Jun 15, 1994 - squall lines associated with a tropical storm [Fu and. Holt, 1982], and atmospheric convective rolls [Thomp- son et al., 1983a; Fu and Holt, 1982] ...
JOURNALOF GEOPHYSICALRESEARCH,VOL. 99, NO. C6, PAGES12,613-12,621, JUNE 15, 1994
Atmospheric boundary layer rolls observed by the synthetic aperture radar aboard the ERS-1 satellite W. Alpers Institut flit Meereskunde,Universit/it Hamburg, Hamburg, Germany B. Brfimmer
Meteorologisches Institut, Universit/itHamburg,Hamburg,Germany
Abstract. Two syntheticapertureradar (SAR) imagesacquiredby the European RemoteSensingSatelliteERS-1overthe Jade-Weser estuaryin the GermanBight of the North Sea on January 2 and 20, 1992, are analyzed. The images show sea surfacemanifestationsof atmosphericboundary layer rolls. This is inferred from the orientationof the quasi-periodicsea surfacepatterns which are aligned
approximatelywith the wind direction,from the ratio of the wavelengthof the patternsto the heightof the boundarylayer,andfromthe conditionsencountered in the atmospheric boundarylayer as measuredquasi-simultaneously by radiosondes. The atmosphericboundarylayer rolls weregeneratedby a dynamicinstabilityon January2 andby a thermalinstabilityon January20. For the first time, quantitative estimatesof variationsof the wind velocity at the sea surfaceassociatedwith the
atmospheric rolls are extractedfrom a spaceborne radar SAR image. It is shown that wind velocitiesderivedfrom SAR image intensity variations are in agreement with
theoretical
estimates.
1. Introduction It is well known that mesoscaleatmospheric phenomena over the ocean affect the sea surface roughness and thus become detectable by radar. Examples
Sea. They showquasi-periodicpatterns on the sea surfacealignedapproximatelywith the wind whichare interpreted as sea surfacemanifestationsof atmospheric boundary layer rolls.
Atmosphericboundarylayer rolls are helicalcirculaare atmosphericgravity waves[Thomsonet al., 1992], tion patternsin the atmosphericboundarylayer which squalllines associated with a tropical storm [Fu and are superimposed on the mean wind field, i.e., the priHolt, 1982],and atmospheric convective rolls[Thompson et al., 1983a;Fu and Holt, 1982]. Associatedwith maryflow (Figure 1). They canbe generatedeitherby thesemesoscaleatmosphericphenomenais a varyingsea
thermalinstability(Rayleigh-Bdnard instability)when
surface wind field which modulates the short-scale sea
the layer is heated from below or cooledfrom above
surfaceroughness andthusthe radarechoor normalized radar crosssection(NRCS) of the seasurface.If the relationshipbetweenthe NRCS and the oceansurface wind vectoris known, variationsin the radar imagein-
or by dynamicinstability(inflectionpoint instability)
to NRCSvariations)canbe converted into variationsin
1985],theoretical analyses [e.g.,Brown,1970]andnu-
when the wind velocity changeswith height in such a way that an inflectionpoint occursin the wind component normal to the roll axis, as, e.g., in the Ekman wind tensity(whichfor linearradarsystems areproportional profile. Observations[e.g., LeMone, 1973; Briimmer,
merical model simulations[Deardorff, 1972; Mason, radar(SAR) aboardthefirstEuropeanRemoteSensing 1983]give a fairly goodview of the roll-scaleflow or Satellite,ERS-1, whichoperatesat 5.3 GHz (C band) secondaryflow pattern. The roll axesare orientedbeand at vertical polarizationfor transmissionand recep- tween the directions of the mean surface wind and the wind abovethe boundarylayer. Usually tion (i.e. VV polarization), thisrelationship is givenby geostrophic the boundary layer is cappedby an inversionso that the so-calledCMOD4 modelfunction(seesection5). In this paper we analyze two ERS-1 SAR images the depthsof the boundarylayer and the roll layer cowhichwereacquiredon January2 and 20, 1992,overthe incide. In the case of a thermal instability, the aspect Jade-Weserestuary in the German Bight of the North ratio, i.e., the horizontalwavelengthof the roll pattern A dividedby the roll heighth, is 2.8 according to the linear P•ayleigh-Bdnard convection.The mostfrequently Copyright1994 by the AmericanGeophysicalUnion. observedvaluesrangebetween2 and 4 [LeMone,1973; Kelly, 1984; Kuettner, 1971]. In the caseof an inflecPaper number 94JC00421. tion point instability, the aspect ratio is about 2 ac0148-0227/94/94JC00421505.00 the seasurfacewind velocity. For the synthetic aperture
12,613
ALPERSAND BR•IMMER: SAR IMAGES OF ATMOSPHERICBOUNDARYLAYER ROLLS
12,614
cloud street
geostr
A
•.•,-mean
sunace
/'•' wind y
Verticalwindcomponentof secondaryflow
B
x/
-
•.y
Uz
• Horizontalwindcomponentsof secondaryflow
Figure 1. Schematicplot of the secondary flow pattern associated with atmosphericboundary layerrolls. (A) Perspective viewof the three-dimensional flow;(B) variationof the verticalcomponentuz of the wind velocityalongthe y direction;(C) variationof the horizontalcomponents u• and Uy (in the x, y plane). cording to the stability analysescarried out for Ekman
2. ERS-1 SAR Images
wind profilesunder neutrally stable conditions[Lilly, 1966;Brown,1970]and accordingto the numericalsimFigures2 and 3 showERS-1 SAR imagesof the Jadeulationsof Deardorff[1972].Observations of inflection Weser estuary of the German Bight of the North Sea point instability rolls in the atmosphereare rare, so that a range of most frequently observedaspectratios cannot be given. If the moisture conditionsare favorable, cloud streets may be formed in the upward rising branchesof the roll circulation. Atmospheric boundary layer rolls contribute significantlyto the vertical exchangeof momentum, heat, and moisture in the atmosphere.At higher altitudes
their relative
contribution
to the total vertical
fluxes in the atmospherecan be even larger than that
of the turbulentfluxes[Mason,1983;Br•'mmer,1985]. Sea surface manifestations of atmosphericboundary layer rolls were first noted on L band SAR imagesacquired over the Atlantic Ocean off the coast of Florida during the Marineland Experiment in 1975 from a Na-
tional Aeronauticsand SpaceAdministration(NASA) airplane[Thompsonet al., 1983a].Later suchfeatures were also observedin L band (1.275 GHz) SAR imagesfrom the Seasatsatellite[Fu andHolt, 1982],from the spaceshuttle Columbia during the Shuttle Imag-
ing Radar-A (SIR-A) experiment[Ford et al., 1983]. However, none of these radars were well calibrated, and therefore it was not possibleto extract quantitative information
on sea surface wind variations
from the radar
images. In this investigationwe derive quantitative estimates of the sea surface wind velocity variations associated with convectivemotionsof atmosphericboundaryrolls from two C band SAR imagesof the European ERS-1 satellite.
It is shown that
the velocities
derived from
the SAR image intensity or NRCS variations are consistent with theoretical estimates and previous in situ measurements.
that were acquired on January 20, 1992, at 1025 UTC
(ERS-1orbit 2685,frame2529)andonJanuary2, 1992, at 1025UTC (ERS-1 orbit 2427,frame 2529),respectively, during descendingpassesof the satellite. The center coordinatesof both scenesare 53ø40tN,8ø04rE, and the imaged area is 74 km by 48 km. The SAR scenesshowdigitallyprocessed three-lookimages(Eu-
ropeanSpaceAgency(ESA) product:SAR.PRI) with a nominal geometricresolutionof 22 m by 22 m. The SAR imageswereprecisionprocessed by the UK Processing and Archiving Facility at Farnborough,England, for the ESA. They wereprovidedon digitaltape (Exabyte) with a pixel size of 12.5 m and a pixel depth of 16 bits. On both SAR images,quasi-periodicstreakscan be delineatedon the sea surface. They are more pronouncedon the imageof January20 (Figure 2) than on the imageof January2 (Figure3). On January20 the averagedirectionof the streaksis 800 againstN, and on January 2 it is 780 against N. Image intensity (gray level) scansperpendicularto the streak lines were carried out along lines A and B shownin Figures2 and 3. This was doneby usingthe digital data and by averagingover 32 parallel scanlines
(corresponding to a width of 400 m) and over8 pixels in the directionof the scanlines. In Figures4 and 5 the image intensity variationsare plotted alongtheselines from S to N in a decibelscale. The image intensityis convertedinto NRCS by usingthe conversion (calibration) factor providedby ESA. In the image of January 20, the distancebetweenadjacent brightlines(wavelength)variesbetween1.8 and 3.9 km (Figure 4), and the averagewavelengthis 2.75 km. In the image of January 2, the wavelengthvaries
ALPERS AND BR•TMMER: SAR IMAGES OF ATMOSPHERIC BOUNDARY LAYER ROLLS
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