The Helsinki Mesoscale Testbed

The Finnish Meteorological Institute and the Vaisala meteorological measurements company have established a new mesoscale observational network in Southern Finland. The Helsinki Testbed (HTB) program is designed to provide new information on observing systems and strategies, mesoscale weather phenomena, urban and regional modeling, and applications in a high-latitude coastal environment (60-61N, 24-26E). All four seasons are distinctly separated in this northern environment. Typically, the mean monthly temperature varies between -7°C and +17°C, annual precipitation is 600 mm, snow cover lasts for 100 days and the sea is frozen for two months. This paper describes the HTB program -- and the accomplishments to date -- in detail and also serves as an open invitation to researchers, operations groups, and users.

The HTB focuses on meteorological observations and forecasting directed towards meso-gamma-scale phenomena that typically last from a few minutes to several hours. These weather events are often too small to be detected adequately by traditional observing networks. In coastal Finland, such weather events include temperature inversions, sea breeze, fog and low stratus, snow bands, urban heat island and convective storms (e.g "lake-effect" blizzards). These and related phenomena like lightning and hail are often hazardous and cause substantial damage. For instance, fog causes considerable disruption of land, sea and air traffic. The evolution of the sea breeze plays an important role in the dispersion of atmospheric constituents. The Helsinki capital area offers a representative study region for urban air quality research and boundary-layer modeling, especially in stable nocturnal conditions, which are dominant in the area (especially in winter when stable stratification can exist throughout the day). The Helsinki Testbed also provides comprehensive data sets for many other research purposes including e.g. computerized nowcasting methods and mesoscale analyses, remote sensing of different precipitation types, road surface and related weather modeling, and development and verification of numerical weather prediction models. The program enables testing of scientific theories, new technologies and end-user applications. It also serves as an open test platform for extensive development projects, and it provides a framework for atmospheric information systems integration. While facilitating experimentation with complete systems, the risk is minimized with respect to the operating environment.

The Helsinki Testbed also supports the development and testing of new observational instruments, systems and methods. First live tests of the HTB took place in August 2005, and provided real-time support to the World Championships in Athletics. Measurements are being accomplished through five thematic, month-long, measurement campaigns. The last campaign is scheduled for August 2006. During these campaigns, the existing Finnish weather observation network will be supplemented with a large number of new in situ and remote sensing observations.

The area of interest 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. Each mast has at least two measurement heights, typically 2m and mast top. Additionally, the amount of radio soundings and ceilometer measurements has been increased, a wind profiler has been installed, and data from five satellites and five local weather radars are available; all data are combined in a data warehouse. In the course of the project, all measured and NWP model data will be made available on the Internet, see http://testbed.fmi.fi. The testbed is an open program, which means collaborative and independent measurements are encouraged.

All interested users are welcome to use the data or to implement additional observational, modeling or application tools. Guidance and data policies are provided at the HTB website.