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| CASE:Helsinki Test Bed | |
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Helsinki Testbed concentrates on small-scale weather observation and forecast networks in high-latitude conditions.
The Finnish Meteorological Institute, Vaisala and partners have established a particularly dense weather observation network in the Greater Helsinki area in Finland. The Helsinki Testbed promotes the measurement and understanding of fine-scale weather phenomena, i.e. local weather. Of special interest are weather events that take place within a 50-kilometer (30-mile) region, and last up to a few hours.
The Helsinki Testbed helps in developing and testing new instruments and methods for weather observation. The activities concentrate on specific measurement campaigns, organized around chosen research themes. The campaigns utilize an already existing, approximately 150 x 150 kilometer (93 x 93 mile) wide observation network, which has been enhanced with a variety of new weather observation equipment from Vaisala.
Wet World Championships in Athletics
The first measurement campaign focused on gathering data for short-term forecasts. It was timed to coincide with the 10th World Championships in Athletics, held in Helsinki in August 2005. Several additional weather observation sites had been installed, for example along the marathon route and at the Olympic Stadium, in cooperation with the Finnish Athletics Federation and the City of Helsinki.
Unluckily for the athletes and spectators, the skies opened up for the big event. According to the Testbed measurement data, it rained a total of 38 mm, or 1.5 million liters, inside the Olympic Stadium during the day and evening events. Similar brief heavy rains occur in Finland only once every ten years on average.
Focus on precipitation types
The November measurement campaign focused on different precipitation types. Rain, snow and their mixture were measured in 350 measurement points with different methods and equipment.
When the temperature is close to 0 °C, precipitation can take different forms even within short distances: for instance, it can be snowing in one part of Helsinki while it is raining in another. Precipitation type depends on the temperature in the upper atmosphere, as well as on the temperature and humidity close to the ground. This is why it can snow in dry weather even in +4 °C. Supercooled liquid precipitation can occur in freezing temperatures if the upper atmospheric layers produce such conditions. Rain in freezing temperatures is particularly troublesome for road traffic and aviation, as the water quickly freezes as it hits a cold surface, such as a highway.
In November, the temperature in the Greater Helsinki area is often close to 0 °C, and precipitation falls as rain or snow, or mixture of both. The relatively great differences in temperature at sea and in inland areas cause significant differences in the type of precipitation that occurs in different areas within Greater Helsinki.
Under scrutiny: stable boundary layer
The January-February 2006 campaign investigated the atmospheric layer closest to the ground, particularly the lowest few hundred meters. In winter conditions, this layer is mostly stable, meaning that the air closest to the ground does not mix with air in the upper layers. Typically in these conditions, temperature rises vertically from the ground up. This phenomenon is called temperature inversion.
Moderate and cold temperature inversions close to the ground have a strong effect on the spreading of air impurities and radioactive particles. During inversions, the impurities concentrate close to the ground, thus deteriorating air quality at times.
The January-February campaign of the Testbed project enabled the versatile measurement and study of the strength, height and development of temperature inversions in the Greater Helsinki area.
Two more confirmed campaigns remain after this - one on sea breeze in May 2006, and the other on convection in August 2006.
Author: Marikka Metso, Vaisala, Helsinki, Finland
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