E. B. Kraus The Bahama Bank Planetary Rosenstiel School of Marine and Atmospheric Science Division of Atmospheric Science Boundary Layer Experiment University of Miami 17 Aprii-io May 1971 Coral Gables, Fla.

1C03 Abstract The Bahama Banks Experiment was conceived pri- marily to obtain additional data from the marine atmo- A field program to study the marine planetary boundary sphere that might be used to investigate the validity—- layer was carried out jointly by scientists and students and the limitations—of various existing, steady-state from the Universities of British Columbia, Hamburg theories. At the same time, we tried to organize measure- and Miami and by personnel from the NOAA Sea-Air ments in a way which would permit us to approach the Interaction Laboratory. The following article describes study of the time-dependent case. briefly the basic purpose, location, timing and execution of this work. 2. Participants The experiment involved informal cooperation between 1. Introduction several groups, each with its particular area of compe- The convergence, of horizontal transports in the tence which complemented that of the other, but also planetary boundary layer is an essential link in the with individual goals that could be pursued inde- dynamics of atmospheric and oceanic circulations. In pendently. We envisaged deliberately a relatively inex- the tropical atmosphere, it affects not only the filling or pensive operation, believing that the disadvantages of deepening of depressions, but also the amount of latent improvisation were balanced in this case by greater heat with which they are supplied. In the oceans, flexibility in the choice of place and time and by the divergence of the integrated boundary layer mass trans- possibility of repeating the work if that was found port causes upwelling with all its mechanic, climatic and desirable. ecological consequences, including the transfer of vor- The work was carried out by groups from three Uni- ticity to the ocean interior. versities—Miami, Hamburg and British Columbia—with Boundary layer transports, particularly in the atmo- the active help of persons from the NOAA Sea-Air Inter- sphere, have been the subject of many investigations. In action Laboratory. The University of Miami group spite of prolonged and intense research, however, it has concentrated on the development of pressure sensors not been found possible so far to match theoretical and the establishment of an array that could be used to results systematically with observed wind profiles in the establish time-series of the surface geostrophic and planetary boundary layer. Although hundreds of pilot perhaps gradient wind components with an accuracy of balloons and rawinsonde observations are broadcast and about 50 cm sec"1. Time-series of the actual surface wind published daily all over the world and although wind were obtained simultaneously. Both the geostrophic and profiles over the sea have been an object of several the actual wind were sampled at 80-sec intervals. special expeditions (e.g., Charnock et al., 1956), authors From the Meteorological Department of the Univer- of recent theoretical papers (Deardorff 1970; Swinbank sity of Hamburg came a team of eight scientists, students 1969) still compare their predictions with the forty-year and technicians headed by Dr. H. Hoeber. This group old "Leipzig wind profile" (see Lettau, 1950). concentrated on the sampling of the wind profiles in Part of the trouble is that most theoretical studies the marine planetary boundary layer by double-theodo- presuppose conditions that are statistically stationary, lite pilot balloon observations. Being concerned with horizontally homogeneous and hydrostatically neutral the structure and the time-dependent behavior of the throughout the planetary boundary layer. It is very rare boundary layer over the sea, their program called for for assumptions of this type to be realized, even approxi- ascents to be carried out day and night at about 15- mately, in nature. Theories that presuppose such rare minute intervals. conditions cannot be reconciled easily with field obser- Dr. M. Miyake and his associates from the Univer- vations. On the other hand, most of the available ob- sity of British Columbia were interested primarily in servational material is not very suitable for the study of the different characteristics of the covariance spectrum transients with characteristic time scales that do not between the vertical velocity and the associated anoma- match the sampling frequency. lies of the horizontal velocity, temperature and humidity i Contribution No. 1422 from the University of Miami, a few meters above the sea surface. To establish these Rosenstiel School of Marine and Atmospheric Science. quantities they used a three-dimensional sonic anemom-

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eler and a Lyman-Alpha humidiometer (Miyake and Mc- transport theory that the local momentum flux is propor- Bean, 1970). tional to the local velocity shear multiplied by some Willard W. Shinners from die NOAA Sea-Air Inter- local coefficient of eddy viscosity. This only transfers the action Laboratory and his colleagues released special problem, however, because the local eddy viscosity radiosondes at six-hourly intervals. These were tracked depends again on the velocity shear everywhere else. by single theodolites for wind information. Time-lapse, Another approach is based on dimensional consider- all-sky movies were taken also during the whole duration ations. Following an earlier study by Kazansky and of the experiment. Monin (1960), it was argued by Czanady (1967), by Gill Additional investigations which made use of the avail- (1968) and by Blackadar and Tennekes (1968) that the able boats and background data but did not form part turbulent flux in hydrostatically neutral conditions can of the main experiment were carried out by Dr. D. J. be related to two length scales. Immediately above the Latham, who dealt with the flux of electric charge surface, in the so-called constant stress region, eddies from the sea, and by Dr. Kirby Hanson who was in- remain influenced by the surface geometry, which may terested in the flux of radiation above and below the be characterized (hopefully) by the so-called surface interface. We did not seek a wider extension of the pro- roughness length Zo. Farther away in an outer region of gram as it was believed that the resulting logistic com- the boundary layer they are related to G and to a length plications would outweigh the benefit of additional, u*/f where w*—the friction velocity—is defined by: complementary data that might have been collected. To = pu* u*. 3. The planetary boundary layer The argument does not yield explicit values for the A boundary layer is defined as a region in which vari- velocity distribution in the outer region. However, if ations normal to a surface of discontinuity or wall one assumes that there is no discontinuity between the surface are much larger than variations parallel to it. two regions, one can eliminate any explicit reference to This is almost always true for the layers above and the velocity and obtain a relationship between the fric- below the sea surface. Thus, to a first order of approxi- tion velocity u* and the geostrophic wind G, mation, one can there neglect the horizontal variations of the mean horizontal velocities. The averaged equa- ft (u*, G;z0,f, A, B,) = 0. (3) tions of horizontal motion then establish a relation Details can be found in the quoted papers. The quan- between the mean velocity vector U, the Coriolis force as tities A and B are empirical parameters. With u* deter- specified by the parameter f, the geostrophic wind vector mined from Eq. (3) one can establish the total hori- G, which is simply a convenient representation of the zontal mass transport in the boundary layer without pressure gradient, and the convergence of the vertical great difficulty. Because the surface wind stress on the flux of horizontal momentum x. sea is produced mainly by short capillary or capillary-

F(U, G, dt/dz;f)z=zn = 0. (1) gravity waves, Zo appears to remain nearly constant over a wide range of wind velocity. In storms it may con- The relation (1) is applicable in its explicit form to any ceivably increase with u*2. particular level z = z and all the variables refer to the n It is now seen how the various programs in the same level. Bahama Banks experiment interact. Observations of U The momentum flux or Reynolds stress which occurs and G by the Hamburg and Miami groups may allow in Eq. (1) depends in turn on the velocity distribution. us to deduce dt/dz as a function of height and time with In hydrostatically neutral conditions it does not depend the aid of Eq. (1). The actual stress can then be obtained directly on anything else. It follows that the mean by a process of integration. Similarly the observations of velocity field should be fully determinable to first w* by the Canadians together with the known values of G order by the values of the pressure gradient or geo- may allow us to establish the validity of the relation (3) strophic wind alone. The difficulty arises from the fact, and the numerical value of the parameters A and B. that while the equation of horizontal motion (1) does All this does not imply that observations, like those connect the local mean velocity with the local pressure and stress gradients, the value of the latter is not de- carried out on the Banks, can yield a definite solution of termined locally but depends on the velocity distribution the planetary boundary problem. In particular, the throughout the fluid. studies that led to explicit forms of Eq. (3) involve reliance on speculative arguments about the character-

Tz=zn = 4>[U(*)]o<*

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4. Place, time and execution We chose the Grand Bahama Bank as a site mainly be- cause it is close to Miami—an air ticket to Bimini costs only twelve dollars—and because we wanted to avoid the use of deep water, instrument buoys. The Banks cover a vast area in which air flows unobstructed over the sea. The shallow water depth—about five meters on the average—suppresses long gravity waves, but as these do not seem to contribute directly to the wind stress in any significant amount, this was not considered a crucial impediment in an experiment with FIG. 1. Typical record of diurnal, tidal surface air pressure a rationale based on first-order approximations. variation on Orange Rock (1 volt corresponds to 1 mb). To study the pressure field we installed an array of five sensitive, recording pressure transducers. The array tidal pressure range of nearly four millibars is associated had the shape of a diamond, with east-west and south- with changes of almost one meter per second in the north diagonals and a side length of about 100 km as north-south geostrophic wind component. When condi- shown in Fig. 2. The desirable accuracy of the sensors tions are disturbed synoptically, this tidal effect becomes was prescribed by the relatively small-scale lengths and relatively less important, but other transients then be- low latitude. As we wanted to establish the geostrophic come more pronounced instead and some of these are wind components with an accuracy of 50 cm sec"1, it was likely to interact with the time scale of 7r// •—• 16 lir, necessary to establish pressure differences with an error which characterizes the adjustment of a planetary bound- smaller than 40 dynes cm-2 (=0.04 mb). This corresponds, ary layer in the latitude of the Bahamas. however, to a geopotential difference of less than 40 In non-neutral boundary layers, the parameters A and cm. With the relative local tide heights known only B in Eq. (3) can be considered functions of the hydro- approximately, we were not able to establish the geo- static stability. Investigations of this matter by Clarke potential height of our sensors much more accurately (1970) and others involve a stipulated constant lapse than that. It is proposed, therefore, to adjust the mean rate. In reality this again is rarely true over the oceans. or zero reading for each station by fitting it to a series A low-level inversion above near neutral surface layers of synoptic analyses on a somewhat larger scale. For usually makes the lapse rate highly nonlinear. Deardorff's this purpose, we have backed up our measurements with integrations as well as observations suggest that it is the an array of standard microbarographs and have ar- inversion height and not the value of u*/f which deter- ranged also to obtain all the available synoptic surface mines the dominant scale length in this case. This height data. generally is not stationary in time. Geisler and Kraus The central station of the pressure array was estab- (1969) have treated this subject with the assumption lished on Turtle Rocks, a chain of small, uninhabited that momentum, as well as potential density, is dis- coral islands which extend for about 1500 m in a north- tributed uniformly along the vertical within the mixed south direction. The mean height of the islands is less layer, except for the immediate vicinity of the interface. They found an increasing departure of the layer wind from geostrophic equilibrium with decreasing inversion height, but actual values depend on the representative- ness of their rather crude approximations. We may learn more about this matter from a comparison be- tween the pilot balloon data and the radiosonde measurements obtained by the NOAA team, though the six-hourly interval between radiosonde ascents makes this prospect rather uncertain. For all these reasons it can be seen that there is room for improved observational programs in the marine planetary boundary layers. Significant progress may come from an application of remote sensing techniques which would allow continuous observation of the actual perturbation pattern in the mixed layers and in the limiting inversions and thermoclines. In the meantime, we do hope that the results obtained from our program will allow some confirmation and modest refinement of FIG. 2. Array of pressure and wind sensors. Static locations the established, empirical and dimensional relationships. are marked by triangles.

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than 2 m with a maximum height of 5 m. Their width Acknowledgments. If the Bahama Banks Experiment is nowhere more than about 50 m along the edge of turns out to be profitable, it will have been due to the Grand Bahama Bank some five miles south of the students, technicians and scientists who applied Bimini. The length of the island chain was used by the themselves to it with a cheerful spirit, in conditions that University of Hamburg group as a baseline for their were sometimes rough and at other times tedious; two recording theodolites. Balloons were released from releasing balloons and manning theodolites through a ship positioned on the Banks some 3 km to the east— 12-hr watches day and night; maintaining equipment first the R.V. Gerda and then the R.V. Calanus of and being pounded in small boats as they went about the University of Miami. The U.B.C. group installed its their tasks. recording gear next to the pressure recorder on the We are indebted to Mr. Ostapoff for his helpful coop- ' central island of the chain, where power was made eration and for arranging the participation of the available from a portable generator. The sensors were German and Canadian groups. The American Museum exposed on a mast in the water about 60 or 70 m east of Natural History made the facilities of the Lerner of the rock edge. Radiosondes, sky camera and radiation Marine Laboratory in Bimini under the direction of Dr. measurements were carried out from N. Bimini. Mathewson available to us. This involved personal ac- We wanted to investigate primarily the boundary commodations, laboratories and boats—including the layer in undisturbed, trade wind conditions. From the wayward Katie. Some other equipment was lent to climatic records, it appeared that April and May were us by NCAR. particularly favorable for this purpose. During winter, The work of the University of Miami group was sup- westerlies are not uncommon, and the area is affected ported by NSF under grants GA 23169 and GA 28275, occasionally by the passage of fronts; in summer, it is that of the University of Hamburg by the German Re- affected by tropical perturbations. search Council (Deutsche Forschunggesellschaft) under As usual, the weather did not conform entirely to their GARP program; the University of British Colum- expectations. Spring 1971 proved to be one of the bia team was supported by the Canadian Research coldest and latest on record in the southern United Council and NOAA's Sea-Air Interaction Laboratory. States. In the Bahamas we still experienced westerlies and fronts in late April, which is unusual. One night, References with a pre-frontal southwesterly of 30 kt, the tide and Blackadar, A. K., and H. Tennekes, 1968: Asymptotic simi- surf rose almost to the level of one of the spray- larity in neutral barotropic planetary boundary layers. drenched theodolite stations. Unfortunately, some water J. Atmos. Sci., 25, 1015-1020. damaged the theodolite recording gear which was re- Charnock, H., J. R. D. Francis and P. A. Sheppard, 1956: An paired only after several days of hard effort. investigation of wind structure in the trades: Anegada, The most dramatic incident of the expedition oc- 1953. Phil. Trans. Roy. Soc. London, A249, 179-234. Clarke, R. H., 1970: Observational studies in the atmospheric curred at the very end. For the last three days, when boundary layer. Quart. J. Roy. Meteor. Soc., 96, 407, 91- we had no regular research vessel available, we chartered 114. a covered workboat—the Katie—as a balloon launching Csanady, G. T., 1967: On the resistance law of a turbulent platform. She was at anchor station, and there was Ekman layer. J. Atmos. Sci., 24, 467-471. only one man, John Pifer, on board to release the bal- Deardorff, J. W., 1970: A three-dimensional numerical in- loons. The Katie leaked. At night she took in water vestigation of the idealized planetary boundary layer. faster than the pumps could operate. Pifer radioed for Geophys. Fluid Dyn., 1, 4, 377-410. help at 3 a.m., only to be told by the theodolite crews , 1970: Preliminary results from numerical integrations that his next balloon was due in 7 min. With an un- of the unstable planetary boundary layer. J. Atmos. Sci., becoming lack of devotion, he inflated his rubber raft 27, 1211-1213. instead. A few minutes later, the Katie settled in 15 ft of Geisler, J. E., and E. B. Kraus, 1969: The well-mixed Ekman boundary layer. Deep-Sea Res., Supp. to Vol. 16, 73-84. water. Apparently, this was not the first time she had Gill, A. E., 1968: Similarity theory and geostrophic adjust- done that, though we did not know about it before. ment. Quart. J. Roy. Meteor. Soc., 94, 586-588. In spite of such tribulations, we were able to obtain Kazansky, A. B., and A. S. Monin, 1960: A turbulent regime a series of promising data. In particular, there were above the ground atmospheric layer. Izv. Acad. Sci., six days in early May when all systems were working, U.S.S.R. Geoph. Ser. No. 1, 110-112, English translation, and all groups obtained their measurements simultane- A.G.U. ously, in the sort of trade wind conditions which we had Kraus, E. B., 1972: Atmosphere-Ocean Interactions. Oxford expected. The fact that, earlier in April, we had been University Press, 265 pp. Miyake, M., and G. McBean, 1970: On the measurement of able to collect data in westerly, frontal conditions vertical humidity transport over land. Boundary-Layer may actually contribute to the value of the whole ex- Met., 1, 88-101. periment. Full evaluation of the record will necessarily Swinbank, W. C., 1969: Structure of wind and the shearing take some time, though we hope to have preliminary stress in the planetary boundary layer. NCAR Manuscript results availably before the end of this year. No. 69-60.

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