1. Introduction
An aerosol and trace gas measurement program has been conducted at the summit of Whistler Mountain in British Columbia, Canada, since 2002. A central issue at Whistler and other mountaintop observatories is the ability to discriminate between times when the site is representative of the lower free troposphere (FT) and times when it is influenced by planetary boundary layer (PBL) air (e.g., Baltensperger et al. 1997; Henne et al. 2008). In a recent paper by Gallagher et al. (2011), condensation nuclei (CN) and meteorological datasets from the mountaintop were used as a basis for assessing the frequency and patterns of influence from the PBL at the Whistler site. In that analysis, it was found that days with diurnal increases in concentrations of CN and water vapor were usually associated with synoptic-scale conditions that supported convective mixing, that is, vertical transport of valley-based PBL air. These conditions were found to occur mainly in the spring and summer months, but the highly variable synoptic-scale weather of this coastal site makes the occurrence of PBL influence at the summit irregular, and it was concluded that careful analysis of all relevant datasets should be performed for periods during which the origin of sampled entities is of concern.
Evaluation of vertical mixing at Whistler was previously aided by the availability of local radiosonde data from a wintertime special observing period in 2009. Such vertical profile data can provide reasonable estimates of mixed-layer heights, from which a determination of the airmass character (FT or PBL influenced) can be made. Another approach using radiosonde data is to compare water vapor mixing ratios from the observatory with free-tropospheric observations from the same altitude (e.g., Weiss-Penzias et al. 2006). Enhanced water vapor at the observatory as compared with the free troposphere is assumed to be indicative of PBL air reaching the site. The distance to the nearest ongoing upper-air sites (>250 km) makes routine application of such an approach unreliable for Whistler, especially during changing synoptic-scale conditions.
Studies of boundary layer evolution over mountains have been conducted using airborne lidar in the Swiss Alps. Nyeki et al. (2000) presented a case study in which the convective boundary layer (CBL) was found to extend to above 4 km MSL on a summer afternoon, with a fairly uniform top above the mountains. Further analysis of the same lidar dataset by De Wekker et al. (2004) used model output to show that the aerosol layer (AL) over the mountains extended higher than the CBL (i.e., well-mixed unstable layer); they attributed the high AL to thermally driven mountain venting processes. Henne et al. (2004) used the term “injection layer” to describe the somewhat stable area above the conventional CBL into which aerosols from the PBL are vented. For the purposes of this study, the AL as inferred from lidar data is considered to be representative of the height of PBL influence.
In April of 2010 a ground-based, upward-pointing lidar system was installed in Whistler Valley. This instrument, which operates continuously except during precipitation and aircraft overflights, provides vertical profile data adjacent to Whistler Mountain that can be used to aid airmass discrimination at the summit. In this paper, lidar imagery together with corresponding meteorological and chemical data are evaluated for three brief case studies to demonstrate a range of PBL-influence scenarios at the observatory. Conclusions from Gallagher et al. (2011) about temporal patterns of CN concentration and the nature of PBL influence at the site are corroborated by the vertical aerosol structure as derived from the lidar data.
2. Data
The observatory atop Whistler Mountain (50.06°N, 122.96°W; 2182 m MSL) is located about 100 km north of Vancouver, British Columbia, in the Coast Mountains. A description of the site and its climate are provided in Gallagher et al. (2011). Details of the instrumentation can be found in Macdonald et al. (2011) and Gallagher et al. (2011). Datasets of temperature T, pressure P, relative humidity RH, CN number concentration, and ozone (O3) taken at 1-min intervals were quality checked and converted to hourly means for analysis. Water vapor mixing ratio was calculated from T, P, and RH.
The lidar system at Whistler is part of Environment Canada’s Canadian Operational Research Aerosol Lidar Network (CORALNet). A description of the CORALNet system and its applications can be found in McKendry et al. (2011). The Whistler lidar samples at a vertical resolution of 3 m with a usable range of 15 km. Atmospheric profiles are taken every 10 s; for display purposes, images have been temporally averaged to a resolution of 120 s. Overlap functions may vary slightly, but full overlap is always achieved at an altitude of 150 m or less above the instrument. The lidar is collocated with the “NAV CANADA” surface weather station at 660 m MSL in Whistler (Fig. 1). The horizontal distance between the lidar beam and the summit observatory is approximately 8 km.
Map of the Whistler area showing locations of the air-chemistry observatory on Whistler Mountain and the CORALNet lidar, 8 km north. Spatial data source: Canadian Digital Elevation Data from GeoBase (online at http://www.geobase.ca).
Citation: Journal of Applied Meteorology and Climatology 51, 10; 10.1175/JAMC-D-12-067.1
Synoptic weather maps were produced using National Centers for Environmental Prediction (NCEP) reanalysis datasets from the Physical Sciences Division of the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (online at http://www.esrl.noaa.gov/psd/data/gridded/reanalysis). Upper-air observations from Kelowna, British Columbia (255 km east of Whistler), were acquired from the University of Wyoming’s radiosonde data archive (online at http://weather.uwyo.edu/upperair/).
3. Results
Backscatter ratio data from the CORALNet lidar provide depictions of relative aerosol concentration over Whistler Valley; displayed as time–height plots, these data are useful for evaluation of PBL evolution in different conditions. Several automated and semiautomated methods have been developed to calculate mixing height on the basis of the vertical gradient in lidar backscatter intensity typically found between the PBL and the FT (e.g., Steyn et al. 1999; Cohn and Angevine 2000; van der Kamp and McKendry 2010). A simple mixing-height algorithm has been developed for CORALNet whereby the top of the mixed layer is defined as the lowest point in a given profile at which the measured backscatter ratio falls below a manually selected threshold value. The threshold value is based on the difference between the molecular backscatter ratio and backscatter values from air near the surface that is clearly within the PBL. For this study, a threshold value of 3 times the molecular backscatter ratio was applied to each case. The resulting mixing-height estimations are represented by a pink line superimposed on the lidar imagery in Figs. 2, 3, and 4. When this boundary appears above the summit level of Whistler Mountain (1.52 km above the lidar, marked by a red line across the images), the mountaintop observatory is assumed to be within the AL. When the boundary is below the summit, the observatory is assumed to be in the FT. Three examples are presented below to examine different scenarios of PBL evolution that occur at this location.
Time series of data from Whistler on 11 May 2010 showing (a) 1064-nm backscatter ratio data from the CORALNet-Whistler lidar from the surface to 7 km AGL, (b) CN number concentration and water vapor mixing ratio at the summit observatory, and (c) O3 concentration at the summit. The horizontal red line across the lidar image indicates the summit height of Whistler Mountain.
Citation: Journal of Applied Meteorology and Climatology 51, 10; 10.1175/JAMC-D-12-067.1
As in Fig. 2, but for 8 Jul 2010.
Citation: Journal of Applied Meteorology and Climatology 51, 10; 10.1175/JAMC-D-12-067.1
As in Fig. 2, but for 3 Nov 2010.
Citation: Journal of Applied Meteorology and Climatology 51, 10; 10.1175/JAMC-D-12-067.1
a. Diurnal CBL growth: 11 May 2010
A time series of backscatter data from 11 May 2010 is shown in Fig. 2a. Altitudes given on the y axis are above instrument level. The lidar operates at wavelengths of 1064 and 532 nm; only the 1064-nm data are shown here, but results are consistent between the two wavelengths.
From the image, the early morning PBL is somewhat indistinct but the mountaintop appears to be above the layer of relatively high aerosol concentration. Presumably there was a stable nocturnal boundary layer in the valley at this time. Starting around 0900 LST, CBL growth is evident, with the approximated mixing height reaching a maximum altitude of nearly 3 km at around 1700 LST. Areas of no data (appearing black in the image) in the upper part of the column between 1600 and 1900 LST correspond to cumuliform clouds that formed at the top of the CBL. From this image, it can be inferred that the observatory was representative of the FT until midmorning, after which convective updrafts brought PBL air to the site throughout the afternoon and early evening. In late evening as convection subsided, the AL top was lowering but was still above the mountaintop height at midnight.
The diurnal course of CN concentration and water vapor mixing ratio at the summit (Fig. 2b) supports the above interpretation. Daily minima occurred in the morning, followed by diurnal increases to maxima in the early evening. These surface-based entities increased in concentration through the day with timing that closely corresponds to that of the CBL growth observed by the lidar. Concentrations dropped gradually late in the evening as convective uplift subsided.
The trend in O3 on this day (Fig. 2c) is consistent with the PBL-influence scenario described in Macdonald et al. (2011). A midmorning decrease in O3 concentration signaled the arrival of PBL air, which was followed by an increase that can be attributed to photochemical production.
The synoptic situation on 11 May is summarized by the 500-hPa and sea level pressure (SLP) maps in Fig. 5a. High pressure was building into the area from the Pacific Ocean, both at the surface and aloft, while a well-defined surface trough extended northward from Utah into southern British Columbia. Cold air aloft on the downstream side of the upper ridge in conjunction with the surface trough provided an environment favorable for convection. Sounding indices from Kelowna that afternoon reflected the regional instability (e.g., negative Showalter and lifted indices and total totals = 55). Thus, the diurnal changes in the PBL as observed by the lidar can be explained by convective vertical motions. In this case, the lidar image provides a clear depiction of diurnal PBL impingement at the mountaintop site, which is confirmed by the diurnal course of water vapor, aerosol, and ozone.
NCEP reanalysis data, averaged over the four synoptic times available for the LST day, showing (left) 500-hPa geopotential heights (m) and (right) SLP values (hPa) for (a) 11 May, (b) 8 Jul, and (c) 3 Nov 2010. A solid triangle symbol marks the location of Whistler Mountain.
Citation: Journal of Applied Meteorology and Climatology 51, 10; 10.1175/JAMC-D-12-067.1
b. Persistent deep AL: 8 July 2010
The lidar backscatter image in Fig. 3a represents a day on which the summit of Whistler was apparently within a surface-based AL at all hours. The lower backscatter values of the morning hours at the observatory level (as compared with late afternoon and evening) may represent residual aerosol from the previous day’s convective activity. Renewed convection in the afternoon mixed additional aerosol upward and expanded the mixed layer to above 3 km AGL by 1600 LST.
Water vapor and CN concentrations (Fig. 3b) were high all day (above 5 g kg−1 and 1000 cm−3, respectively), with only modest diurnal increases. Mean daily values for these entities are the highest of the three cases presented here. The O3 mixing ratios (Fig. 3c) also showed limited diurnal variation with a daily mean value of 36 ppbv, which is the lowest of the three cases. This is consistent with PBL influence and the summertime seasonal minimum in O3 observed by Macdonald et al. (2011).
Synoptic maps for 8 July are given in Fig. 5b. This was the warmest day during an extended dry period, with a high temperature of 33.5°C reported in Whistler. A high pressure ridge aloft was situated over British Columbia while a surface thermal trough extended along the coast. Typical of a summer dry period in British Columbia, this pattern featured mostly clear skies and slightly stable convective indices (e.g., Showalter and lifted indices ≈ +1 at Kelowna). Despite the positive indices, strong insolation and light synoptic-scale pressure gradients supported thermally driven mesoscale wind circulations and convective mixing. Slope winds and mountain venting likely contributed to the deep AL observed during this time.
For this midsummer case, strong surface heating maintained convective activity into the evening hours and light winds allowed residual aerosol and other PBL constituents to remain aloft when the atmosphere stabilized. Thus, the mountain observatory was not representative of the FT at any time during this particular day. This case supports the idea that the dampened diurnal cycles and relatively high median aerosol concentrations of summer shown in Gallagher et al. (2011) are largely due to the frequent presence of nocturnal residual layers during summer high pressure systems. Similar conclusions have been arrived at for the Jungfraujoch observatory in Switzerland (Nyeki et al. 1998; Collaud Coen et al. 2011) and Mount Cimone, Italy (Marinoni et al. 2008).
c. Shallow AL: 3 November 2010
In contrast to the previous two examples, the lidar image from 3 November (Fig. 4a) depicts a relatively shallow AL, the top of which appears to have lowered through early afternoon. The synoptic pattern (Fig. 5c) was dominated by strong high pressure at the surface and aloft over the British Columbia interior, placing Whistler in a zone of warm-air advection aloft. Broken cirrus cloud cover was reported at Whistler with surface winds from the north. (Some midlevel clouds appear as strong returns above 3 km in the lidar image.) NCEP reanalysis data from 700 hPa indicated weak subsidence over Whistler in the morning hours, corresponding to when the AL top was lowering. Regional upper-air soundings indicated strong stability with daytime inversion layers present below 800 hPa.
The atmospheric stability and strong low-level flow in this case acted to restrict CBL growth at Whistler. Water vapor mixing ratios at the summit (Fig. 4b) generally decreased through midafternoon, which is consistent with dynamic subsidence. Both water vapor and CN concentration did, however, increase late in the day whereas O3 (Fig. 4c) decreased throughout the afternoon. These latter observations suggest that some PBL air reached the observatory, albeit starting later in the day than is typical. The dark blue hues in the backscatter ratio data encompassing the summit height beginning at approximately 1700 LST (Fig. 4a) do indicate a modest increase in aerosol concentration. Although no convective clouds were reported at Whistler on this day, upslope flows from diurnal heating may have been sufficient to overcome the synoptic-scale stability for a few hours.
4. Conclusions
The three examples presented in this paper are indicative of the variety of PBL manifestations that can be captured by the Whistler lidar and illustrate how such information can assist the discrimination of FT and PBL influences at the mountaintop observatory. In each case, interpretation of lidar-derived mixing height and evolution is consistent with the approach of Gallagher et al. (2011) that was based on assessment of mixing ratios and condensation nuclei. The observed trends in O3 at the site are also consistent with these interpretations. Consideration of the synoptic weather situation can usually account for the variations in observed diurnal evolution of the AL.
Lidar data from the July case support the idea that, during summertime periods with intense surface heating, PBL influence can persist at the summit through the nighttime. The example from May best fits the typical diurnal cycle in convective activity and PBL influence that occurs often throughout the warm season. The November case presents a less certain scenario: a first look at the lidar imagery might lead one to assume that the site was in the FT all day, while the in situ data from the summit suggest that PBL air arrived late in the day. This last case reaffirms the notion that, when practical, all applicable datasets should be evaluated to determine which atmospheric layer the observatory best represents during the time of interest.
The daily cases analyzed here were selected to represent certain situations thought to be common at Whistler. Future work will include analysis of a longer-term dataset (e.g., a calendar year), which will provide an opportunity to assess further the utility of lidar for airmass discrimination. The high frequency of low clouds and precipitation at Whistler is expected be a limiting factor in the application of lidar data for this purpose; extended fair-weather periods do occur, however, especially during the summer season. The lidar can also help to identify plumes from regional and long-range pollution transport events. The study of such events is part of the ongoing research at the Whistler observatory.
Acknowledgments
This research has been supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Foundation for Climate and Atmospheric Sciences. We are grateful to our site operators, Juniper Buller and Anton Horvath, and to Whistler Blackcomb for their support of atmospheric monitoring programs on Whistler Mountain. This manuscript was improved by the helpful comments of two anonymous reviewers.
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