Thunderstorm Intensity as Determined from Satellite Data

Digital infrared data from a geosynchronous satellite (SMS 2) on 6 May 1975 are used to study thunderstorm vertical growth rates and cloud top structure in relation to the occurrence of severe weather (tornadoes, hail and high wind) on the ground. All thunderstorms from South Dakota to Texas along a north–south oriented cold front are monitored for a 4 h period with 5 min interval data. An examination of five cloud elements having eight tornadoes indicates that in seven of eight cases the first report of the tornado took place during, or just after, a period of cloud top ascent. This vertical velocity is applicable to an area of 15 km on a side. Thunderstorm growth rate, as determined by the rate of blackbody temperature isotherm expansion and minimum cloud top temperature, are shown to be correlated with reports of severe weather on the ground. A time analysis indicates that the derived parameters reach critical values soon enough to provide a potential warning lead time of approximately 30 min. Equations are derived relating the thunderstorm growth rate to vertical velocity and outflow layer divergence. Severe thunderstorm elements are shown to have mean vertical velocities approximately twice as large as the non-severe elements. The outflow layer divergence is calculated to be 1 × 10−3 s−1 for the severe thunderstorms.