Finnish Meteorological Institute Vaisala

Helsinki Testbed second measurement period concentrates on precipitation type

2.11.2005

Finnish Meteorological Institute and Vaisala Oyj with partners are starting second measurement campaign in the project, in which very dense meteorological observation network is used. The campaigns bring information on small scale weather phenomena for research purposes and demonstrate technology integration. Measurements in November focus in precipitation type studies

Rain type is often obvious. In freezing temperatures well below zero, precipitation is in solid phase (snow). In midsummer, precipitation is entirely in liquid form. When temperature is near to zero degrees of Celsius, type can change considerably in a short distance. For example, it may snow in Helsinki Malmi while in Market square rains water at the same time. Rain type does not depend merely on temperature near ground: in dry air it can snow even in +4 Centigrades. In freezing temperatures it may rain water if conditions are favorable in upper air layers. In November, temperature is often near zero and rain is occasionally got as liquid water, occasionally in solid phase and sometimes as combination of these (sleet). In late autumn, comparitively large temperature differences between the sea and inland may cause significant differences in rain type in greater Helsinki area.

Freezing rain is especially problematic in road and air traffic operations. In this case liquid water freezes immediately when it faces cold surface. Freezing of a wing changes flying properties of an air plane. In addition to aviation, important users of rain type information are road and real estate maintenance authorities and companies, as well as citizens exposed to slipperiness injuries.

In November campaign rain type is studied with many different methods. Temperature and moisture are measured with dense observation network. Radio acoustic sounding system has been installed at Malmi airport and other frequent radio soundings give temperature information in upper air layers. Falling rain particles are also measured with a weather radar specifically developed for this purpose.

Observations of the experiment can be seen during November on the Internet: http://testbed.fmi.fi.

Further information:
Researcher Jani Poutiainen, tel. (09) 1929 4140, Jani.Poutiainen@fmi.fi