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| Titel |
Investigating land-atmosphere coupling and convective triggering associated with the moistening of the northern North American Great Plains |
| VerfasserIn |
Tobias Gerken, Gabriel Bromley, Paul Stoy |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250147061
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| Publikation (Nr.) |
 EGU/EGU2017-11160.pdf |
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| Zusammenfassung |
| Parts of the North American northern Great Plains have undergone a 6 W m−2 decrease in
summertime radiative forcing. At the same time agricultural practices have shifted from
keeping fields fallow during the summer (“summer fallow”) towards no-till cropping
systems that increase summertime evapotranspiration and decrease soil carbon loss.
MERRA (Modern-Era Retrospective analysis for Research and Applications) for the
area near Fort Peck, Montana, (a FLUXNET site established in 2000) shows a
decrease of summertime (June-August) sensible heat fluxes ranging from -3.6 to
-8.5 W m−2 decade−1, which is associated with an increase of latent heat fluxes of similar
magnitude (5.2–9.1 W m−2 decade−1). While net radiation changed little, increasing
downward longwave radiation (2.2–4.6 W m−2 decade−1) due to greater cloud cover, was
mostly compensated by reduced solar irradiance. The result was a strong decrease of
summer Bowen ratios from 1.5–2 in 1980 to approximately 1–1.25 in 2015. At
the same time, atmospheric soundings have shown significant increases in both
convective available convective energy (CAPE) and convective inhibition (CIN)
for the same time span. Overall, these findings are consistent with the effects on
increased summertime evapotranspiration due to reduction in summer fallow that
should lead to smaller Bowen ratios and a larger build-up of moist static energy as
expressed in higher values of CAPE. In order to further investigate the impact of
the surface energy balance and flux partitioning on convective development and
local land-atmosphere coupling in the North American prairies, a 1-dimensional
mixed-layer model is used to compare the evolution of mixed-layer heights to the lifted
condensation level, a necessary but not sufficient condition for the occurrence of
convective precipitation. Using summertime eddy covariance data from Fort Peck
and atmospheric soundings from the nearby Glasgow airport, we establish that the
mixed-layer model adequately captures mixed-layer heights and timing of locally
triggered convection at the site. The model is then used to quantify the sensitivity of
mixing-layer height, CAPE and convective triggering potential, in response to changes in
surface flux partitioning between latent and sensible heat due to changes in soil
moisture and agricultural management. Results are used to establish the exact nature or
land-atmosphere coupling associated with moistening of the atmospheric boundary-layer and
increases in convective triggering and will contribute to disentangling local and
regional effects on trends in observed precipitation in the northern Great Plains. |
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