Geoscience Reference
In-Depth Information
Measuring snowfall with short wavelengths can bring us to the edge of assumption of the
radar equation for Rayleigh scattering: are the particles much smaller than the radar
wavelength ?
In this chapter, snow will be discussed from viewpoint of a radar meteorologist. Many
topics are relevant for operational weather service, others more for the researcher.
Increasing use of polarimetric radars is bringing new perspectives to measuring snow with
radars.
2. Vertical structure of snowfall
With the usual measuring geometry of a scanning weather radar we have to take into
account the vertical structure of precipitation. In warm weather, we measure rain near
ground, wet snow above it and dry snow on the top, as can be seen in Fig. 1. A typical
reflectivity structure is related to the temperature structure so that we have a maximum just
below 0 ºC isotherm. Above it, in the snowfall area, reflectivity decreases with an even
gradient of approximately 7.5 dBZ/km. This decrease is related to four factors:
at higher altitudes, it is colder and snow crystals are typically smaller in diameter
at higher altitudes, the absolute humidity is smaller so the mass of snow per cubic
kilometer of cloud is smaller there
crystals fall down while they grow, so older crystals which have had time to grow large
are more likely to be located at lower altitudes
near the cloud top there may be effects of partial beam overshooting
In the precipitation system of a warm front, the two first factors create also horizontal
gradients: the leading edge is in colder and drier air.
When the snowflakes melt, the surface gets wet first while the inner parts are still of dry
snow. The partially-melted, wet snowflakes have approximately the size and fallspeed of
snowflakes, but the dielectric properties of water surfaces. Hence the radar reflectivity peaks
in the melting layer, a phenomenon also known as the bright band. In the hands of an
inexperienced user of radar data, this could lead to an overestimation of precipitation
intensity. In a modern weather radar service, the overestimation is corrected using
knowledge of the vertical profile of reflectivity (Koistinen et al., 2003). Recently, Giangrande
et al. (2005) and Boodoo et al. (2010) have shown, that the parameters of dual-polarization
radars can be used effectively to follow the temporal and spatial variation of the melting
layer height and thickness. This is especially important in cold and temperate climates,
where much of precipitation is associated with fronts, because in frontal situations the
temperature gradients are sharp.
In Fig. 2 we see RHI and PPI images in a snowstorm in Finland 2 February 2010. Cloud tops
are observed between 6 and 8 km, and reflectivity is growing downwards from there. No
bright band is observed, as there is no melting. Temperatures in cloud tops are near -35..-40
ºC, at ground -5..-7 ºC (based on Tallinn and Jokioinen 00 UTC soundings). The effect of
vertical gradient is obvious in the RHI image, but the gradient in PPI is related to two
factors: the vertical gradient and the horizontal variation of intensity.
