| A detailed knowledge of rain drop size densities is an essential presumption with respect to
remote sensing of precipitation. Since maritime and continental aerosol is significantly
different yielding to differences in cloud drop size densities, maritime and continental rain
drop size densities may be different, too. In fact only a little is known about differences in
rain drop size densities between land and sea due to a lack of suitable data over the sea. To fill
in this gap measurements were performed during the recent 10 years at different locations
in Germany and on board of research vessels over the Baltic Sea, the North Sea,
Atlantic, Indian, and Pacific Ocean. Measurements were done by using an optical
disdrometer (ODM 470, Großklaus et al., 1998), which is designed especially to perform
precipitation measurements on moving ships and under high wind speeds. Temporal
resolution of measurements is generally 1 minute, total number of time series is about
220000.
To investigate differences in drop size densities over land and over sea measurements
have been divided into four classes on the basis of prevailing continental or maritime
influence: land measurements, coastal measurements, measurements in areas of
semi-enclosed seas, and open sea measurements. In general differences in drop size densities
are small between different areas. A Kolmogoroff Smirnoff test does not give any significant
difference between drop size densities over land, coastal areas, semi-enclosed, and open seas
at an error rate of 5%. Thus, it can be concluded that there are no systematic differences
between maritime and continental drop size densities. The best fit of drop size densities is an
exponential decay curve,
N(D ) = 6510m -3mm -1mm0.14h- 0.14-
R-0.14-
exp(- 4.4mm0.25h-0.25-
R- 0.25-
D mm -1),
it is estimated by using the method of least squares. N(D) is the drop size density normalized
by the resolution of the optical disdrometer, D the diameter of rain drops in mm, and R the
precipitation rate in mmh-1.The precipitation rate dependent factor in the exponential is
similar to that given by Marshall and Palmer (1948). The intercept parameter, in the original
Marshall Palmer formulation not depending on the rain rate, is also of the same
order.
A number of recent publications have shown that drop size densities of convective and
stratiform rain show significant differences, too. Several procedures have been developed in
the past to decide, whether precipitation is of stratiform or convective character. Here two of
them are used, temporal variability in rain rates and additional information by weather radar
data for the South Western Baltic Sea.
Main result is that differences in drop size densities depend on the integration time of
measurements. For integration times of 1 minute no significant differences can be detected
at an error rate of 5%, while integration times of 10 minutes yield to significant
differences between drop size densities of prevailing stratiform and convective
rain.
References:
Großklaus, M., K.Uhlig, and L.Hasse, 1998: An Optical Disdrometer for Use in
High Wind Speeds, Journal of Atmospheric and Oceanic Technology Vol. 15(4).
1051–1059
Marshall, J.S. and W.M.K.Palmer, 1948: The distribution of rain drops with size, J.
Meteor., 5, 165-166 |