Geoscience Reference
In-Depth Information
For radar-based nowcasting applications, lake effect snow is a challenge firstly, because the
snow storms do not move, thus motion vectors can be misleading, and secondly because
their shallow and intense nature can cause beam overshooting problems..
7. Operational applications
Snowflakes are beautiful and interesting, but most people who buy radars want also to do
something useful with the data. The most frequently asked questions are when, where and
how much. These have been solved with more and less advanced accuracies over the years.
Advanced questions to be still researched are related to properties of snow, most of all its
density.
7.1 Accumulated snowfall, Z/S
When we talk of precipitation, we often think about rainfall, and ignore snow. There are
two main approaches to accumulated snow: 1) snow water equivalent (SWE): how large
is the mass of snow per unit area and 2) the (increase of) thickness of snow layer (TSL).
SWE is important for hydrological applications, and it is also usually the parameters
measured at surface station rain gauges (in manual gauges, the snow is melted and
volume of resulting water is measured). SWE has also applications in estimates of
snowload of buildings and tree crowns. Thickness of snow layer has applications in road
maintenance, biology and recreational activities, and it is a tricky thing to estimate for
everyone, not just radar meteorologists. A bulk equation TSL=10*SWE (both TSL and
SWE expressed in mm), is often used, even though we all know that the density of snow
on ground varies a lot. Matters are further complicated if we try to accumulate TSL over
a longer period, because snow on ground is changing shape, density and even location
(by blowing snow).
The radar equation (see Zrnic, this volume, for details) includes the parameter|K|,
dielectricity of scattering particles, which has different values for ice and water. In
operational signal processing, this is assumed to be always the water-value, so we should
call the measured parameter Z
e
(where the “e” stands for “equivalent”). The error caused by
assuming the same dielectricity is compensated in using a different ZS relation for snow,
and including the effect of dielectiricity there.
The density of snow crystals and aggregates varies as a function of structure from 50 to 900
kg m
3
, with higher values expected for solid ice structures and wetted particles. The size
distributions of ice crystals and snow aggregates can be represented by exponential and
gamma functions, and the total number of concentrations is on the order of 1-10
4
m
-3
for
aggregates, 10-10
9
m
3
for individual crystals at colder temperatures (T < -20 ºC), and often
as high as 10
4
m
-3
at warmer temperatures (Pruppacher & Klett, 1996). The diameter of large
crystals can be up to 1-5 mm, while the diameter of aggregates can grow to 20-50 mm,
occasionally even more. The shapes of aggregates vary from approximately spherical to
extremely oblate, and the approximate shapes of crystals can vary from extreme prolates
and oblates to essentially spheres (Pruppacher & Klett, 1996). Most individual crystals tend
to fall with their largest dimension horizontally oriented unless there are pronounced
electric fields. Aggregates also can fall in a horizontally oriented manner or may tumble
(Straka, 2005).
