Precipitation production in three Alberta Thunderstorms

Abstract

Radar reflectivity patterns of three large, long-lasting Alberta thunderstorms were analysed to determine the precipitation content and the outflow rate as functions of time. These quantities were then used to calculate the rate at which precipitation is generated and the characteristic time of the precipitation process, as functions of time. The maximum hourly-average precipitation content was approximately 0.5 Tg for one storm (Storm A) and 1 Tgfor each of the other two. The maximum hourly-average outflow rate was approximately 0.5 Gg s -1 for Storm A and 0.8 Gg s -1 for the others. Each storm had two fairly well defined periods of peak precipitation production lasting about half an hour and separated by about 45 min. The characteristic time, defined as the ratio of the instantaneous precipitation content to the outflow rate, was somewhat longer on the average for one storm than for the other two, but in no case was far from 20 min, which is approximately the time required for rain to develop in cumulonimbus clouds. The total amounts of rain produced by the three storms ranged from 4 to 7 Tg. RESUME Les configurations de la réflectivité du radar de trois tempêtes considérables et de longue durée en Alberta ont été analysées pour déterminer le contenu en précipitation et le taux du débit en fonction du temps. Ces valeurs ont été utilisées pour calculer, en fonction du temps, le taux avec lequel la précipitation est générée, et le temps caractéristique du processus de précipitation. Pour l'une des tempêtes (tempête A), le maximum de la moyenne horaire du contenu de la précipitation était de 0.5 Tg approximativement, et était de 1 Tg pour chacune des deux autres. Le maximum du taux horaire moyen du débit était de 0.5 Gg s -1 approximativement pour la tempête A et de 0.8 Gg s -1 pour les autres. La production de précipitation de chaque tempête possédait deux maximums bien défini, les maximums duraient une demi-heure environ et un intervalle de 45 min les séparait. Le temps caractéristique, défini comme étant le rapport entre le contenu en précipitation instantanée et le taux du débit, était en général quelque peu plus long pour l'une que pour les deux autres, mais en aucun cas, n'était trop éloigné de 20 min, période approximative, nécessaire à la formation de la pluie dans les nuages du type cumulonimbus. La hauteur totale de pluie générée par les trois tempêtes variait de 4 à 7 Tg.

Radar observations of precipitation production in thunderstorms

“…It is not clear whether this fluctuation is a systematic feature or simply "noise" in the observations. A similar variation in rain production with a period of 30-45 min was also recorded in Alberta thunderstorms (Rogers and Sakellariou, 1986). Time variations of storm properties.…”

“…Table 4 summarizes average values for Lowveld storms, together with those reported for New England storms (Geotis, 1971) and Alberta storms (Rogers and Sakellariou, 1986). The values for New England storms represent average values obtained for 8 severe thunderstorms observed with an S-band radar.…”

“…The data reported in the literature about precipitation production were reviewed by Rogers and Sakellariou (1986). The estimates of precipitation content and outflow rate cover a large range of values because convective storms vary greatly in size and intensity.…”

“…Estimates of integrated storm parameters were reported for storms of different intensities occurring over different geographical locations : hailstorms over Alberta (Pell, 1971), a huge convective storm complex over Quebec (Holtz, 1968), a severe local storm over Oklahoma (McLaughlin, 1967), intense thunderstorms over New England (Geotis, 1971), a hailstorm over Oklahoma (Fankhauser, 1971), a thunderstorm of "moderate intensity" over Colorado (Foote and Fankhauser, 1973) and large thunderstorms over Alberta (Rogers and Sakellariou, 1986). Before reliable radar data were available, integrated storm parameters had been estimated using data from radiosondes and rain gauges at the surface (Braham, 1952).…”

“…Other sources of errors are due to partial beam filling, the 30-dBZ threshold and the high 3° base elevation angle. Geotis (1971) and Rogers and Sakellariou (1986), who used similar methods, suggested that the calculated values of P(t) and O(t) were within a factor of 2 of their correct values. The accuracy of our estimates is likely to be similar.…”