146 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

A Cloudform-Stability Scale for Tropical Oceanic Areas *

REID A. BRYSON Department of Meteorology, University of Wisconsin, Madison, Wis.

ABSTRACT On the basis of experience in Puerto Rico and the western Pacific, a scale for rating con- vective cloudforms in terms of the thermodynamic stability represented was developed. This scale provides a quantitative measure for the study of diurnal variation of cloudform, and for weather pattern diagnosis.

I. THE NATURE OF CONVECTIVE CLOUD OVER face. The significance of the changes lies in the THE TROPICAL OCEANS size of the cumuli, the visibility reduction due to HE distribution of stratiform and cumuli- rain, and associated wind and turbulence condi- form low cloud over the tropical oceans is tions. Tcharacterized by mutual exclusion. In any The most pronounced synoptic variation to be given season, at a given location, there is a great found in the cumulus areas of the tropical oceans preponderance of one type or the other; only is the day to day variation in the size, density, rarely is there considerable admixture. Large and structure of the cumuli. The range of cloud- areas may be delineated within which one class of form is from no clouds or isolated cumulus humilis cloud may dominate the skies for the entire year, through what is called "trade cumulus" and has no for example stratiform cloud at places such as mid-latitude counterpart, to cumulus congestus, Ascension Island, or the Galapagos. On the other then cumulonimbus. A careful distinction must hand, true stratocumulus is very rare at such be made between this morphology and that of places as Guam, Ponape, Truk, Kwajalein, or continental cumuli. The familiar land case is as Tarawa. Of 52 weeks of 1945-1946 in the Mari- follows: during the night, no low clouds, giving anas, only one was marked by frequent decks of way to a few cumuli humili by mid-morning. true stratocumulus. These small cumuli have distinct, sharp upper A brief examination of the "Atlas of Climatic surfaces. As surface heating proceeds, the num- Charts of the Oceans" [1] will show the only re- ber of cloud cells increases, and the upper sur- gions of abundant stratiform cloud to be those faces, still sharply defined, begin to form cabbage- located over cold currents on the west coast of like protuberances. The cloud is now called cu- continents: the Humboldt, California, and Ben- mulus congestus, but the dividing line between guela particularly. As for the rest of the oceanic humilis and congestus is rather obscure. Further tropics, cumulus dominates. Cumulonimbus and rapid development of the protuberances takes nimbus are most frequent along the "equatorial place, an anvil is produced, and the end of the front" and where the major meridional trough sys- development is reached. Deterioration of the tems intersect the equatorial trough. Quite uni- entire system follows with the approach of night. formly the stratiform areas exhibit 5-7 tenths The sequence is quite different over the ocean. cloud cover while in the remainder, the cumulus While over the land the diurnal changes of cloud areas, 3-5 tenths prevail. are large and the synoptic effect is superimposed, Does this mean then that over the tropical seas over the sea far from land the diurnal variation there is endless monotony of either somewhat is very nearly negligible (as will be demonstrated) more than half the sky covered by stratocumulus, and the primary cloud sequence is due to the syn- or slightly less than half covered by cumulus, the optic variation. Over the land, the various con- only breaks in this condition being associated with vective cloudforms are produced mainly by varia- passages of occasional typhoons or the equatorial tions in the surface temperature, which produce in front ? Significant changes in the weather do take turn variations in lapse rate. Over the seas, the place from day to day, but they are not associated surface temperature remains nearly constant, the primarily with profound changes in cloud amount, variations in lapse rate which differentiate the nor with changes from one airmass to another, nor different cloud conditions being the result of ver- with appreciable temperature changes at the sur- tical shrinking and stretching—or convergence— operating on a conditionally unstable airmass. In * Part of a doctoral dissertation in Meteorology sub- the continental case the morphological sequence mitted at the University of Chicago.

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reflects marked stability changes due to surface heating and cooling. Evidently this fundamental difference in process must mean a markedly different cloud sequence. In the land case there is a critical point beyond which surface heating will produce congestus. Rising currents due to the unstable conditions in the lowest layers are restricted by a stable layer until this critical point is reached. When the sta- bility is no longer sufficient to prevent further rise the congestus rapidly develops. On the other hand, the corresponding point in the oceanic case is that at which the rising currents begin to develop; hence the transition is diffuse and an entirely new cloud type, the trade cumulus, enters FIG. 2. Stability index 4, as observed south of Puerto the sequence (FIG. 1). Rico during winter of 1944. Note soft outlines and blocky, though somewhat sheared, appearance. Stability II. THE OCEANIC CLOUDFORM SEQUENCE 6 in distance. The original type photographs of all the various classes The typical oceanic sequence is as follows, as- of the stability scale are unfortunately not available. suming an initial condition of strong horizontal divergence in the lower troposphere: 2. Slightly less subsidence does not force the 1. With strong divergence, subsidence takes inversion below the condensation level, and small place and the lower layers are capped with a amounts of very flat cumulus humilis develop, due slight inversion (the so-called trade inversion). to turbulent overturn. Under these conditions there are widespread stra- 3. With still less divergence or elimination of tocumulus decks off the west coasts of continents, its effect by convergence, the stable layer is found but in the area with which this paper is concerned at greater elevation, but the condensation level there are no low clouds or only a few fractocumuli remains quite fixed, so that the vertical extent of near islands. the cloud increases. Nevertheless, the vertical dimension remains less than the horizontal and a distinct humilis character remains. 4. As the effect of the initial divergence and subsidence is nearly eliminated and the state of neutral stability approached the ratio of height to width approaches unity. The tops and edges of the cloud soften; the humilis character diminishes (FIG. 2). 5. The trend towards soft outlines is the domi- nant feature of the neutral case. Over the oceans trade cumulus seem to represent simply a very slow overturning of the layer between the conden- sation level and the top of the moist layer. The slowness of this overturn is attested by the very slight bump that is experienced by aircraft in passing through the cloud. These soft, poorly- defined cloud masses of blocky outline do not fit into the international classification, and hence are never properly encoded in the synoptic report. This is not the fault of the observer for by regula- tion and necessity he must use the land sequence —which contains no provision for the occurrence of trade cumulus ( FIG. 3 ). FIG. 1. Morphology of cumuliform low cloud observed 6. With passage to the case of equilibrium over land and over the sea. slightly unstable as a result of continued conver-

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scud and very low stratus are abundant. This convection type occurs but rarely except in ty- phoons and markedly convergent areas of the "equatorial front," yet over the tropical oceans far from land it is nearly as frequent as completely clear skies. It should not be assumed that these cloudforms represent convergence or divergence per se. They are the result of that process acting on the tropical airmass and thus represent a cumulative effect. Clearly a sudden change from strong divergence to strong convergence will not immediately pro- duce cumulonimbi from a clear sky, but the time lag will not be great. The intrinsic effect of the FIG. 3. Typical "trade cumulus," stability index 5, convergence is to change the stability of the air- observed south of Puerto Rico during the winter of 1944. mass, the cloud then adjusting to the new condi- Compare with FIGURE 1. The extension of the haze to tion. The form of the cloud may then be consid- the top of the cloud layer is evidence that the cloud repre- ered a measure of the atmospheric stability. sents slow overturn of the entire moist layer rather than buildup by penetration of a stable layer. The reverse case, or the transition from insta- bility to stability, may be divided into two cases. If the last stages in the development have been gence, the outlines of the cloud again become more due to an unstable upper troposphere, then dissi- definite, the contours rounded rather than blocky, pation will be similar to the breakup of the di- vertical velocities greater, the tops higher. The urnal buildup over land until approximately the term congestus is applicable to this case. point is reached in cloudform at which the buoyant 7. Continued convergence produces greater in- lifting started. In the more common case, how- stability and thus towering cumulus congestus ever, the decline of the clouds will be similar to comparable in shape and size to the "thunder- the reverse of their rise except for the abundant heads" of the middle latitude continental areas, remnants of both stratiform and cumuliform mid- though they are not as close to the cumulonimbus dle and high cloud decks produced by the cumulo- stage as are these familiar clouds. nimbi. This does not mean that the cumuli shrink 8. Depending on the stability of the upper tro- in size, but rather that further successive cloud posphere, either buoyancy or de-stabilization due developments in the area reach less and less ad- to continued convergence may extend the cloud vanced stages. development beyond this point. This often hap- pens because of the prevailing moist-adiabatic at- mosphere (or conditional equilibrium) over much of the tropics. The cumulonimbi produced may still cover little more than half of the sky. These are the usual middle- and high-cloud factories of the tropics as well, for soon after the cumulonim- bus stage is reached, the scud and stratocumulus patches which began to appear in the previous case become appreciable, and altocumulus, alto- stratus, cirrocumulus, and cirrostratus shelves mark the upper levels which are characterized by changes in stability (FIG. 4). P. The extreme case, only produced by very strong convergence, consists of organized systems of cumulonimbi rising like pillars through many decks of middle and high cloud. The lower por- FIG. 4. Stability index 8. At this stage of develop- tions of the major convective centers in such a ment scud, stringers of small cumuli, and stratiform layers system are joined by lines and patches of smaller are associated with the centers of cumulonimbus devel- cumuli and stratocumulus, and under the bases opment.

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III. DEVELOPMENT OF THE CLOUDFORM TABLE I. CLOUDFORM-STABILITY SCALE STABILITY SCALE Rat- Description Usual Usual Sometime in 1943 United States Army weather ing amount thickness officers in Panama, in order to forecast the time 1 Very great stability—clear or only 0-1 0-500' of afternoon showers, developed a scale of stability a few isolated fumuli. similar to that which will be described in the fol- lowing paragraphs. The occurrence of afternoon 2 Marked stability—incipient humi- 1-2 500- showers in that area is associated with diurnal lis or sctd fumuli. 1000' heating in the mountain areas and subsequent 3 Stability—humilis in ideal devel- 2-3 1000- drift of the showers coastward. The first step in opment, definite bases and tops, 2000' forecasting then is to estimate whether the heating no tops extending above general during the day will be sufficient to produce show- level, vertical dimension less than horizontal. ers. Lacking soundings, this estimate depends on judgment of the stability conditions. For con- 4 Slight stability—humilis, but softer 3-4 2000- venience the visual evidence gleaned from obser- in form and with vertical dimen- 4000' vation of the morning sky was classified according sion approaching equality with horizontal. to the indicated stability conditions and the classes numbered in order of decreasing stability. STA- 5 Neutral—trade cumulus of block- 3-4 4000- BILITY 1 might then indicate no shower activity like form and soft edges, no sign 6000' during the day, while STABILITY 2 might be asso- of humilis or congestus form. ciated with weak, late development, and so forth. 6 Slight instability—trade cumulus 3-5 5000- Late in 1943 this idea was expanded at the but tops beginning to show hard 7000' Institute of Tropical Meteorology, Rio Piedras, rounded outline, piled-up rather Puerto Rico, as a more general measure of atmos- than blocklike in form. pheric stability. The scale developed by Allen 7 Instability—well-developed con- 3-5 7000- and Bryson is as follows [2] : gestus, isolated cumulonimbi, sctd 10000' 1. Extreme stability throughout the atmosphere. lgt to occnl mdt showers. —Poor visibility, haze layers, stratiform clouds 8 Marked instability—mixed con- 5-8 10000- only or none at all. gestus and cumulonimbus, occnl 28000' 2. Extreme stability in one layer.—One stable thunderstorms, low ceilings, sctd layer such as at the top of a layer of cold-air ad- mdt, occnl hvy showers. vection, flat-topped cumulus humilis, haze layer, 9 Very great instability—organized 8-10 25000- perhaps altostratus or altocumulus, sometimes cumulonimbus systems, thunder- 65000' stratocumulus cumulogenitus. storms, low ceilings, very hvy 3. Slight stability.—Haze layer, trade cumulus showers, waterspouts. over the ocean, limited tops to the cumulus and often stratocumulus. appreciable effect on the cloud development, and 4. Slight instability.—Good visibility, no haze refers explicitly to mountains. Over much of the layer, cumulus congestus and chimney cumulus, tropics, the oceanic parts, these conditions are not cumulonimbi over the mountains, occasional light met, and the scale is not applicable. For this rea- showers. son a new scale was developed by the author for 5. Extreme instability.—Organized cumulonim- use by the AAF Weather Central on Saipan dur- bus systems with associated cirrus, altostratus and ing late 1944. The details of the scale are pre- stratocumulus decks, turbulence, frequent moder- sented in TABLE I. It will be noted that the divi- ate to heavy showers, often with light rain between sions on the scale are comparable to the numbered showers. cloudform sequence steps described in Sect. II This scale was intended for the same type of above. use as the scale developed in Panama. Clearly Of the thousands of observations which were such use implies a large diurnal variation in the taken, only a few exceptions were found to the clouds, controlled to a certain extent by the syn- rule that clouds or arrays of clouds have dimen- optic conditions through their non-diurnal effect sions within the range assigned in TABLE I for on the lapse rate. Again the scale is designed clouds of that description. One exception, com- for use not in the purely oceanic tropics, but rather moner than the rest though still rather rare, is where the land mass is large enough to have an that of the excessively large cumulonimbi which

Unauthenticated | Downloaded 10/03/21 10:12 PM UTC 150 BULLETIN AMERICAN METEOROLOGICAL SOCIETY are sometimes observed in the tropics. Whereas TABLE II. PRELIMINARY ESTIMATE OF THE MAXIMUM the common cumulonimbus of the tropical oceans RATE OF PRECIPITATION THAT MIGHT BE EXPECTED may reach a height of 40,000 feet or so, these FROM CLOUDS OF A GIVEN STABILITY RATING PASSING DIRECTLY OVER THE STATION* larger clouds may reach 60,000 feet, as measured by triangulation. Further, these larger convection 5 no rain 6 trace centers may be ten times as great in horizontal 7 10 inch per hour dimension as the ordinary cumulonimbus. Dur- 8 50 inch per hour ing August 1945 the author observed three cumu- 9 over .50 inch per hour lonimbi west of Guam which reached to an esti- * Obtained by comparison of the hourly rainfall rates mated 30,000 feet, were brilliantly white in the observed at the AAF Weather Central at Guam with the sunlight, and stood out very plainly against a coincident cloudform index. grey-blue horizon sky. Above and beyond the three clouds was the edge of a sheet of rather stability. (The effect of darkness will be dis- dense cirrostratus. Closer examination of the sky cussed in a succeeding article to appear in a later revealed that the grey-blue back-drop was in real- issue of this BULLETIN.) ity an enormous cumulonimbus! So large was From the fact that hourly observations of the this cloud, from which the dense cirrostratus ex- cloudform-stability index were made from No- tended, that the author completely missed it in vember 1944 until sometime in the autumn of focussing on the three smaller clouds, yet these 1945, it may be seen that an entirely different smaller clouds were large by mid-latitude stand- purpose was served than that for which the earlier ards. Much observation and research needs to be forms of the scale had been intended. Referring done on these remarkable clouds. It has been pointed out verbally by Prof. Erik Palmen that to the preceding discussion of the oceanic cumulus soundings in the tropics frequently show a sort of sequence, the scale was used as a measure of the semi-tropopause or break in the lapse rate at about stability not in terms of how the diurnal pattern 12 km or so, while the true tropopause is found at might be affected, but rather as an observation about 18 km.* It will be noted that the smaller which per se might sum up the array of cloud cumulonimbi reach about to 12 km while the heights and forms which at any moment charac- larger approach the tropopause. terize the sky into one easily handled parameter. All observations of the cloudform-stability index No account was taken in the development of the were made by officers specially qualified in tropical scale of the middle and high clouds other than in weather, most of whom graduated from the Insti- connection with the larger congestus and cumulo- tute of Tropical Meteorology. In order to avoid nimbus clouds, for in the operations with which subjectivity as far as possible, these officers occa- the Weather Central dealt they played very little sionally checked observations with each other, part. though not as frequently as desirable. After check Preliminary study of the rates of precipitation periods of several days of simultaneous observa- that might be expected from clouds of a given tion it was possible for two individuals to arrive stability index indicates that towards the un- at the same index rating for the "cloud-scape." stable end of the scale each rating has a charac- Stability was estimated to the nearest half point teristic precipitation rate (TABLE II). Since on the scale, though there was a strong tendency cloud size, or depth of the convergent layer, is to favor the whole numbers of the scale. This nearly fixed for each rating, this characteristic tendency seems most pronounced for the middle rainfall intensity is a measure of the instability and lower values of the scale, and less so for the as well. high and very low values. Probably this repre- A discussion of the observations made with this sents a fault of the scale—either the stages in index in the Marianas will appear in a later com- development represented by the successive num- bers are too crowded in the center of the range, munication to this BULLETIN. or there is a larger gap between successive stages REFERENCES in the larger clouds. [1] Atlas of Climatic Charts of the Oceans. United Subjectivity, though present, is not serious, es- States Department of Agriculture, Weather Bu- pecially with regard to the general trend of the reau No. 1247, 1938. [2] Allen, P. W., and Bryson, R. A.: Tropical Micro- *Cf.: Graves, Bull. Amer. Met. SocFeb. 1951, pp. analysis. University of Chicago, Institute of Trop- 54-60.—Ed, ical Meteorology, Lecture Notes No. 1, 1944.

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