JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D18, PAGES 23,087-23,101, SEPTEMBER 27, 1998

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1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 103, NO. D18, PAGES 23,087-23,101, SEPTEMBER 27, 1998 Snw breezes in the breal frest Christpher M. Taylr, Richard J. Harding nstitute f Hydrlgy, Wallingfrd, England Rger A. Pielke St., Pier L. Vidale, and Rbert L. Walk Clrad State University, Frt Cllins Jhn W. Pmery Natinal Hydrlgy Research nstitute, Saskatn, Saskatchewan, Canada Abstract. This study examines thermally induced flws (r"snw breezes") assciated with snw cver in the breal frests f Canada. Observatins frm a lake less than 4 km acrss were made as part f the Breal Ecsystem-Atmsphere Study (BOREAS) winter field campaign. These are interpreted with the aid f idealized three-dimensinal messcale mdel simulatins representing the frest-lake cntrast. Typically, strng frest-lake temperature cntrasts develp in the lwest 50 rn f the atmsphere during the mrning. The resulting pressure gradients induce lw-level nshre wind cmpnents acrss the lake. This snw breeze persists int the afternn prvided that large-scale winds remain light. A characteristic snw breeze signature is clearly evident in wind bservatins averaged ver 27 days f data, in agreement with mdel simulatins. The study suggests that snw breezes will regularly develp ver the many larger lakes and ther unvegetated areas in the regin. 1. ntrductin transprt mechanism requiring parameterizatin in general circulatin mdels [Pielke et al., 1991; Lynn et al., 1995; Variatins in the land surface can have a marked influence Zeng and Pielke, 1995]. Other papers have examined the rle n the prperties f the planetary bundary layer (PBL). The f NCMCs n the initiatin and develpment f strms [Sun effect f cntrasting surface fluxes f heat, misture, and and Ogura, 1979; Chang and Wetzel, 1991, Chen and mmentum depends n spatial scale [Shuttlewrth, 1988]. At Avissar, 1994]. As nted by Segal and Arritt [1992], scales f less than a few kilmeters (r micrscale) [Raupach bservatinal studies f NCMCs are scarce [Segal et al., and Finnigan, 1995], the effect f variability in fluxes n the 1989; Segal et al., 1991a; Mahrt et al., 1994; Smith et al., atmsphere may be blended ut near the surface [Masn, 1994]. This is partly due t the shrtage f apprpriate 1988]. On the ther hand, the entire depth f the PBL can be messcale bservatins but als thrugh the masking f a affected by messcale hetergeneity. n this case if the circulatin signal by tpgraphic and large-scale flws. surface variability is assciated with strng hrizntal Zhng and Dran [1997] argue hwever that the lack f gradients in sensible heat flux, thermally frced "nn clear bservatinal evidence indicates that under real wrld classical messcale circulatins" (NCMCs) can develp cnditins, NCMCs are nt s widespread as predicted by [Segal and Arritt, 1992]. idealized mdeling studies. Nnclassical messcale circulatins have been the fcus The Breal Ecsystem-Atmsphere Study (BOREAS) f a number f idealized studies [e.g., Okuchi et al., 1984; [Sellers et al., 1995] prvides the pprtunity t examine the Pinty et al., 1989; Mahfuf et al., 1987;?ielke et al., 1991; ccurrence f NCMCs in the breal frests f Canada. Segal et al., 1988; Dalu et al., 1991; Segal et al., 1991b]. Summer airbrne bservatins [Sun et al., 1997] have shwn These have illustrated hw messcale patterns f sil hw lakes within the frest prvide surface heating cntrasts misture, snw, and vegetatin cver can generate and sufficient t induce lake breezes. Detailed mdeling studies mdulate PBL flw. The develpment f NCMCs is very using realistic surface parameters [Vidale et al., 1997] have sensitive t the cntrast in surface heat fluxes between supprted these result findings. The study f hetergeneity in adjacent patches, the length scale f the patches, and the the BOREAS regin is extended in this wrk t the winter large-scale wind. n particular, messcale circulatins are and spring mnths. At this time, the regin is snw cvered, favred by light ambient wind cnditins and strngly with lakes frzen ver, inslatin lw, and temperatures as cntrasting patches several tens f kilmeters acrss [e.g., lw as -40øC. n this case, hetergeneity is prvided by the Segal and Arritt, 1992]. A number f authrs have argued cntrasting characteristics f the frest with pen snw cver that vertical fluxes assciated with NCMCs are an imprtant lying n lakes and in ther unvegetated areas such as pen wetlands and ver agricultural and recreatinal develpment. Cpyright 1998 by the American Gephysical Unin. These areas may be cnsidered t cver abut 30% f the Paper number 98JD regin. This raises the pssibility f NCMCs induced by the /98/98JD presence f pen snw cver within the frest, r "snw 23,087

2 23,088 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST breezes". "Frzen lake breezes" may als ccur in the regin when snw has melted ff the lake surfaces. As frzen lake breezes are similar t snw breezes [Segal and Kubesh 1996], nly the latter will be treated here. Observatinal evidence fr snw breezes is presented here frm measurements taken during a field campaign in March and April 1996 as part f BOREAS. Fr lgistical reasns the bservatins were made acrss ne f the numerus relatively small lakes, apprximately 4 km acrss. At this length scale, snw breezes are expected t be weak and may be masked by ther atmspheric features. A messcale mdel was therefre used t interpret the bservatins. The bservatinal study area is described in sectin 2, alng with an utline f the measurements taken. Characteristic differences in the surface energy balances f frests and frzen lakes are shwn. Sectin 3 prvides a summary f the numerical mdel, with an emphasis n the parameterizatin f surface fluxes frm snw-cvered surfaces. A case study f a snw breeze case under light wind cnditins is presented in sectin 4. The bservatins are interpreted with the aid f a mdel simulatin. Observatins and idealized simulatins are analyzed in sectin 5 under a range f wind cnditins. This gives an indicatin f hw frequently snw breezes ccur in the regin. 2. Descriptin f Surface and Observatins 2.1. Observatinal Netwrk Arund Namekus Lake The bservatin campaign tk place n Namekus Lake, Saskatchewan (53ø52'N, 106ø8'W) in Canada between March 7 and April 11, The study area is within Prince Albert Natinal Park, depicted in Figure 1. Namekus Lake is rughly circular, with a 3.5 km diameter, and is set amng a mixed-wd frest cntaining jack pine, black spruce, and aspen. Within the Natinal Park, very little frest has been cleared, thugh clear-cuts are extensive elsewhere in the regin. There are a number f smaller neighbring lakes, and the larger Waskesiu Lake 10 km t the nrthwest. The surrunding terrain is fairly flat, althugh a small hill rises 100 m abve the lake 3 km t the nrthwest. Observatins are als available frm a jack pine frest site near Beanrap Creek, sme 7 km nrthwest f Namekus Lake. Spatial variability in the surface layer was sampled at three sites acrss Namekus Lake (Figure lb). Three anemmeters and wind vanes were installed at a height f 2 m abve the snw surface apprximately 500 m frm the nrthwestern, suthwestern, and eastern shres. n additin, temperature, radiatin and turbulent surface fluxes were recrded at the nrthwestern site. Further measurements f radiatin, wind, turbulent fluxes, and temperature were made at the nearby Beartrap frest site. Over a perid f 10 days shwing lakes (hatched) and height cnturs every 125 feet (38 m). The bservatinal sites at Beartrap (BT) creek and (March 12-21), a tethered balln system (AR nc., Namekus Lake (NL) are marked. (b) Observatinal netwrk Bulder, Clrad) was installed n the lake at a lcatin ver Namekus Lake shwing the tethered balln site (TB) 750 m frm the nrthwestern shre. This system prvided and the lcatins f the three anemmeters near the nrthtemperature and wind speed and directin measurements western, suthwestern, and eastern shres. every 10 s t heights f several hundred meters. T ensure a stable platfrm fr the measurements, ascents fr were nly made when winds were generally less than 6 ms -. index f 2.2 m2/m 2, and canpy cverage f 82%. The Observatins were pssible n 7 ut f the 10 days. surface energy balance has been characterized by Harding and Pmery [1996] and Pmery and Din [1996] frm measurements made in Despite the presence f snw 2.2. Winter Surface Energy Balances The Beartrap frest site is in a stand f mature jack pine (Pinus banksia) with a height f typically 20 m, leaf area b N km 5 Figure 1. (a) Study area in Prince Albert Natinal Park n the frzen frest flr thrughut the winter mnths, the net albed f the frest is lw ( ), depending n slar angle. Fllwing snwfall, substantial latent heat fluxes

3 TAYLOR ET AL.' SNOW BREEZES N THE BOREAL FOREST 23,089 can arise frm sublimatin f intercepted snw n the canpy. During the cld, lw-radiatin midwinter, intercepted snw can remain n the lwer canpy fr several weeks. Hwever, in general, turbulent fluxes abve the canpy are t is imprtant t nte that this expressin assumes that dminated by sensible heat. Fr example, during the daytime intercepted snw has a negligible effect n the energy perid in the mnths f March and April 1996, 40% f the balance. This cnditin restricts the use f the scheme t incming slar radiatin abve the canpy was cnverted t days when there is n snw n the upper canpy, in particusensible heat fluxes, including days when intercepted snw lar in the days fllwing snwfall. was present. The large sensible heat fluxes are due t a high The tp snw level energy balance is expressed in terms rughness length, frzen sils preventing transpiratin and f water cntent per unit area f snw (and liquid water if evapratin, and pr cupling between the trest flr and melting) W and a variable Q representing the ttal thermal the PBL. energy assciated with the snw (and pssibly water) at temperature T,., including a latent heat term t accunt fr changes f phase. n additin t the net lng and shrt wave radiatin terms L s and S,, the balance includes sensible and latent heat fluxes H,. and ) E,.,. t the canpy space and a heat flux term t the next highest snw level G. The lake energy balance cntrast strngly with that f the trest [Harding and Pmery, 1996]. Observatins n Namekus Lake shw that albeds during the winter range between 0.7 and 0.9, while rughness lengths typically take values -1 mm, depending n the wetness f the snw and ice expsure. Blwing snw ccurs n pen surfaces when wind speeds exceed a threshld value. Average threshld speeds f 7.7 ms - at 10 m have been reprted by Li and Pmery [1997] fr dry snw, increasing when the snw is wet. During blwing snw events, turbulent transfers with the surface are greatly enhanced, resulting in episdic negative sensible heat fluxes as large as 100 W m -2. Averaged ver the entire 1996 campaign hwever, dwnward daytime sensible heat fluxes are small, nly 1.4% f the incming shrt wave energy. 3. Numerical Mdel n this study, the Reginal Atmspheric Mdeling System (RAMS) [Pielke et al., 1992] is adpted t examine the atmspheric interactins between the frest and the frzen lake. This three-dimensinal messcale mdel has been used widely t investigate surface-induced circulatins [e.g., Pielke et al., 1991]. The mdel was run with a hrizntal grid length f 375 m ver a dmain f 38 x 46 grid pints. A series f fully interactive nested grids at carser reslutin extend the dmain ver 100 km in the hrizntal. The vertical grid spacing increases frm 5 m abve the surface by a factr f 1.1 with each level up t a height f 4 km. Water vapr is carried by the mdel, but n cndensatin prcesses are permitted. Subgrid turbulent mixing is based n a first-rder defrmatin parameterizatin [Smagrinsky, 1963]. The mixing length and stability crrectins emplyed in the vertical diffusin cefficients are taken frm McNider and Pielke [1981]. A mre detailed descriptin f the land surface scheme is utlined belw Surface Scheme The land ecsystem-atmsphere feedback (LEAF-2) mdel [Walk et al., 1998]) was used. This includes a multilayer snw and sil mdel, with a tw surce surface flux scheme. Of particular relevance here is the parameterizatin f surface fluxes frm breal land cver types during the winter. The temperatures f a vegetatin layer T v and a snw-cvered grund layer T.,. are treated separately and cupled t a canpy space air temperature and mixing rati T. and Z,.. The vegetatin temperature in the mdel is fund frm a balance f net shrt and lng wave radiatin (S v and L v respectively) and sensible heat flux frm the vegetatin t the canpy (Hvc) with an areal heat capacity f the frest C v prviding a thermal inertia term. -Cv t = -l c - Lv - S (1) _ QW = H + X,E, - L - S - G (2) The fluxes t the canpy air space frm the snw and the vegetatin are given by E. = pcp (Zs-Z,.) (3) r d H c : pcp (T -T ) (4) r d H,., = pcp (T -T ) (5) where p and cp are the density and heat capacity f the air, and the resistances r, and r d depend n characteristics f the surface. Radiatin transfer within the frest canpy is a cmplex subject [Pntery and Din, 1996]. Develped fr a range f vegetatin types, the scheme in LEAF-2 treats the radiatin rather simplistically, ignring factrs such as the strng dependence f albed n Sun angle. The net shrt wave radiatin fluxes absrbed by the vegetatin S v and snw S. are given by S v = S d veg (1 - %. + c (1 - veg)) (6) Sa -- Sd ( 1 - veg ) ( 1 - t ) (7) where S d is dwnward slar radiatin, % and % are the albeds f the vegetatin and snw, respectively, and the vegetatin cvers a fractin veg f the surface area. gnring multiple reflectins and assuming that snw has an emissivity f unity, the net lng wave cmpnents fr the vegetatin L v and snw L,. are L v = Ldœvveg + {JœvVeg(T. 4-2rv 4) (8) L = Ld( l - veg) - ( 7., 4( 1 - veg) '" + aœvveg(tv4 _ Ts4) (9) The canpy air temperature and mixing rati are cupled t the first atmspheric level using similarity thery, with transfers characterized by a net rughness length fr the surface, z0. Fr the case f a hmgeneusurface such as the snw-cvered lake, the tw surce scheme abve cllapses t a single snw energy balance with veg=o. The stability crrectins f Luis et al. [1981] are adpted, and

4 23,090 TAYLOR ET AL' SNOW BREEZES N THE BOREAL FOREST Table 1. Surface Mdel Parameters describing Jack Pine Frest and Snw-Cvered Lake. av % veg z 0 (m) r d r b Frest /u, 35. Lake these agree clsely with the frmulatin f Male and Granger [1978] which has been widely used fr snw cver. The varius parameter values and resistance frmulatins fr the frest and lake surfaces are given in Table 1 and have been derived frm bservatins [Harding and Pmery, 1996]. The simplifying assumptins made in this implementatin f LEAF-2 fr the breal frest mean that the seasnal curse f snw accumulatin will be prly simulated. Hwever, cmparisn with bservatins suggest that the diurnal curse f turbulent fluxes abve the canpy are well represented. n additin, the use f a cnstant rughness length ver the lake is inapprpriate fr cnditins f blwing snw Mdel nitializatin The bjective f the numerical mdeling is t explre the diurnal evlutin f the PBL arund an idealized circular frzen lake f diameter 3.75 km, surrunded by a unifrmly t flat frest. These dimensins are intended t represent Namekus Lake. The idealized cnditins were adpted as mre realistic surface parameter maps at the spatial reslutin required were nt available. The atmsphere was initialized hmgeneusly in the hrizntal frm a single vertical prfile f temperature, humidity, and wind. The mdel was run frm 0600 t 2000 g 50- LT (1200 t 0200 UTC), cvering the daytime perid when lake and frest heat fluxes are strngly cntrasting. The snw was mdeled with three layers assumed t have a cnstant density f 250 kg m -3 and a ttal depth f 300 mm. On the timescales cnsidered here, the atmsphere is insensitive t the details f snw initializatin. 4. Case Study, March 12, 1996 n this sectin, an bservatinal case study f a snw breeze at Namekus Lake is presented. Of the 10 days when the tethered balln system was perating, March 12 was chsen because it presented the mst favrable atmspheric cnditins fr snw breezes. T cmplement the bservatins, the numerical mdel is used t examine the frest-lake interactins under these meterlgical cnditins Observatins With temperatures at the nearby Beartrap frest site reaching 12øC, March 12 was the third day in successin when snwmelt ccurred. The sky was initially clear, with scattered shallw cumulus appearing in late mrning. Nrthwesterly winds -3 ms '1 strengthened t a speed f 7 ms 4 by 1400, preventing subsequent balln ascents. The balln measurements prvide an indicatin f the vertical structure f the PBL rughly 750 m dwnwind f the frest-lake transitin. Hurly ascents up t 200 m were made between 1000 and 1400, with a number f shallwer prfiles recrded in the intervening times. The evlutin f the stable layer abve the lake is illustrated by the hurly prfiles in Figure 2. At 1000 the temperature prfile indicates strngly stable cnditins up t a height f 60 m, with the largesthermal gradients ccurring in the lwest 10 m. n the fllwing hur, warm air advected ff the frest gradually erded the stable layer frm abve. Subsequently, the ptential temperature prfiles were fairly well mixed abve 10 m. Hwever, beneath the lwest balln measurement (2 m), the prfile remained extremely stable thrughut the day, with the frzen lake surface temperature unable t rise abve 0øC (equivalent t a ptential temperature f 4.4øC). This intense temperature gradient was visible as a "heat haze" just abve the lake surface during the afternn. The balln bservatins f ffshre wind prfiles are less easy t interpret than the temperature data. This is due primarily t the transience f the wind field. Time-averaging techniques were attempted t prduce a smther cmpsite prfile. Hwever, the averaging perid required fr this ften exceeded the time in which the temperature prfile culd be regarded as apprximately cnstant. nstead, the nce-hurly "snapshts" are shwn in Figure 2. The representativeness f these wind cnditins is prvided qualitatively by reference t the balln ascents made between the hurs. looo ffshre w nd (m/s) ffshre w nd (m/s) ' ptential temperature (C) ffshre wind (m/s) , ! ptential temperature (C) t! / i / % ptenhal temperature (C) ffshre wtnd (m/s) ptential temperature (C) Figure 2. Hurly prfiles f ptential temperature (øc; slid) and ffshre wind (ms'l; dashed) n March 12. A three pint filter has been applied t the wind data.

5 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST 23,091 The 1000 prfile is typical f the early mrning cnditins an underpredictin f the clear-sky dwnward shrt wave and shws a well-defined regin f weak nshre flw cmpnent at the surface f up t 100 W m -2. Despite these beneath 50 m. This crrespnds t the strngly stable layer and cntrastsharply with repeatedly bserved ffshre flw - 4 ms -j alft. The lw-level nshre cmpnent gave way t ffshre winds during the day as the temperature prfile became mre well mixed. Hwever, it shuld be nted that differences in frcing, the LEAF-2 scheme prduces a similar partitin f fluxes (Figure 4) t the measurements. As bserved, the rise in mdel sensible heat flux lags the net radiatin by an hur r s when the Sun cmes up, due t the thermal inertia f the surface. During perids f clear during a perid f lighter winds between 1145 and 1245 (- 2 ms - at 60 m), ther prfiles indicated a strengthening f the skies the mdeled vatins. sensible heat flux is similar t the bserinversin between 2 m and 20 m. This was accmpanied by Flux bservatins ver the lake are nt available fr weak nshre wind cmpnents beneath 20 m, as is evident March 12, but values frm the simulated lake 750 m f¾m the 1200 prfile in Figure 2. The spatial variability in lw-level wind acrss the lake can be seen in Figure 3. This figure shws the hurly averaged wind vectrs frm the SW and E sites fr the perid when balln prfiles were available (nte that the wind vane at the nrthwestern site was nt functining n dwnwind f the shre are shwn in Figure 4. The sensible heat flux is very clse t zer during the initially calm perid and becmes slightly mre negative as the near surface wind speed and temperature rise. The magnitude f the mdeled lake heat flux is slightly sensitive t the initial cnditins f the snwpack. Hwever, it is clear frm Figure 4 that even this day). Fr cmparisn, the time-average winds bserved n such a warm day, when temperature gradients abve the by the tethered balln between 45 m and 200 m are als shwn. Prir t 1300, a marked lw-level nshre cmpnent t the wind is evident at the SW and E sites. This is in cntrast t the nrthwesterlies bserved abve the stable layer by the tethered balln. The bservatinsuggesthat the flw abve the lake is snw are large, the cntrast in surface heating between frest and lake is determined by the absrptin f radiatin at the frest canpy. The mdeled evlutin f the PBL ver the cntrasting surfaces can be seen in Figure 5. At 1000 (Figure 5a), sensible heating erdes the initially stable layer t a depth f influenced by a snw breeze. A strng inversin frms when 50 m abve the frest. Abve the lake surface hwever, a warm frest air is advected ver the lake. This prevents cl strng inversin persists in the absence f lcal surface air at the lake surface frm mixing upward and induces a near-surface pressure gradient. The wind at lw levels is heating. As a result, marked hrizntal temperature gradients develp acrss the shreline (Figure 5b). The thermal cntrasts are cnstrained t the lwest ten meters in the partly decupled frm the nrthwesterlies alft, and nshre flw develps in respnse t the high pressure ver the lake. vertical by the initial stable layer. The hrizntal temperature Hwever, when the wind speed alft increases, the stable layer is erded and the snw breeze dissipates. This qualitatgradients perturb the mixed layer westerly mmentum, with the lwest 15 m characterized by nshre flw. The ive descriptin f March 12 is nw examined quantitatively symmetry f the snw breeze is distrted in the directin f using the numerical mdel. the large-scale wind. This is due t advectin strengthening (weakening) the thermal gradients at the upwind (dwnwind) edge f the lake Mdel Simulatin By 1300, turbulent mixing has erded the inversin frm The mdel is initialized frm the 0600 (1200 UTC) abve, with stable cnditins restricted t the dwnwind side atmspheric sunding at the Pas, Manitba. Lcated 300 km f the lake (Figure 5c). Acrss the upwind shre the t the east f Namekus Lake, this was the clsest available lw-level flw accelerates in respnse t the decrease in bservatin. Abve 500 m the prfile is mderately stable, surface rughness. N snw breeze effect is apparent here, while n the clder eastern side, the wind directin remains primarily nshre. Fllwing a further 3 hurs in the simulatin, the thermally induced pressure gradients have with wind speeds increasing frm 5 ms - t 7.5 ms - at 3 km. Fr the purpses f mdel initializatin the strngly stable prfile at the Pas belw 500 m is warmed thrughut by several degrees. This is dne t match lcal bservatins f temperature abve the frest canpy and tethered balln measurements f the residual layer abve 100 m at The prfile (Figure 2) indicates temperatures f 10øC at 150 m, implying an intense lw-level inversin. The snw is initialized with a surface temperature f-7.5øc, increasing t -4.5øC at the bttm f the 300 mm pack in agreement with available bservatins. been advected dwnwind f the lake. Althugh the lw-level wind still decelerates acrss the eastern half f the lake in respnse t the thermal frcing, the directin remains clse t westerly everywhere Cmparisn f Observatins With Mdel n this sectin the available bservatins are cmpared with the numerical mdel t verify that the simulatin The bserved net radiatin and sensible heat fluxes frm captures the basic features f the PBL n March 12. A the Beartrap frest site are shwn in Figure 4. The turbulent cmparisn f vertical temperature and mmentum prfiles sensible heat flux represents a little less than 50% f the net at 1000 and 1300 is shwn in Figure 6. The mdel diagnsradiatin absrbed by the frest canpy and underlying snw. tics are taken frm the grid pint dented by a circle in With bserved hurly latent heat fluxes never exceeding 40 Figure 5, 750 m dwnwind f the shreline. The transitin W m -2(nt shwn), the remaining energy is used t warm in stability between 1000 and 1300 is clear in bth mdel the frest canpy and snwpack, with melt rates in the and bservatins (Figure 6a). Hwever, the stable layer afternn crrespnding t ver 100 W m -2. The numerical bserved in the lwest 25 m at 1300 is nt reprduced in the mdel prducesignificant differences in dwnward radiatin mdel. This inversin develped during the relatively calm at the surface cmpared t the bservatins. This is due t perid between 1145 and 1245 and was nt apparent in bth the absence f cluds in the idealized simulatin, and subsequent ascents.

6 23,092 TAYLOR ET AL.' SNOW BREEZES N THE BOREAL FOREST ,[ m/s Figure 3. Hurly averaged wind vectrs frm the functining anemmeters n March 12. Als shwn fr each hur is the wind vectr averaged frm balln bservatins between 45 m and 200 m. The mdel PBL gains mre sensible heat frm the surface 0.3 K. Later in the afternn, the temperature bias in the than bservatins wuld suggest (Figure 4). This is particu- simulated PBL becmes mre prnunced (- 1 K). larly s in the afternn when cnditins becme cludy, Frm Figure 6b, lw-level nshre flw ms 4 is and the mdel significantly verestimates dwnwelling fund in bth the mdel and the bservatins at Als, radiatin. Hwever, by 1300 a simple heat budget calculatin the transitin t ffshre flw ms ' abve the stable suggests that when mixed up t the inversin at 1 km, this layer at 50 m is well reprduced by the mdel. This encurgives a warm bias in the mdel ptential temperature f nly aging cmparisn suggests that the mdeled surface-induced

7 _ ß TAYLOR ET AL.' SNOW BREEZES N THE BOREAL FOREST 23, net bs. '-'" ': nhet s?el _../,/½"'"" "'-,%,%,... H mdel,lake),,., // :;'... 14',,,...,.,..'..½...?.:.._..., / ' ' ' ' ' ' ' ' lcal time 4.4. Summary f Simulatin Figure 4. Observed and mdeled net radiatin and sensible heat flux (H) ver the frest. The mdeled sensible heat flux nitialized with a prnunced lw-level inversin acrss the dmain, the mdel illustrates hw the flw develps at the lake site is shwn fr reference. pressure gradients ver the lake are f a similar magnitude t thse encuntered n Namekus Lake n March 12. By 1300 the mdel indicates a well mixed mmentum prfile, with a magnitude similar t a lcal average f the rather nisy bserved prfile. Cmparisn between mdeled and bserved cntrasts in lw-level temperature are shwn in Figure 7. Prir t 1130, temperatures the frest site are several degrees warmer than at the site 500 m dwnwind f the frest (nrthwestern (AWS) in Figure 7a) due t the variability in surface fluxes. This bserved thermal gradient diminishes during early afternn. Hwever, a cntrast- 3øC develps between the tw lake sites arund midday, despite being nly 2 km apart. Bth f these hrizntal temperature cntrasts are well represented by the mdel, althugh it shuld be remembered that the simulatin develps a warm bias in the PBL. wind speed ( /s) :'* "" ' :..." i.. '::' " "... '"":'" 10-!i?..: '"'"'-'-":'-'---'-"..-'-' '-.... '. F": ' ii?..: :...:. i i i :. : :. ;i:....: `...:..i::... : iii... ' " i!:-.'-: i½:i,:,...:..:.:i: ii?.:.../...: i½ig ;.!;i½i?.:...:. : 47i ,, -....:.:. -!½.-.,...:. :...: i..-...,,...:...,...,... i,...!;ii?. i?.-.,,,....½...:.,.. ; i!.?-'---'".----" ;i?-'"-': ':---...:-'-?-"-"--" :- ;-.--'i!!:'-'---' i::i i ::--'Z i:: :F'"'-'-"-' :...'..-:- :i :""--".- '"...' : i! ""'...-/.." i.".-ii"-: --'"-': : ::"':"' i! ½?..-': ii:--" '...:.--, :.. -..;?..:-.'...'-../..½?.'.'-: ;..._?.:.."?..?.:?... i?..., ;:i 4.:... iii. "...:.`. ii: ½;.);.".:.? i,?ii"":" ":- -½:,,,i,?: '::' "',i!;;'""'. "'":"'... "...'-,, ;' '... ;i "::':'" '-.. "½--' %41;;:' '"'"'"'-.."-'½..-:-: :...D;;' ""'--'"'" '"' "' '"'"' '": : '... :i ':., ' '-' '"':" ' ß iir:'""-:'- ":'-.-.': '.' :di :.-":. i{/ -'"' " :'.:!! "%'"-'""----'"-'- - '"'"" ' ' ':½. /'-" - :' ' ---' ::-.,xr -... t, -'%-""'"'"'" '""'" "":'"" ' ;.'... 1 ' '" ' ; ;::?:;? iii½: :-.'."..[.".4::.':..-'.'i ::. :,. /..; ½¾, :::,:: :: :i:i:i:!i!! i!:.... t... ;:!i! i.'-'.' :!!i!ii.:..' ½..:.::'!! i.-'.:.-' ; '".:.-.?:!,;; 4... ii4!?'"' ;if'"'"" "":""" : iii ':"".::,:iiiii;;!', i:½ "" "-:"""': :i "' i:½"'":""'""'"..--: '""'"""'""" '"" '!4::' '""' '-'"-" """""" "... iiii!!:,,,,::,...,21 '--' - :!:iii!i:'.;'-'...'...'-' i '" '......'} ' ' '" ' ",,:;::!. iii½-'. i i-.-'i':*""' ' :' -:"? ;'- -'":'... :'-,'- ß..- ":- E 3 i?:.-.:' i?..":...::?...':";?:: ;?:i::;: ::;' "' i ½.?:'":-:- --..: :??. :? '"' F'"'"' ' '-' ' " "' **ii ' b ;'*':?:" ::'"'... wind speed (m/s) i!;½. ii'--'"------'-... ' '"'"'"'"'"'"'"'""-'... : '.'..'"..':..l., x (km) i i i i,:: ii: i::ii!... i...ii:i,:i...,... /,...`.`... :..... i i... :., : E":"'"'::" ::--- '--"' ',ii i:;... '"ii::'"""" ii((i"'""':-"-"!i!" '"'"'" " ::' ', ' i:""'"" : ' i:!i"'"' '-'- """ ß t d,. iii':" :: "'"" "' ii.:.';:.:...:.,:.....:..-..:..;.;....;..;. ;.;.;..; '"' -':-... ii i...: :...:':... ":"... :'i*" ' ' '"'; "'" i:"'""":' '":'... : iil l, wind speed, (m/s) x (km) Figure 5. Mdel simulatin shwing ptential temperature cnturs (øc) and three dimensinal wind vectrs at (a, b) 1000, (c, d) 1300, and (e, f) The vertical crs sectins (a, c, e) are at y=3 km, and the plan views (b, d, f) are at 2.5 m, with the t¾zen lake indicated by the slid line. The circle and the triangle in the plan views indicate the lcatin f the NW and SW AWS relative t the mdeled wind directin.

8 23,094 TAYLOR ET AL.' SNOW BREEZES N THE BOREAL FOREST acrss the lake under typical spring-melt cnditins. Over the trest the inversin is destryed by strng heating frm the canpy. The warm and well mixed air is advected ver very stable air extending several tens f meters abve the lake. A snw breeze develps, with nshre flw exceeding 1 ms - arund 1200 in the lwest 10 m. Hwever, the advected frest air gradually erdes the lake inversin frm abve. By the afternn, prfiles are generally well mixed dwn t a few meters frm the lake surface, depending n dwnwind distance frm the frest. A prly reslved but extremely stable layer remains just abve the melting snw. Hrizntal temperature gradients f several degrees persist acrss the lake thrughuthe afternn, leading t a weak snw breeze effect at the dwnwind side f the lake. Mdeled hrizntal and vertical temperature gradients cmpare favrably with the bservatins. Reasnable agreement is als fund between the available wind measurements and the mdel. These cmparisns lend cnsiderable weight t the ability f the mdel t capture the key atmspheric prcesses under these cnditins. 5. Observatins and Simulatins Under Typical Cnditins 5.1 Observatins f Lw-Level Flw n the previus sectin we have chsen a single day t examine the develpment f a snw breeze given light winds ' a r?.., bs mdel mdel bs ,,?, ' ' ' ptential temperature (C) bs "- -; bs \ -- _ --.. [... mdel ,,. -:...t "... mdel 1300 / : ' 4 ;-, ffshre wind (m/s) Figure 6. Observed and mdeled prfiles f (a) ptential temperature and (b) ffshre wind 750 m dwnwind f the frest-lake transitin at 1000 and and high inslatin. We nw seek t generalize these findings by analyzing the typical diurnal cycle f the lw-level flw under a range f atmsphericnditins. While the balln bservatins can prvide useful measurements f the vertical structure abve ne pint n the lake, ascents were nly made n 7 days. nstead, we rely n the anemmeter netwrk which prvided a cmplete data set f hurly wind speed and directin at a height f 2 m in three lcatins fr 27 days (March 15 t April 11). This extends 8[:!:g! :.:.:. -::.: :<.:...x.:...:`:-+-.: <<.`. :.:.:...m > `..:..`.,-.: ; :??..:.::...x.+::.:...-:..-.>x.., :x.::r.. :....::..:.:...x.:...:..-.:.-...::::.:.:,-..k x+:..-.-.:,,.g >..,...x:... :<.. >:. g:..,....,,.¾s...-..; [(.:.:i-:-. :.x.x.,.::,..:.:., -' i!x.:.:...:. :<.:..:::::::. ::: - ii:.8::_.-:¾.q::>.::.:::: int the perid when the snw and ice n the lake surface were rapidly melting and includes a range f typical atmspheric cnditins. Lw-level wind speed and radiatin F... -.,.,,,... :. :. ii,...i::... :..!.:.,.::,...:. :...,.. i.:.:... i...,,.._,.....,._.:.!.. <.,,.....:<...j ::..;.,,,.._.,...:..,,.,... cnditins are summarized by the bar charts in Figure 8. A daily averaged wind speed between 2 and 4 ms 4 was... recrded n 16 f the 27 days. The daytime average incming slar radiatin exceeded 300 W m -2 with a similar frequency. Fr reference, the case study day f March 12 had smewhat weaker surface winds (1.6 ms") and lwer 6....¾. ::<:½..-.u:.-.. :.:: ¾.::,¾:¾..:i.?..-:: =========================================================... :::..-:'..! :. : :.,.::::.,.:!: ',---,<.,.'-' 4..- :..,.:.,.'.-'.. i " iii?:? i:½ii?'":< "" '": '"""::":' 'ø"'"'... "' '"'"""'"'" '"'"'-'"- - ' ' : ' wind seed m/ x (km) Figure. (cntinue. d) inslatin (240 W m -2) than the average fr the perid. A maximum daily average wind speed f 6.6 ms - was recrded n March 16, with blwing snw bserved n the lake thrughut the day. Shrter blwing snw episdes are likely t have ccurred n a number f ther days in the data set. Figure 9 presents the mean wind speed at the nrthwestern site ver the perid as a functin f time f day. There is a prnuncediurnal cycle, with speeds increasing by almst 3 ms 4 during the perid Fr the remainder f the afternn, wind speeds remain relatively cnstant befre

9 ß TAYLOR ET AL.' SNOW BREEZES N THE BOREAL FOREST 23,095 a a b :::::::::::::::::::::,:.:+:.:.:.:,:.:.:. ::::::::::::::::::::: ß lo E /:,f :::::::::.::: ::::::::::::::::::::::::::::::.::.:+:.:.:.:.: ::.:.:.:+:.:+:.:.,::,,,:,;.:.:.:,: :+:.:.:.:,:,::.:._ 20 c _/,,,"::.'"',,,,',/,."... NW AWS ß "..-' SW AWS ' ' frest site b ' ' ' ' ' ' wind speed (m/s) 9 1 O racmrag slar radiatin (W/re"2) Figure 8. Histgrams shwing fi'equency f (a) wind speed and (b) incming slar radiatin cnditins during the perid March 15 - April 11, The wind speeds are meaned ver all three sites and fr every hur in the day, while the slar radiatin is meaned ver every hur between 0600 and _.... NW mdel " --.."/ SW mdel frest mdel ' ' lcal time Figure 7. Evlutin f telnperature abve stirface as (a) bserved by AWS at the frest and tw lake sites and (b) mdeled at an upwind frest pint and at the crrespnding lcatins n the lake. drpping rapidly between 1900 and Winds then remain light thrughut the night, until - 2 hurs alter sunrise. Fr the study f snw breezes the spatial variability in flw acrss the lake is mre imprtant than the mean wind speedß This variability can be characterized by the hrizntal divergence f the wind, Ou/Ox+Ov/r-)y. Given wind cmpnents u,, u:, and u 3 in the x directin lcated at (x,,y ), (x:,y2), and (x3,y3) respectively, and apprximating u as a linearly varying functin f x and y yields the fllwing three equatins' u = % + x, + [3y / 2 = /'/0 + O( X 2 + [.1/2 (10) % = % + x. + [3."'3 These equatins can be slved t find, equivalent t Ou/Ox. Adpting a similar prcedure n the v cmpnent f the wind vectr, the hrizntal divergence can be expressed as (11) day (Figure 9). This prduces a strng diurnal signal in phase with the wind speed. Di',ergcnce increases after ()800 and reaches a maximum an hur befre slar nn (1300). As the maximum wind speed is reached in the aftern{xm, divergence ver the lake drps steadily, with a iapid transitin arund 2000 t cnvergent ncturnal c,m!i!ins. The bservatins are nw reanalyzed acc >rtting t large-scale wind cnditins. n the absence f wind speed bservatins well abve the lake and fres! cancq)y the tata are classified accrding t the daily average wind speeds frm the three anemmeters. Average diurnal cycles f the wind speed and divergence are presented in Figtire 10 fr the calmest and windlest five days f the bservatin perid. These crrespnd t daily averaged 2 n wind speeds f less than 2. l ms and greater than 4.1 ns. rcspcctix, cly. Ft r tl e calmest days the amplitude and phase f lh½ diurnal wind speed signal is very similar t the average wind speed while remaining abut 2 ms - lighter. On the windlest days, diurnal trends in wind speed are less clear, thugh an increase in late mrning is still evident. Examining the divergence patterns 01 T' y (x 2 -x ) % (x -x ) +y (x,-x 2) Using hurly values f u and v fi'm each f the three sites, the divergence is calculated using the abve expressin and then averaged ver all 27 days as a functin f tiine f lcal hme Figure 9. Diurnal cycle f wind speed (ms- and divergence (x 104 S -1) averaged ver 27 days.

10 23,096 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST a days. Here the perturbatin wind vectrs are nt clearly related t the directin f the nearest shreline. 8 * 0 (1) b,,, Simulatins f Lw-Level Flw Under dealized Atmspheric Cnditins The numerical mdel is initialized here under idealized atmsphericnditins t help interpret the bservatins in the previus sectin. A cnstant ptential temperature gradient i)o/i)z and gestrphic wind prfile are adpted fr this study. Snw and vegetatin temperatures are set at T O, the temperature at the lwest atmspheric level f 2.5 m, and the frest canpy is assumed t be snw free. n the simulatins described belw, values f i)o/i)z=o.015 K m -l, T0=-21øC, and 50% relative humidity are used, with incming slar radiatin calculated by the shrt wave parameterizatin fr March 21. The sensitivity f PBL flws t wind speed is illustrated with westerly gestrphic winds Ug f 2, 4, and 15 ms 4. Nte that even in the 15 ms - simulatin, wind speeds at 10 m abve the lake remain belw the average dry snw threshld fr blwing snw [Li and Pmery, 1997]. Figure 12 illustrates the hrizntal wind and temperature fields ver the lake at 2.5 m. As expected, the lw-level flw is sensitive t the value f %. n the light wind case ms- ), hrizntal temperature differences exceeding 2øC persist in the lwest 25 m. Cnsequently, the westerly flw * 0 (1) 5 calmest days 0.5 rn/s lcal time Figure 10. Observed wind speed (ms'l) and divergence (s 4) ver the lake as a functin f time f day averaged ver (a) the five calmest and (b) the five windiest days. The average values ver all 27 days are shwn (thin lines) fr reference. a calmest days 0.5 rn/s fr the calmest and windiest days, several features are apparent. On the calmer days the amplitude f the diurnal signal is mre marked than n average. This is due t bth strngly cnvergent flw at night and mre divergent daytime cnditins. On the windy days, there is a crrelatin between strngly divergent flw and wind speed. The nature f the strngly divergent and cnvergent flws apparent in Figure 10 can be interpreted frm analysis f the 5 windiest days 2 rn/s wind vectrs acrss the lake. At each f the three sites, wind cmpnents are averaged ver the data set fr every hur f the day. These three sets f cmpnents are then linearly cmbined t prduce a lake average wind vectr, frm which each f the three statins has a lcal deviatin. The average and lcal deviatin vectrs acrss the lake are pltted in Figure 1 l a and 1 lb fr the five calmest days in the data set. c 0100-!100 i i i Fr clarity these have been averaged ver tw perids f 10 0!an 5 hurs each, cvering the strng daytime divergence and Figure 11. Wind vectrs averaged ver (a) daytime n the nighttime cnvergence apparent in Figure 10a. five calmest days, (b) night time n the five calmest days, The lcal perturbatin vectrs in Figure 1 l a are directed and (c) the mrning perid n the five windiest days. The nshre during the day, a situatin which is reversed at night large vectr at the center f the lake shws the mean (Figure 1 lb). This cntrasts with the vectrs in Figure 1 l c cnditins frm the three sites, while the lcal perturbatins fr the early mrning divergent perid n the five windiest are indicated at their respective lcatins n the lake.

11 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST 23,097 :::::::::::::::::::::::::::::::::::::::::::::: :.:::.,, ;:-.;: ':::...:._. -.-::::... ::'.:. ::::::::::::::::::::::::::::::::::::::::::::::::... ::i::::: '-.'. :'-, ":i-'-,.. :i:'.','.. ii!'"::... :,-".'--.-".:iil :::'":.--:ii::i::iii ::.':: :?::-'?:i :.::...'. ::iiii ::::::::::::::::::::::::::::::: i':'"::': ii ::?:?:.:-=...": ::iii :i:: 3 6 x (km) is strngly mdified by a snw breeze which develps in the lwest 100 m abve the lake. Onshre flw is fund all acrss the lake, with the strngest cmpnents at the upwind edge. n the high wind case (Figure 12c, %=15 ms-l), hrizntal temperature cntrasts f typically øc are fund thrughuthe lwest 50 m, and the flw directin acrss the lake is unifrm. Wind speeds increase dwnwind f the transitin between the aerdynamically rugh frest and the smth frzen lake. n this case, the clder lake air is advected dwnwind int the frest, destrying the thermally induced pressure gradients which drive the snw breeze. The flw fr the intermediate wind speed simulatin (Figure 12b, Ug=4 ms -1) is qualitatively similar t the March 12 case study. A weak snw breeze develps in the lwest 25 m in the mrning. By cntrast with the %=2 ms - case, thermal gradients are weakened by turbulent mixing and advected eastward, resulting in nly small perturbatins t the flw dwnwind during the afternn. The difference in snw breeze develpment between gestrphic winds f 2 and 4 ms - is in qualitative agreement with the simple scaling arguments prpsed by $egal and Arritt [1992]. Given a midday surface heating cntrast f 200 W m -2 arund a patch f length scale 4 km, thermally induced flws cunter gestrphic winds f less than apprximately 2.5 ms -. The evlutin f the mdel flw ver the lake is charac- terized in Figure 13, shwing the average wind speed at 2.5 m averaged ver every lake pint fr the duratin f the simulatin. The lw-level wind speed fr all three simulatins shws marked diurnal variatins. Under light winds (Ug=2 ms -1) the pattern is assciated with the snw breeze, giving a maximum speed at 1500, apprximately 2 hurs after slar nn. When Ug= 15 ms -l, the wind speed increases rapidly during the mrning and remains clse t 5 ms -l thrughut the afternn. This behavir can be understd by examining the stability f the prfile abve the lake, as illustrated by the plt f average lw-level Richardsn number (Ri). Arund 0900, warm frest air is advected ver the lake, capping the cler air beneath and preventing mmentum transprt dwn frm alft. Gradually, the lake inversin is erded frm abve by vertical wind shear with Ri decreasing tward a neutral value f zer. Under these cnditins mmentum is transprte dwn t the surface and wind speeds are large. n the evening, a stable layer redevelps and near-surface wind speeds cnsequently decrease. Fr the simulatin where %=4 ms -l, winds are dminated by the snw breeze in the mrning and, increasingly, stability effects in the afternn. The resulting areally averaged wind speed reflects this cmplex behavir. Als shwn in Figure 13 is the vertical cnvergence (-Ow/Oz) ver the lwest atmspheric level, averaged ver every lake grid pint. By cntinuity, this quantity is apprximately equal t the hrizntal divergence ver the lake (Ou/Ox + Ov/Oy) assuming that density gradients are small. Fr Ug=2 and 4 ms -, the shape f the curve is clsely linked t the evlutin f the snw breeze. This is illustrated in Figure 14 which shws vertical crss sectins f vertical velcity ver the lake at Fr Ug=2 ms -, the subsidence wind speed (m/s) 10 x (km) Figure 12. Hrizntal wind vectrs and ptential temperature cnturs (every øc) at a height f 2.5 m abve the surface at The edge f the lake is dented by the thick cntur. (a) Ug=2ms -l.(b)%=4 ms -l.(c) Ug=15ms -.

12 _. 23,098 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST :: :i i:i :i:i:i :i:i:::::::::::::::::::::::!. : ::!:i: :i:!:i:i:i:i:i:i:i!..`..<.:. :!::: ::i i :.`.::::::::::::::::i: : ::!: i 'l-: ',!i : :?:,i.-'.. ::?:.-:-' /: :"'-:':':-- --'--.-:- '-.-'.'...' : : : '::...:.... :?::... : il; ':-': :. '-" :"" :' ' ' " :' :...: i'"' '"'"'"' <":':':"' '":'"-':'--. ::--" :; : :i : :i ii!ii: ii ii!.:!ii i!:.&...::i:ii!ii!ii!ii: : : ::: :: :::>..:i:... i!::ilili:.:.....,, :: ::.:.:::: ::::.,..... :::!:!:::i:::i:>.:.:, :.. :.. ::.. : :.::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::.'-:: :::.-'&:::::::::.<::: ::::-::: ::-::.:.i..:-:: O Ug=2m/s.., ß" "'"" 12 - Ug=4m/s Ug=15m/s / t,, r!,, 6- O "" " 1250 i 3 6 x (kin) b J ß lcal time Figure 13. (a) Wind speed at 2.5 m and (b) vertical cnvergence ver lwest 5 m, averaged ver all lake grid pints as a functin f time f day fr three values f gestrphic wind speed. The evlutin f the lake-averaged Richardsn number (œj) between 2.5 and 7.6 m is shwn fr reference in Figure ] 3a fr = 15 ms -. N :::>... : ::: :: ::::::: ::::: : :: :.:..;..e...:?.!: ::::.::::: :::: ::::. :. :::::..y:::... :: ; ::.: :::::.:::: =================================================== ::: :[...! ::::.<::::.:::::::::::::::::::::::... :.. :.:... :::: : :. : ::.-.::::' :-:::[:::::-:J8 :.1? ½:-:: ::-:. -: :-. :::: :::::::::::::::::::::::::::::::::::::::::::::: :l:i:i i:i:!:?i. :.'::- /:- :2 ::)?::- iii ' -'--: -:: :, : ['""'-" ' assciated with the snw breeze is lcated ver the lake. Fr ug=4 ms -i hwever, the circulatin the upwind edge f the lake has been destryed. Weaker nshre flw remains n the dwnwind side f the lake at 1400 (evident frm Figure 12b), and it is this that prduces the dwndraught lcated ver the eastern half f the lake and beynd in Figure 14b. Because this weaker circulatin has been advected dwnwind ver the frest the vertical cnvergence averaged ver the lake is further reduced relative t the Ug=2 ms - case. n bth simulatins the vertical velcity perturbatins extend t the tp f the PBL as can be seen frm the (shaded) ptential temperature cnturs, and their intensity is reflected in the near-surface values. The shrelines in the high wind simulatin (Figure 14c) are assciated with shallwer vertical mtin in the ppsite directins t the snw breeze cases. N :i:.>.-::::.::!:::!::: i!:!:::.-'-'!!; '.'?.- :!!!..".' [: i i:i i! i i i i ii iii!i! i i! ii i! i! i i:. i i i i:! i i! i i!::..-.'!: :i:!:i:!:i:i:i:i i:i:!:i::.:i:i:i: :. :t: :;:i: :i: :i : i:: i?: :: i:: :::::::::::::::::::::::::::::::::: i ii:i i ii:: i':?:!i:: :::: : i ii:: ili!! ii:il.! :! i'..':iiii::i i:: i :::::::: ::!::.½: :::-.:::::::!i!:: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::.::::::::::::: :::5:::::::::! ::::::::::::5 ::::::::';::1.>:::::::::::::.::::::::::::::';.':! :::::::::::::::::?::: ::::::::::::::::::::>.':::';5:: ::::::::: ::::.-::: ' '"""-""" ' ::i::i ::i:i?: :::: ::::::::::: :::- :::::::::: ::."::::: ::::."-'::: :::::..'.::: :: : :: ::::: :: ::?-': 5:: ::::: : ::;::.>::::::: ::::::::::: :.::::::: ::: ::::::: '-' ::::::::::::: :J.:: :..>.':. ::::5:: -.'::: :::::::::::::. :: ::::::::: :.: :.: :: : ::::::::: -;.':.: :::::::::::::5 >.':?:.>.: ::::::::::::::::::::::::::::::::::::::::::: ::5::::: :::::::::: : '.-:.-::::::::::::: :'..-: ::::::::: ::: ::: : :::::::::: -: ::::::::::: ::::: : ::::::::.::::::::: :..-'. :::::: ::::::::::::: :::::::: : :. ::::: :::::::::: : ::: ::::::::::::::::::::::::::::::: :: ::.>.. : ::::. : :::::: ::::::::: :: ::::::::::.: 5:::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::. :::: :<..:: ::::::5 ::-::.': :. :..;.':: :½: :. j ::::: : :::::::::::::::::::..'.: ::::: : [-':-':::::..< ::. ::::::::::::::::::::::::::: :::- ::.::::::i: :::: ::::':5.'::::::: ::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::: : ::::::::::::.-.'::: ::::::::::::::::::::::::::::::::: :::::::::..>..:: :.:.'? :½::::::: :::::...' :::::::: : ::: 5..:.'. ::::::::. : :::...'.. '.). ::..: :::: :}.': :::.?.,.:::::::::: ::::.:.-: /::: :.-..-:: ::::::::::: : ::::::::..-.:..,..::: :::::..-:. :. :::::.::::::: ::::::::::::::::::::::::::::::::::.,.-:: : :: :-, : ::::: :::::::: :-..: :::::::::: ::::::::::::::::::::::::::::: ::: : : ::::::::::::::::::: t:::::::: ::::::::: :::::::::.: ::..'.-: >.'::: ::::::::: ::::::: ::::..<i::z-.: ::::::: ::: ::::::: :::: :::::::::- : ::::5 ::: ::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :. :.<::?.::.-...)::::>::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: : :: ::::::::::::::::: ::: ::: 5::: :::: :::::: ::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :: ::- :.-':: 5: :. : :::::::::::::::::::::::::: :::::::. : :::;.' :?.?.-:::::::::.:::½:::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::: : ::.>.' :::- ::::::: : :. : ':.<.:: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::: Figure 14. Vertical velcity cnturs every 4 cms - alng the axis y=3 km at 1400 fr (a)%=2 ms -,(b) Ug=4 ms -, and (c) Ug=15 ms -. Negative values are shwn dashed. The lcatin f the lake is marked by the thick line at z=0 m, and ptential temperature cnturs every 2 K (shwn by shading) indicate the depth f the mixed layer. c 25O ::::::::::::::::::::::::::::::::::: :::. ::::::::::::.::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ================================ ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: x (kin)

13 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST 23,099 A dwndraught at the upwind shre results frm the acceleratin f the lw-level flw as it passes ver the smther lake. The wind subsequently decelerates as it reaches the rugh frest, accmpanied by ascent. The bserved divergence frm the three anemmeters (Figures 9 and 10) represents an areally averaged lw-level vertical velcity ver the lake and can be cmpared with the three simulatins (Figure 13). n ding s, ne must remember that the mdel flw is sensitive t the chice f impsed gestrphic wind. Hwever, wind cnditins abve the surface layer in the bservatins are bth unknwn and time evlving. Frm a qualitative viewpint the diurnal cycle f the bservatins is a feature that is captured in the idealized simulatins. Under light winds, hrizntal divergence rises sharply. This is assciated with nshre flw in bth bservatins (Figure 11a) and the mdel (Figure 12a). Quantitatively, the bserved divergence maximum under light winds is cnsiderably smaller than in the idealized simulatin Other simulatins with alternative temperature prfiles have been made. The lw-level flw was mdified nly slightly by halving the value f O/ z, fr example. The effect f raising daytime atmspheric temperatures abve 0øC was als investigated by taking T0=-øC. Again, the trends in lw-level wind speed and divergence remained with ug=2 ms - and f similar magnitude t the case when unmdified, with nly slight changes in their amplitudes. The ug=4 ms -l. Withut knwledge f cnditins in the upper snw breezes simulated are barely affected by this because PBL n bservatin days, interpretatin f this cmparisn the cntrast in surface heating between the frest and the is difficult. Assuming that the mdel results are reasnable, lake is fairly insensitive t the initial temperature. The it suggests that snw breezes d nt typically persist ver thermal gradient is driven by the frest heat flux, which in Namekus Lake thrugh the entire day, as in the ux=2 ms -1 this case is cntrlled by radiatin rather than temperature. case. The simulatin with ug=4 ms -1 may be mre typical, This demnstrates that the snw breeze is insensitive t with mrning snw breezes weakening and being advected whether the lake surface temperature is melting r is well dwnwind in the afternn. t shuld be nted hwever, that when wind speeds drpped and inslatin was high, belw 0øC. divergence f the magnitude simulated in the u =2 ms - case was recrded ver several hurs. Hwever, such light synptic winds did nt persist fr lnger than a few hurs during the bservatin perid. The bserved ncturnal cnvergence ver the lake n the five calmest days is nt reprduced by the idealized simulatins, and its rigin is unclear. Measured fluxes ver the lake and trest d nt indicate any clear difference in heat fluxes after sunset, bth remaining clse t zer. The data shw that ffshre flw after sunset cincides with sudden decreases in wind speed. These events may be assciated with a decupling f the lake surface layer frm the PBL. Under such calm cnditins, it is pssible that weak tpgraphic flws can develp. Under strng wind cnditins, bth the mdel and the bservatins suggest that divergence increases with wind speed. They als indicate that the lcal flw perturbatins giving rise t divergence are largely in the directin f the wind alft (Figures 11 and 12c). The lw-level wind speed and divergence are less than in the bservatinal data set. This may be due t the bservatins being made under ver Namekus Lake, sensitivities f the mdel t varius atmspheric feature shuld be highlighted. n particular, the lw-level divergence ver the lake is very sensitive t the chice f gestrphic wind speed. Fr decreasing values f gestrphic wind belw %=5 ms -, divergence increases sharply, assciated with a mre intense snw breeze. At Ug=2 ms -, the dwndraught remains cherent ver the lake thrughut the day. Hwever, cmparisn with bservatins suggest that snw breezes ver Namekus Lake tend t be weaker than this, particularly in the afternn. Fr values f Ug abve 5 ms - the maximum wind speed and divergence abve the lake rise slwly with increasingestrphic wind speed. The snw breeze is sensitive t incming radiatin cnditins which drive the frest heat flux. n the idealized experiments, the sky is clud free, while bserved shrt wave fluxes are ften reduced by cluds. n additin, the presence f snw n the frest canpy reduces the bserved sensible heat flux n a number f days. This results in a mdeled heat flux that is 20% larger than the bservatins when averaged ver all 27 days. Hwever, the mdeled and bserved sensible heat fluxes cmpare very well under clear skies. Additinal simulatins were made under midwinter radiatin cnditins (December 21). These indicate a weak snw breeze persists when ux=2 ms -. t is unclear hw frequent such midwinter snw breezes culd ccur ver Namekus Lake. This is due t differences in the mid-winter frest energy budget cmpared with the mdel and, in particular, the rle f interceptin and albed at lw Sun angles. Fr calm days in March hwever, the existence f snw breezes in lw-inslatin cnditins was cnfirmed by bservatinal evidence frm bth the tethered balln prfiles and the anemmeter netwrk. These indicated snw breeze develpment when the mixed layer wind speed windier PBL cnditins than thse represented by ux=15 drpped t 1 ms - fr several hurs. This ccurred n a day ms- t may als be related t the effect f blwing snw when incming slar radiatin remained arund 200 Wm -2, episdes in the bservatins. The mdel is nt valid under apprximately 30% f typical clear-sky values fr March. such cnditins, when sensible heat fluxes int the lake may The value f 1 mm fr the lake rughness length was be cnsiderably greater. A feature f the bservatins that is adpted as a reasnablestimate frm a lng time series f reprduced by the mdel is the increase in lw-level wind bservatins. Hwever, the rughness f the snw-cvered speed (and divergence) during the mrning. n the mdel this lake des vary ver time, due t ageing f the snw, is related t the vertical transprt f mmentum dwn t the develpment f drifts, etc. Simulatins using an alternative lake surface, cntrlled by stability effects assciated with value f rughness length f 10 mm prduced very similar the nearby frest. divergent flws. One shuld nte that areas f pen wetlands 5.3. Sensitivity f Simulatins t nitial Cnditins T place the interpretatin f the bservatins frm the and encraching farmland can als be characterized by rughness lengths f this magnitude when snw is n the grund. n this case, the energy balance is similar t that f idealized simulatins in the cntext f typical flw cnditins a snw-cvered lake. Snw breezes f a similar magnitude

14 23,100 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST are therefre likely t develp ver patches f clear-cut as well as arund lakes. 6. Cnclusins This study has shwn hw snw breezes affect the lw-level wind field ver a frzen, snw-cvered lake less than 4 km acrss within the breal frest. Examining the average diurnal cnditins ver a perid f 27 days in March and April, a snw breeze signal is evident during the mrning perid. This thermally induced influence n the lw-level wind weakens in the afternn. With clear skies, snw-free canpies, and under calmer cnditins (wind speeds - 2 ms4), the snw breeze will persist thrughut the day. Hwever, snw breeze effects are nt in evidence n the windiest days. n this case, the lw-level wind accelerates acrss the lake due t the cntrast in rughness between the frest and the lake. There is n clear effect f ambient temperature n the develpment f snw breezes acrss Namekus Lake, despite the extremely stable cnditins assciated with temperatures abve 0øC ver the lake. The intensity f NCMCs is expected t be very sensitive t the length scale f hetergeneity. Fr example, bservatins by Segal and Kubesh [1996] indicate that thermally induced winds as strng as 6 ms ' develp arund the much larger frzen lake Winnipeg. This study has shwn that circulatins, albeit weak nes, still develp ver a snw-cvered lake nly a few kilmeters acrss. This is due t the extreme cntrast in surface features ver the breal frest during winter. The hetergeneity may be naturally ccurring, due t frzen lakes and wetlands, r man-made as in the case f frest cleared fr agriculture. Between them, these surfaces accunt fr typically 25% f the landscape in this part f Canada and ccur at a range f length scales. This suggests that snw breezes are widespread thrughut the regin during the winter. Acknwledgments. This wrk was supprted financially by the NERC thrugh its TGER (Terrestrial nitiatives in Glbal Envirnmental Research) Prgram, award GST/91/.2/2A, thrugh BOREAS cntract NAG-52302, the Prince Albert Mdel Frest Assciatin nc., Atmspheric Envirnment Service, Climate Research Netwrk, Mackenzie Basin Glbal Energy and Water Cycling Experiment, Natinal Hydrlgy Research nstitute, Envirnment Canada, and the Divisin f Hydrlgy, University f Saskatchewan, Saskatn. The authrs wuld als like t thank Newell Hedstrm, Dell Bayne, and Mike Strud fr their assistance in the field. References Luis, J. F., M. Tiedke, and J. F. Geleyn, A shrt histry f the PBL parameterizatin at the ECMWF, in Wrkshp n Planetary Bundary Layer Parameterizatin, 59-80, Eur. Cent. fr Medium-Range Weather Frecasts, Reading, England, Lynn, B. H., D. Rind, and R. Avissar, The imprtance f messcale circulatins generated by subgrid-scale landscape hetergeneities in general-circulatin mdels, J. Clim., 8, , Mahfuf, J. F., E. Richard, and P. 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Meterl. Sc., 76, , Shuttlewrth, W. J., Macrhydrlgy---The new challenge fr prcess hydrlgy, J. Hydrl., 100, 31-56, Guerrini, Linear impact f thermal inhmgeneities n messcale Smagrinsky, J., General circulatin experiments with the primitive atmspheric flw with zer synptic wind, Ann. Gephys., 9, , Harding, R. J., and J. W. Pmery, Energy-balance f the winter breal landscape, J. Clim., 9, , equatins, 1, The basic experiment, Mn. Weather Rev., 91, , Smith, E. A., M. M. K. Wai, H. J. Cper, and M. T. Rubes, Linking bundary-layer circulatins and surface prcesses during Li, L. and J.W. Pmery, Estimates f threshld wind speeds fr FFE-89, 1, Observatinal analysis, J. Atms. Sci., 51, snw transprt using meterlgical data, J. Appl. Meterl., 36, , , Sun, J.L., D.H. Lenschw, L. Mahrt, T.L. Crawfrd, K.J. Davis,

15 TAYLOR ET AL.: SNOW BREEZES N THE BOREAL FOREST 23,101 S.P. Oncley, J.. MacPhersn, Q. Wang, R.J. Dbsy, and R.L. Desjardins, Lake-induced atmspheric circulatins during BOREAS, J. Gephy. Res., 102, 29,155-29,166, Sun, W. Y., and Y. Ogura, Bundary-layer frcing as a pssible trigger t a squall-line frmatin, J. Atms. Sci., 36, , Vidale, P.L., R.A. Pielke, L.T. Steyaert, and A. Barr, Case study mdeling f turbulent and messcale fluxes ver the BOREAS regin, J. Gephy. Res., 102, 29,167-29,188, Walk, R.L., et al., Cupled atmsphere-terrestrial ecsystemhydrlgy mdels fr envirnmental mdeling. Class Reprt g9, Dept. f Atms. Sci., Clrad State Univ., Frt Cllins, CO Zeng, X. B., and R. A. Pielke, Landscape-induced atmspheric flw and its parameterizatin in large-scale numerical-mdels, J. Clim., 8, , Zhng, S., and J. C. Dran, A study f the effects f spatially varying fluxes n clud frmatin and bundary layer prperties using data frm the Suthern Great Plains Clud and Radiatin Testbed, J. Clim., 10, , C. M. Taylr and R. J. Harding, nstitute f Hydrlgy, Crwmarsh Giffrd, Wallingfrd, Oxfrdshire, U.K. ( ; cmt@ih.ac.uk; rjh@ih.ac.uk) R. A. Pielke, P. L. Vidale and R. L. Walk, Department f Atmspheric Science, Clrad State University, Frt Cllins, CO ( tara@rmulan.atms.clstate.edu; w ti bet. atm s. cl s rate, edu) J. W. Pmery, Natinal Hydrlgy Research nstitute, Saskatn, Canada ( pmeryj@nhrisu.nhrc.sk.ec.gc.ca (Received Octber 21, 1997; revised April 6, 1998; accepted May 8, 1998.)