Example: Melting layers (bright bands)

Temperature/radar image/hydrometeors in melting layer

When snow melts into rain, the region where this melting occurs often has a stronger reflectivity than snow above or rain below; this region was hence given the name of "bright band". The bright band (shown above as a layer of warmer colors) occurs just below the height of the 0°C level (temperature measurements with height are shown on the left). On the right side of the image are measurements of the shape of hydrometeors observed: below the bright band, raindrops (the black disks) are present; above, snowflakes (the weird black shapes) can be seen; within the bright band, a mixture of these two are observed.

The bright band is caused by the melting of snow into rain: snow falls slowly (so there is a lot of them) but they are made of ice that reflects less than water. When they start melting, snowflakes get covered by water and look like large raindrops so they reflect a lot. When melting finishes, the soon-to-be raindrop shrinks and speed up; their size and their number per unit volume diminishes and the reflectivity decreases.

Bright band imagesAt McGill, considerable research on the bright band has been done because it attenuates radar waves significantly, which means it would also attenuate microwave communication signal significantly too: when comparing measurements made by the profiler (unaffected by precipitation) and the VertiX radar, the measurements in snow (behind the bright band) can be much weaker for the shorter wavelength VertiX (VPR) than for the profiler when rainfall is moderate or strong (03:45-04:00 on the enlarged image).

In general, the snowflakes melt in one layer and then it is over. On occasions however, the melting is not complete before the melting snowflakes reenter a colder region and refreeze again; the final melting will then occur at a lower level. When this situation occurs, two or even sometime three bright bands can be observed. In the example on the right, the situation is so complex that it defies any simple explanation.

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