JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. C7, PAGES 16,391-16,400, JULY 15, 1996

Viewing a cold air outbreak using satellite-based synthetic aperture radar and advanced very high resolution radiometer imagery

Pierre D. Moured

Applied Physics Laboratory, College of Ocean and Fishery Sciences University of Washington, Seattle

Bernard A. Welter

Science Applications International Corporation, Bellevue, Washington

Abstract. In this phenomenological study we compare nearly simultaneous visible- wavelength advanced very high resolution radiometer (AVHRR) and synthetic aperture radar (SAR) images of a cold air outbreak that occurred off the ice pack in the Bering Sea. The visible image shows the spatial evolution of the cloud structure from the ice edge to several hundred kilometers offshore. Near the ice edge, the clouds form regular linear features, known as cloud streets. These streets are spaced about 5 km apart, 5 times the depth of the boundary layer. By 200 km offshore, the visible image shows nascent, closed-cell, mesoscale cellular convection (MCC), with connected disks and smears of white clouds surrounded by arcs and rings of dark, cloud-free regions. The diameters of the disks range between 10 and 15 km. The SAR image shows the same, broadly defined spatial pattern but with a substantial increase in detail uncorrelated with the AVHRR image. In the area of the cloud streets shown in the AVHRR image are two-dimensional, elongated regions of enhanced radar backscatter, called SAR streaks, with the same cross-wind spacing as the cloud streets. This suggests the presence of quasi-two-dimensional roll vortices in the atmospheric boundary layer. Downstream, in the area of proto-MCC, are broadly spaced expanses of enhanced backscatter that form two-dimensional patterns such as connected arcs that have structure and scales similar to the spaces between the clouds. The spatial evolution of the streaks and streets broadly conforms to the patterns of downdrafts and updrafts, respectively, exhibited in modeling studies of cold air outbreaks. However, the comparison with modeling studies is more successful for the cloud streets than for the SAR streaks. We suggest that this difference is because the clouds in the AVHRR images give a time-integrated signal of large-scale, quasi-two-dimensional atmospheric turbulence, whereas mesoscale radar patterns of surface roughness give a relatively instantaneous view of fundamentally three-dimensional structure within the atmospheric boundary layer.

General Introduction

Walter [1980], Atlas et al. [1986], Miura [1986], and Hein and Brown [1988], among others, describe obser- vations of the spatial evolution of cloud structure in cold air outbreaks. Starting near the coastline if not upstream of the coastline, linear cloud patterns known as "cloud streets" are aligned with the mean boundary- layer wind. Within a hundred kilometers of the coast, the cloud streets broaden and thicken, and their ini- tially linear features begin to waver in the horizon- tal plane. By a few hundred kilometers offshore, the cloud patterns are markedly three dimensional, often

Copyright 1996 by the American Geophysical Union.

Paper number 96JC01123. 0148-0227/96/96JC-01123509.00

forming disks of clouds surrounded by cloud-free rings (so-called "closed-cell" mesoscale cellular convection, or MCC) or rings of clouds that surround cloud-free disks ("open-cell" MCC). This regularity in cloud patterns has inspired the view that large-scale eddies (LE) are organized features of atmospheric boundary-layer tur- bulence. How well does this perspective hold up when considering the instantaneous response of the ocean sur- face to wind-borne stresses captured by synthetic aper- ture radar (SAR) imagery relative to the integrative effect of visible cloud patterns in advanced very high resolution radiometer (AVHRR) images? This question becomes particularly pressing when one considers nu- merical studies (Mason and Thompson [1987] among others) that predict that the LE structure is more inter- mittent than would be suggested by the cloud patterns.

Here we present an observational study of a cold air outbreak in which we compare nearly simultaneous SAR

16,391

16,392 MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY

and AVHRR images. We conclude that (1) there is a parallel evolution of boundary-layer-scale cloud streets and cloud disks on the one hand and SAR streaks and

SAR arcs on the other, both of which are consistent with existing numerical simulations of cold air outbreaks, and (2) the instantaneous SAR images show that the mesoscale LE structure is, indeed, more three dimen- sional and intermittent than is suggested by the cloud patterns.

SAR Introduction

Several papers in the literature note the existence of streaks in SAR images that scale with the boundary- layer depth. Of relevance here are, for example, both Getling [1986] and Alpers and Briimmer [1994], who in- fer that kilometer-scale streaks observed in SAR images (SAR streaks) can be caused by the creation of stress patterns on the ocean surface by quasi-two-dimensional atmospheric roll vortices. They make this inference be- cause of the similarity between the streaks and theo- retical descriptions of such vortices lEtling and Brown, 1993]. Johannessen et al. [1994] go a step further, not- ing the correspondence between the linear patterns and scales in the near-shore region of a cold air outbreak within nearly simultaneous AVHRR and SAR images. However, they do not address the downwind develop- ment of the cloud and SAR patterns.

Thompson et al. [1983] report simultaneous obser- vations from a high-altitude NASA aircraft of linear cloud features (cloud streets) and linear areas of en- hanced SAR backscatter on the ocean surface created

during a cold air outbreak off Florida. To date, their pa- per contains the most detailed, published analysis of a cold air outbreak involving SAR images. Their descrip- tions support the view that large atmospheric eddies of roll-vortex form can cause SAR streaks, although their analysis leaves several questions unanswered and raises others. Since our study addresses these questions, we briefly review their work.

Thompson et al. [1983] observed that the linear cloud patterns became visible some 30 km offshore and extended at least another 220 km away from shore. The clouds were aligned with the mean wind. Close to the coast, their cross-wind spacing was about 2-4 times the cloud height (which was itself about a kilome- ter), whereas downstream the features appeared more broadly spaced. Even farther offshore, beyond the Gulf Stream, the cloud streets grew and merged, forming "nearly solid masses with sizes up to several tens of kilometers" [Thompson et al., 1983, p. 1173].

In agreement with the visible image, the SAR image showed no organized patterns close to the shore, where there were no clouds. In the region where the cloud streets started to form, the SAR image showed streaks comprised of "kilometer-sized areas...appear[ing] in quasi-parallel patterns aligned with the wind" [Thomp-

son et al., 1983, p. 1173]. These streaks were most intense over the Gulf Stream, where the clouds were most organized. However, the authors did not report the spacing of the backscatter areas, only their size. Also, in contrast to the cloud streets in cold air out- breaks, which evolve spatially as a function of distance offshore, these areas of increased backscatter were de- scribed as growing in strength as a function of fetch without mention of scale or changes in scale. Moreover, Thompson et al. report only that the radar signatures persisted beyond the Gulf Stream, with no description of their structure.

Their brief study does not provide a detailed descrip- tion of the SAR patterns, a quantification of the rela- tionship between the SAR streaks and the cloud streets in a cold air outbreak, or an interpretation of these pat- terns in terms of the physics of a cold air outbreak. Several questions remain, including "How do the length scales, both size and spacing, of the streets and streaks compare as a function of distance from the coast?," "When the cloud streets evolve into more complex fea- tures, in what manner do the SAR streaks evolve?," and '"How regular are the SAR signatures of large eddies compared with their signatures in AVHRR images?" Also, a more general question arises: "Is the view of large eddies as two-dimensional roll structures, as sug- gested by AVHRR studies of cloud streets, consistent with the SAR patterns?" This paper addresses those questions.

The AVHRR Image and Supporting Meteorological Data

On February 27-28, 1992, the Bering Sea was un- der the influence of cold northerly winds flowing off the ice edge. Figure I is a map of the area under consid- eration. A rawinsonde from St. Paul Island (57.15øN, 170.22øW, south of the ice pack and about 400 km from the area of interest) at 0000 UTC provided a sounding which shows a well-mixed boundary layer with a strong inversion at 1250 m (Figure 2). These in situ data give a qualitative picture of the structure in the region of the SAR image because the data were collected approx- imately the same distance downstream of the ice edge as the center of the imaged region and because the biggest change in boundary-layer structure within a cold air outbreak, barring the presence of synoptic-scale fronts, is in the downstream direction [Walter, 1980]. Depth- averaged, mean boundary-layer winds at St. Paul Is- land were about 12 m s -1 from 340 ø . Figure 3 is a visible AVHRR image of a portion of this area taken at 0046 UTC February 28, 1992; the image is centered on the ice edge and the water south of the ice. (The ice reaches across the top and forms the large, roughly triangular patch in the upper right-hand corner of the image. Within the ice, north and east of the rectan- gular box, is the elongated contour of St. Matthew

MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY 16,393

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100 o 170 o 160 ø Figure 1. Map of the North Pacific, the geographic area under consideration.

Island.) The box south and west of St. Matthew Is- land circumscribes the area of a composite image :made up of two geo-coded SAR images taken 2 hours before the AVHRR image. Analysis of this region of overlap between the SAR and AVHRR coverage is the focus of this paper.

The synoptic-scale winds do not parallel the long sides of the overlap region; instead, they are primarily parallel to the meridian that cuts vertically through the region, which, in turn, is parallel to the cloud streets. The S AR box is therefore oriented such that its long axis is at an angle to the linear cloud patterns. Also, the cloud patterns evolve to first order as a function of distance away from the ice edge [Walter, 1980], and here the ice edge is at an angle to the long axis of the S AR image. For these reasons, we cannot describe the spatial evolutioh of the cloud streets or SAR streaks simply as a function of distance away from the top of the S AR image. Instead, we divide the overlap region shown in the AVHRR image into four sectors, using the 58øN latitude and 174øW longitude lines. Figure 4 shows a diagram of those sectors. For the most part,

the NW sector is upwind of the SW sector and part of the SE sector, while only the westernmost part of the NE sector is upwind of the SE sector.

In the AVHRR image, the NE sector of the overlap re- gion contains linear cloud streets, while the NW sector contains cloud streets that have begun to loose their linearity. Upwind of the NW sector, outside the area covered by the SAR image, the cloud patterns are more like those seen in the NE sector of the overlap region. (Note again that the NE sector is closer to the coastline than the NW sector.) In the NW sector, the streets undulate in the horizontal plane. By the bottom of this sector and into the SW sector, we see the beginnings of closed-cell MCC, which appears as light, thick bands or contiguous disks of clouds. The spacing between centers of these thick cloud segments ranges from 10 to 15 km, and they grow even larger downwind of the region in the SAR image. These partial disks are separated by cloud-free segments. Near the top left-hand corner of the overlap region, west of the 174øW longitude line, the cloud streets appear to bend in unison as a function of distance from the ice edge. Just within the overlap

16,394 MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY

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Figure 2. Profiles of (a) potential temperature, (b) wind speed, and (c) wind direction at St. Paul Island. Note the strong inversion at 1250 m.

region, along the top portion of the 174øW longitude line, the cloud streets are indistinct, whereas east and west of this area they are well resolved. This variation in the cloud streets (and in the surface roughness, as discussed below) could result from the feedback onto the quasi-two-dimensional rolls of some local combina- tion of mesoscale variations in geostrophic adjustment and the equivalent of a land breeze [Overland et al., 1983; Fairall and Markson, 1987]. Finally, in the SE section, downwind of the indistinct cloud region, the

clouds again appear in an undulating form like that in the NW sector.

The SAR Image

Plate I shows a composite, low-resolution (ERS 1, C band) SAR image taken at 2240 UTC on February 27, 1992, of an area just south of St. Matthew Island cor- responding to the boxed region in Figure 3. (See the Appendix for details of the image processing.) The pix- els measure 100 m to a side, and the image is 954 pixels wide by 1898 pixels long, or 95.4 km by 189.8 kin. As with the AVHRR image in Figure 3, we have divided the SAR image into four sectors, shown in Figure 4. All distances are measured from the top (north) part of the image.

The dark, triangular feature in the upper right-hand corner of the NE sector of Plate I is ice, which is also manifest in the visible image. This ice fills out more of the northeast corner the SAR image than of the boxed section of the AVHRR image because clouds can hide ice in AVHRR images. Within the top 90 km of the NE sector, especially along its western edge, are elongated, two-dimensional features of enhanced backscatter from

the water's surface. The cross-wind spacing of these fea- tures is about 5 km (see "A" near the top of the figure); their average width is about 500 m, and their length can be up to several times their width. Just south of the 95-km mark in the western portion of the NE sector, the bands become thicker and more irregular. The re- gions of enhanced backscatter are more broadly spaced than upwind of this area, as well as more fragmented and spatially intermittent.

In addition to the downwind development of the SAR streaks in the NE sector, there is some east-west vari- ation. To the east, within the lower half of the sector, there are few regions of enhanced backscatter. Alpers and Briimmer [1995] describe regions of high winds that do not have significant SAR streaks. This would appear to be another example of this phenomenon. We note an explanation for this in the conclusions.

Now consider the NW sector of the SAR image. Near the top, the SAR streaks are spaced as in the western part of the NE sector (5 km apart, elongated along the mean wind direction, etc.) but are more fragmented. This area corresponds to the region in the AVHRR image where the cloud streets bend together. Look- ing more closely, we find that within the top 35 km of the NW sector, the SAR streaks are more broken up and unrelated than downstream, where, centered on the 47-km mark, the boundary-layer-scale SAR streaks are relatively elongated and spatially coherent. Far- ther south along the western edge of the SAR im- age, between 70 and 95 kin, there is another reduc- tion in the coherency of the streaks. Between 100 and 140 kin, regions of enhanced backscatter again be- come prominent. These streaks are thicker and more broadly spaced than the upstream strea.ks and also form

MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY 16,395

.

...

.

Figure 3. An advanced very high resolution radiometer (AVHRR) image of the Bering Sea (at visible wavelengths) undergoing a cold air outbreak. The top of the image is toward the north. Dominating the upper right-hand corner of the image is pack ice. St. Matthew Island is near the top and center of the image and also appears in Figure 1. There are cloud streets in a variety of states of evolution over the Bering sea to the south of the ice. The rectangular region marks the area imaged by the ERS I synthetic aperture radar (SAR) system 2 hours before this AVHRR image was made. It is about 190 km long and 95 km wide and is cut vertically by the 174 ø W latitude line and horizontally by the 58 ø N longitude line. Both the latitude and longitude lines are spaced by 2 ø .

a richer, more two-dimensional pattern: their perime- ters are more crenelated, and they form loosely con- nected arcs rather than straight, parallel lines. Also, the regions of high backscatter are more coherent and thicker than those of low backscatter. Near the bottom

of the NW sector and continuing into the SW sector, many of the bands partially circumscribe cores of es-

pecially low radar backscatter. The spacing between adjacent bands has increased to an average of 12 km (see "B" near the bottom of Plate 1) with a fair bit of variability.

To summarize, the SAR image shows a complex spa- tial structure with a distinct pattern of spatial evolution that is a function of fetch. Near the ice edge, the SAR

16,396 MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY

NE

NW

Figure 4. A schematic of the S AR image with defini- tions of the sectors used in the text. These sectors are

formed by the intersection of the latitude and longitude lines with the imaged region.

field contains elongated, parallel segments of enhanced backscatter. In the middle third of the image, down- wind of a confused section of patterned backscatter, the regions of enhanced backscatter thicken and become more widely spaced. Farther downwind, the enhanced- backscatter features are more arclike, partially circum- scribing low-backscatter regions. This broadly painted, fetch-dependent pattern of radar backscatter has addi- tional complexity. Moving from north to south through the western part of the SAR image, for example, there appears to be a wavelike pattern superimposed on the linear backscatter features in the form of enhanced then

suppressed backscatter with an east-west axis and a wavelength of about 45 km. Finally, moving east to west in the eastern portion of this image, there are re- gions without significant backscatter and only hints of linear structure.

A Comparison of the SAR and AVHRR Images

In comparing the SAR and AVHRR images, keep in mind that the pixel size for the SAR image is 100 m, whereas that for the AVHRR image is about 1000 m. Thus there could well be structure in the cloud pat- terns that is not resolved by the AVHRR images (see Haul [1985] for an example). Also, the cloud signature represents the time-integrated effect of at least several minutes worth of net uplifting by a range of tempo- rally evolving, turbulent motions. In contrast, the SAR pattern shows the comparatively instantaneous ocean- surface response to variations in surface stress on the gravity-capillary field, as well the signatures of break- ing waves. Both processes have timescales of several seconds.

A working hypothesis for interpreting mesoscale pat- terns in radar backscatter from the ocean surface is

that those patterns can be caused by the response of

the ocean surface to net circulation within the ma-

rine planetary boundary layer. One source already dis- cussed would be the surface stress fields caused by roll vortices [Gerling, 1986] or MCC [Mitnik, 1992]. An- other would be the action of atmospheric gravity waves [Thomson et al., 1992; Vachon et al., 1994]. Also impor- tant is the fact that variations in SAR backscatter can

be caused by changes in wind magnitude or direction. Indeed, Freilich and Dunbar [1993] point out that for the ERS I C band radar, a 1-dB variation in measured radar backscatter, averaged over 1 ø grids, can be due to changes in local wind direction relative to the sensor direction, rather than to changes in wind speed. Thus, in principle, all the structure in this averaged SAR im- age could be due to changes in wind direction rather than changes in wind speed. (Mourad [1996] discusses this possibility in more detail.) Complicating interpre- tations of SAR backscatter in terms of patterns of only atmospheric processes are biological effects [Nilsson and Tildesley, 1995] and internal and surface waves (see the special J GR-Oceans issue introduced by Hughes and Gasparovic [1988]; also Hasselmann and Hasselmann [1991]), anything that affects the response of gravity- capillary waves or breaking waves to wind stress.

With these caveats in mind and the 2-hour time dif-

ference between when the images were taken, one can- not expect a one-to-one correspondence between the SAR and AVHRR patterns. However, the agreement of the broad trends in the SAR and AVHRR images both confirms the persistence of organized, mesoscale structures in the turbulent atmospheric boundary layer and elucidates that structure.

SAR Streaks Versus Cloud Streets

First, consider the top half of the boxed region in the AVHRR image. It contains a well-organized linear ar- ray of cloud patterns that suggests the action of a large eddy field made up of (longitudinal) roll vortices. Now consider the top half of the SAR image. It contains two-dimensional regions of enhanced backscatter simi- lar to those described by Thompson et al. [1983]. These features are about 500 m to I km wide (the depth of the boundary layer is a little over I km) and up to sev- eral kilometers long. They are manifest in the same region and have the same cross-wind spacing as the cloud streets. However, there is no unbroken line of backscatter that corresponds to the lines of clouds. In- stead, there is a linear aggregation of two-dimensional, enhanced-backscatter regions, suggestive of the pres- ence of fundamentally three-dimensional turbulent pro- cesses in the atmospheric boundary layer organized by quasi-two-dimensional large eddies, presumably the roll vortices that create the cloud streets.

Now compare the corresponding NW sections of the SAR and AVHRR images. This region is farther from the coastline than the NE section. The wavelike struc-

ture with east-west crests and a spacing of 45 km in

0 km,

47.5 km

95 km

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relotive bockscotter (dB) Plate 1. Composite of two ERS 1 SAR (C band) images of the boxed region in Figures 1 and 3 (Copyright ESA 1992). A linear trend has been removed, and the result has been spatially averaged. This image was taken 2 hours before the AVHRR image in Figure 3 The long axis of the imaged region is 14 ø clockwise from north, which is toward the top of the image. Contained in this image is a series of parallel aggregations of elongated regions of enhanced backscatter. Both the cross-wind length scales and the spatial complexity of these patterns grow as a function of distance from the top of the image, which is closest to the ice edge. "A" marks the 5-km spacing between SAR streaks in the region of cloud streets; "B" marks a spacing of 12 km between SAR arcs in the region of nascent mesoscale cellular convection.

16,398 MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY

the SAR image is not duplicated in the AVHRR im- age. Instead, the spacing of the cloud streets simply in- creases with distance downwind of the ice edge, and the clouds themselves thicken and undulate as a function of

this distance. In the middle third of the SAR image, where the AVHRR image shows wavering cloud streets, the parallel, linear aggregations of radar backscatter break up. Downwind of the fragmented area centered on the 95-km mark, the regions of enhanced backscat- ter thicken, become more widely spaced, and ultimately become less linear (i.e., more two-dimensional). In the corresponding area in the AVHRR image, the clouds be- come less linear, forming thicker, more two-dimensional features.

Within the top two thirds of the SAR image, the cloud streets evolve as a function of fetch in ways reminiscent of the updraft patterns predicted for two- dimensional roll vortices evolving in a cold air out- break, as analyzed in a number of numerical modeling studies [Etling and Raasch, 1987; Sykes et al., 1988; Chlond, 1992]. The general correspondence between streets and updrafts has been noted in those refer- ences. What is new here is that the corresponding SAR streaks evolve in ways similar to the downdraft patterns calculated in the same studies, although with signifi- cantly greater complexity. In the numerical studies just listed, the spacing between both the updrafts and the downdrafts of the roll vortices increases with time (the proxy for downstream distance in the numerical studies just cited) such that both the downdrafts and updrafts widen, the downdrafts more significantly than the up- drafts. (For an example, consider Figures 6 and l l of Sykes et al. [1988] or Figures 3 and 4 of Chlond [1992].) However, the qualitative similarity in spatial evolution between the SAR patterns and the downdrafts of the numerical simulations is weaker than that between the

downwind evolution of the cloud streets and the up- drafts.

SAR Arcs Versus Mesoscale Cellular

Convection

Near the bottom of the composite SAR image, we see narrow arcs and junctions of enhanced backscatter surrounding broader regions of low backscatter. In the corresponding area in the AVHRR image, we see cloud- free arcs adjacent to undulating cloud streets or con- nected disks of clouds. The SAR arcs and junctions are similar in thickness and length to the cloud-free regions in the AVHRR image. Also, the sizes of the regions of low backscatter partially surrounded by arcs of high backscatter are comparable to the sizes of the cloud disks. This is similar to the SAR-backscatter patterns predicted by Mitnik [1992] associated with closed-cell MCC. In his model, the flow pattern is the sum of a ring of downward motion surrounding a core of upward motion (the latter forming below the cloud), plus a con- stant advection term. Therefore the strong inference is

that the regions of enhanced backscatter near the bot- tom of the image are caused by the surface stress pat- terns created by the downdrafts within aggregations of proto-MCC. Consistent with this interpretation are the regions of very low backscatter that we interpret as cor- responding to the updrafts of the MCC, since Atlas et al. [1995], among others, have inferred that precipita- tion reduces C band radar backscatter and rain tends to fall out of clouds.

Conclusions

We have compared nearly simultaneous AVHRR (vis- ible) and SAR images taken of an area off the ice pack in the Bering Sea during a cold air outbreak. The broad patterns exhibited within the AVHRR and SAR images are closely related both to each other and to the patterns of updrafts and downdrafts predicted by numerical models of cold air outbreaks that focus on

the action of large eddies in the atmospheric bound- ary layer. (However, there are significant differences in the details.) The visible image shows the evolu- tion of cloud structure from the ice edge out to sev- eral hundred kilometers offshore. Near shore, the clouds are in the form of regularly spaced, long, parallel lines known as cloud streets. These streets are about 5 km

apart, 5 times the boundary-layer depth, and gener- ally accepted to be caused by longitudinal roll vortices. By 200 km offshore, the image shows the beginnings of closed-cell MCC in the form of generally contiguous disks or smears of white clouds partially surrounded by dark, cloud-free segments. The spacing between disks ranges between l0 and 15 km. The SAR image shows a similar pattern in the spatial evolution of the backscat- ter cross section: linear aggregations of elongated, two- dimensional regions of enhanced backscatter evolving into more broadly spaced and more complex arcs and junctions. The linear aggregations suggest longitudi- nal roll vortices and simultaneous, significantly three- dimensional substructure, whereas the arcs and junc- tions suggest fundamentally three-dimensional circula- tions that are consistent with closed-cell MCC.

Our comparison indicates that SAR images can be used to analyze the evolution of atmospheric turbulent structure during a cold air outbreak. However, the SAR image under consideration is more intermittent in its structure and therefore more complex in its interpre- tation than the cloud image. For example, the "roll vortex" signature in the SAR image is more tenuous than that in the cloud patterns. Also, as noted earlier, there are areas where roll signatures exist within the AVHRR image but do not exist within the SAR im- age. This is probably because the cloud patterns show the time-integrated effects of turbulent structure that spans the boundary layer depth (such as roll vortices), while S AR patterns highlight the instantaneous forms of atmospheric coherent structures that act within the surface layer. (It would be edifying to compare SAR

MOURAD AND WALTER: SAR VERSUS AVHRR IMAGERY 16,399

images of the same region taken only moments apart. Regrettably, this is currently impossible.) These forms include roll vortices and their substructure but also

other turbulent circulations, some of which reside pri- marily in the surface layer. (See Mourad [1996] for a follow-on to this line of reasoning.) Also, there is a wide range of atmospheric and oceanic effects in ad- dition to atmospheric boundary layer turbulence that can influence the roughness of the ocean surface. For example, our image analysis suggests larger mesoscale or sub-synoptic-scale influences on the boundary layer, only some of which appear in the cloud patterns.

Therefore both the modeling of SAR patterns created by atmospheric mesoscale structures and the remote sensing of atmospheric boundary-layer properties based on SAR images must take into account the immediacy of the SAR imaging procedure relative to the averag- ing effects caused by the processes that form clouds, as well as the complexity of turbulence in the atmospheric boundary layer.

Appendix: Processing the SAR Image

To construct the composite SAR image shown in Plate 1, we took two adjacent SAR images (numbers 11748200 and 11749200) and connected their digital representations. The images were transmitted with a 10-km area of overlap in the latitudinal direction and a relative longitudinal displacement of 5 km. We removed the redundant 10-kin-wide horizontal patch from the bottom (south) side of the top image, removed 2.5 km from the right-hand side of the top image, and removed 2.5 km from the left-hand side of the bottom image. The resulting image was 954 pixels wide by 1898 pixels long, or 95.4 km by 189.8 kin. This is the region that is marked on Figures 1 and 3 and presented in Plate 1. We have removed from the image a linear trend in the backscatter that extends between the long sides of the. composite SAR image. (This linear trend had a slope of 0.035 dB km -x. It started with the average value of the left-hand column of pixels in the image, calculated without the ice, which is -8.5 dB.)

The resulting composite image was low-pass filtered using a kernel whose shape approximated a circle and whose weights fell off with distance from the center of the circle like a Gaussian. In particular, the size of the filter is 17 x 17 pixels (except near the edges, where the Gaussian was truncated), and the weights are pro- portional to exp (-Jr/L]2), where L = 4 and r - dis- tance from the central pixel. Weights less than or equal to exp (-[9/L] 2) are set to zero. The weights are nor- malized so that their sum is 1. This averaging gives a smoothing function whose most active part acts over a square kilometer, which is the pixel size of AVHRR im- ages. The resulting image was then displayed in color as follows. There are 256 values in the color table (0 through 255). Pixels less than -1 dB are displayed as color - 0 (dark blue here). Pixels greater than +1 dB

are displayed as color = 255 (flame red here). Pixels be- tween 4-1 dB are linearly mapped to the interval [0,255] to get the display color. Over 98% of the pixels have rr values between 4-1 dB.

Acknowledgments. We thank Harry Stern of the Ap- plied Physics Laboratory for processing the SAR image and helping with the processing of the AVHRR image. We thank Michael Horgan of SAIC for his processing of the AVHRR image. PDM was funded by the Office of Naval Research under contract N00014-94-1-0186. BAW was funded by the Office of Naval Research under contracts N00014-92-C-0077 and N00014-91-C-0056.

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P. D. Mourad, Applied Physics Laboratory, University of Washington, 1013 N.E. 40th Street, Seattle, WA 98105. (e-marl: pierre@apl.washington.edu)

B. A. Walter, Science Applications International Corpo- ration, 13400 B Northup Way, Suite 36, Bellevue, WA 98005 (e-mail: walter@salty. nw.saic.com)

(Received July 24, 1995; revised April 3, 1996; accepted April 8, 1996.)