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Developing a universal method for snow type classification is even more challenging than
finding a representative ZR relation: reliable surface observations of snowflake type are rare,
and if they are performed at longer distances from the radar, the snowflakes can change
between the radar measurement and the surface observation. Correction for vertical profile
of reflectivity is a standard procedure, but correction for vertical profile of snow type is still
strongly hypothetical.
6. Characteristic properties of typical snowfall situations
In general, precipitation events can be split to orographic, frontal and convective
precipitation. Especially snowfall is often related to warm fronts and lake effect induced
convection.
6.1 Warm fronts
Much of snowfall is related to frontal systems of extratropical systems. In their analysis and
forecasting, the value of weather radar data lies primarily in the mesoscale structure: detection
of the mesoscale bands of heavy snowfall is needed for accurate short term forecasting. The
banded structure leads to rapid changes in visibility, and areal differences of accumulated
snowfall. Their dynamical structure is complicated, related to negative equivalent potential
vorticity (EPV) mainly associated with conditional symmetric instability (CSI), and not always
perfectly forecasted by numerical weather prediction models. Hence, identification and
extrapolation of movement of these bands using a radar can improve short-range forecasts of
extreme events significantly (Nicosia and Grumm, 1999).
Snowfall from warm fronts is also a challenge for a radar meteorologist: forgetting the three-
dimensional structure of the frontal system can lead to embarrassing misinterpretation.
In satellite images, we can see the leading edge of frontal system (“warm front shield”) and
educated meteorologists already know, that arrival of this edge does not mean onset of
precipitation. In Fig. 6 the shield at 2 km extends 70 km ahead the surface precipitation. I
sincerely hope that everyone using different radar products remembers this, too: the leading
edge in products like TOPS, MAX, VIL or even medium-level CAPPI does not indicate the
precipitation on ground level. See Fig. 7.
The sloping edge of precipitation area can also be seen in PPIs. In upper panels of Fig. 8, the
gap in the centre of the image indicates area where radar beam was below the warm front
shield. The gap gets smaller when the surface front approaches the radar. Also, the gap is
not a circle but an oval, also indicating the slope of the cloud base.
Warm fronts are ideal for producing Doppler wind profiles (VAD and VVP), because the
wind field is usually uniform. In lower right panel of Fig. 8 we have a time series of VVP
wind and reflectivity. In this case, the wind shear related to the warm advection is not very
strong, and it is hard to distinguish it from wind shear related to friction in the boundary
layer. Sharper turning of wind is of interest to aviation weather service. We can also see the
shield of overhanging precipitation with approaching front (05-06 UTC) from “+”-signs
indicating missing wind barbs. Warm advection can also be seen indirectly: part of the
increase of reflectivity at low altitudes around 08 UTC is probably related to temperatures
rising to near zero, and snowflakes growing larger.
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