Theodore V. Blanc,* The Naval Research Laboratory's Air-Sea William J. Plant/+4 and William C. Kellert Interaction Blimp Experiment Naval Research Laboratory Washington, DC 20375
Abstract been at a small number of locations under a limited variety of oceanic environments. Many of these mea- The rationale is given for a unique experiment in which microwave surements were made with restricted upwind fetch scatterometer and surface flux measurements are to be made from lengths, most were made over relatively shallow water, a blimp to develop an improved scatterometer model function. A principal goal of the effort is to obtain a more accurate understand- and only a few were made with a full appreciation ing of the relationship between the surface fluxes and the micro- of how the measurements were affected by distortions wave power backscattered from the surface of the ocean. The induced by the observation platform (Blanc 1983a, limitations of previous overwater surface flux and scatterometer 1985). For these reasons, the Naval Research Labo- measurements are reviewed. The accuracy of various flux mea- ratory (NRL) has initiated a basic research effort to surement techniques are compared. Evidence shows that if direct surface flux measurements are to be accurate to better than 20%, advance the state of flux measurement techniques over the measurements should be made at an altitude of about 5 m to the ocean. 10 m from a platform that is free of flow distortion. The improved The need for improved measurements on a global surface flux measurements are required to test proposed scattero- scale has resulted in considerable effort being ex- meter theories and to determine whether the radar backscatter is pended over the last two decades to develop various principally a function of surface stress or wind speed. It is con- cluded that scatterometer measurements accompanied by eddy- remote-sensing techniques for possible use from earth- correlation technique flux measurements must be made from a orbiting satellites (e.g., Brown et al. 1982; Bernstein platform that is highly mobile and which enables the measure- and Chilton 1985). In particular, the development of ments to be made over a variety of oceanic conditions. To meet a satellite-based technique for remotely measuring the these requirements, the Naval Research Laboratory is undertaking surface stress has received much attention because a series of air-sea interaction experiments in which a sonic ane- mometer and other flux measurement instrumentation are sus- of the limited availability of wind measurements over pended 60 m beneath a blimp flying at an altitude of 70 m while large regions of the ocean that are not frequented by multiple scatterometer measurements are made from the blimp's commercial shipping. The lack of observations in these gondola. Experiments are planned for a wide range of oceanic regions has had a severe impact on the accuracy of environments beginning off the central east coast of the United global weather and ocean wave forecasts. An active States in 1990. microwave system, called a scatterometer, presently seems to be the best candidate for the routine collec- tion of global oceanic wind measurements. A scat- 1. Introduction terometer operates by directing microwave radiation towards the surface of the ocean and measuring the The flux of momentum, heat, and humidity over the microwave power scattered back in the direction from sea must be measured to properly characterize the which it came. From space such a system could op- complex exchange process that occurs between the erate 24 hours a day in virtually any weather, except ocean and the atmosphere. Because three-quarters of heavy rain, to obtain wind information on a global the earth is covered by ocean, understanding this scale. coupled two-way interaction is essential for improv- The manner in which the ocean surface backscat- ing the forecasting of global weather and ocean wave ters microwave radiation is only partially understood conditions. The momentum flux (force per unit area) at the present time. At the relatively large 20° to 60° imparted by the wind to the ocean is commonly called incidence angles typically used in scatterometry, a the surface stress. Up to the present, only a few thou- perfectly flat surface would specularly reflect all of sand hours of high-quality overwater flux measure- the radiated power away from the direction of the ments of any kind have been made, and these have incident beam. The intensity of the radiation scat- tered back to a scatterometer is, therefore, related to the roughness of the ocean. Experiments have shown that a large portion (50% to 80%) of the backscatter * Atmospheric Physics Branch (Code 4110) is the result of Bragg scattering, a resonant-type in- t Space Sensing Branch (Code 8310) t New Address: Ocean Engineering Department, Woods Hole teraction between centimeter-long microwave radia- Oceanographic Institution, Woods Hole, MA 02543 tion and the wind-induced, centimeter-size, gravity
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Unauthenticated | Downloaded 10/10/21 09:04 PM UTC Bulletin American Meteorological Society 355 capillary waves that are propagating in the direction of the radiation on the sea surface (Plant 1986). Al- though other less understood types of scattering such as wedge effects, spray, and bubbles (Lyzenga et al. 1983; Phillips 1988) undoubtedly contribute to the total backscattered signal, assessment of their impor- tance is complicated by the fact that they, like Bragg scattering, vary with wind intensity and incidence an- gle of the radiation. A topic of keen interest within the scatterometry research community is whether the intensity of the backscattered power is more directly the result of surface stress or wind speed. Although the surface stress is largely (about 65%) a function of the average wind speed, it is also influenced by other variables, such as the preexisting sea state. Different amounts of stress, therefore, can be imparted to the ocean at the same wind speed. Given an accurate direct surface stress measurement, which requires making fast-response measurements of all three com- ponents of the wind vector, it is possible to determine an accurate average wind speed. The converse, how- FIG. 1. The relative microwave backscattered power as a func- tion of bulk-determined surface stress for unstable and near-neutral ever, is not true. stability conditions over the Gulf of Mexico. Accurate surface flux measurements are critical to the validation of most scatterometer theories. Such problems involved in developing a more accurate measurements are difficult to obtain over the ocean. model function. The figure shows approximately 240 Very few scatterometer measurements have been made 21.3-min averaged X-band (3.2 cm, 9.4 GHz) micro- simultaneously with direct, high-quality measure- wave backscatter observations and coincident sur- ments of the surface stress. Much of the existing data face determinations made over the Gulf of Mexico base consists of scatterometer results calibrated with (Keller et al. 1985). The measurements were made indirect measurements of the surface stress obtained during a two-week period in the late fall from a tower with the bulk method (e.g., Liu and Large 1981). The located in 32-m-deep water. The antenna system for bulk method is a technique that estimates the fluxes the vertically polarized measurements was pointed from rudimentary measurements of average wind down at an angle of 45° and rotated to face approx- speed, air temperature, humidity, and water temper- imately into the wind. Data in which the antenna's ature. The bulk estimate of the surface stress is based misalignment with the wind was greater than 40° were on the general observation that the stress tends to rejected, and the remaining misalignment influence increase in rough proportionality to the square of the on the measured backscattered signal was corrected average wind speed. Blanc (1987) used a best-case using the model function of Wentz et al. (1984). The scenario to show that, over the range of surface stress meteorological observations were averaged over a 5- magnitudes encountered during these calibration ex- min period at the start of the clock hour and were periments, the uncertainty of the individual bulk-de- made at an altitude of 24.7 m from the same tower rived stress estimates typically ranged from 35% to as the scatterometer measurements. From this infor- well in excess of 100% of their true value. An over- mation, the values for the "standard" meteorological view and history of scatterometry is given by Moore altitude of 10 m were computed. The wind speed at and Fung (1979); Taylor (1985) summarizes many of 10 m ranged from 1 m • s~1 to 13 m • s~\ and the the problems inherent in previous surface stress de- air-water temperature difference at 10 m varied from terminations used to calibrate scatterometer measure- -16°C to +2°C. The surface stress, or downward ments. It is not surprising then, that the relationship momentum flux, and the stability were computed by between the scatterometer output and surface stress using the Large and Pond (1981, 1982) bulk flux is less exact than desired. A principal goal of the scheme. Stress values for which the Monin-Obukhov Naval Research Laboratory effort is to remedy this stability at 10 m (Blanc 1985) were less than -0.16 situation. are labeled as unstable. Most observations were made A scatterometer model function algorithm is re- under near-neutral stability; none were made under quired to translate the scatterometer output into the highly stable conditions. Assuming that the meteoro- desired surface stress or wind speed observation. The logical observations are no worse than those taken data in figure 1 are presented to illustrate some of the from weatherships, Blanc (1987) estimated the sur-
Unauthenticated | Downloaded 10/10/21 09:04 PM UTC 356 Vol. 70, No. 4, April 1989 face stress and stability determinations to have an cidence angle relative to nadir, the direction the scat- absolute accuracy of approximately 50% and 100%, terometer antenna is pointed relative to the wind, and respectively. If a straight-line polynomial curve fit were the polarization of the radiation. The antenna-to-wind used as an algorithm to represent the relationship be- angle is commonly called the azimuth angle and is tween backscattered power and surface stress shown equal to zero when the antenna is looking into the in figure 1, the maximum deviation in the data would wind. The polarization of the radiation is traditionally represent an uncertainty of about 75% in the inferred defined in terms of the direction of the transmitted surface stress. How much of the scatter is due to the electric field. As a general rule, the received back- inaccuracies of the antenna-wind misalignment cor- scattered power is higher for vertical than horizontal rection procedure? How strongly are the surface stress polarization and increases with increasing wind in- determinations influenced by the flow distortion tensity or decreasing incidence angle. It varies with around the observation platform? Is the altitude of the azimuth angle such that the backscatter minima oc- meteorological measurements appropriate for surface cur near the two crosswind directions. Ku-band (2.2 flux determinations? Is the length of the individual cm, 14 GHz) is the most widely used wavelength for meteorological observations of sufficient duration? spaceborne scatterometry. What is the influence of the water depth and local Scatterometer model functions usually relate the surface conditions? If surface stress were measured normalized radar cross section to the surface stress more accurately, would the scatterometer output cor- or a wind speed at an altitude of 19.5 m. Historically relate better with stress or wind speed? Is the apparent this altitude has been used instead of the standard stability influence real? What happens under stable 10-m meteorological height because it was the mean conditions? Clearly, if these and other questions are anemometer altitude for the ships studied in a land- to be answered, improved flux measurements will mark paper by Pierson and Moskowitz (1964). For a need to be made over a wide range of oceanic en- neutrally stable wind speed of 10 m • s~\ Wentz et vironments simultaneously with the scatterometer ob- al. (1984) found that when the incidence angle was servations. changed from 20° to 55°, the typical radar cross sec- tion integrated over all azimuth angles varied from -2 to -18 dB for vertical polarizations and from -3 2. Scientific rationale to -24 dB for horizontal polarizations. Jones et al. (1977) found that when the azimuth angle was a. Scatterometer model functions changed for a given incidence angle, the cross sec- The data presented in figure 1 show only the relative tion varied in a sinusoidal fashion and resembled a microwave backscattered power received by a par- second harmonic function of the azimuth angle. An ticular scatterometer system. To relate scatterometer example of radar cross section variation with azimuth measurements made with different antennas, wave- angle is shown in figure 2. lengths, and altitudes, the scatterometer output is Most scatterometer model functions assume the usually expressed in terms of the normalized radar normalized radar cross section (
FIG. 2. A typical normalized radar cross section shown as a function of azimuth angle from Li et al. (1988). The K^band horizontally polarized scatterometer observations were made from an airplane at an incidence angle of 50° under constant wind speed conditions. A -10 dB change in cross section represents a change in backscattered power equal to one order of magitude. An azimuth angle of 0° indicates that the antenna is looking upwind and an angle of 180° that it is looking downwind. information. More recently, Durden and Vesecky and ships. Each imposes a particular set of limitations (1985), Plant (1986), and Donelan and Pierson (1987) on the type and quality of the measurements. Rotary- have proposed different forms of the model function wing aircraft are unsuitable for such measurements based on the physical principles of Bragg and spec- because of the highly turbulent down-wash generated ular scattering theory. These physically based models by the aircraft's propellers. consider atmospheric stability, water temperature, Although the results presented by Nicholls et al. and dominant surface wave spectra and generally (1983) and Friehe et al. (1988) for a limited number propose a wind speed dependence that is more com- of flux measurements made simultaneously by two plex than a simple power law. airplanes flown side-by-side at an altitude of 90 m The substantial amount of scatter evident in the and 170 m appear to agree very well, airplane mea- figure 1 results is typical of the existing data sets that surements compare less favorably with measure- are based on bulk or other indirect determinations of ments made closer to the ocean surface. Friehe et al. the surface stress and stability. Validation of the phys- (1984) found, for example, that buoy and airplane ically based scatterometer model functions will re- wind speed measurements corrected for altitude quire scatterometer measurements accompanied by could disagree by as much as 30%. This would im- much more accurate measurements of the air-sea sur- ply a surface stress discrepancy of about 60%. Be- face fluxes. In addition to the surface stress (momen- cause of safety and operational constraints, fixed-wing tum flux), measurements of the temperature and aircraft generally cannot be flown low enough to en- humidity fluxes are required to determine the atmo- sure that the measurements are made within the mar- spheric stability. ine atmosphere's surface layer—the approximately 50-m-high region that most influences the ocean. Buoys require highly ruggedized sensor packages b. Surface flux measurements that must survive unattended in the extremely hostile Until the current NRL effort, only four basic types of marine environment while consuming a very limited surface-based platforms had been used for air-sea flux amount of electrical energy. Highly accurate fast-re- measurements: fixed-wing aircraft, buoys, towers, sponse flux sensors usually cannot survive for long Unauthenticated | Downloaded 10/10/21 09:04 PM UTC 358 Vol. 70, No. 4, April 1989 under such adverse conditions. Augstein and Wuck- found that the two disagree by no more than 55%. nitz (1969) found flux measurements to be degraded An additional source of uncertainty exists with the by wave-induced motions of buoys, and Wucknitz three semiempirical flux methods because each (1980) concluded that the measurements can be se- method can be implemented by a variety of schemes. riously distorted by the wind flow around a buoy's For example, depending on which of two principal superstructure. Although considered mobile, buoys profile schemes is selected, Lo and McBean (1978) are usually not easily moved from place to place and found that the same profile observations can yield require a considerable amount of expensive logistical flux determinations differing by as much as 40%. De- support such as oceangoing tugboats and divers. pending on which of 10 published bulk-coefficient Towers are not usually mobile, are available in only schemes is selected, Blanc (1985) found that the same a limited number of locations, and are typically sit- set of ship observations can yield bulk flux determi- uated in relatively shallow water. Thornthwaite et al. nations differing by more than 100%. A similar com- (1965) and Wieringa (1980) demonstrated that the parison among the various dissipation schemes has upwind flow distortion produced by even the rela- yet to be made. tively open structure of a tower can make flux mea- The single most obvious source of flux measure- surements extremely difficult. (See Dobson et al. 1980 ment errors is that produced by the sensors them- and Blanc 1983b for more details.) A survey of tower selves. McBean (1972), Horst (1973), Francey and and mast flow distortion studies is given in Blanc Sahashi (1979), and Finkelstein et al. (1986) have (1986a). demonstrated that the accuracy of flux measurements An analysis of ship model wind tunnel tests found clearly depends on the accuracy of the sensors. In that some ships can induce wind speed measurement the marine environment, propeller, vane, or hot-wire errors as large as 50% at the standard forward-mast devices appear to be particularly susceptible to prob- anemometer locations because of the massive ob- lems because of salt-spray. Although there are some struction that the above-water structure of the ship limitations to the use of sonic anemometers in rain poses to the wind field (Blanc 1986a). Shinners (1970) and heavy fog, they are generally accepted as being argues that because ships absorb solar radiation and the best devices presently available for making the usually contain large internal heat sources, they tend fast-response, three-component wind vector mea- to thermally "pollute" the surrounding atmospheric surements required for direct eddy-correlation sur- and oceanic environment. Additional information face flux measurements. A more detailed discussion about the distortion of flux measurements made from of the difficulties of using flux instrumentation in the ships may be found in Blanc (1986b). marine environment is presented in Dobson et al. There are four principal measurement techniques (1980) and Blanc (1983b). for determining the surface fluxes: eddy-correlation, The appropriate altitude at which to make over- profile, dissipation, and bulk methods. The eddy cor- water surface flux measurements is an important con- relation technique is a direct measure of the flux and sideration. Smith and MacPherson (1987) compared is the most accurate. It requires making accurate and aircraft measurements made at 50 m to buoy mea- very fast measurements of the three-component wind surements and found that the surface stress determi- vector. The other three techniques are semiempirical, nations could disagree by as much as 60%. Panofsky or indirect, measurements (Dobson et al. 1980). As and Dutton (1984) caution that the height of the a general rule, the more accurate the flux measuring atmospheric surface layer can frequently be less than technique, the more difficult it is to execute in the 10 m, particularly under stable conditions. This has field and the more sensitive it is to platform-induced been experimentally confirmed by Hogstrom (1988) distortions. and others. At the other extreme, wind profile mea- J. C. Kaimal (pers. com.) has found that side-by- surements made by Krugermeyer et al. (1978) and side eddy-correlation surface stress measurements Hasse et al. (1978) showed that if the surface stress made with similar properly designed sonic anemo- measurements are made at an altitude that is less than meters usually agree to better than about 5%. Miyake two or three times the wave height, they will be con- et al. (1970) and Wucknitz (1976) compared simul- taminated by the local flow distortion induced by the taneous over-water profile and eddy-correlation stress large-scale undulating wave field. The minimum measurements and found the two to disagree by no measurement altitude is also a function of the sensing more than 25%. Large and Pond (1981) and Smith volume of the flux array and the atmospheric stabil- and Anderson (1984), in similar comparisons be- ity. An analysis conducted by Kaimal (1975) deter- tween dissipation and eddy-correlation measure- mined that the minimum altitude occurs during ments, found the two to disagree by no more than unstable conditions and that for a typical sonic-ane- 40%. A similar comparison of bulk and eddy-corre- mometer arrangement the altitude should be no less lation stress determinations reported by Smith (1980) than 4 or 5 m. As a general rule, the more accurate
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the measurement technique, the more critical the se- m • s_1 could be obtained in one-fourth the time from lection of the appropriate measurement altitude. a platform moving into the wind at an air speed of Although the atmospheric surface layer itself is tac- 20 m • s~1. The ground speed of the moving platform itly assumed to be a region of uniformly distributed would be 15 m • s_1. (For a more detailed discussion flux, Dyer and Hicks (1972) have noted that the con- of the Taylor hypothesis, see Panofsky and Dutton cept of a constant flux layer allows vertical varia- 1984.) tions of about 10% or 20%. Vertical variations in the Whether air-sea interaction flux measurements are flux of this magnitude have been experimentally con- made for their own sake or to develop an improved firmed with eddy-correlation measurements reported scatterometer model function, the following six po- by Kaimal (1969), Haugen et al. (1971), and Hogs- tential difficulties are inherent to all overwater surface trom (1988). Prudence, therefore, dictates that sonic flux measurement efforts: (1) the variety of accessible anemometer surface flux measurements be made as oceanic environments, (2) the distortions produced close to the surface as possible, somewhere between by the observation platform, (3) the limitations of the about 5 and 10 m depending on the waveheight and measurement technique, (4) the inaccuracies of the the stability. The recent findings of Geernaert (1988) sensors, (5) the appropriateness of the measurement would appear to confirm this strategy. altitude, and (6) the sufficiency of the measurement Previous field experiments, for example those by duration. Geernaert et al. (1987) and Katsaros et al. (1987) made We concluded that development of a more accu- from stationary locations, typically used sonic ane- rate scatterometer model function will require sonic mometer surface flux measurements made over pe- anemometer (eddy-correlation) flux measurements, riods of 10 min to 60 min duration. The required free of flow distortion, for durations of approximately duration of flux measurements has been examined by 60 min, from a platform that is highly mobile and Haugen et al. (1971), Wyngaard (1973), and Kaimal that enables the flux measurements to be made within (1975). They found the duration to be a function of a few meters of the ocean's surface over a wide range measurement technique and atmospheric stability. As of water depth, sea state, and surface wave condi- a general rule, the more accurate the measurement tions. technique or the more unstable the atmosphere, the longer the required measurement period. Their anal- yses determined that the minimum period needed to obtain a single eddy-correlation flux measurement 3. Experimental approach from a stationary location was 60 min. If, on the other hand, a single measurement were to be extended be- To overcome or minimize the flux measurement dif- yond a period of about 120 min, the findings of Fied- ficulties, Blanc (1985) proposed the use of a blimp, ler and Panofsky (1970) suggest the measurement or airship, as a "sky hook" for making over-water would begin to be distorted by diurnal variations. surface flux measurements simultaneously with local Although most experimenters prefer to use shorter pe- scatterometer measurements. Haugen et al. (1975), riods of duration than 60 min to simplify their data Ogawa and Ohara (1982), and Lapworth and Mason analysis procedure and maximize the size of their (1988) have demonstrated that overland eddy-corre- data bases, they do so at the risk of increasing the lation flux measurements can be made from tethered uncertainty of their final results. balloons. Wind tunnel boundary layer studies con- The required duration for flux measurements can ducted on a scale replica of an Airship Industries Model be decreased by making the measurements from a 600 airship by Wills and Cole (1986) demonstrated moving platform. Consider two flux measurements, that if a flux-sensor array was suspended four blimp initiated from the same location, in which one mea- diameters beneath the vehicle, the flux measure- surement is kept stationary and the other measure- ments would be unaffected by the presence of the ment is made at the same altitude from a platform blimp. By using the work of Wyngaard and Zhang that is moved into the wind. If we accept Taylor's (1985) and Conklin et al. (1988) as guidelines, we "frozen-turbulence" hypothesis that the fluctuations plan to correct our sonic anemometer measurements in the atmosphere are carried by the mean wind and for the local sonic transducer-induced flow distortion change slowly compared to their movement, it can by a method described by Kaimal and Gaynor (1983). be seen that the reduction in the measurement du- The temperature and humidity fluxes will be mea- ration is proportional to the ratio of the mean wind sured to determine the atmospheric surface layer sta- speed observed at the stationary location and the air bility. A Ku- and/or X-band scatterometer system is to speed of the moving platform. Virtually the same ob- be mounted to the airship's gondola. servation that required a measurement duration of 60 Figure 3 shows one of the airship platforms being min from a stationary location at a wind speed of 5 considered for the NRL air-sea interaction experi-
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FIG. 3. The Airship Industries' Model 600 airship at take off. The vectored fan propellers at the rear of the gondola provide the platform with an enhanced maneuvering capability. It can operate within a maximum radius of about 500 km from a base facility.
merits. It is approximately 60 m long, 15 m in di- study the poorly understood phenomena of cloud-top ameter, and has a lift capacity capable of carrying entrainment. An airship would provide an ideal plat- the crew and two scientists, with 1000 kg of equip- form for conducting such a study. A meteorological ment on a 1000-km round trip. The gondola's main sensor array could be lowered from a hovering air- cabin has approximately 34 m3 of space available for ship into the cloud-top interfacial layer where the scientific equipment and personnel. An experienced entrainment takes place. The importance of making pilot of the airship has indicated that with a minimum Lagrangian measurements in studying atmospheric airspeed of 4 m • s~\ a sensor array on a 60-m tether diffusion is discussed by Hanna (1981). could be kept at an altitude of 5-10 m (to within Figure 4 depicts the suspended meteorological flux- about 2 m). Because the airship would allow flux sensor array consisting of a sonic anemometer, a fast measurements to be made from within a few meters thermometer, a fast humidity sensor, and an inertial- of the ocean's surface to an altitude of 3 km while reference sensing system. The inertial-reference sys- either stationary or moving at air speeds up to 25 tem contains three orthogonally mounted acceler- m • s~1, it is ideal for comparing tower, ship, buoy, ometers fixed to the sonic anemometer's frame of and fixed-wing aircraft measurements, as well as for reference and two gyroscopes to measure the pitch, profiling the entire atmospheric boundary layer. roll, and yaw of the array. The flux and inertial data In addition to making the conventional Eulerian- are passed through 10-Hz antialiasing filters into an type atmospheric measurements, an airship has the onboard computer and digitized 20 times per second unique capability of being able to drift with the mean (Kaimal 1975). The sensor array will also contain a wind and potentially could be used for making La- laser altitude and wave height measuring device for grangian-type measurements. As described by Al- making wave measurements and to enable the airship brecht et al. (1988), a substantial experimental and pilot to fly the array at a desired altitude. A group of modeling effort is presently underway to better un- slow-response sensors will be used to measure the derstand the radiative and microphysical effects in- ambient wind speed, wind direction, air temperature, volved in the evolution of marine stratocumulus humidity, and barometric pressure relative to the ar- clouds. A future emphasis within this effort will be to ray. Other devices will measure the sensor array's
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FIG. 4. An artist's conception of a prototype design for the meteorological flux sensor array to be suspended beneath a hovering airship. The fast flux sensors are placed as far forward as possible to minimize any flow distortion from the rest of the array.
compass heading, the infrared surface temperature of measurements at azimuth angles of 0°, 90°, 180°, and the water, and the bulk (or "bucket") water temper- 270°. The array will be configured so that the inci- ature. dence angle at the center of each antenna's footprint Once every 0.1 s, the inertial data will be used to will be 45°. An incoherent pulsed-microwave source correct information from the sonic anemometer flux will be range-gated to allow measurements at inci- sensor for the movement and tilt of the sensor array. dence angles of 25°, 35°, 45°, and 55° from each This is to be done to an accuracy of 0.1°, or better, antenna. To minimize the beam spread at each in- as prescribed by Kaimal and Haugen (1969). To en- cidence angle, the beam pulse width will be kept to sure that this has been done properly, we plan to less than 100 ns. Inclinometers will be attached to initially fly the blimp-borne system alongside a sta- the antenna array to allow the backscattered signal tionary system at the Boulder Atmospheric Observa- to be corrected for the pitch and roll of the airship. tory (Kaimal and Gaynor 1983) to independently verify It is planned that measurements will generally be taken our procedure. with the airship heading into the prevailing wind. A The scatterometer system will employ eight micro- magnetic-direction sensor will be used to measure wave antennas rigidly mounted to the exterior of the the difference between the airship's heading and the airship's gondola. Each antenna will be of a fan-beam wind direction as measured from the suspended design allowing an approximately 40° vertical and meteorological array. A highly accurate altimeter will 2.5° horizontal beam width. The antenna array will be used to measure the height of the antenna array. consist of four pairs of antennas, each pair containing The pulsed system will be timed to enable all four one vertically and one horizontally polarized an- incidence angle range cells of a given antenna to be tenna. The antenna array will point forward, star- collected within about 1 jus. A switching system will board, aft, and port of the airship. With the airship be used to direct the operation around the antenna heading into the wind, this will allow scatterometer array so that all eight antennas will be sampled every
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0.2 s. Since the Doppler bandwidth of the return sig- first integrated-system flight tests are planned for nal will be much greater than the data sampling rate, 1990. It is anticipated that a total of about 1000 flight each sample will be an independent measurement of hours, based from a number of locations during a five- the surface backscatter. The system is configured to year-period, will be required to ensure that the mea- allow 32 essentially simultaneous scatterometer mea- surements are made over a sufficiently diverse vari- surements to be made at a single time—two polari- ety of environmental conditions. zations, four azimuth, and four incident angles. Simultaneous flux and scatterometer measure- ments are to be made from the blimp, while both 5. Summary underway and at stationary locations. To ensure that each flux measurement is based on a statistically Existing experimental evidence demonstrates that a meaningful data sample, the autocorrelation criterion new and radically different approach is required to described in Blackman and Tukey (1958) and Bendat develop an improved scatterometer model function. and Piersol (1980) will be used to objectively deter- More accurate surface flux measurements are re- mine the appropriate measurement duration. Gaynor quired to validate most scatterometer theories and to and Mandics (1978) and Khalsa (1980) have studied determine whether the microwave power backscat- the periodicity of the marine atmospheric surface layer tered from the surface of the ocean is more directly and found that under unstable conditions it typically related to surface stress or wind speed. To achieve has a period of about 10 min. To optimize the num- these goals, the Naval Research Laboratory is under- ber of flux measurements, the example of Donelan taking a series of experiments in which coincident et al. (1974) will be followed and a sliding-measure- scatterometer and surface flux measurements are ment-period arrangement initiated at 10-min intervals made from a blimp over a wide variety of oceanic will be used. For example, if data are taken at a sta- environments. The use of a blimp will enable the tionary location continuously for 4 hours and each surface flux measurements to be made within a few flux measurement made during this time needs to be meters of the sea surface free of the distortions as- of 60-min duration, this arrangement would allow us sociated with conventional observation platforms. to have a total of 19 measurements.
Acknowledgments. The fruition of the project was made possible 4. Field sites and schedule by the moral support provided by J. A. Businger, L. Cavaleri, M. A. Donelan, C. W. Fairall, M. H. Freilich, P. Frenzen, L. M. Ham- marstrom, L. Hasse, S. A. Hsu, R. W. James, K. B. Katsaros, J. C. There are many World War ll-vintage Navy blimp Kaimal, W. G. Large, T. J. Lockhart, D. H. Lenschow, E. C. Mon- aerodrome facilities still in existence throughout the ahan, V. E. Noble, W. A. Oost, R. E. Payne, W. J. Pierson, J. H. world. For example, such facilities can be found in Richter, L. H. Ruhnke, R. J. Serafin, S. D. Smith, P. A. Taylor, and J. A. B. Wills. We are indebted to these colleagues for their un- England at Cardington and in the United States near flagging encouragement over the last five years. These experiments San Francisco, New York, Key West, and Norfolk, to are funded in part by grants from the Naval Research Laboratory's name a few. These and other locations afford excel- Director of Research, the Space Oceanography Program of the lent access to good and potentially diverse oceanic Naval Research Laboratory, and the Ocean Engineering Remote- sites. The blimp facility near Norfolk at Elizabeth City, Sensing Program of the Office of Naval Research. North Carolina, was selected as the base for the first experiments because the oceanic and atmospheric References climatology of the area is well documented (e.g., Saunders 1977). As can be seen from a map of the Albrecht, B. A., D. A. Randall, and S. Nicholls. 1988. Observa- selected region shown in figure 5, the proximity of tions of marine stratocumulus clouds during FIRE. Bull. Am. the Gulf Stream to the coast allows ready access to Meteorol. Soc. 69: 618-626. both warm-water (unstable) and cold-water (stable) Augstein, E., and J. Wucknitz. 1969. The quality of wind speed environments. Water depths within the region range measurements on a semistabilized buoy. "Meteor" Forschung- from a fraction of a meter to in excess of a kilometer. sergeb, Reihe B. No. 3: 27-32. Bendat, J. S., and A. G. Piersol. 1980. Engineering applications The diversity of experiment sites will facilitate the of correlation and spectral analysis. New York: John Wiley and study of the surface layer modifications produced by Sons. discontinuities in the water temperature field and the Bernstein, R. L., and D. B. Chelton. 1985. Large-scale sea surface influences of wind speed, current, water depth, sea temperature variability from satellite and shipboard measure- state, rain, surface spray, and snow on the scattero- ment. J. Geophys. Res. 90(C6): 11, 619-630. Blackman, R. B., and J. W. Tukey. 1958. The measurement of meter observations. power spectra from the point of view of communications engi- Land-based trials are presently underway to test and neering. New York: Dover Publications. calibrate the individual measurement systems. The Blanc, T. V. 1983a. An error analysis of profile flux, stability, and Unauthenticated | Downloaded 10/10/21 09:04 PM UTC Bulletin American Meteorological Society 363
FIG. 5. The region of the central east coast of the United States selected as the location for the first experiments. The dashed lines indicate the water depth. The operations will be based at the blimp facility near Elizabeth City, North Carolina. The region affords access to a diverse variety of oceanic experiment sites.
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