Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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High Frequency Single Board Doppler Minisodar for Rain, Hail, Snow,
Graupel and Mixed Phase Precipitation Measurements
Shixuan Pang and Hartmut Graßl
Max-Planck-Institut für Meteorologie, Bundesstrasse 55, D-20146 Hamburg, Germany
As a main ground-based precipitation remote sensing
method the conventional microwave radars may suffer
from large uncertainties arising from an unknown vertical
wind and a significant difference of refractive indices
between ice and liquid water. The former causes a rain
rate observation error up to a factor of 2, while the latter
causes a huge uncertainty up to a factor of 8 for the case
of rainfall with hailstones (e.g. Wilson 1970: Lee 1988;
Gossard et al. 1990).
However, for acoustic precipitation remote sensing there
are no such problems, because:
(1) Acoustic refractive index is about 1000 times stronger
than that of microwaves, therefore, a high frequency
Doppler sodar can measure both the turbulence spectrum
(vertical mean wind measurement) and precipitation
spectrum simultaneously, the vertical wind and spectrum
broadening effects can thus be removed.
(2) Acoustic refractive indices of ice and water are almost
identical, whereas for microwave the one of ice is only
about 20% of that for water. Therefore, the rain sodar
retrieval can be directly applied to the event of rainfall
with hailstones without any necessity to distinguish rain
or hail, dry or wet etc. However, if hailstones are
predominant, which can be easily identified by the
location of the Doppler spectrum peak, the ice density of
0.9g/cm3 has to be included in the retrieval. For large
hailstones the Mie calculation has to be applied to the
retrieval, in fact, we use the Rayleigh-Gans
approximation instead.
(3) Acoustic wave attenuation in ice and water is very
small and can be neglected, while the microwave
attenuations in ice and water are large and very different
for e.g. X-band wavelength and higher frequency.
Moreover, they show also a strong temperature
dependence. Obviously, the difficulty arises from the
strong dependence of microwave wave attenuation on the
precipitation intensity.
However, major limitations and difficulties for a high
frequency sodar are (1) the strong acoustic attenuation by
air, which significantly limits the sounding height, (2)
sensitive to ambient acoustic noise and mechanical
vibration and (3) acoustic disturbance for the neighbor.
Based on DSP (Digital Signal Processing) technique we
have developed a single board high frequency Doppler
minisodar with high reliability, accuracy, resolution and
flexibility but at very low cost. To develop a new sodar
using software as much as possible instead of hardware is
the main guide for our design. Since 25 Oct 2002 a nonattended
automatic single board minisodar is continually
operating at Westermarkelsdorf Weather Station in north
Germany.
During stratiform rain events the minisodar observation
shows good agreement with a Joss-Waldvogel
disdrometer and an optical rain gauge developed by the
Institute for Marine Science at Kiel University. However,
for heavy showers the minisodar observes higher rain
rates than all other sensors. The comparison between
drop size distributions measured by the minisodar and J-
W disdrometer clearly indicates that such an
underestimate by J-W disdrometer is probably due to the
loss of small drops and the neglect of hailstones.
The snow and graupel acoustic retrievals are quite
different from the one of rain and hail. We are developing
a statistical method, at first, to identify the kind of
precipitation by means of a pattern analysis of measured
spectrum, then to retrieve with different algorithms the
corresponding particle size distribution, and precipitation
rate.
The continually non-attended automatic in situ operation
of the minisodar indicates its capability for all kinds of
precipitation measurements. The high frequency
minisodar could become a routine device for all kinds of
precipitation ground truth measurements.
Reference
Wilson, J.W., Integration of radar and raingage data for
improved rainfall measurement, J. Appl. Meteorol.,
Vol.9, 489-497, 1970
Lee, A.C.L., The influence of vertical air velocity on the
remote microwave measurement of rain,
J. Atmos. Oceanic Technol., Vol.5, 727-735, 1988
Gossard, E.E., R.G. Strauch and R.R. Rogers, Evolution
of dropsize distributions in liquid precipitation
observed by ground based Doppler radar, J. Atmos.
Oceanic Technol., Vol.7, 815-828, 1990