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The method was applied to a 655-km2 area surrounding Roundtop Mountain, Quebec, Canada, during a carefully selected sample period from the summer of 1993. Field measurements of fog water volume were used to estimate the cloud-base height and the rate of change of the LWC with height. Topographic data were used both as a forcing function in the wind flow model and as a means of defining the three-dimensional geometry for deposition calculations. The goal is the development of a simple model that can be used over large geographic areas.
Microwind 35 Full Version 20
DECEMBER 1996 WALMSLEY ET AL.A Method for Estimating the Hydrologic Input from Fog in Mountainous TerrainJOHN L. WALMSLEY AND ROBERT S. SCHEMENAUERAtmospheric Environment Service, Downsview, Ontario, Canada HOWARD A. BRIDGMANDepartment of Geography, University of Newcastle, Newcastle, New South Wales, Australia(Manuscript received 3 October 1995, in final form 10 May 1996)ABSTRACT A methodology for obtaining estimates of the spatial distribution of fog water volume collected by a treecanopy in complex terrain is described. The method includes assumptions about the shape and spacing of thetrees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid watercontent (LWC) within ground-based clouds. The method was applied to a 655-km2 area surrounding Roundtop Mountain, Quebec, Canada, during acarefully selected sample period from the summer of 1993. Field measurements of fog water volume wereused to estimate the cloud-base height and the rate of change of the LWC with height. Topographic data wereused both as a forcing function in the wind flow model and as a means of defining the three-dimensionalgeometry for deposition calculations. The goal is the development of a simple model that can be used overlarge geographic areas. Results of the application are presented over various domains ranging from 2 to 164 km2 in size. Spatialvariations in the wind velocity field just above the canopy were found to be related to the main terrain features(summits, ridges, and valleys). The fog water deposition rate was specified as a linear function both of terrainheight above cloud base and of wind speed. Near the summit of Roundtop Mountain, variations in terrain heightwere more pronounced than those of treetop wind speeds. Spatial patterns of fog water deposition, therefore,strongly reflected the pattern of topographic contours, with some modifications being apparent due to spatialvariations in wind speed. Calculated deposition values ranged up to 0.69 mm h- and were found to be typicalof measured values in the literature.1. Introduction Mountaintops are frequently enveloped in fog as aresult of the advection of clouds over and around thehigher elevations. The fog is used by plant species inforests as a source of moisture and nutrients, and toassist in their growth and development. This is welldocumented, particularly for tropical forests (e.g., Kerfoot 1968). The fog droplets, however, can also containhighly concentrated solutions of various pollutants.This problem has been reviewed by McLaughlin(1985), Schemenauer (1986), Lovett and Kinsman(1990), Vong et al. (1991), and others. In 1985 the Chemistry of High Elevation Fog(CHEF) experiment began on two mountains in southern Quebec, Canada (Schemenauer 1986), out of concern that much of the wet deposition at higher elevations may come from fog water. The CHEF project was Corresponding author address: Dr. John L. Walmsley, Atmospheric Environment Service, 4905 Dufferin Street, Downsview, ONM3H 5T4, Canada.E-mail: jwalmsley @row.on.doe.calinked with the Mountain Cloud Chemistry Project(MCCP) in the Appalachian Mountains. Both projectswere designed to measure concentrations of inorganicions in fog and precipitation, as well as their relationship to meteorological conditions, and to assess theirimpact on high-elevation forests. Results from theMCCP have been published or used by several authors(e.g., Mohnen and Kadlecek 1989; DeFelice and Saxena 1991; Lin and Saxena 1991; Mueller et al. 1991).CHEF wet chemistry data were given in Schemenaueret al. (1995). These and other studies of fog and cloudwater chemistry, with samples from high-elevationsites or from aircraft, have consistently shown higherconcentrations of the major ions in fog water than inprecipitation. The primary reason for this is that themajor entry point for particulate and gaseous pollutantsis through cloud base. As cloud droplets rise in updraftregions of the cloud, they grow by condensation, whichdilutes the concentrations of major ions such as sulfate.There is normally, therefore, a vertical gradient in theconcentration of pollutants in cloud water, with higherconcentrations near cloud base. It is typically samplesfrom near cloud base that are collected as fog at mountain sites. In contrast, the precipitation that is sampledc 1996 American Meteorological Society2238 JOURNAL OF APPLIED METEOROLOGY -OLUME35at these sites reflects the integrated cloud chemistrydownward from the top of the cloud, since the raindrops, for example, have usually formed through a process beginning with an ice crystal at higher altitudesand followed by growth by sublimation, condensation,and coalescence during the particle's fall through thecloud. Bridgman et al. (1994, hereafter BWS94) presenteda topographic description of Roundtop Mountain inQuebec and the surrounding region (Fig2 1 ). The MSMicro/3 model (hereafter MS-Micro) used to ,obtainthe wind velocity fields has been described in Walmsley et al. (1986). The ultimate purpose is to provide anestimate of the spatial distribution of acidic ion deposition over Roundtop Mountain. In the present paper,we describe the next step toward that goal--the estimation of spatial variations in fog water volume. Schemenauer (1986) found that the summit (970 m) ofRoundtop and the adjacent ridge (845 m), where elevations are above mean sea level (MSL), were in cloudabout 44% and 38% of the time, respectively, duringthe year. Significant opportunities are, therefore, available for the collection of fog water. We present a sample calculation and compare it with field measurementstaken during the summer of 1993.2. Measurement program Three sites on or near Roundtop were fully instrumented to measure meteorological parameters and collect precipitation and fog water. Full details were presented in Schemenauer (1986) and Schemenauer et al.(1995). Sites S and R (Fig. 2) and a valley site 10 kmto the west were equipped with Campbell Scientificmeteorological stations to measure wind speed, winddirection, precipitation, and other parameters. The datawere sampled at 5-s intervals and stored as 15-min avRS'00 -.,-,%_.., ,...oo+-' /coeOc '":5 '00 Fro. 1. The Roundtop Mountain complex from an azimuthal direction of 225-. The contour interval is 50 m, and the grid resolutionis 100 m. The locations of the CHEF ridge and summit sites areindicated. The x and y coordinates are displacements (m) in the UTMgrid from the position 690 000 m'east, 4 994 000 m north in UTMzone 18. The vertical scale is MSL height. The source is BWS94,with improved digitization of topographic contours. Roundtop Mountain-6400 -3200 0 3200 64000 3200 3200 E >,, -3200 -3200 -6400 -6400 -i400 -3200 0 3200 6400 x (m) FIG. 2. Same as Fig. 1 except a contour map of MSL heights witha contour interval of 100 m. The CHEF summit (S) and ridge (R)sites, Mont Gagnon (G), and Mont Brock (B) are indicated.erages. Table 1 gives newly corrected locations andelevations of the three sites. Uncertainties in elevationare indicated. Locations are believed to be accurate to+20 m (i.e., _+0.6" latitude or _+0.9" longitude). Fog water was collected by a Canadian-built passivecollector (AES/ASRC--Atmospheric EnvironmentService/Atmospheric Sciences Research Center), essentially identical to that used in the MCCP. The collector was also used successfully in collecting fog waterfor chemical analysis in Chile (Schemenauer and Cereceda 1992a), Australia (Post et al. 1991), and othercountries. The instrument stands about 1 m high andcontains approximately 370 vertical Teflon fibers, 50cm long and 0.53 mm thick, spaced 3 mm apart. TheAES/ASRC collector surface is a vertically mountedcylinder of 48.5-cm height and 25.3-cm diameter, giving surface and cross-sectional areas of 0.386 and 0.123m:, respectively. Droplets carried to the instrument inthe airflow impact on the fibers, and fog water runsdown into a polyethylene collection bottle. Collectionrates depend on wind speed. The AES/ASRC closelysimulates fog water collection by coniferous trees(DeFelice and Saxena 1990; Joslin et al. 1990), with acollection efficiency of 0.82 to 0.87 when wind speedsare between 5.6 and 10.3 m s - (Lin and Saxena 1991 ).(In the present study, cross-sectional areas of both thecollector and the treetops were used for calculations.) A second fog water collector, a vertically mountedplane of 1 m x 1 m dimensions, the surface of whichis composed of a polypropylene mesh, was orientedtoward the west. The measured collection efficiencynear the center of the plane was found to be 66% (Schemenauer and Cereceda 1994), based on earlier workDECEMBER 1996 WALMSLEY ET AL. 2239by Schemenauer and Joe (1989), but this decreasedtoward the edges. Its overall collection efficiency wasestimated to be 50%. No correction was made for variations in collection efficiency with wind direction, assuch corrections were believed to be small for the casepresented here (i.e., wind direction 264-). At sites R and S, the instruments were placed justabove treetop height, with full exposure on all sides.When the wind at R was from the south-southeast, therewas some terrain sheltering from the summit. In thevalley, the meteorological instruments were placed ona 3-m pole in a roughly triangular-shaped forest clearing, the sides of which were approximately 160 m. Theanemometer location was about 50 m from the northwest corner of the clearing; sheltering effects were expected for winds from the sector 2000-3600-70-. Thiswill be discussed further in section 5a.3. Estimating fog water volume The results of a number of different studies haveestablished that fog water is deposited on the canopytop from stratiform clouds by several major pathways:mixing by turbulent eddies interacting with the treetops, sedimentation of droplets, and advectional impaction on surfaces [see Saxena et al. (1989) for areview]. Of these, impaction is most important, especially on the canopy top in a windy environment (>2m s - ). Note that Mueller et al. ( 1991 ) used the termadvective cloud water flux to mean the component ofwind normal to the terrain. They found this contributionto be small; we assume it is zero. In this paper, on theother hand, the term advectional impaction means theadvection of wind through the vertical surfaces of thetreetop cones. In maintaining that advectional imPaction is important, we are, in fact, in basic agreementwith Mueller et al. (1991). Any apparent discrepancyis due to a different definition of advection. The amount of fog water deposition is highly sitedependent, based on five factors: canopy structure, horizontal wind speed, the collection efficiency of the treetops, the liquid water content (LWC) of the fog, andthe variation of fog frequency with altitude. In this section, the assumptions needed to perform the calculations are discussed. Details appear in appendix A. For the Roundtop Mountain calculations, the fivefactors were handled as follows. I0 1 2 3 4 Horizontol distonce FIG. 3. Vertical cross section of three model trees. Trees are cylindrical (diameter b) and topped by a cone (height h, angle qb). Heightof trees up to the base of the cones, shown here as 5 m, is arbitrary.See appendix A for details. 1 ) Figure 3 depicts the assumptions about the treetop geometry. Each tree was assumed to be a verticalcylinder topped by a cone (b 60-, h = 1.5 m, Tv= 1.30 m2, Tn = 3.00 m2). (Refer to appendix A formore details.) The term canopy means the top of thecylinders, whereas canopy top means the treetop cones.The horizontal spacing between the tree centers was1.73 m (in reasonable agreement with visual observations), and the horizontal tree density was 3333 ha-.In (AS) the ratio Tv/Tn is 0.433. 2) Spatial variations in wind speed and directioncaused by variations in terrain were calculated by MSMicro, as applied in BWS94. MS-Micro wind fieldswere assumed to apply at the treetop level--that is, 1.5m above the base of the treetop cones. The wind flowmodel assumes that the flow follows streamlines. It operates in a terrain-following coordinate system and produces two components of horizontal wind velocity. Inthe present study, it was assumed that close to the forestcanopy the streamlines and, hence, the flow were par T^BIE 1. Locations and elevations of CHEF summit and ridge sites on Roundtop Mountain and CHEF valley site near Sutton, Quebec (Schemenauer 1986, with modifications). Elevation Latitude Longitude UTM east UTM north Site (m MSL) (N) (W) (m) (m)Summit 970 - 2 45-04'52" 72-32'51" 693 025 4 994 625Ridge 845 _+ 5 45-05'17" 72-33'09" 692 625 4 995 400Valley 245 - 2 45004'32" 72040'33" 682 950 4 993 8252240 JOURNAL OF APPLallel to the canopy top. Similarly, Coe et al. (1991)assumed that trajectories of the fog water droplets followed the airflow streamlines. Mueller et al. (]L991)discussed the fact that streamlines are not always parallel to the surface. In the present case, however, at aheight of 1.5 m above the forest canopy, the flow wasunlikely to be very far from parallel. 3) The collection efficiency of the treetops was as sumed to be represented by the samples from the AES / ASRC fog water collector (]oslin et al. 1990). 4) The LWC of the fog could not be directly deter mined from collector sample amounts because of the variation in collector efficiency with wind speed and droplet size (Mueller and Imhoff 1989). The cloud LWC at different elevations was calculated from cloud base height observations and an assumption thai: LWC above cloud base was 38% of the value calculated for adiabatic ascent (Leaitch et al. 1986). When compared with the LWC of the fog, as given by Saxena et al. (1989), reasonable agreement was found. 5) In general, the variation of fog frequency with altitude was assumed to be a linear function of MSL height (Schemenauer 1986). For the particular 4-h case examined in the present study, however, the frequency was assumed to be 100% everywhere abo.ve cloud base. A possible sixth factor affecting deposition is droplet size. Coe et al. (1991) investigated the influence of droplet size on deposition velocity and concluded that, to a good approximation, the liquid water deposition velocity could be set to the momentum deposition ve locity. That is, the effect of droplet size could be ne glected. Gallagher et al. (1992), on the other hand, found that for moderate wind speeds the turbulent de position velocities were strong functions of droplet size. Nevertheless, since detailed droplet size measure ments for Roundtop Mountain were lacking, these ef fects were not considered in the present study. The cal culations presented here examine fog water deposition over an area of 164 km2. Ultimately, calculations of deposition over much larger areas are needed. In such calculations, field measurements of droplet sizes will not be available and an assumption that fog water de position on large scales is independent of droplet size will be necessary. The fog water flux, defined as the product of LWC and wind speed (Joslin et al. 1990), describes the fog water deposition on the canopy top. The fog water flux increased with height above cloud base, due to in creases in both LWC and wind speed.4. Selection of the example period To verify the methodology, an observation periodwas needed during which fog was present bat no precipitation occurred. Periods in which the winds werereasonably steady in both speed and direction, were preferred, in order to satisfy the requirements of theIED METEOROLOGYVOLUME 35steady-state MS-Micro model. Data collected during a3-day period, 15-17 July 1993 [Julian days (JD) 196198], contained candidate periods, which were examined in more detail. Figure 4 displays time series plotsof wind speed and direction at the CHEF ridge andvalley sites, together with the fog water collection rateand output from a fog detector at the ridge site. Periodswith fog and without precipitation are identified in Table 2, together with wind speed and direction statisticsfor those periods. The second period (JD 197) was rejected because fog was only continuous for about 1 h during the 4-h period. The first-period (JD 196) had very light wind speeds and unacceptable variations in wind directions at the valley site. During the third period (0430-1130 EST JD 198), valley winds were slightly stronger but variations in wind direction were high, due to a brief wind shift (Fig. 4b). This period was subdivided to obtain the fourth period (0730-1130 EST JD 198), which satisfied all the selection criteria--fog, no pre cipitation, and steady winds.5. Preparation of input dataa. Upwind wind speed and direction The MS-Micro model is initialized with a wind velocity at 10 m above uniform vegetation in flat terrain.In the present case, the CHEF valley site wind measurements were used, despite the fact that they presented certain problems. The valley site anemometerwas located at a height of 3 m within a forest clearing.Although the clearing was grass covered, suggesting aroughness length of 0.05-0.1 m, the "very rough" category of Wieringa (1992) best described the situation.Accordingly, a local roughness length of 0.5 m wasassumed. The surrounding forest was well representedby Wieringa's "closed" category, with a roughnesslength of 1 m. In addition, there were believed to besignificant sheltering effects from a 10-m-high peakedroof house located approximately 30-40 m west of theanemometer and a 9-m-high forest about 50-60 maway in the southwest, west-southwest, and west sectors. The shelter-correction model of Taylor and Salmon (1993) was applied. The resulting correction facttors were clearly overestimates of what would be expected, even for the conditions at the CHEF valley site.The shelter-correction model is apparently not valid forobstacles in the very near field. For other wind directions (e.g., sectors east-northeast to south-southwest)with fetches of about 75-200 m, correction factors produced by the shelter-correction model ranged from 2.1to 1.2, respectively. A correction factor of 1.4 wouldseem to be a reasonable, though possibly conservative,estimate for winds in the period selected--that is, from264- (see Table 2). Accordingly, the following corrections were applied to the valley wind speed to provide appropriate inputDECEMBER 1996 WALMSLEY ET AL. 2241(o)01200 (c)1.51.0 0.0 1200 (e)? 4.0o 2.52.00.5 0.0 200 Ridge........ Volley JD 196 JO 197I I - :M :': :! i '*' ',':': ,',?;,,i "l v.j b.; ' 19'20 ' '26'40 ' 5560 4080 Time (rain)JD 196No precip. - /.. b'., ' 1920 '26'40-- Plonor collector........ AES/ASRC - JD 197 o_pecip.3360 ' 40'80Time (rain)JD 196No preclp. - :: it 19'20 26'40-- Plonor collector........ AES/ASRC JD 197 _. r?'3go '40'80Time (rain)JD 198(b)560 300 - 240 No precip. j [-- --l C , 120 I ,: ._c I 602:'":" U' '"! '48b0' ' ' 55200 i, 1 , , 1200 1920 2640JD 198NO preclp.400 5520(d)4000 5000 2000 10000 ,1200 26'40 JD 196 NO precip. I I t I I I I I I: .A 1920-- Ridge........ Volley JD 197No precip.'3360' '40'80Time rain) JD 198No precip.4800 5520 JD 197o_ precip.' 3360 4080Time (rain) JD 198 No precip. F -- -IiI I , , 48005520JD 198No precip.' 48'00' 550 ' FtG. 4. Time series plots spanning intervals when fog waspresent at the CHEF ridge site. Plots begin at 0000 EST 14July 1993. Solid vertical lines are drawn at midnight, andJu