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Abstract:
Large uncertainties still exist in the global methane
budget with clear disagreements between bottom-up
and top-down estimates, limiting confidence in climate
projections. This is particularly true in the Arctic,
which is warming rapidly while storing vast amounts
of organic carbon that could potentially be released
as carbon dioxide and methane, adding a new greenhouse
gas source of unknown magnitude. Regional
scale methane emission estimates and functional relationships
between potential drivers and methane
fluxes are currently unavailable.
The Airborne Measurements of Methane Fluxes
(AIRMETH) campaigns are designed to quantitatively
and spatially explicitly address this question.
While ground-based eddy covariance (EC) measurements
provide continuous in-situ observations of the
surface-atmosphere exchange of energy and matter,
they are rare in the Arctic permafrost zone and site
selection is bound by logistical constraints among others.
Consequently, these observations cover only small
areas that are not necessarily representative of the
region of interest. Airborne measurements can overcome
this limitation by covering distances of hundreds
of kilometers over time periods of a few hours.
During the AIRMETH-2012 campaign aboard the
research aircraft POLAR 5 we measured turbulent
exchange fluxes of energy and methane along thousands
of kilometers covering the North Slope of Alaska.
Time-frequency (wavelet) analysis, footprint modeling,
and machine learning techniques are used to extract
spatially resolved turbulence statistics and fluxes, spatially
resolved contributions of land cover and biophysical
surface properties to each flux observation, as well
as regionally valid functional relationships between
environmental drivers and observed fluxes that can
explain spatial flux patterns and – if available in temporal
resolution – allow for spatio-temporal scaling of
the observations.
Here we present a 100 m resolution gridded methane
flux map for the North Slope of Alaska, covering
about 90.000 km2. We show that surface properties
like elevation, temperature, and NDVI along with meteorological
drivers such as shortwave radiation, water
vapor mixing ratio, and horizontal wind speed are
sufficient to explain and project the measured fluxes.
The median methane flux for the campaign period
(end of June/beginning of July) was 19.4 mg m−2 d−1
after excluding all values with 30 % standard error.
The largest fluxes were observed along the coast and
in the Arctic coastal plain, decreasing towards the
Brooks Range.
