On the Cause of Downdrafts from Dry Thunderstorms over the Plateau Area of the United States

IN their excellent study of The Thunderstorm, Byers and Braham [1] make the following statement regarding the formation of the thunderstorm downdraft:— "The processes contributing to formation and maintenance of the thunderstorm downdraft appear to be more complicated than those producing the updraft. In general, it is possible to determine the probable relative importance of contributing factors and fit together a sequence of events which could lead to the formation of vigorous downdrafts. Unfortunately, present data do not permit the conclusion that the series of events outlined below represent the only way in which downdrafts may be formed, although it must be admitted that the proposed process fits observed data in a satisfactory manner. ,, "The key to development of thunderstorm downdrafts is entraining within the updraft during the cumulus stage of development. As a consequence of entrainment, the lapse rate within the updraft becomes approximately that of the environment. The result is that any air forced downward by the drag of liquid water within the updraft will undergo a temperature change at the moist-adiabatic rate and very quickly become more dense than the air of the environment. From this point the air would continue to descend because of its density as well as the drag force of the drops; and, as long as sufficient water remains to keep the descending air saturated, it will reach the surface colder than the environment surface air. Rain falling from the updraft supplies the necessary water. It is thus seen that the thunderstorm downdraft develops in the very region where the updrafts have existed and that its development is dependent upon the drag force of the contained liquid water drops. " Since the physical research in connection with The Thunderstorm Project was conducted in Florida and Ohio under conditions where air moisture is normally rather high (low cloud bases), it is logical to ask if Byers and Braham's results can be applied or extended to explain observed characteristics of thunderstorms in air containing less moisture. During the summer season, the Plateau Region is the breeding ground for frequent widespread airmass thunderstorms. Characteristically, most of these storms develop in air which contains much less moisture than the normal storm over the eastern half of the United States. It is not at all uncommon to observe active thunderstorms with cloud bases 14,000 feet to 16,000 feet above sea level (10, 000 to 12,000 ft. above the average terrain). Indeed, the bases of these thunderstorm clouds are so high that it is not uncommon for very light rain to occur, or no rain at all to reach the ground from them. These storms nevertheless are generally marked by very strong downdrajts. Indeed, it is not at all uncommon to experience downdraft winds of 30 to 40 mph from these storms, and occasionally the winds reach destructive force of better than 50 mph. To plainly indicate the typical hour to hour development of this type of situation, the observational record is given for several such conditions at Missoula, Montana. (See examples on W. B. Form 1130A Rev., FIG. 2.) Many other examples could be taken from the records of any weather station in the Plateau Region. Even if the "rain drop friction or drag" is responsible for the initiation of the thunderstorm downdraft, it is felt that other factors should be stressed to explain fully these strong surface winds when little or no rain reaches the ground. There is good evidence to suggest that these winds may be caused by dense (cool) air which develops below the Cb cloud as a result of progressive evaporation of the rain which does fall. This hypothesis may be developed by considering the thermodynamics of the downdraft in the levels below those discussed by Byers and Braham [1, p. 38]. In the case of the typical airmass dry thunderstorm, the lapse rate between the surface and the cloud base is very close to the dry adiabatic. This has been checked by the writer repeatedly both by the use of raob data, and by the calculation of cloud base using temperature and dewpoint values, and then checking the computed cloud height against measured height. This means, then, perhaps 10,000 feet of air with very near the dry adiabatic lapse rate. Under these conditions, if air