Abstract
Abstract. The three-hour life-cycle of the isolated thunderstorm on 15 July 2007 during the Convective and Orographically-induced Precipitation Study (COPS) is documented in detail, with a special emphasis on the rapid development and mature phases. Remote sensing techniques as 5-min rapid scans from geostationary satellites, combined velocity retrievals from up to four Doppler-radars, the polarimetric determination of hydrometeors and spatio-temporal occurrences of lightning strokes are employed to arrive at a quantification of the physical parameters of this, during the COPS period, singular event. Inner cloud flow fields are available from radar multiple Doppler analyses at four consecutive times separated by 15 min-intervals. They contain horizontal winds of around 15 m s−1 and updrafts exceeding 5 m s−1, the latter collocated with lightning strokes. Reflectivity and polarimetric data indicate the existence of hail at the 2 km level around 14:40. Furthermore, polarimetric and Doppler radar variables indicate intense hydrometeor variability and turbulence corresponding to an enhanced variance of the retrieved 3-D wind fields. Profiles of flow and hydrometeor statistics over the entire cloud volume provide reference data for high-resolution numerical weather prediction runs in research mode. The study embarks from two movie-loops of geostationary satellite imagery (as Supplement), which provide an intuitive distinction of six phases making up the entire life-cycle of the thunderstorm. It concludes with a triple-image loop, juxtaposing a close-up of the cloud motion as seen by Meteosat, simulated brightness temperature (as a proxy for clouds seen by the infrared satellite channel), and a perspective view on the model generated system of cloud cells. The simulation suggests that several updrafts fed from a low level convergence line eventually removed the convective inhibition and set deep convection in motion. A shear line in the radial velocity relative to the Feldberg radar site shows good agreement beween observation and simulation, whereas the onset location of deep convection exhibits a horizontal discrepancy of 15 km. A quantitative schematic of the isolated thunderstorm synthesizes all retrieved characteristics.
Highlights
Thunderstorms constitute an archetype of a fierce meteorological event accompanied by a fleet of potential dangers, e.g. severe gusts, high precipitation rates, large hail, and lightning
In mid-latitudes, thunderstorms are often connected to dynamically active frontal zones of cyclones or they develop in isolation when atmospheric conditions favour their initiation, especially on hot summer days. The latter category exhibits a typical life cycle, for which three phases can be distinguished (Cotton and Anthes, 1989, p. 456): the initiation of convection by surface heating and horizontal convergence at a variety of locations becomes marked by towering cumulus clouds; the mature stage sees the merger of several cumulus elements into a large convective system with the formation of various hydrometeors, the occurrence of lightning discharges compensating previous charge separations, and the onset of precipitation; during the dissipating phase downdrafts prevail in the Published by Copernicus Publications on behalf of the European Geosciences Union
Two loops are provided as Supplement to give an intuitive awareness of the horizontal dimensions and the duration of the thundercloud development as well as its relation to other cloud systems during 15 July 2007: (i) an overview for western and central Europe concatenating 145 images between 06:00 and 18:00 with a nominal display speed of 14 frames per second (4,200-fold speed-up), and (ii) a close up for the wider Convective and Orographically-induced Precipitation Study (COPS) region consisting of 97 images between 10:00 and 18:00 at half the speed (7 fps; 2,100-fold speed up)
Summary
Thunderstorms constitute an archetype of a fierce meteorological event accompanied by a fleet of potential dangers, e.g. severe gusts, high precipitation rates, large hail, and lightning. Two loops are provided as Supplement to give an intuitive awareness of the horizontal dimensions and the duration of the thundercloud development as well as its relation to other cloud systems during 15 July 2007: (i) an overview for western and central Europe concatenating 145 images (movie1.mpg) between 06:00 and 18:00 (all times in UTC) with a nominal display speed of 14 frames per second (4,200-fold speed-up), and (ii) a close up for the wider COPS region consisting of 97 images between 10:00 and 18:00 (movie2.mpg) at half the speed (7 fps; 2,100-fold speed up) These are considered as the overall features of the event, which provide the frame for the detailed data analyses of this study: (i) well ahead of a frontal cloud band across France small cumulus clouds develop around 12:00 only over the mountains of the Vosges, the Black Forest and the Alps; (ii) the formation of a nearly contiguous, 100-km-long cloud line over the eastern slopes of the Black Forest by 13:00; (iii) the gradual enhancements of the cloud line’s southern and northern portions until 14:00; (iv) the sudden vertical development at the middle location at 14:30, which induces a dark shadow to its east; (v) the formation of a wide and curved anvil around 15:00, and (vi) its detachment as well as north-eastern progression relative to a few remaining cumuli at low levels.
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