2002 Annual

Monday, 14 January 2002
The impact of land-atmosphere interactions on the Benson, MN tornado of 11 June 2001
Daniel Cheresnick, CIMMS/Univ. of Oklahoma, Norman, OK; and J. Basara
On 11 June 2001 a strong tornado struck the town of Benson, Minnesota. The synoptic conditions were favorable for supercell thunderstorms in a limited area along a northwest to southeast oriented frontal boundary. In the hours preceding the outbreak of violent thunderstorms, the severe weather environment was modified due, in part, to an increase of moisture in the planetary boundary layer. The expansive agricultural regions of northern Iowa and southern Minnesota produced an airmass of increased moisture through evapotranspiration from the land surface. This air mass, containing increased atmospheric moisture, was advected northward into Minnesota and resulted in a pooling of moisture near Benson. At the same time, a tropical cyclone was situated along the northern coast of the Gulf of Mexico. The resulting northerly flow along the Texas coast cut off the typical source region of increased atmospheric moisture (i.e., deep moisture from the Gulf of Mexico).

The increase in moisture due to land-atmosphere interactions influenced several critical parameters for severe thunderstorms. First, the increased moisture diminished the strength of the capping inversion. The combination of increased lift along the front, and the additional moisture were sufficient for convective development. In addition, instability in the atmosphere was dramatically increased. Modified soundings of the region approximately 150 km south of the frontal boundary consistently produced convective available potential energy (CAPE) values of approximately 3000 J/kg. However a similar modified sounding with surface data collected at the Benson Municipal Airport in the hour before the tornado resulted in CAPE values in excess of 7000 J/kg. Finally, the modified sounding using data from the Benson ASOS station yielded a lifted condensation level (LCL) and level of free convection (LFC) which were significantly lower than those soundings modified for the region 150 km south of Benson. The end result was a local atmosphere impacted by land-atmosphere interactions and an enhanced potential for violent, tornadic thunderstorms.

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