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Kananaskis Valley Winds in Summer

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  • 1 Department of Forestry and Rural Development, Otlawa, Canada
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Abstract

Anemograph charts from three stations in a north-south valley were analyzed to find the degree to which average diurnal variations were explainable on the basis of valley wind theory and local topography. Prominent diurnal cycles of the cross-valley component were found in the monthly averages at each station. At one station it was a morning-evening slope-wind cycle; at the other two, a day-night cycle up and down a sub-valley. The component along the main valley showed greater complexity, which is partially attributed to gradient wind interference in the afternoon when convective activity is greatest. The diurnal patterns for a group of selected clear days were similar to, but slightly sharper than, those of the monthly average charts.

Abstract

Anemograph charts from three stations in a north-south valley were analyzed to find the degree to which average diurnal variations were explainable on the basis of valley wind theory and local topography. Prominent diurnal cycles of the cross-valley component were found in the monthly averages at each station. At one station it was a morning-evening slope-wind cycle; at the other two, a day-night cycle up and down a sub-valley. The component along the main valley showed greater complexity, which is partially attributed to gradient wind interference in the afternoon when convective activity is greatest. The diurnal patterns for a group of selected clear days were similar to, but slightly sharper than, those of the monthly average charts.

348 JOURNAL OF At'I~LIEI3 ;vl/~l~t~t~-~ts~ v,~u ....Kananaskis Valley Winds in S~_mmer L. B. MAcH^TTIEiDepartment of Forestry and Rural Devdopment, Ottawa, Canada(Manuscript received 18 August 1967)ABSTRACT Anemograph charts from three stations in a north-south valley were analyzed to find the degree to whichaverage diurnal variations were explainable on the basis of valley wind theory and local topography. Prominent diurnal cycles of the cross-valley component were found in the monthly averages at each station. Atone station it was a morning-evening slope-wind cycle; at the other two, a day-night cycle up and down asub-valley. The component along the main valley showed greater complexity, which is partially attributed togradient wind interference in the afternoon when convective activity is greatest. The diurnal patterns for agroup of selected clear days were similar to, but slightly sharper than, those of the monthly average charts.1. Introduction Surface wind has a significant effect on the spreadof forest fires, pollen, seeds and insects. It directs theadvective component of energy in evapotranspiration.It is well known that the local surface wind often differsfrom the gradient shown on the surface synoptic chart;the precise manner in which topography causes suchdifferences is only partially established. Geiger (1965) has reviewed the theory of winds in amountain valley and reproduced Defant's diagram.Davidson and Rao (1958) and Buettner and Thyer(1966) discussed the inaccuracies of this diagram andassociated theory in the light of their own and others'observations. Urfer-Henneberger (1964) confirmed thatslope winds are a continuing rather than a transientfeature, and that they are unsymmetrical and inharmony with the unsymmetrical exposure of the valleysides to insolation. Most of the above investigations used observationsfrom selected clear days only, in order to study thevalley wind system undistorted by synoptic-scalepressure gradients. This paper describes what windsoccur (regardless of synoptic situation), then discusses: a) the reasons for the main features of the observed wind patterns, and b) the degree to which the monthly patterns reflect those for low-gradient clear days.Z. Area, instruments and method The surface wind observations were made in thenorthward draining Kananaskis valley of southwesternAlberta in 1960, as part of a study of the variation withtopography of meteorological factors affecting forestflammability. Fig. 1 is a contour map of the area. Adescription of the Kananaskis valley was given in aprevious report (MacHattie, 1966). The wind data are derived from three anemographsof Meteorological Service of Canada design, t.x~e 45(Department of Transport, 1951). These show a stepon the chart for the passage of each mile of wind; thewind direction (to 8 points) is recorded at the time ofeach step.TA~tJ~ 1. Anemometer locations and exposures. Station elevation Above Above sea valleyStation level floorname (ft) (ft)Period of recordAnemometer expOSureHeadquarters 4560 50Meadow 4730 0Marmot 5680 950All year1 July-9 September6 July--28 August*48 ft above ground on grassy knoll south of 40-ft pine stand33 ft above grassy river flat33 ft above ground in a 15-ft pine stand on a flattened ridge. * Except 11, 12, 15 July and $, 9, 23 August.On loan from the Meteorolo~cal Service of Canada.1968 L. B. M A c H A T T I E 349 115-15,51005,50-50 115015'114-58' 51o05,' 50o50' 114058' Scale in Miles 2 I 0 2 4 ~----------J I I I~:~c,. ~. Co~tou~ -~a~t sSow~n~ t~e three ~,~memctcr s~t. es i~ t,~e ~o~-r ~,nana~i~ -a~--. The contour interval is 500 ft. The anemometers were exposed at the locationsmarked in Fig. 1 and described in Table 1. The exposureswere not ideal, but their limitations are not thought tocompromise the conclusions drawn below. Hourly abstracts of average speed and direction weremade from the anemograph charts. The hourly valueswere then resolved into components along and acrossthe valley direction. The valley direction was takenas north-south at Meadow and Marmot, northeastsouthwest at Headquarters. To represent the diurnal wind pattern during lowgradient clear weather, 13 days were found for which: a) the geostrophic wind (on both 700-rob and sea level synoptic charts) was generally less than 15 mph, and b) daily sunshine was 9 hr or more.Thermograph charts were also examined to make surethe temperature pattern was not abnormal. The dayswhich met these criteria were 3-6 and 14-18 July and350 JOURNAL OF APPLIED METEOROLOGY Vono~m7FREQUENCY [days) HEADQUARTERS ALONG VALLEY class VALLEY :t ',i ............. ~m, !'11 'llllJJllltltll - -'.-==.==:~=~=== == 10 30 NW JULY[:REQUENCY 20 r~ J (days) [ ..../ N~..~__..~/~ -J 20 ",' " ..... "., ~ %....'~: .... ,o ....... ,. ..... SPEEDIO 10~~**'*'''''*' "".'"'-.,." ~.~o~ , , ~ ,~, ~,, ,~ ~"/'~;~~ , .... J, ' ,'~ ' ','i '"?~ *; .... ~,' .... ,,~ ...... ,~ ..... ;,, HOUR HOUR '~',o I II1~11[ Iltlt~llll :::::::::::::::::::::::: -.~ ,o-~i: ': ' '~'' AUGUST -20 ~2o . NW ~.x ~SW x I' -,~. ~, _. ~-~ ~,o,.~ ...... ..,~ NE ,., ~'-. SE ..""'..'" ". .. _ -O I0~0 ' ..... '~ :~,~,~~,~,~~,~: (mph) ~,,~,!,, ,~!llJ Jl~lllll I o I ~6 1~ 1~ o o6 12 18 24 HOUR HOUR MEADOW 0 - . . . . , . - . . . , ~ . . . . . ..... o ,,~,,llljljljlllll,~, p (~o- ' ....... :~:~::'::::'' 10~3o -~ - 20 j .~. '*." '%. ''-- ~ ~ ~/ / "~ ~ ~ ~ ~ ~ ~'"'"'"' " W *'*~" ........ -,.. ..... ~,o- ~ ~ ~ ~ ~"~ l:~ ~ ~ 0 i ~ ~ ; ~ '1' ' ' ' ' i - . ~ . - , ~ ; ~ ~ 1 0 ~ 1~ I~ 06 1~ 18 24 FLOUR HOUR ~j**~lllljjjjj~ljl~j ...... .. . . ......... j- .:..-..' :::~:[:: ..... '~0 ~ - 10- -~0 -20 S ~ -20 .....~.... .,o ........ ~'~ ~.r'"'; ... ?',~.' .. ........ - / ~-~ %* *,,' ', ..... ~ t- '-.,' ....... ' ". -O~ ...... ~ J ~ -' 10 ~ ~ ~ l, ,, ~ j j~ Ill ~~ j ~ J I j j 0 ; ' ' ' t '1' ' ' ' ' '1~ i , , J ~1~ - ~ ~ i ~1 ~ ~6 1~2 1~8 24 0 ~ 12 18 24 HOUR HOURMARMOTFREOUENCY [days) SPEEO 0 (mph 10FREQUENCY[days) SPEED 10 ~mphl C SPEED 0 (mph)FREQUENCY , (days)SPEED lO(mph) 0SPEED 0FREQUENCY (days)SPEED I0.(mph) OO~ l J2 1~8 24JULY00-30 Fro. 2. Hourly frequencies of wind components and their average speeds for along-valley (left column)and cross-valley components (right column). Data for 31 days per month are used except at Marmot.1ONE1968 L. B. MAcHATTIE 351 0SPEED(mph) I0FREQUENCY (doys) SPEEDI0' mph) 0 0 SPEED (mph)loFREQUENCY (doys) 1G SPEED (~)oALONG VALLEY-13-I0 NIP /'~ ~. ~., ~ / -- J ~/ \~~~/-- '~\\HEADQUARTERS CROSS VALLEY 0 ''; ;:'i.:-:.'[: i: 10 13 '"' ........... ' NW ~10 ""%. / -'"'""' %. **ooo.----%o / - :; '. ..' -5 ); '......,.:. ~' .......... SE lOFIG. 3. l~ourly wind components for 13 selected clear days, displayed as in Fig. 2.8-11 August, inclusive. Average hourly components~vere computed for this 13-day period at Meadow andHeadquarters, but not at Marmot because it wasinoperative on 6 of the 13 days.3. ResultsMonthly average wind components are shown inin Fig. 2. Meadow has a consistent down-valley wind at night.The up-valley wind increases in the daytime to maximain both speed and frequency, but only briefly does itreach the frequency of the down-valley daytime wind.The cross-valley components show east to be dominantin the morning and west in the late afternoon; in eachcase the wind blows toward the more intensely insolated slope. In contrast to Meadow, the graphs for Headquarters(Fig. 2) do not show an appreciable oscillation up anddown the Kananaskis valley. They do show a distinctcross-valley diurnal cycle. Because this cycle is inversein direction to what insolation effects in the Kananaskisvalley proper would give, and because it is a day-nightrather than a morning-afternoon cycle, we concludethat it must be the Lusk Creek sub-valley which isthe controlling influence at the Headquarters site.The northwest-southeast components thus show diurnalwinds up and down the Lusk Creek sub-valley. At Marmot the pronounced diurnal variation ineast-west components is apparently an oscillation upand down the Marmot Creek sub-valley. A reason issuggested below for the early switchover from east towest in July. Fig. 3 presents data for the selected clear days. Itsmain features are similar to Fig. 2. This indicates thatthe local temperature and pressure differences whichstem from radiational variations within the valley area dominant influence on the average summer day, aswell as on those clear days especially selected to maximize such effects. There are some differences, however, between theclear-day and the monthly graphs. The frequency ofup-valley winds in the daytime is greater in Fig. 3than in Fig. 2 at both Headquarters and Meadow,especially in the late afternoon. Apart from a downvalley intrusion 1300-1500 (discussed below), the clearday hodograph for Meadow would show a counterclockwise circuit of wind direction; neglecting Coriolisacceleration, this is what elementary theory wouldpredict for a northward draining valley.4. Gradient wind influence There are several features in Figs. 2 and 3 whichappear to derive from gradient wind influence. Consider the north-south components at Meadow in July.The frequency of up-valley winds rises rapidly for 3 hrafter sunrise, but then decreases to a minimum in midafternoon. It is suggested that increasing convectionduring the morning resulted in increasing amounts ofgradient wind momentum being brought down intothe valley. By noon, this was usually sufficient toreverse the up-valley wind. When convection decreasedagain in late afternoon and the coupling betweensurface and gradient wind level disintegrated, thefrequency of up-valley winds increased briefly untilsunset initiated the return of nocturnal down-valleywinds. A similar pattern is evident in the graph for selectedclear days. Because the gradient winds were weakeron these days (than on the monthly averages), theperiod of gradient wind dominance was briefer. The above explanation assumes that the gradientwind effect includes a significant south component.Fig. 4 shows the frequencies of gradient wind directionsat the 700-mb level, which is at the height of the higher$52 JOURNAL OF APPLIED METEOROLOGY' VOLU,X~F, 7mountain peaks in this area in smnmer. The directionswere taken from twice daily maps prepared by theCentral Analysis Office in Montreal. For most days thegradient direction lay in the 190--310- sector. Somebacking of the actual 700-mb wind from the directionof the map contours would be expected due to frictionover the mountains. (Pilot balloon observations mademainly at sunrise on 15 days, 5 July-2 September 1956from a 6000-ft MSL bluff 2 mi north of Meadowstation showed an average backing of 20-, i.e., a20- difference between the 700-rob gradient directionand the observed wind at 10,000 ft.) The differencein direction between the map gradient and the actualwind at 700 mb would be expected to vary diurnally,reaching a maximum at the time convection exertedthe greatest retardatior~ on the 700-rob wind speed. Itis concluded that the gradient wind momentum broughtdown into the valley as a consequence of convectionwas from the southwest direction, on the average. The difference between July and August north-southgraphs for Meadow is attributed to less intense convection in August than July, due both to lower solarelevation and to fewer hours of sunshine; only 191 hrof bright sunshine were recorded in August, comparedto 313 hr in July. Similarly, the west component of the gradient wind(coupled with greater convective activity in July)is thought to explain the switchover :from east to westbeing earlier in the afternoon in July, than in August,at both Meadow and Marmot sites.5. Conclusions In the north-south Kananaskis valley, surface windcomponents across the direction of the main valleyshow a more pronounced diurnal cycle than componentsalong the valley, both ola selected clear days (with lowsynoptic pressure gradient) and on monthly averages. It appears that the effects of sub-valleys (and theslope effect in the main valley) on surface wind are lesssusceptible to overriding by synoptic-scale influencesthan the flow along the main valley. Though mountain and valley winds arise from radiational influences on mountain slopes, it is possible forthese local wind patterns to be less apparent on clear JULY Z2 so 180 210 240 210 300 330 360 030 060 Variable ~ 3 13 CLEAR DAYS 5 210 240 270 300 330 i 36O DIRECTION (degrees) Fro. 4. Histogram of 700-rob contour directions over theKananaskis valley measured from twice-daily charts (0000and 1200 GMT).days in high summer than on days when incomingradiation is somewhat less intense. This is becauseconvective activity and, hence, the coupling of surfacewind with the synoptic gradient wind tend to bestronger on days of more intense radiation. Daytime up-valley winds will naturally suffer moregradient wind interference than nocturnal down-valleywinds.REFERENCESBuettner, l~onrad J. K., and Norman Thyer, 1966: Valley winds in the Mount Rainier area. Arch. Meteor. Geophys. Biokli~n., 14, 125-147.Davidson, Ben, and P. Krishna Rao, 1958: Preliminary report on valley wind studies in Vermont, 1957. New York Uni versity College of Engineering, Res. Paper, 54 pp.Department of Transport, 1951: Wind equipment M.S.C. pattern. Meteorological Branch Circular 2039, 34 pp.Geiger, Rudolf, 1965: Climate Near the Ground. Cambridge, Harvard University Press, 611 pp.MacHattie, L. B., 1966: Relative humidity in Rocky Mountain forests of southern Alberta in smnmer. Canada, Department of Forestry Information Rept. FF-X-1, 54 pp.Urfer-Henneberger, Charlotte, 1964: Wind- und Temperaturver Mltnisse an ungest6rten Sch6nwettertagen in Dischmatal bei Davos. Schweiz. Amstalt Forst. Vers. Mitt., 40, 389-441.

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