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improves (scattering gets smaller). Hence, any reflectivity-to-visibility -equations depend
heavily on particle size distribution.
Rasmussen and Cole (2002) have given to visibility the equation Vis = k / ND
2,
where k
is a constant related to snow type. Having the two equations available, it would be
tempting to “just solve the k” and get a reflectivity - visibility equation. However, the
“constant” k can get plethora of values depending on the crystal type, the degree of
riming, the degree of aggregation, and the degree of wetness of the crystals. Rasmussen
et al (1999) derived ratios of visibility and liquid equivalent snowfall rate for 27 crystal
types and two aggregate types. In their study, typical variations in visibility for a given
liquid equivalent snowfall rate ranged from a factor of 3 to a factor of 10, depending on
the storm.
After this, the next attempt would be try to “calibrate” the factor k for each storm.
However, Rasmussen et al. (1999) also noted that k has a wide degree of scatter also
during a given storm. As we know from other studies, the type of snow crystals
depends on temperature and humidity it has experienced during its growth time.
Snowstorms are often related to weather situations (such as warm fronts) with strong
gradients of temperature and humidity. Hence, changes of crystal type during a storm
are natural.
Another factor making comparison of reflectivity and visibility is the illumination. In same
snowfall intensity, visibility in night (how far can you see a light source) can be twice as
good as in daylight.
8. Conclusion
The four main properties a radar meteorologist should remember about snow are:
It falls from shallow clouds, so you can't see it from far.
It falls slowly, so it may advect after we measure it.
It has different scattering properties from rain, so your precipitation estimates may be
inaccurate if ZR relations are applied for snowfall
Snowflakes come in many sizes and shapes, and their scattering properties may vary, so
more information may be acquired using dual polarization.
9. Acknowledgment
The author wishes to thank Aulikki Lehkonen for finding the illustrative weather situations
for examples, and Tuomo Lauri and Pekka Rossi for their constructive comments to the
manuscript.
10. References
Barthazy E. & Schefold, R. (2006). Fall velocity of snowflakes of different riming degree and
crystal types.
Atm. Res.,
391-398
