Updrafts crucial: Clouds in the southern hemisphere more precisely
understood

Date:
January 26, 2022
Source:
Leibniz Institute for Tropospheric Research (TROPOS)
Summary:
Clouds in the southern hemisphere reflect more sunlight than those
in the northern hemisphere. The reason is a more frequent occurrence
of liquid water droplets, which results from an interplay between
updrafts and a cleaner environment.



FULL STORY ========================================================================== Clouds in the southern hemisphere reflect more sunlight than those
in the northern hemisphere. The reason is a more frequent occurrence
of liquid water droplets, which results from an interplay between
updrafts and a cleaner environment. In a study published in the journal Atmospheric Chemistry and Physics, a team of researchers led by the
Leibniz Institute for Tropospheric Research (TROPOS) found a stronger
influence of updrafts than expected. The new results were made possible
by long-term measurements in Leipzig (Germany), Limassol (Cyprus) and
Punta Arenas (Chile).


========================================================================== Covering three years, the measurements in Punta Arenas are the longest
dataset on cloud properties obtained with ground-based lidar and radar
in the Southern Ocean. From 2018 to 2021, an international team from
University of Magallanes (UMAG), TROPOS and University of Leipzig
had conducted extensive observations of aerosols, clouds, wind and precipitation in the very south of Chile as part of the DACAPO-PESO
field experiment. The researchers used two datasets from the northern hemisphere locations of Leipzig and Cyprus to put their findings into
global context. Data from the CyCARE field campaign on Cyprus were
collected in the years 2016 to 2018 in collaboration with researchers
from the Cyprus University of Technology and the ERATOSTHENES Centre of Excellence in Limassol.

The main objective of the measurements in the pristine environment at the southern tip of South America was to study the atmosphere and to learn
more about the interactions between aerosols and clouds in a region where
there is hardly any long-term data available so far. To address the lack
of observations, TROPOS brought the two containers of the LACROS mobile atmospheric observatory together with instrumentation of the University of Leipzig to Punta Arenas. There, the observations were conducted together
with UMAG's Laboratory for Atmospheric Research. The instrumentation of
LACROS consists of multiple lidars, radars, radiometers, sun photometers
and others.

These measurements were supplemented by aerosol filter samples from
Cerro Mirador, a 600 m high hill close by.

Originally, the measurements were planned for one year as a contribution
to the "Year of Polar Prediction in the Southern Hemisphere"
(YOPP-SH). But due to the global COVID-19 pandemic and the resulting
travel restrictions, the measurements were extended by two years and
only finished at the end of 2021.

"Scientifically, this delay was of great benefit," says Kevin Ohneiser,
PhD student at TROPOS. Because the huge "Black Summer" wildfires of
2019/20 occurred in Australia during this period. Their smoke was
transported more than 10,000 kilometres across the Pacific to South
America and could be observed there up to heights of 25 km with the
laser-based investigations until the measurements were completed at the
end of 2021. Since the air in southern Chile is usually very clean,
this type of air pollution was immediately noticeable and underlines
the global influence of the large forest fires on climate.

"With DACAPO-PESO, we have filled a gap in measurements that has long
existed for the southern hemisphere. The freely available data can now
help to improve current climate models," explains Dr Boris Barja from
UMAG, who played a decisive role on-site in ensuring that the instruments
could be in continuous operation despite the Covid-19-related travel restrictions.

With more than 10 follow-up projects, 20 conference papers and 10 research publications to date, the project has been very successful scientifically.

Further publications are close to finalization. Teresa Vogl, Phd student
from University of Leipzig, for instance, is currently working on a method
to characterize precipitation formation with a machine learning-based
cloud radar algorithm.

The recently achieved main purpose of the project was however to
investigate the differences of thin cloud layers over Leipzig, Limassol
and Punta Arenas.

Due to a high percentage of oceans covering the Earth in the southern hemisphere, the atmosphere in this region is cleaner, i.e. contains
fewer aerosol particles. This difference is especially strong in the
free troposphere -- the air masses at higher altitudes, unaffected from
local pollution sources.

"Fewer particles mean fewer ice nuclei in the atmosphere. But it is
precisely these that are needed to cause cloud droplets to freeze into ice crystals at temperatures between 0 and -40DEGC. Therefore, clouds ice up
much less in the mid-latitudes of the southern hemisphere and contain more liquid water at the same temperatures. This means that they influence the incident sunlight and also the thermal radiation emitted from the Earth's surface differently than in the north. This is one explanation why global climate models are still not able to represent the radiation balance of
the southern hemisphere with sufficient accuracy," summarises Dr Patric
Seifert from TROPOS. In the temperature range between -24 and -8DEGC,
the lack of ice nuclei caused the clouds over Punta Arenas to form ice
on average 10 to 40 percent less often than the clouds over Leipzig. The
ice mass produced by the liquid water clouds is also reduced by at least
a factor of 2.

However, contrary to previous studies, differences in atmospheric
pollution are not the only cause of the observed contrasts, especially
at even lower temperatures. The investigations in southern Chile showed
that the clouds are often influenced by so-called gravity waves. The
strong westerly wind from the Pacific collides with the Andes mountains,
is displaced upwards and creates these gravity waves. "By measuring
the up- and downward winds within the clouds, we were able to detect
clouds that had been influenced by these waves and filter them out of
the overall statistics. This allowed us to show that these gravity waves,
and not the lack of ice nuclei, are mainly responsible for the excess of
cloud droplets at temperatures below -25DEGC," explains Dr Martin Radenz
from TROPOS, who recently obtained the Phd degree for his work on this
subject. " However, it is currently unclear whether this phenomenon only influences clouds in southern Chile. How important are gravity waves
for the formation of clouds and precipitation in other regions of the
Southern Ocean? How often do gravity waves occur over the open ocean,
which covers most of the Earth's surface between 30 and 70 degrees south
and is currently only observed by satellites? Further measurements of air motion in clouds are needed to further constrain the role of ice nuclei
in the apparent excess of liquid water in clouds. In the near future,
we plan to work with our partners to investigate these questions at other locations in the Southern Hemisphere, such as Antarctica and New Zealand,
and ideally also from aboard research vessels.

Because from space those observations are not possible at the moment."
The two LACROS containers will be back at TROPOS in Leipzig at the end
of January and will then be prepared for their next deployment. Within
the framework of ACTRIS-D, the German contribution to the European
research infrastructure for aerosols, clouds and trace gases, three new instruments will then be integrated. With a new sun photometer, microwave radiometer and 94 Ghz cloud radar, LACROS will go to the Swiss Alps in
November to investigate artificially generated ice clouds. Tilo Arnhold ========================================================================== Story Source: Materials provided by Leibniz_Institute_for_Tropospheric_Research_(TROPOS).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Martin Radenz, Johannes Bu"hl, Patric Seifert, Holger Baars, Ronny
Engelmann, Boris Barja Gonza'lez, Rodanthi-Elisabeth Mamouri, Fe'lix
Zamorano, Albert Ansmann. Hemispheric contrasts in ice formation
in stratiform mixed-phase clouds: disentangling the role of aerosol
and dynamics with ground-based remote sensing. Atmospheric Chemistry
and Physics, 2021; 21 (23): 17969 DOI: 10.5194/acp-21-17969-2021 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220126144156.htm

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