WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

- 57 -

The distrometer (see Fig. 12)

Drop size spectra in PEP are mea:,ured lid th the distrometer developed by Joss and
Waldvogel (1967). The transducer is exposed to rain and produces an electric pulse for each
raindrop impacting on a sensitive surfi3ce. The smallest detectable drop has a diameter of
0.3 mm and the largest that does not si3turatE! the instrument has a diameter of < 5mm. The
corresponding output pulse heights are 0.3 mV and 10 V, respectively. A simple and fixed
relation exists between the drop diameters and the output pulse amplitude. The sampling
area F of the instrument is 50 cm2. ThB maximum counting rate is several hundred drops per
second. A 20-channel pulse-height analyser is used to separate the pulses of the spectro-
meter into 20 different drop size clasBes. The last channel contains all drops having dia-
meters> 5mm. The drop size classes arB chosen for the following reasons: ..! the diameter
intervals must have a minimal width to give a statistically meaningful number of drops
during a sampling time of about 1 min. (Joss and Waldvogel, 1969), ~ the ratio nOlO should
be about equal for all classes, and ~ the diameter intervals should be described with sim-
ple decimal numbers.

r-f--J- -}-1
I SPIS/f/ve I
J swiaCL!
I l~~
loonput pu~
I feeclbad ( J\..
,I
I I
1 N I
L____!::.~~er I

puncheol
ptJpO" tape

. .: ::_ .:.~: 0'. : ~ ~

paper tope ptmeh

proce.ssor

QnO/Y.5er

pulse heigh! analy.sis
Wlt!:rnDry, f,"";"'g

signal [,Hering
CD"'press'OI?

Fig. 12: Schematic diagram of the electromechanlcal ralnarop spect:romet:sr.

Each pulse arriving at the anlyser input is allocated to one of the 20 channels. The number
of pulses in each channel is added up and stored in a memory contained in the analyser. Every
minute the number ni of drops in each size interval is recorded in the ASCII-code on punched
paper tape or other digital processing 13quipmemt.

Example

Suppose ni = 1 drop per unit time t (= 'Is) is falling on the sampling area F (50 cm2) at its
terminal velocity v and produces a pulse in ch,mnel No.1 which registers drops with diameters
between 0.3 and 0.4 mm. The mean diameter is therefore Di = 0.35 mm and ~Di= 0.1 mm. The num-
ber of drops per m3 and millimeter N(OiJ can be calculated by

N(D ) ni (- 1429 -3 -1 f' . ~
i FotovonDi' - m rnm , 'or 1 = -'I'

where v(Oi) = 1.4 ms-1 has been used. Mason, 1971).