All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 163 23 2
PDF Downloads 69 9 0

Some Aspects of a Severe, Right-Moving Thunderstorm Deduced from Mesonetwork Rawinsonde Observations

View More View Less
  • 1 National Severe Storms Laboratory, ESSA, Norman, Okla
Full access

Abstract

On 24 May 1968 a rawinsonde observation was obtained in the updraft of a growing cell on the south flank of a thunderstorm which was turning to the right of winds and later produced large hail and funnel clouds. Moist adiabatic ascent was indicated with maximum excess temperature of 10C at 500 mb. Winds above 4.5 km deviated as much as 64° from environmental winds measured 15 n mi upwind. Diversion of mid-tropospheric flow is attributed to horizontal accelerators produced mainly by excess hydrostatic pressure within the updraft. At 5 km, a 23 m sec−1 updraft deduced from a hydrostatically-calculated ascent rate is one-third less than that obtained from an entraining jet model and two-thirds less than the parcel theory estimate. Excess hydrostatic pressure is considered as a contributing factor in reducing bouyancy. Although ambient vorticity through an attitude of 7 km was mostly anticyclonic, cyclonic rotation in the southwest portion of the storm was observed by radar and corroborated by visual observation of the cloud base. Evaluation of the vorticity equation for parcels entering the updraft base shows one component of the tilting term was generating cyclonic vorticity at a rate sufficient to maintain a parent tornadic cyclone. Obstacle flow and cyclonic rotation suggest existence of a right-deflecting force on the updraft, but the data are insufficient for quantitative evaluation of this effect. Sounding processing techniques are briefly described in an appendix.

Abstract

On 24 May 1968 a rawinsonde observation was obtained in the updraft of a growing cell on the south flank of a thunderstorm which was turning to the right of winds and later produced large hail and funnel clouds. Moist adiabatic ascent was indicated with maximum excess temperature of 10C at 500 mb. Winds above 4.5 km deviated as much as 64° from environmental winds measured 15 n mi upwind. Diversion of mid-tropospheric flow is attributed to horizontal accelerators produced mainly by excess hydrostatic pressure within the updraft. At 5 km, a 23 m sec−1 updraft deduced from a hydrostatically-calculated ascent rate is one-third less than that obtained from an entraining jet model and two-thirds less than the parcel theory estimate. Excess hydrostatic pressure is considered as a contributing factor in reducing bouyancy. Although ambient vorticity through an attitude of 7 km was mostly anticyclonic, cyclonic rotation in the southwest portion of the storm was observed by radar and corroborated by visual observation of the cloud base. Evaluation of the vorticity equation for parcels entering the updraft base shows one component of the tilting term was generating cyclonic vorticity at a rate sufficient to maintain a parent tornadic cyclone. Obstacle flow and cyclonic rotation suggest existence of a right-deflecting force on the updraft, but the data are insufficient for quantitative evaluation of this effect. Sounding processing techniques are briefly described in an appendix.

Save