Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Measured Drop Size Distributions: Differences over Land and Sea
Marco Clemens and Karl Bumke
Leibniz-Institute of Marine Science at the University of Kiel IFM-GEOMAR, Düsternbrooker Weg 20, 24105 Kiel, Germany
mclemens@ifm-geomar.de
1. Introduction
Weather radar technology allows to get detailed information
about actual precipitation fields. For the BALTEX area
weather radars are not only used over land. NORDRAD, the
Nordic weather radar network, is also applied over the Baltic
Sea for the monitoring of precipitation. The necessary radar
nowcasting algorithms lack the knowledge of the relation
between radar reflectivity Z and rain rate R, which strongly
depends on drop size distributions.
A study of Strümpel (2001) shows that correlation lengths of
precipitation fields derived from radar measurements change
significantly at the transition zone from land to sea. Subject
of the present study is whether changes in correlation
lengths are caused by differences in drop size spectra over
land and sea. That implies an investigation of the properties
of different instruments which are suitable for drop size
spectra and precipitation measurements.
2. Optical disdrometer
The optical disdrometer (Großklaus et al., 1998) uses the
principle of light extinction by rain drops passing an
illuminated cylindrical volume of 22 mm in diameter and
120 mm length. This sensitive volume is kept perpendicular
to the local wind direction by a wind vane. The size and the
residence time within the sensitive volume of each drop is
measured separately. That allows to estimate drop size
distributions and, when the falling velocities of the drops are
known, to calculate precipitation rates. Both, drop size
spectra and precipitation rates, are corrected for fringe
effects caused by drops just grazing the sensitive volume of
the optical disdrometer and for coincidences. Due to its
construction the optical disdrometer is also particularly
suitable for measurements under high wind conditions.
3. Comparison to an impact disdrometer
Simultaneous measurements of the IfM optical disdrometer
(OD) with other instruments like the Joss-Waldvogel (JW)
disdrometer (Joss and Waldvogel, 1967) and the Micro-
Rain-Radar (MRR), a vertical looking device (Peters et al.,
2002), were performed at several sites within the frame of
APOLAS (accurate areal precipitation measurements over
land and sea) in DEKLIM.
Figure 1 gives a comparison of precipitation rates measured
by the OD and the JW. The precipitation rates, based on 1
minute measurements, are highly correlated. In contrast
there are large differences in spectral rain rates (Figure 2).
That might be due to differences in the spectral resolution of
both instruments. The JW sorts drops in 20 size intervals
ranging from 0.3 to 5.5 mm in diameter with a non-uniform
width ranging from 0.1 up to 0.5 mm diameter increasing
with drop size. The OD counts drops in 121 intervals of
diameter, ranging from 0.37 to 6.37 mm in constant intervals
of 0.05 mm. Since smallest resolvable drops are of the same
order, this cannot give an explanation for the missing small
drops in the JW measurements. But Tokay et al. (2001) have
also reported that the JW underreported small drops. Further
MRR measurements give significantly more small drops
than the JW. Another feature in the JW spectra (Figure 2)
is a gap between 1.3 and 2.0 mm in diameter. This
indicates that the JW possibly mismatches drops to their
correct size range. These are obviously not caused by the
limited spectral resolution of the JW. This is supported by
Figure 3, which shows for the same situation the spectral
rain rates for the optical disdrometer artificially reduced in
spectral resolution to make it comparable to the JW.
Figure 1. Comparison of 1 minute measurements of
precipitation rates in mm h -1 of the optical disdrometer
with measurements of a Joss-Waldvogel disdrometer in
Westermarkelsdorf in October 2002. The line gives the 1:1
relation. The correlation coefficient is 0.98, mean wind
speed is about 2ms -1 , and number of measurements is 707.
Figure 2. Spectral rain rates for measurements in
Westermarkelsdorf with a JW (dashed line) and OD (full
line) for precipitation rates ranging from 5 to 10mmh -1 .
Spectral rain rates are normalized to classes of drop
diameter of 1 mm width.
Additionally the influence of the wind speed on the
measurements with the JW was investigated. Figures 4(a)
and (b) depict a comparison between the OD and the JW.