Diploma Thesis
Messtechnische und messmethodische Einflussfaktoren bei der Schließung der Energiebilanz am Erdboden
Claudia Liebethal (11/2000-04/2001)
Support: Thomas Foken
(Claudia Bruckmeier)
This thesis investigates the effects of correction
procedures for flux measurements (dome- and f-correction, Moore-correction
and Webb-correction) on the closure of the energy bal-ance near the ground.
Other reasons for the non-closure of the energy balance are not dis-cussed
(e.g. heat storage in soil). This analysis is based on data, which were
recorded dur-ing the Energy Balance Experiment EBEX-2000. All data had
to pass a selection procedure. It had to fulfill quality criteria as well
as meteorological criteria. The experi-ment EBEX-2000 was carried out under
special meteorological conditions. These include high values of radiation,
temperature and humidity.
The dome- and the f-correction eliminate the influence
of the silicon dome of pyrgeome-ters and the longwave part of solar radiation
on measurements of the downwelling long-wave radiation. These corrections
were able to lower the residuum of the energy balance equation by about
7 % with our data set. This rather high value is due to the intense radia-tion
during EBEX-2000. The Moore-correction, which has to be applied to sensible
and latent heat flux, compen-sates spectral losses. Spectral losses are
caused by time constants of sensors, sensor design and arrangement and
by data handling. The Moore-correction is carried out by parame-trizing
these effects as well as the theoretical shapes of the energy spectra.
It should be assured that the spectral model used for the parametrization
accords with the actual mete-orological conditions. The Moore-correction
explained about 10 % of the residuum (for EBEX-data set). Sensor separation
proved to have the highest influence on spectral losses. The Webb-correction
considers an oversimplified calculation of constituent fluxes, which are
often equated with the covariance of the vertical wind velocity and the
partial density of the constituent. This procedure neglects the influence
of relevant terms in the exact equation for constituent fluxes. Different
approaches of the Webb-correction produce nearly identical correction values
for the data set recorded during EBEX-2000. They all reduce the residuum
by less than 3 %. Nevertheless, regarding other constituents (e.g. CO2
or O3), the Webb-correction may be quite significant. Combining all of
the correction procedures mentioned above rises the energy balance clo-sure
from 64 % to 72 %. This corresponds to a reduction of the residuum by 20
%. Conse-quently, correction procedures can be seen as a very important
step towards energy balance closure.