The HTB focuses on meteorological observations and forecasting directed towards meso-gamma-scale phenomena that are often too small to be detected adequately by traditional observing networks. The domain of the HTB covers much of southern Finland and the Gulf of Finland. Most of the new observation sites are located in an area of about 150 x 150 km that includes Helsinki. In particular, more than 40 communication masts, 60–100-m high, are equipped with new weather transmitters capable of measuring temperature, humidity, air pressure, rain, and wind speed and direction. Additionally, the number of radio soundings and ceilometer measurements has been increased, a wind profiler and dual-polarimetric radar have been installed, and data from four Doppler weather radars are available. Most of the data is freely available on the Internet, and the HTB is an open program, which means collaborative and independent measurements are encouraged. The HTB supports the development and testing of new observational instruments, systems and methods in concentrated field experiments.
One aspect of mesoscale meteorology that the HTB is well suited to study is fronts moving through southern Finland. Analysis of several fronts to date indicate nonclassical structures and evolutions that require explanation. For example, on 31 October 2007, the radar data and data from several instrumented towers indicated a structure reminiscent of a density current, although other characteristics of the front argue against this interpretation. For example, the front occurred in a region of near-continuous rain, as observed by the radar and by a collocated disdrometer and a precipitation occurrence sensor. Thus, evaporation into the cold pool is unlikely to have contributed toward the cold pool. Masted data can also be used to show that the cold pool was less than 1 km deep, with all the cold air underneath the melting layer. Consequently, this case illustrates one of the problems with inferring the dynamics of a front from its morphology. Unexplained near-surface prefrontal warming, seen in other fronts across the world, remains unexplained. This case, and others to be presented at the conference, are a precursor to a larger study of the interaction between fronts and the near-surface boundary layer to be conducted in southern Finland over the next few years.