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

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 s

V O L 3 2 , N o 2 , F E B R U A R Y , 1 9 5 1 6 1

A Formation of Pileus-like Veil Clouds Over Cape Cod, Massachusetts lf 2

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

con-trast is indicated as the critical factor in this unusual cloud formation This conclusion is

fur-ther supported by an airplane study of the cumulus structure

IN 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,

tenu-ous, pileus-like veils These first appeared

en-tirely alone so that they resembled many

"eye-brows" 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

vigor-ous cumuli, which occur frequently when

convec-tive 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

1 Contribution No 534 from the Woods Hole

Oceano-graphic Institution

2 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

Nonr-174(00) with the Office of Naval Research and the

Illi-nois 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 superposi-tion of such convergence upon the more direct consequences of heating and mixing the air, similar

to the "Lanai-type" Hawaiian cloud streets dis-cussed 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 2 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 tem-perature 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 de-scription 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 condi-tional instability and a rapid decrease of the dew-point temperature with height The convection condensation level (as defined by Spilhaus and Miller [3]) 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

FIG 1 (a and b) Photographs looking northward from Falmouth Airport at about 2 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

V O L 3 2 , N o 2 , F E B R U A R Y , 1 9 5 1 6 3

ratio constant only up to 1000 ft, gradually

de-creasing 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

un-usually pronounced for the area and time of year

and probably accounts for the form and sequence

of condensation phenomena observed It is

hy-pothesized that when the air flowing from the

southeast over the Cape encountered the

inward-moving sea breeze on the northern shore, it

under-went convergence and bodily lifting Due to the

strong moisture gradient, its lower layers

ap-proached 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

life-time was only 5-7 minutes in toto, and by their

notable lack of development into swelling cumulus despite the slight stability aloft

Without further observational evidence, how-ever, 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 fleet-ingly 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 cumu-lus 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 2 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

64

FIG 3 The Nantucket sounding for 11 a.m E.D.T on July 24 is given by the heavy lines, T

and Td P , 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

"sea-breeze front" when the sea "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

sev-eral 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

cor-responded to a convergence of 1 per hour,3

com-parable to the magnitude of the cumulus-stage

convergence found on the Florida peninsula by

the Thunderstorm Project [7] An average

con-vergence of this magnitude maintained in a

one-kilometer-thick airmass for one hour gives an

av-erage 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 1 km

lifting required to saturate the lowest points on

3 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 2 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 pene-trate farther inland than the point at which the marine air becomes heated to the temperature of the inland air gives results consistent with an aver-age 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 2) Synoptic surface observations showed that upon arrival at Falmouth Airport, the lowest air had warmed 2.2C° above the Vineyard Sound water tempera-ture 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 tempera-ture at Otis Field at 3:25 p.m E.D.T (FIG 2), 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

V O L 3 2 , N o 2 , F E B R U A R Y , 1 9 5 1

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

Extrapo-lating the latter warming rate (and the Otis Field

6 5

wind speed of 10 mph) 6 miles further along the surface wind to the cloud boundary gives a tem-perature of about 26 °C at the sea-breeze front Since the water temperature just north of the Cape

FIG 4 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 peratures In the 2800 ft traverse, the displacement between the observer's recording of "under cloud" and the tem-perature 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 re-spect to the shear vector on the 3800 ft traverse, despite reversal in direction of flight

(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

ex-pected 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 4 of the cumulus

clouds themselves corroborates the above theory

concerning their origin Horizontal traverses

with accelerometer and recording dry- and

wet-bulb 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,

4 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 2), in excellent agreement with the

Danzig observations [4]

The accelerometer and temperature records

made on this same 2800 ft traverse are reproduced

in FIGURE 4 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

dis-cerned only weakly at a distance of 300-400 ft

un-der 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

south-westerly at about 2800 ft, if any appreciable

amounts of air from lower levels were entering the

clouds, they would show some component of

hori-zontal 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

4 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]

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 con-fined 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 sistent with the importance of the sea-breeze con-vergence 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 alti-tude right on up into the individual clouds Fur-ther observational evidence under many conditions

of the air beneath cumulus is greatly needed, how-ever, 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 Re-search 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 Me-teorol Observ., Forschungsarbeiten, Hefte 8, 10, and 11

[6] Braak, C M 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 Proj-ect United States Government Printing Office, Washington, D C., p 21

[8] Vine, A C., 1945: Accelerometer for Air Turbu-lence Measurements. Memorandum on file at the Woods Hole Oceanographic Institution

[9] Wyman, J and Collaborators, 1946: Vertical Mo-tion and Exchange of Heat and Water Between the Air and the Sea in the Region of the Trades. Re-port 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