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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 354 Vol. 70, No. 4, April 1989 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.

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