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. Unauthenticated | Downloaded 10/03/21 10:12 PM UTC VOT,. 32, No. 4, APRIL, 1951 147 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- Unauthenticated | Downloaded 10/03/21 10:12 PM UTC 148 BULLETIN AMERICAN METEOROLOGICAL SOCIETY 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.