Thursday, 18 July 2002
Using the inverse method to obtain area averaged turbulent fluxes from airborne measurements at one low altitude
During LITFASS98 airborne measurements from the helicopter-borne
turbulence probe Helipod and from the research aircraft Dornier 128
were taken to determine area-averaged vertical turbulent fluxes.
For LITFASS98 a square-shaped relatively small flight pattern of
10 km x 10 km for the Helipod and 15 km x 15 km for the Dornier
aircraft was flown at three altitudes above heterogenous ground.
The lowest flight path was at 140 m altitude for the
Helipod and at 245 m for the Dornier aircraft.
The main goal was to determine vertical latent and sensible heat fluxes
through inverse modeling using only measurements form the lowest
flight path. The inverse method uses appropriate model parameters that
are based on theoretical assumptions to fit measured data.
The output of the inverse model provides the gradient
and the temporal development of the mean potential
temperature and of the humidity. Inserting the gradients into the
budget equation and using linear extrapolation to the ground
values for the turbulent fluxes can be obtained.
In a second step turbulent fluxes from inverse modeling were compared
to turbulent fluxes retrieved with an ordinary method based on
measurements on three heights and on ground measurements.
It appears that using the inverse modeling method has the advantage of
not needing the approximation of a linear extrapolation for the complete
boundary layer as it is necessary for retrieving turbulent fluxes from
airborne measurements in three heights. Also the inverse modeling
method does not require further measurements (e.g. ground or mast
measurements). Simply one square in low altitude has to be flown.
In future the approach to use inverse modeling will be extended and
improved on further airborne measurements. Future aims include the
possibility to be able to obtain fluxes of momentum as well as
horizontal fluxes of latent and sensible heat.
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