[15200477 - Bulletin of the American Meteorological Society] A Formation of Pileus-like Veil Clouds Over Cape Cod, Massachusetts ,

VOL 32, No 2, FEBRUARY, 1951 61 A Formation of Pileus-like Veil Clouds Over Cape Cod, Massachusetts lf JOANNE STARR MALKUS Woods Hole Oceanographic Institution and Illinois Institute of Technology ANDREW F BUNKER Woods Hole Oceanographic Institution AND KENNETH MCCASLAND Woods Hole Oceanographic Institution ABSTRACT A case of low-altitude veil clouds from which cumulus later grew is studied by means of airplane, photographic, and synoptic data Convergence due to the land-sea temperature contrast is indicated as the critical factor in this unusual cloud formation This conclusion is further supported by an airplane study of the cumulus structure N the course of an observational program to study convection and cumulus clouds in the region of Cape Cod and the adjacent islands, a particularly interesting case was encountered on July 24, 1950 Although superficially typical in many ways of the average manner of cumulus formation over the Cape, an unusual and striking feature of this case was the formation at altitudes of 3000 ft or lower of large numbers of dark, tenuous, pileus-like veils These first appeared entirely alone so that they resembled many "eyebrows" in the sky (see FIG 1) and fifteen to twenty minutes later small cumuli began to sprout, the veils remaining visible at their bases This situation should be contrasted with the more usual examples of pileus formation at the tops of vigorous cumuli, which occur frequently when convective clouds push upwards against an inversion By means of airplane traverses and soundings in the cloud area in conjunction with local synoptic data, the present formation can be understood as a somewhat specialized outgrowth of convergence caused by land-sea temperature contrast Previous observational work by the writers [1] has demonstrated that the cumulus streets formed by Cape Cod and adjacent islands are intimately related to the heating of the land surfaces relative to the surrounding waters In the cases previously I Contribution No 534 from the Woods Hole Oceanographic Institution The observational work discussed in this paper was carried on under Contract No N6-onr 277, Task Order No II, NR-082-021 by the Office of Naval Research and the Woods Hole Oceanographic Institution, and analysis of the results was completed under Contract No Nonr174(00) with the Office of Naval Research and the Illinois Institute of Technology studied, the cloud streets have been observed to form and decay at times coincident with the rise and decline of solar heating That the sea-breeze circulation is also a consequence of differential heating is well known and the present case provides evidence that some, at least, of the weaker cloud streets may owe their existence to the superposition of such convergence upon the more direct consequences of heating and mixing the air, similar to the "Lanai-type" Hawaiian cloud streets discussed by Leopold [2] In the case of July 24, the cloud streets which formed out of the pileus-shaped veils were oriented along the northern shore of Cape Cod as illustrated in FIGURE The whole area was in a moderately strong high-pressure cell with the polar front quite far to the south, and a weak gradient flow from a southeasterly direction prevailed until the onset of the sea-breeze effect The water temperature was almost identical with that of the lower air as it started inland and began its rapid heating by the sandy soil of the Cape A description of the initial character of this air is given by the 11 a.m E.D.T Nantucket sounding (FIG 3), the striking features of which are weak conditional instability and a rapid decrease of the dewpoint temperature with height The convection condensation level (as defined by Spilhaus and Miller [ ] ) for the lowest 3000-ft layer is, however, higher than 10,000 ft, and only with the addition of sufficient moisture to extend the surface value of the mixing ratio up to 3000 ft is it lowered to the observed cloud base An airplane sounding made over Falmouth Airport at 3:15 p.m E.D.T., however, showed a well-mixed layer with mixing Unauthenticated | Downloaded 01/26/22 07:59 PM UTC 62 BULLETIN AMERICAN METEOROLOGICAL SOCIETY FIG (a and b ) Photographs looking northward from Falmouth Airport at about p.m E.D.T showing how small cumuli were already beginning to sprout from the veil clouds Plane flight into the cloud area revealed that the veils were all at altitudes close to 3000 ft lb is a section of a photograph taken at nearly the same time as la, enlarged to show some of the veil clouds which had not yet sprouted cumuli Unauthenticated | Downloaded 01/26/22 07:59 PM UTC VOL 32, No 2, FEBRUARY, 1951 ratio constant only up to 1000 ft, gradually decreasing from there to the level of cloud base (see FIG 3) On the other hand, the rapid drying out with height shown by the Nantucket sounding is unusually pronounced for the area and time of year and probably accounts for the form and sequence of condensation phenomena observed It is hypothesized that when the air flowing from the southeast over the Cape encountered the inwardmoving sea breeze on the northern shore, it underwent convergence and bodily lifting Due to the strong moisture gradient, its lower layers approached saturation first and became visible as the dark, eyebrow-shaped veils Once such a layer became saturated, large buoyancy forces were available to it, so that the cumuli built up from the veils as bases That the cumuli were penetrating extremely dry air is revealed by the time-lapse motion pictures which showed their average lifetime was only 5-7 minutes in toto, and by their 63 notable lack of development into swelling cumulus despite the slight stability aloft Without further observational evidence, however, it might still be suggested that the "eyebrows" need not have been integrally related to the air structure and cloud-formation process but were due perhaps to some accidental concentration of highly hygroscopic nuclei on that particular day Fortunately, the pileus-shaped veils recurred fleetingly in the same spot on October 17, 1950 The similarity in lapse rates on the two days, including the marked drying out with height, was too great to be coincidental and supports the contention that the veils were the first step in a somewhat unusual method of cumulus formation It should be pointed out that the more common summer cumulus in this area is associated with pronounced conditional instability, and is not preceded by veil formation Considerable observational evidence also exists to show that on July 24th convergence of the re- FIG North-south cross-section of Cape Cod from Sandwich to Falmouth, showing the major features of the case studied The Otis Field pilot balloon observation, however, was made at 11 a.m E.D.T and by the time of flight at 2:30 p.m (time for which the section is drawn) a southwesterly wind extended down to about 3000 ft Unauthenticated | Downloaded 01/26/22 07:59 PM UTC 64 BULLETIN AMERICAN METEOROLOGICAL SOCIETY FIG The Nantucket sounding for 11 a.m E.D.T on July 24 is given by the heavy lines, T and TdP, the temperature and dewpoint curves, respectively The mixing ratio in gm/kg is given beside each point on the temperature curve Nantucket is about 25 miles upwind (over water) from the Falmouth shore of the Cape The airplane temperature sounding made over Falmouth Airport at 3:15 p.m E.D.T is shown by the crosses quired magnitude actually did take place The appearance of the veils at 1:45 p.m E.D.T in a rather sudden burst is consistent with the accepted picture [4, 5] of the abrupt arrival of a "seabreeze front" when the sea breeze is opposed by the gradient flow That such convergence has a sharp maximum along a rather narrow line, often marked by rows of cumuli, has been found in several studies [2, 6] A marked decrease in surface wind between Falmouth and Otis Field (see FIG 2) is customary on such days, and in this case corresponded to a convergence of per hour,3 comparable to the magnitude of the cumulus-stage convergence found on the Florida peninsula by the Thunderstorm Project [7] An average convergence of this magnitude maintained in a onekilometer-thick airmass for one hour gives an average lifting of the airmass of roughly 500 m Since the convergence at the sea-breeze front on July 24th probably far exceeded this figure, the km lifting required to saturate the lowest points on Results of detailed surface wind observations in many places on Cape Cod on a day similar to the one studied here showed that the components of surface wind normal to the section shown in FIGURE would, if anything, add to the convergence calculated the Nantucket sounding could easily have occurred along this boundary Unfortunately, no wind measurements were made on this particular day at the northern shore of the Cape However, following Wexler [4], the assumption that the sea-breeze front cannot penetrate farther inland than the point at which the marine air becomes heated to the temperature of the inland air gives results consistent with an average inflow of 5-7 mph from the northern beach to the inshore limit of the cumulus formation, about 6-7 miles inland (illustrated in FIG ) Synoptic surface observations showed that upon arrival at Falmouth Airport, the lowest air had warmed 2.2C° above the Vineyard Sound water temperature of 20°C Since the distance, parallel to the wind direction, from shore to airport is 2.2 miles and the wind speed was 15 mph, this represents a heating rate of 15C° per hour Using the temperature at Otis Field at 3:25 p.m E.D.T (FIG ) , and a mean wind speed of 12 mph between shore and Otis Field, the average rate of heating between the Sound and Otis Field (6 miles) is about 8C° per hour, both these figures being in excellent agreement with similar measurements made in Unauthenticated | Downloaded 01/26/22 07:59 PM UTC VOL 32, No 2, FEBRUARY, 1951 sea-breeze air near Danzig This means that the rate of heating between the airport and Otis Field had dropped to about 4C° per hour Extrapolating the latter warming rate (and the Otis Field 65 wind speed of 10 mph) miles further along the surface wind to the cloud boundary gives a temperature of about 26 °C at the sea-breeze front Since the water temperature just north of the Cape FIG Data obtained from horizontal airplane traverses in the cloud area The dry-bulb temperature trace is indicated by T d and the wet-bulb trace by T w The amplitude of the accelerations indicates the development of small scale turbulence but gives no direct information about the presence of drafts, which show up better in the temperatures In the 2800 ft traverse, the displacement between the observer's recording of "under cloud" and the temperature peaks is almost certainly due to the slope of the updrafts with height toward the northeast along the wind shear vector (verified by the time-lapse pictures) rather than due to the asymmetries between draft and liquid cloud discussed by Malkus [10] which must be very sifoall near cloud base, although located similarly with respect to the shear vector These asymmetries, in both turbulence and temperature (slight), appear in the same location with respect to the shear vector on the 3800 ft traverse, despite reversal in direction of flight Unauthenticated | Downloaded 01/26/22 07:59 PM UTC 66 (Cape Cod Bay) is commonly nearly 2C° colder than that in Vineyard Sound to its south, this means that the fresher marine air flowing in from the north (sea-breeze front) must be warmed about 8C° in the 5-7 miles Since the average heating rate over this land mileage might again be expected to be about 8C° per hour, the inflow speed of 5-7 mph observed there on similar sunny days is consistent with all phases of the observations in the present case A detailed airplane microstudy of the cumulus clouds themselves corroborates the above theory concerning their origin Horizontal traverses with accelerometer and recording dry- and wetbulb thermometer were made below and through the clouds An interesting fact revealed by the traverse at the constant altitude of 2800 ft toward 345° was the increase in height of the cloud base from less than 100 ft above the plane at the start of the traverse to 300-400 ft above it at the end, miles later If the cloud base can be identified with the top of the newest marine air, this gives a slope of the sea-breeze front a little steeper than 1/100 along the cross-section perpendicular to the shore (FIG ) , in excellent agreement with the Danzig observations [4] The accelerometer and temperature records made on this same 2800 ft traverse are reproduced in F I G U R E While the cloud "roots" are quite apparent in the turbulence and dry- and wet-bulb traces 100 ft below the cloud base, they can be discerned only weakly at a distance of 300-400 ft under the clouds Further evidence that little of the cloud air was being carried up from the lower levels is given by the cloud direction, which was noted by both plane and ground observers to be from the southwest, the same as the wind direction at cloud level Since the wind underwent a marked turning from northerly at the ground up to southwesterly at about 2800 ft, if any appreciable amounts of air from lower levels were entering the clouds, they would show some component of horizontal momentum in a direction different from that of the ambient wind This effect is discussed quantitatively by the writers elsewhere [1] The fact that the updrafts associated with the The aircraft, how it was flown, and the details of the instrumentation are described in other reports of the Woods Hole Oceanographic Institution [1, 8, 9] BULLETIN AMERICAN METEOROLOGICAL SOCIETY cumulus (to be distinguished from the gradual, more uniform lifting of the entire airmass, which would consist of vertical motions too slow to show up directly on the aircraft records) were thus confined largely to the cloud layer, and that warm, moist bubbles or columns of rising air could not be traced far below the level of the visible cloud seems consistent with the importance of the sea-breeze convergence and lifting on this occasion In ordinary fine-weather cumulus formation over heated land, glider pilots have frequently been able to follow the rising columns from a few hundred feet altitude right on up into the individual clouds Further observational evidence under many conditions of the air beneath cumulus is greatly needed, however, to confirm this point The writers would like to thank Captain John Glascow, U.S.A.F., of the Otis Air Force Base Weather Station for kindly making available much of the synoptic data used in this study REFERENCES [1] Malkus, J S., Bunker, A F., and McCasland, K., 1949: Observational Studies of Convection Tech Rep No 3, submitted to the Office of Naval Research under Contract No N6onr-277, Task Order No II, NR-082-021 Reference No 49-51 of the Woods Hole Oceanographic Institution [2] Leopold, L B., 1949: The Interaction of Trade Wind and Sea-Breeze, Hawaii, Jour Meteor., Vol 6, No 5, pp 312-320 [3] Spilhaus, A F and Miller, J E., 1942: Workbook in Meteorology McGraw-Hill Book Company, Inc., New York, New York, pp 133-134 [4] Wexler, R., 1946: Theory and Observation of Land and Sea Breezes Bulletin of the AMS, Vol 27, No 6, pp 272-287 [5] Koschmeider, H and Hornickel, K., 1936, 1941, 1942: Danziger Seewind Studien I—III Dansiger Meteorol Observ., Forschungsarbeiten, Hefte 8, 10, and 11 [6] Braak, CM 1928: Beobachtungen iiber den Seewind, Ann d Hydr., Jg 56, pp 190-192 [7] Byers, H R and Braham, R R., Jr., 1949: The Thunderstorm Report of the Thunderstorm P r o j ect United States Government Printing Office, Washington, D C., p 21 [8] Vine, A C., 1945: Accelerometer for Air Turbulence Measurements Memorandum on file at the Woods Hole Oceanographic Institution [9] Wyman, J and Collaborators, 1946: Vertical Motion and Exchange of Heat and Water Between the Air and the Sea in the Region of the Trades Report of the Woods Hole Oceanographic Institution (unpublished) [10] Malkus, J S., 1949: Effects of Wind Shear on Some Aspects of Convection Trans Amer Geophys Un., Vol 30, No 1, pp 19-25 Unauthenticated | Downloaded 01/26/22 07:59 PM UTC ... from the veil clouds Plane flight into the cloud area revealed that the veils were all at altitudes close to 3000 ft lb is a section of a photograph taken at nearly the same time as la, enlarged... the Falmouth shore of the Cape The airplane temperature sounding made over Falmouth Airport at 3:15 p.m E.D.T is shown by the crosses quired magnitude actually did take place The appearance of. .. Since the water temperature just north of the Cape FIG Data obtained from horizontal airplane traverses in the cloud area The dry-bulb temperature trace is indicated by T d and the wet-bulb trace