Geoscience Reference
In-Depth Information
Radar estimates of ice water content of crystals and aggregates are greatly complicated by
the multitude of crystal sizes and shapes, various crystal and aggregate densities, and
dielectric constants, among others. Of all the snow types, the determination of the amount of
wet aggregates is probably the most difficult (Straka, 2005).
For liquid precipitation, the classical ZR relation was published by Marshall and Palmer
1948. For snow, similar classical paper is probably that of Sekhon and Srivastava (1970).
However, when dropsize distributions are known to vary a lot, snow particle distributions
vary even more. Applying correction for vertical profile of reflectivity before the ZR relation
is crucial, but does not eliminate all uncertainties.
7.2 Nowcasting snow
The simplest application of radar images for nowcasting is to display a time series as an
animation, and visually follow its speed and direction of movement. Second level of
complication is to estimate the future movement with some vector field, which can be
derived from observed movement, NWP, or even Doppler velocity field (note this is not
recommended but some people do it). Compared to summertime convective precipitation,
snow has some advantages and some disadvantages in this respect. Because snow is seldom
related to convection, it has less diurnal variation, and hence frontal snowstorms can
sometimes be tracked and extrapolated for several hours with fairly good accuracy. On the
other hand, snowstorms tend to be shallow, and hence the geometrical factors can cause
error in speed estimates, and even causes of total miss (snowstorms hiding under the lowest
radar measurement).
Because snowflakes fall slowly, they can advect remarkable distances after the radar
measurement. If we measure at height of 800 m, and the snowflakes fall 1 m/s, they reach
the ground 800 seconds later, and if wind blows 10 m/s, the location can be 8 km downwind
from the radar measurement. From height of 1800 m, the flakes fall for half an hour, and
from height of 4 km more than an hour, and for a distance in order of 40 to 60 km. This
affects all studies comparing radar measurements to “ground truth”, and it can be annoying
for nowcasting, too. On the other hand, for an optimist it is a source of information:
basically, we have already measured the snowflakes which will fall e.g. to the runway half
an hour later. Lauri et al. (2012) have discussed the effect of advection in snowfall
measurement.
7.3 Visibility in snow
Aviation meteorology uses abbreviation LVP (low visibility procedure) and we often read
this as “fog and stratus”). However, even snowfall reduces visibility in significant amounts.
Unlike in fog, the visibility in snowfall often fluctuates rapidly and significantly, and hence
use of radar data to aid nowcasting would be beneficial.
Visibility is related to scattering of visible light, radar reflectivity is related to scattering of
microwaves. In case of particles in typical sizes of snowflakes and snow crystals, these two
behave differently: if the amount of snow in air stays same, but crystals join to larger
aggregates, radar reflectivity grows (following the ND
6
equation) while the optical visibility
