Alignment of mountain wave patterns above ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 104, NO. D8, PAGES 9199-9212, APRIL 27, 1999

Alignment of mountain wave patterns above Wales' A VHF radar study during 1990-1998 Richard M. Worthington Department of Physics,University of Wales, Aberystwyth

Abstract. Mountain waves must be parameterized in numerical models of the atmosphere,but long-term obervationsof their characteristicsare sparse. In this

study, a large data set from the VHF radar at Aberystwyth(52.42øN,4.00øW), during the years 1990-1998, is usedto investigatethe relation betweenbackground wind direction, local topography, and the mountain wave alignment. The horizontal wave vector of the mountain waves is, on average,found to be biased l0 ø anticlockwisefrom the wind direction in the lower troposphere near 2 km altitude, and biased 20ø clockwisefrom the wind direction near the ground.

This can be explainedif the "wave-launching height" (wherethe backgroundwind speedand directionvaluescan be assumedresponsiblefor generatingthe waves), lies within the boundary layer, where the wind has only partly rotated between its direction near ground level and its direction in the free troposphere above the boundary layer. On average,and for most backgroundwind directionsat Aberystwyth, the wind direction within the boundary layer is a more important influenceon wave alignmentthan the orientation of upstreammountain ridges. The relative heightof the mountainpeaksand the boundarylayer may affectthe average alignment of mountain wavesand therefore the wave stressby as much as 30ø.

1.

Introduction

avoidthis problemby smoothingtopographicmap• ridgesusingan algorithm. Mountain wavesand other gravity wavesare an im- then fitting two-dimensional [1993,Figure2] represents the portant input for numerical models of the atmosphere For example,Bacmeister

[Palmeret al., 1986],and recentstudieshavereported mountainsof the British Islesby two ridges,alignedapthat the azimuth dependenceof the waves should also proximately N-S and NE-SW. However, when a subbe incorporated[Gregoryet al., 1998;Medvedevet al., stantial part of the three-dimensional structure of the 1998].Formountainwavesgeneratedby two-dimension-mountainshas been discarded,it is not certain that the al mountain ridges, the azimuth of their horizontal horizontal wave vector of the mountain waves must lie wave vector is expected to be perpendicular to the exactly perpendicularto theseartificial two-dimensional

ridges[e.g.,Mitchell et al., 1990;Smith,1976];in con- ridges. Somemountain ridgesdo fit closelythe twotrast, isolated mountain peaks produce a "ship wave" type pattern, containing a range of wave vectors. For more azimuthally isotropic mountains, the wave vector may instead point into the low-level wind direction

dimensionalapproximation, but real mountain struc-

ture is often much more complex(e.g., Figure 1); as discussed by Worthington[1999],neitherthe assumption of two-dimensional mountainridges[Bacmeister, (e.g., $hutts[1992]for mountainwavesaboveWales). 1993]nor isolatedmountainpeakscreatingship wave

The most appropriate choice for the "low-level wind" patterns, appears appropriate for the land around the is uncertain, and often any available wind measure- Aberystwyth radar. Previousstudiesof mountainwavealignmentand its ment in the lower troposphere is used, such as the minimum height measurableby VHF radar, typically relation to the backgroundwind have usually required

2-3 km [e.g. Prichardet al., 1995]. The assumptionaircraftor modeldata [e.g.,Cacciaet al., 1997;Deanof a specific"wave-launching" altitude [e.g., $hutts, Day et al., 1998; $hutts,1992],and long-termstatisti1997], presumablynear the height of the mountain cal studiesare thereforevery difficult. The only other peaks,remainsarbitrary.Somemodelingstudies[e.g., option has been to examine cloud bands on satellite Bacmeister,1993; Bacmeisteret al., 1994]attempt to images[e.g.Cruette,1976].However,the presentstudy usesa data archivefrom the Aberystwythmesospherestratosphere-troposphere (MST) radar, 1990-1998, to

Copyright 1999 by the American GeophysicalUnion. Paper number 1999JD900054.

0148-0227/ 99/ 1999JD900054509.00

study the relation between wind and mountain wave

directions,basedon the radar techniquedescribedby Worthington[1999]. 9199

9200

WORTHINGTON:

ALIGNMENT

OF MOUNTAIN

WAVES

ABOVE

WALES

N

700m

MST

Figure 1. Map of centralWalesshowingthe land height (in meters)surroundingthe Aberystwyth MST radar, whichis markedby a cross,and the positionsof five U.K. MeteorologicalOffice synopticstations(Aberdaron,Lake Vyrnwy, Sennybridge,Aberporth, and Trawscoed)where surfacewind speed and direction are measured.

2. Aberystwyth The

Natural

MST

Environment

Radar Research

and AtmosphericAdministration(NOAA) visible-light Council

MST

radar at Aberystwyth is a 46.15MHz VHF system, located 152.42øN,4.00øW, and this study usesdata from 1990-1998 while it was operating using at least five beam directions. The one-way beam width is 3ø, peak transmitted power is 160 kW, and pulse length is 8 Its with codingto give height resolutionof 300 or 1150m, coveringan altitude range • 1.7-20 km. Four beams are pointed6ø off vertical in azimuthscloseto NW, NE, SE, and SW, and a fifth beam is pointed vertically. Time resolutionis typically 2.4 or 4.0 min, dependingon the beam configurationbeing usedon a particular day. The land height surroundingthe radar is shownin Figure 1.

3. Case Study of March 11, 1998

satellite image of the British Isles, at 1417 UT on March 11, 1998. Synopticcharts (not shown)indicate that the surfaceairflow is northerly, on the eastern side of a high-pressuresystem. Mountain lee-wave clouds can be seenabove and downstream of Ireland, but near Aberystwyth, which is marked by a crossin Figure 2, there are no mountain

wave cloud bands and the satel-

lite image gives no information about the existenceof mountain

waves above Wales.

Measurementsof vertical wind by the Aberystwyth VHF radar, in Figure 3a, show,however,that the troposphere and lower stratosphere are filled with mountain

waveactivity [e.g.,Prichard et al., 1995; Worthington and Thomas,1996]. The vertical wind amplitudeis variable,oftengreaterthan +0.5 m s-1 and sometimes reachingover+1 m s-1, and the wavesare unsteady.

To illustrate the method being used, a case study The mountain wave alignment can be calculated beis presentedfirst. Figure 2 showsa National Oceanic cause, as outlined in Figure 4, the wavestilt the fine-

WORTHINGTON:

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ALIGNMENT

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OF MOUNTAIN

WAVES ABOVE

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Figure6. Scatter plotsof(a)mountain wave alignment andwinddirection at thelowest altitude measured by radarnear2 km; (b) mountain wavealignment andthewinddirection measured

at thesame height; (c)mountain wave alignment andthesurface winddirection measured by

anemometer; (d) surface and2-kmwinddirection, showing howsurface winddirection isrotated anticlockwise fromthe2-kmwinddirection.Thestraight diagonal linesindicate differences

between wind andwave directions of0ø,+45ø,and+90ø.Asolid lineshows asmoothed average oftheindividual datapoints. "Wind direction" refers tothedirection from where thewind blows,

measured clockwise fromnorth."Mountain wavealignment" refers to thedirection towardwhich thehorizontal component ofthemountain wavevector points, measured clockwise fromnorth.

ridgemodel[Bacmeister, 1993]cannotexplaintheover- almostidentical to Figure5a,showing a biasofthesame all clockwise andanticlockwise biases in Figure5a. signandmagnitude (seeTablesla-lc and2). Evenin the moredetailedplotsin Figures8a and8b,themean 4.2. Further Tests mountainwavealignmenthasapproximately the same To help confirmtheseresults,additionaldata from dependence onwinddirection asshown in Figures 6a

1990-1995, beforethe installation of the surfaceanemo-

and6b. The mountain wavealignments predicted by

meter,areusedto createFigures7 and8. Figure7 re- Bacmeister [1993] fortheBritishIsles,withridgealignpeatsthe analysisof Figure 5, minusthe surfacewind mentsassumed to be nearNE-SW and N-S, are also data,butwithanentirelyindependent setofradardata. shown.Notethat the background winddirections are

The relationbetweenthe mountainwavedirectionand more evenlydistributedin Figures6a and 6b than in

thelocalwindand2-kinwinddirections, inFigure 7a,is Figures 8a and8b,sincetheprevailing windsarewest-

9207

WORTHINGTON:ALIGNMENT OF MOUNTAIN WAVESABOVEWALES

a) 300-JUN

1990 - OCT 1995

x

-300b)

I

--I-- LOCAL WIND 2-km WIND

200-

200

100-

100

0

0

'

-80

-40

0

40

80

0

40

80

Differencebetweenmountain-wave and windazimuth(degrees) Figure7. Same asFigure 5,butforJune1990to October 1995, when surface winddataare not available.

erly,butinrecent years, since 1995(Figures 6aand6b), directionmeasuredby the five nearestU.K. Meteo-

rologicalOfficesynopticstationsthat recorded data hourly (rather than 3-hourly or 6-hourly), at Aberdaron amplitude mountain waves in easterly winds[Worthing-

the radar has often been run in order to observelarge-

(52.78øN, 4.73øW), LakeVyrnwy(52.75øN, 3.47øW), ton and Thomas,1996]. (52.07øN,3.63øW),Aberporth(52.13øN, As a furthercheck,the differenceis plottedbetween Sennybridge

(52.35øN, 3.95øW),asshown the mountainwavealignmentand the surfacewind 4.57øW),andTrawscoed

Localwind (degrees)

2 km wind (degrees) lOO

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Figure8. Same asFigure 6,butforJune1990to October 1995, when surface winddataare notavailable. Theadditional straight linesshow theapproximate wavealignment predicted by theBacmeister [1993] two-dimensional ridgemodel fortheBritishIsles.

9208 Table

WORTHINGTON:

ALIGNMENT

l a. Mean Difference Between Mountain

Direction

OF MOUNTAIN

Wave

and Winds

Years 1995-1998 1990-1995

Surface Wind +22 no data

2-km wind -9 -11

Local Wind -8 -12

WAVES ABOVE WALES

Table l c. Percent of Winds Years 1995-1998 1990-1995

of Mountain

Surface Wind 84 no data

Waves Within

2-km Wind 95 93

60 ø

Local Wind 94 94

Values are in degrees. so the difference

between

wind

and wave azimuths

in

in Figure 1. Their heightsabovesea level are 95,359, Figure 9c becomesmore random and the mean within 307, 134, and 63 m, respectively.The stationsstarted the range-100 ø to 100ø (Figures5a, 7a, 9a, 9c) thereoperatingat variousdates,most recently,Sennybridge fore moves toward zero. Cruette[1976]hasalsostudiedmountainwavealignin 1995, so data from 1995-1998, the sameas Figure 5, are used. The measurements have coarser time and anment, using cloud bands on satellite imagesof Europe, gular resolutions(1 hour and 10ø, respectively)than including Wales. Only the magnitude and not the sign the anemometerat FrongochFarm field station, and of the difference between wave and wind direction was the airflow will sometimesbe modified by nearby hills; studied (Figures 5b, 7b, 9b, and 9d are plotted in however,Figure 9a showsbasicallythe same relation the sameway for comparison).Cruette[1976,p. 517] between the wave and surface wind directions as in found that the mountain wave alignment agreesmuch Figure 5a, whichis reassuring.Note that the biasis ob- better with the wind at 2-3 km or averaged through

servedevenat thehigheststation(LakeVyrnwy,359 m) the troposphere than with the surface wind direction: suggesting that wavealignmentis not determinednear "[cloud]banddirectionseemscloselyrelatedto wind dithe surfaceof the mountains but, rather, higher in the rection[at] altitude,moresothan to the winddirection boundarylayer. Only one higher station existsnearby, at lower levels. More precisely, the band direction is SnowdonSummit(53.07øN,4.08øW)at 1070m, but the very nearly perpendicularto the air flow abovea certain wind data are mostly missingand nevermore frequent altitude dependingon the mountain range." The radar results in Figures 5, 7, and 9 appear consistentwith than 3-hourly. The error in "north alignment" between the Fron- these direct observations and have also been confirmed gochanemometer andthe Aberystwythradaris difficult usingnewsatellitedata (R. M. Worthington,Alignment to measure,althoughit is believedto be