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| Titel |
The influence of warm-season precipitation on the diel cycle of the surface energy balance and carbon dioxide at a Colorado subalpine forest site |
| VerfasserIn |
S. P. Burns, P. D. Blanken, A. A. Turnipseed, J. Hu, R. K. Monson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 23 ; Nr. 12, no. 23 (2015-12-15), S.7349-7377 |
| Datensatznummer |
250118215
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| Publikation (Nr.) |
 copernicus.org/bg-12-7349-2015.pdf |
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| Zusammenfassung |
Precipitation changes the physical and biological characteristics of
an ecosystem. Using a precipitation-based conditional sampling
technique and a 14 year data set from a 25 m micrometeorological
tower in a high-elevation subalpine forest, we examined how
warm-season precipitation affected the above-canopy diel cycle of wind
and turbulence, net radiation Rnet, ecosystem eddy
covariance fluxes (sensible heat H, latent heat LE, and CO2 net
ecosystem exchange NEE) and vertical profiles of scalars (air
temperature Ta, specific humidity q, and CO2 dry
mole fraction χc). This analysis allowed us to
examine how precipitation modified these variables from hourly (i.e.,
the diel cycle) to multi-day time-scales (i.e., typical of a
weather-system frontal passage).
During mid-day we found the following: (i) even though precipitation caused mean
changes on the order of 50–70 % to Rnet, H, and
LE, the surface energy balance (SEB) was relatively
insensitive to precipitation with mid-day closure values ranging
between 90 and 110 %, and (ii) compared to a typical dry day, a day
following a rainy day was characterized by increased ecosystem uptake
of CO2 (NEE increased by ≈ 10 %), enhanced evaporative
cooling (mid-day LE increased by ≈ 30 W m−2),
and a smaller amount of sensible heat transfer (mid-day H decreased
by ≈ 70 W m−2). Based on the mean diel cycle, the
evaporative contribution to total evapotranspiration was, on average,
around 6 % in dry conditions and between 15 and 25 % in
partially wet conditions. Furthermore, increased LE lasted at least
18 h following a rain event. At night, even though precipitation
(and accompanying clouds) reduced the magnitude of Rnet,
LE increased from ≈ 10 to over 20 W m−2 due
to increased evaporation. Any effect of precipitation on the
nocturnal SEB closure and NEE was overshadowed by atmospheric
phenomena such as horizontal advection and decoupling that create
measurement difficulties. Above-canopy mean χc
during wet conditions was found to be about
2–3 μmol mol−1 larger than χc on dry
days. This difference was fairly constant over the full diel cycle
suggesting that it was due to synoptic weather patterns (different air
masses and/or effects of barometric pressure). Finally, the effect of
clouds on the timing and magnitude of daytime ecosystem fluxes is
described. |
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