2011 - Cooperative Institute for Research in Environmental Sciences ...

David Noone
A Modern Approach to Past Climate: Isotope
Meteorology in Greenland
The stable isotopic records
from the Greenland
Ice Sheet are the gold standard
for understanding
climate variations in the
Arctic over the last 100,000
years. While the basic tenets
that underlie interpretation
of isotopic information
appear robust in a
mean sense, meterological
and glaciological processes
can confound simple
interpretations. Processes
of concern are variations
in moisture sources, cloud
processes, surface ablation,
blowing snow and vapor
diffusion in the firn. Layering
of snow at the surface
of the ice sheet (Figure 1) is easily seen in shallow snow pits,
and water vapor transport is known to redistribute the isotopic
signal via diffusion and wind pumping. How the isotopic
signal is changed before the layers are compacted into solid
ice (at about 100 m depth) remains unclear. New laser spectrometers
can measure the isotopic composition of vapor and
thereby provide the key tool to obtain direct evidence for the
processes previously unobservable.
At Summit Camp in Greenland, precipitation spectrometers
to measure the amount, size distribution and approximate
habit of falling and blowing snow, along with turbulence
sensors to measure snow lofting and surface latent heat
flux (ablation and frost), have been installed on a 50-m tower.
Short high-resolution firn cores allow us to reconcile our detailed
measurements and modeling of the source variations,
cloud processes and post-depositional influences with the
glaciological records. The transport of water from the atmosphere
to the surface is associated with snowfall, frosting and
the deposition of fog cloud particles. The rate of deposition
of fog depends on the size of the particles. Figure 3 shows
an example of a fog event in which the early morning fog
droplets form from condensation in the ambient vapor, and
grow to a size of about 30 μm. As ambient temperature rises
at about 6 a.m. local time, the fog dissipates. Once this information
is combined with the isotopic information, a new
understanding of the interplay between low-level vapor, fog
and the isotopic composition of the firn will emerge.
The isotope measurements at Summit are part of an
emerging global observatory to measure water vapor isotopic
composition that has been established by our group with
international partners (Figure 2). The intent of the network
is to provide information about the changing behavior of the
global water cycle in response to climate change. Figure 3: Fog cloud particle size distribution showing the formation
of fog at local midnight on June 27, 2011. Small particles grew from
around 5 μm to a maximum size of about 30 μm at 7 a.m. local time.
42 CIRES Annual Report 2011
Figure 1: David Noone studies the layers of snow at Summit Camp in
Greenland. Layers with different density form due to wind events, and
sublimation and frosting are easily found in the top 1 meter of snow in
Greenland.
Figure 2: Locations of water vapor isotope monitoring stations: Darwin,
Australia; Mauna Lao, Hawaii; Niwot Ridge, Boulder, Colo.; Summit Station,
Greenland; Reykjavik, Iceland; Eureka, Canada; and Barrow, Alaska.