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122 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

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

W. R. KRUMM

Weather Bureau Office, Missoula, Mont.

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

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FIG. 1. Graphically depicting approximate temperature changes as sample of air moves through functions in the development of strong downdraft from dry thunderstorm conditions. from the cloud base were lowered to the ground accounted for by the lowering of the temperature, surface at the moist adiabatic rate (neglecting en- i.e., by added . trainment effects) then the air would reach the No doubt the frictional drag of water droplets surface 30 to 35F° colder than the environment. in and below the cloud would be responsible for an This does not happen in the case of these dry initial momentum of the downdraft from these dry thunderstorms for several obvious reasons: thunderstorms, but from these data and observa- 1. If most of the rain drops evaporate before tions, it appears obvious that the full explanation reaching the surface, then air through which the must take into account the thermodynamics. It rain falls (the downdraft) cannot have descended is suggested that these downdrafts are caused ap- from the cloud base to the surface at the moist adi- proximately as follows (see FIG. 1) : AS the rain abatic rate throughout its movement. falls from the cloud base and evaporates into the 2. Surface in these downdrafts air below the cloud, it cools the air progressively, are normally of the order of 10° to 15°F lower resulting in progressively lower potential tem- than the environment air at the surface. (This peratures (greater density) at successive eleva- would seem to suggest that the maximum cooling tions below the cloud base. It is likely that a effects occur at some distance above the surface, moist adiabatic lapse rate is caused through per- and that because of entrainment progressively haps 5000 or 6000 feet below the cloud base, after higher potential temperatures are resulting as the which there is little rain left for evaporation, and downdraft nears the surface.) consequently a lesser rate of cooling occurs. Un- 3. The surface relative does not in- der circumstances of a progressively lower po- crease sufficiently to become any place near satu- tential temperature than the environment, the rated conditions. As a matter of fact, the relative cooler air would sink, as suggested by Byers [2], humidity usually increases by an amount which is and would probably accelerate in sinking. It is of the order of only 10-15% more than the increase suggested that under conditions of a very deep

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FIG. 2. dry adiabatic environment (10,000 to 12,000 ft.) moisture conditions at the surface in these down- the density difference between the cooler down- drafts) such downdrafts are probably less strong draft air and the warmer environment results in at the surface than at a point just below the level an acceleration of the downdraft. It is suggested of maximum temperature difference. Humphreys further that (in view of observed temperature and [3] passingly hints at a similar explanation.

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After the initial draft of these comments had by the present writer appears to be in full accord been prepared, this writer's attention was called with this statement. to Braham's [4] finding that a large fraction of the It is hoped that the foregoing presentation of water in a thunderstorm is used to maintain the examples and discussion of the thermodynamics cold downdraft through evaporation. Moreover, of "dry" thunderstorms illustrate how the results in discussing the restratification of energy in the of Byers and Braham for thunderstorms in Florida and Ohio may be extended to explain observed as a result of the downdraft cycle, characteristics of thunderstorms in other areas. Braham [4, p. 236] writes, "The importance of this restratification to the thunderstorm energy REFERENCES is illustrated by the thunderstorms which occur in [1] H. R. Byers and Roscoe R. Braham, Jr., The Thunder- semi-arid regions, such as the southwestern United storm. U. S. Weather Bureau, Washington, 1949. States. In these regions, it is not uncommon to [2] H. R. Byers, "Thunderstorms," in:—Compendium of . American Meteorological Society, have a thunderstorm which produces virtually no 1951, p. 684. rainfall. The cold gusty outflow- is very [3] W. J. Humphreys, Physics of the Air. 3rd Ed., Mc- Graw-Hill, New York, 1940. much in evidence, however, indicating that the en- [4] Roscoe R. Braham, Jr., "The Water and Energy tire energy of the storm has resulted from this Budgets of the Thunderstorm and Their Relation to Thunderstorm Development.,, Journal of Mete- energy restratification." The hypothesis outlined orology, vol. 9, No. 4, August 1952, p. 236.

will be required, of which eleven will be provided by NEWS AND NOTES the United States. At the Paris conference the entire North Atlantic meteorological problem was reviewed. It was decided I.C.A.O. Revises Ocean Weather that aeronautical benefits represented 80 per cent of those that would flow out of the program, non-aeronautical Ship Program benefits 20 per cent. Weather crosses the North Atlantic The recent International Civil Aviation Organization from west to east. It was agreed that European states conference on the North Atlantic weather stations, ar- would receive the larger share of non-aeronautical bene- rived at decisions of far-reaching importance. The con- fits and would therefore have to pay 75 per cent of the ference began in Paris on Feb. 9 and ended there on value of this portion. Feb. 25 with the signing of a new agreement which takes Non-aeronautical benefits include those that accrue to the place of one that expires on June 30 of this year. ship owners, such as the sighting of ice floes and bergs, The agreement calls for a network of nine small vessels oceanographic observations of wave heights and sea swells, which will patrol specific areas of the North Atlantic and safeguarding radio frequencies which are indispensable in report on weather conditions to aircraft and shipping. locating ships in distress. There was no allocation of Last year consternation was created when the United costs for meteorological observations because these benefit States notified the Secretary General of I.C.A.O. that it the whole world. would no longer participate in the maintenance of ten The representatives of all the participating states were floating weather stations called for by the North Atlantic worried by the heavy cost of maintaining the floating ocean weather stations program. Harold A. Jones, repre- weather stations, each of which must be served by be- sentative of the United States, declared that his Govern- tween two and three ships. "No funds are available in ment had decided that the services provided by the stations our budget after June," said the delegate of the United were no longer required and that it was not getting its States, "but money will be found for a reasonable and money's worth in benefits. constructive program." He signed the new agreement. The new agreement reached in Paris runs for two Evidently the program seemed to him "reasonable and years, with a possible annual automatic extension there- constructive." Though weather reporting is the chief after. Instead of ten ocean stations there are to be nine function of the North Atlantic program, the ships have after June 30, 1954. By dropping one station, global established a remarkable record for saving lives at sea. operating charges will be reduced annually by $3,800,000. Twelve nations will either operate ships or make cash Despite Jones' declaration about withholding its support, contributions to the maintenance of the network. The the United States will be one of the North American nations are Canada, Denmark, France, Ireland, Israel, countries which will operate four of the nine stations. Italy, the Netherlands, Norway, Sweden, Switzerland, The other five stations will be operated by European the United Kingdom and the United States.—N. Y. Times, states. To man these nine stations, twenty-one ships March 2, 1954.

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