THE SAR MEASUREMENT OF OCEAN SURFACE WINDS: AN OVERVIEW 1 2 Frank Monaldo , Vincent Kerbaol and the SAR Wind Team 1Johns Hopkins Uni versity, Applied Physics Laboratory, Laurel, MD 20723-6099 USA , [email protected] 2BOOST Technologies, 135 rue Claude Chappe, 29280Plouzané, France, Vincent.K [email protected] ABSTRACT Figure 1 is an example of the sort of high-resolution wind field available from SAR imagery. Wind speed in this im- age is computed from RADARSAT-1 SAR imagery over This paper represents a consensus on the state-of-the- the Aleutian Islands. Note the vortices and gap flows gen- art in wind field retrievals using synthetic aperture radar erated by wind flow over the islands visible only in high (SAR) as it emerged during the 2nd Workshop on Coastal resolution imagery. and Marine Applications of SAR in Longyearbyen, Spits- bergen, Norway, held on 8–12 September 2003. This From a practical perspective, the high-resolution wind work is the result of the efforts of many co-authors. The fields from SAR imagery offer the prospect of making length of the author list a compels us to include it as a important contributions to many applications: footnote,1 but the contributions of these co-authors rep- resent far more than a footnote to this paper These con- Weather analysis and prediction: When available on tributions were critical to completeness and accuracy of a timely basis, SAR wind fields can aid local forecast- this paper. ers. Moreover, the understanding of wind field dynamics possible from the routine production of SAR wind field This paper documents the substantial progress that has retrievals in coastal areas will allow meteorologists to been accomplished in SAR wind speed retrieval. The extend and improve coupled atmospheric-ocean numer- consensus is that SARs can estimate wind speed to better ical models allowing enhanced high-resolution wind pre- than 2 m/s and wind direction to 25 ¡ with clear evidence dictions even when SAR data are not available (Young that these values can be improved upon. 2000). Climate Research: An improved understanding of wind field dynamics in general and in coastal areas in particular 1. INTRODUCTION will aid in the climate prediction modeling. It is in coastal areas that potential climate change will have its largest impact on humans. The ability to retrieve wind fields from synthetic aperture Risk Management: Statistical analysis of SAR high- radar images, with the high resolution (sub-kilometer) resolution wind retrievals will aid in the assessment of and wide coverage (500 km) represents an important im- risks relevant to marine engineering, environment pollu- provement for applications where knowledge of the wind tion, security, search and rescue, and defense. field at tine spatial scales is crucial. SAR wind fields have made conspicuous marine atmospheric phenomena that Commercial Consequences: High-resolution winds can were known to exist, but were previously difficult to mea- be used to aid energy production (the placement of wind sure and monitor. For example, we can identify the loca- turbine farms), improve ship routing, and provide data tion of synoptic scale and mesoscale fronts and vortices, useful to naval architects designing ships to operate in including hurricanes and polar and mesoscale cyclones. the coastal environment. Moreover, we can observe a host of additional mesoscale and microscale phenomena such as cellular convection, Other SAR Applications: SAR-derived wind fields can roll vortices, gravity waves, gap flows, barrier jets, and contribute to the usefulness of other SAR applications. von K´arm´anvortices. For example, SAR winds can aid wave field retrieval by providing a first guess wind-wave spectrum. Algorithms for automated detection of oil spills and ships as well as ice identification can be improved with knowledge ofthe local wind field. 1Pablo Clemente-Col´on, Birgitte Furevik, Jochen Horstmann, Johnny Johannessen, Xiaofeng Li, William Pichel, Todd Sikora, Donald The recent availability of calibrated wide-swath SAR Thompson, and Christopher Wackerman. imagery from RADARSAT-1 has inched us closer to down to a resolution of a few kilometers when com- pared to other measurements. Corresponding wind directions have been retrieved to an RMS accuracy of 25 ¡ at a spatial resolution of 25 km. 2. Quasi-operational delivery of SAR winds speeds has been demonstrated with delivery times on the order of 3 hours from time of data acquisition at the satel- lite. After we explain the approaches to SAR wind field re- trieval, we will describe the steps that have been taken toward creating operational products, and finally discuss some important applications and uses of SAR wind fields. This paper is followed by a series of more specific, de- tailed papers on methods for and application of wind field retrievals from SAR. 2. WIND VECTOR RETRIEVAL TECHNIQUES Figure 1. Sample RADARSAT-1 SAR wind speed image crossing the Aleutian Islands with wind speed displayed in pseudo color. Note the von Karm´ an´ vortices in the lee We group wind retrieval techniques into three categories: of a volcano and gap flows caused by land topography. the scatterometer approach, the “kinematic” approach, and emerging approaches. These approaches should be thought of as complimentary in that we anticipate that by the realization of operational measurement of high- applying all techniques possible for a single image, the resolution winds. Although such wind retrievals had final retrieval will improve. been made with SAR imagery from the ERS-1/2 imagery, RADARSAT-1 and now ENVISAT, wide-swath imagery, some of it available in near real time, offer the tantalizing prospect of operational wind products. 2.1. Scatterometry-based approach To address some of the challenges of wide-swath SAR From wind vector to radar cross section: The fun- imagery the Coastal and Marine Applications of Wide damental idea behind the scatterometry-based approach Swath SAR Symposium was held at the Johns Hop- to the measurement of ocean surface wind from SAR kins University Applied Physics Laboratory (JHU/APL) is straightforward. As the wind blows across the sur- 2 in Laurel, Maryland, 23–25 March 1991. Since then, face, it generates surface roughness generally aligned the community has enjoyed the availability of near real with the wind. Consequently the radar backscatter from time data from RADARSAT-1 and has had time to evalu- this roughened surface is related to the wind speed and ate SAR wind speed retrievals against more conventional direction. All active spaceborne wind measurement tech- techniques. Now that wide-swath ENVISAT data are niques rely on this relationship and use various ap- available, it is incumbent on the SAR scientific commu- proaches to infer the wind vector from backscatter mea- nity to assess the state-of-the-art in wind vector retrievals surements. Much of the scatterometry-based research in from SAR and consider how such data can be deployed wind measurement from space consists of understanding in an operational context. This overview paper repre- and describing important details in the wind-vector-to- sents such a consensus on wind vector retrieval as it has radar-cross-section relationship and how the combination emerged from the 2nd Workshop on Coastal and Marine of such active measurements can be used to retrieve wind Applications of SAR in Longyearbyen, Spitsbergen, Nor- speed and sometimes the wind vector. way, held on 8–12 September 2003. ¡ For medium range incident angles (20 ¡ to 60 ), the One important goal of this workshop was to identify the canonical relationship between the wind vector and the progress made since the last workshop in 1999. In this normalized radar cross section takes the form of: paper, we will describe in greater detail the following im- 0 ¡ ¢¤£ portant advances since the last workshop. σ U ¡ φ θ γ θ (1) ¦¨§ θ ¥ θ φ θ φ © ¡ ¢ © ¡ ¢ A ¢ U 1 B U cos C U cos2 1. The accuracy of a “scatterometry” approach to wind where represents σ 0 normalized radar cross section field retrievals has been documented. Wind speed (NRCS), U is statically neutral wind speed normalized retrievals using such an approach achieves root- to 10 m height, φ represents the angle between the radar mean-square (RMS) errors smaller than 2 m/s in look direction and the wind direction, θ is the incident wind speed for the 2 to 20 m/s wind speed range, angle, and A, B, C, and γ are parameters describing the 2An online version of the proceedings can be found at: “geophysical model function” (GMF). The details of this http://www.jhuapl.edu/techdigest/td2101/index.htm. model function can vary depending on the investigator. Some investigators have added a cos3φ term (Shimada speed and 17 ¡ in direction with respect to buoy measure- et al. 2003), while others include an exponent modifying ments (Freilich & Dunbar 1999). the term in the square brackets. However, these modi- fications represent details that, though important, do not The original purpose of the L-band SAR on Seasat was require us to deviate from the general form in Equation 1 not the measurement of wind speed, but rather the mea- for our discussion here. surement of ocean surface wave spectra. Prior to launch, aircraft SAR measurements suggested that long ocean The dominant mechanism for microwave backscatter- surface waves ( 50 m) modulated the surface radar cross ing for moderate incident angles is Bragg scattering, section. Spectral analysis of SAR imagery offered the i.e., the dominant return is proportional to the rough- prospect of measuring the ocean surface wave spectrum. ness of the ocean surface on the scale of the radar wave- length. For spaceborne microwave measurements this Even the earliest SAR images, however, showed patterns wavelength can range from decimeters (L-Band) to cen- of NRCS consistent with variations in the wind field.