Micrometeorological observations of a microburst in Southern Finland

Leena Järvi, Ari Juhani Punkka, David M. Schultz, Tuukka Petäjä, Harri Hohti, Janne Rinne, Toivo Pohja, Markku Kulmala, Pertti Hari, Timo Vesala

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    Abstract

    On the afternoon of 3 July 2004 in Hyytiälä (Juupajoki, Finland), convective cells produced a strong downburst causing forest damage. The SMEAR II field station, situated near the damage site, enabled a unique micrometeorological analysis of a microburst with differences above and inside the canopy. At the time of the event, a squall line associated with a cold front was crossing Hyytiälä with a reflectivity maximum in the middle of the squall line. A bow echo, rear-inflow notch, and probable mesovortex were observed in radar data. The bow echo moved west-north-west, and its apex travelled just north of Hyytiälä. The turbulence data were analysed at two locations above the forest canopy and at one location at sub-canopy. At 1412 EET (Eastern European Time, UTC+2), the horizontal and vertical wind speed increased and the wind veered, reflecting the arrival of a gust front. At the same time, the carbon dioxide concentration increased due to turbulent mixing, the temperature decreased due to cold air flow from aloft and aerosol particle concentration decreased due to rain scavenging. An increase in the number concentration of ultra-fine particles (<10 nm) was detected, supporting the new particle formation either from cloud outflow or due to rain. Five minutes after the gust front (1417 EET), strong horizontal and downward vertical wind speed gusts occurred with maxima of 22 and 15 ms-1, respectively, reflecting the microburst. The turbulence spectra before, during and after the event were consistent with traditional turbulence spectral theory. © Springer Science+Business Media B.V. 2007.
    Original languageEnglish
    Pages (from-to)343-359
    Number of pages16
    JournalBoundary-Layer Meteorology
    Volume125
    Issue number2
    DOIs
    Publication statusPublished - Nov 2007

    Keywords

    • Aerosol particles
    • Convective storm
    • Microburst
    • Micrometeorology
    • Trace gases

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