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Robert F. Adler and Douglas D. Fenn

Abstract

Infrared geosynchronous Satellite data with an interval of 5 min between images are used to estimate thunderstorm top ascent rates on two case study days. A mean vertical velocity of 3.4 m s−1 for 23 clouds is calculated at a height of 8.7 km. This upward motion is representative of an area of approximately 10 km on a side. Thunderstorm mass flux of ∼2×108 kg s−1 is calculated, which compares favorably with previous estimates. There is a significant difference in the mean calculated vertical velocity between elements associated with severe weather reports (w=4.9 m s−1) and those with no such reports (2.4 m s−1).

Calculations were made using a velocity profile for an axially symmetric jet to estimate the peak updraft velocity. For the largest observed w value of 7.8 m s−1 the calculation indicates a peak updraft of ∼50 m s−1.

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Robert F. Adler and Douglas D. Fenn

Abstract

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.

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Robert F. Adler and Douglas D. Fenn

Abstract

Short-interval geosynchronous infrared satellite data are used to examine 11 cases of tornadic thunder-storms with respect to cloud-top temperature (height) variations relative to tornado touchdown times, and in three cases relative to the initial observation of mesocyclones by Doppler radar. The scale of the updrafts observable with the satellite infrared data is ∼10 km. The cases are limited to those with relatively intense tornadoes. In 8 of the A 1 cases there is a period of rapid cloud-top ascent 30–45 min prior to tornado touchdown. This upward growth appears to be associated with the formation of the mesocyclone. This ascent is followed by a period of no growth or even a drop in cloud-top height preceding, or at the time of, tornado touchdown. In the three remaining cases cloud-top ascent is evident in the satellite data at tornado touchdown.

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Robert F. Adler, Douglas D. Fenn, and Douglas A. Moore

Abstract

A dark, spiral feature is noted in the geosynchronous satellite visible image of the top of a thunderstorm which also has a Deppler radar-observed mesocyclone. Although the evidence is not conclusive, the feature may represent cyclonic rotation at cloud top associated with the pre-tornado mesocyclone.

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Robert F. Adler, Michael J. Markus, and Douglas D. Fenn

Abstract

The potential and limitations of detecting severe thunderstorms in the Midwest region of the United States with short‐interval (∼5 min) geosynchronous satellite data are explored. Thunderstorms are defined in the infrared (IR) data as points of relative minimum in brightness temperature (TB) that have good time continuity and exhibit a period of rapid growth (TB decrease or TB isotherm expansion). Thunderstorms so defined are tracked and monitored and parameters related to storm (updraft) intensity, are extracted from the satellite data. These parameters are rates of TB decrease (both in the upper troposphere and the stratosphere), rate of TB isotherm expansion (at TB≤ 226 K), and storm lifetime minimum TB (Tmin, a measure of storm maximum height). Each parameter is shown to be statistically related to the occurrence of severe weather (tornadoes, hail) on four case study days.

The four parameters are combined into a Thunderstorm Index (TI), varying from values of one to nine. Storms with TI≥ 6 have a much higher probability of having severe weather reports and there is a potential warning, lead time of 15 min for the first report of hail and 30 min for the first tornado report. The results are confirmed with an independent case.

The appearance of a “V‐shaped” IR cold feature with an embedded warm point (a cold‐warm couplet) is also shown to be correlated with severe weather reports and with the satellite intensity estimates. Most storms (75%) with the V‐shape have severe weather, but many severe storms (45%) do not have the feature.

Limitations of using satellite IR data to detect severe thunderstorms are detailed, including the difficulty of storm identification during certain stages, limitations due to the coarse field of view on current geosynchronous satellites and limitations due to cloud top TB‐height ambiguities.

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Robert F. Adler, Michael J. Markus, Douglas D. Fenn, Gerard Szejwach, and William E. Shenk

Abstract

Thunderstorm top structure is examined with high spatial resolution radiometric data (visible and infrared) from aircraft overflights together with other storm views, including geosynchronous satellite observations. Results show that overshooting cumuliform towers appear as distinct cold areas in the high resolution 11 μm infrared (IR) aircraft images, but that the geosynchronous satellite observations significantly overestimate the thunderstorm top IR brightness temperature (TB) due to field-of-view effects. Profiles of cloud top height and TB across overshooting features indicate an adiabatic cloud surface lapse rate. However, one-dimensional cloud model results indicate that when comparing thunderstorm top temperature and height at different times or different storms, a temperature-to-height conversion of ∼7 K km−1 is appropriate.

Examination of mature storm evolution indicates that during periods when the updraft is relatively intense the satellite IR “cold point” is aligned with the low-level radar reflectivity maximum, but during periods of updraft weakening and lowering cloud top heights, the satellite TB minimum occurs downwind with cirrus anvil debris. The growth period of a relatively weak cumulonimbus cluster is also examined with aircraft and satellite data.

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