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Unique weather radar to investigate snowfall

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20.03.2006

The Department of Physical Sciences at the University of Helsinki has acquired a state-of-the-art polarimetric weather radar. The new radar is reserved exclusively for research. Its most important meteorological research target is the physics of rain clouds, and scientists intend to focus on snow and sleet in particular. Snowfall and its polarimetric measurements have hardly been studied anywhere else in the world, although in the Finnish conditions, for instance, snowfall is one of the key weather elements.

The first test measurements of the prototype radar built for the University’s radar laboratory provided good evidence for the potential of a polarisation weather radar. The largest road traffic catastrophe in the Helsinki metropolitan area took place on 17 March 2005, with several multiple pile-ups on all Helsinki. The new radar was just undergoing test runs and it immediately revealed the meteorological factors which led to the accidents.

Radar images showed that the road conditions before the accident were dry and cold with only slight snowfall. There was, however, supercooled water in the air, which made the road surfaces slippery immediately prior to the accidents. At the time of the accidents, a narrow zone of heavy snowfall arrived from the south, suddenly reducing visibility. In places, this was followed by a larger area of snowfall with a high content of supercooled water.

Radar signals distinguish rain from snow

As the above example shows, the polarisation weather radar can reveal meteorological factors behind accidents. There is no radar as efficient in every aspect anywhere else in the world. Its exceptional features include an antenna with top-class properties, a versatile and powerful transmitter and an ability to measure two perpendicular polarisations.

The radar works by simultaneously transmitting two differently polarised microwaves to the atmosphere and receiving their reflected signals from the rain. The slight differences in the weak reflected signals with different polarisations enable scientists to deduce whether the precipitation falls as water, snow, hail or sleet, and gives information on the quantity and quality of the various precipitation particles (water drops, ice crystals, snow flakes or hailstones) over distances up to 200 km at all levels of the atmosphere. This is not possible using conventional weather radars, which only show where there is precipitation without being able to distinguish between rain or snow.

The radar’s transmitter is designed so that it can transmit either ordinary microwave pulses or what is called coded pulses. This makes it possible to further improve the radar’s measuring capacity with the help of new mathematical processing methods of radar signals. Indeed, research at the University is also being carried out to develop radar technology and further improve its measuring capacity.

The University’s radar was manufactured in Finland. The properties required from the radar were defined by Timo Puhakka, PhD, radar laboratory manager, and the radar was designed and built by Vaisala Oyj in co-operation with the radar laboratory.

The radar is located at the University’s Kumpula campus, on top of the Physicum building under a round dome, which acts as a landmark for the campus.

Radar meteorology has been one of the focal areas of meteorological research at the University of Helsinki for more than 35 years. The University’s radar laboratory boasts radar meteorological expertise of the highest level and it is widely known internationally.

Minna Meriläinen | Source: alphagalileo
Further information: www.helsinki.fi

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