, shallow cumulus and congestus cloud fractions tend to be underestimated (Nam et al., 2012; Williams & Tselioudis, 2007). While there have been recent improvements (Xie et al., 2019), GCMs struggle to represent the diurnal cycle of convective precipitation correctly, by initializing and deepening convection too quickly (Del Genio & Wu, 2010; Stirling & Stratton, 2012).
4 citations
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01 Jan 2021
4 citations