scholarly journals Quantification of topographic venting of boundary layer air to the free troposphere

2004 ◽  
Vol 4 (2) ◽  
pp. 497-509 ◽  
Author(s):  
S. Henne ◽  
M. Furger ◽  
S. Nyeki ◽  
M. Steinbacher ◽  
B. Neininger ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Production Efficiency ◽  
Weather Conditions ◽  
Free Troposphere ◽  
Fair Weather ◽  
Air Mass ◽  
Air Masses ◽  
Mountainous Regions ◽  
The Alps ◽  
Alpine Valleys

Abstract. Net vertical air mass export by thermally driven flows from the atmospheric boundary layer (ABL) to the free troposphere (FT) above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs) and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient "air pump" that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This could enhance their ozone (O3) production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.

scholarly journals Quantification of topographic venting of boundary layer air to the free troposphere

2003 ◽  
Vol 3 (5) ◽  
pp. 5205-5236 ◽  
Author(s):  
S. Henne ◽  
M. Furger ◽  
S. Nyeki ◽  
M. Steinbacher ◽  
B. Neininger ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Weather Conditions ◽  
Ozone Production ◽  
Free Troposphere ◽  
Fair Weather ◽  
Air Mass ◽  
Air Masses ◽  
Mountainous Regions ◽  
The Alps ◽  
Alpine Valleys

Abstract. Net vertical air mass export by thermally driven f\\/lows from the atmospheric boundary layer (ABL) to the free troposphere (FT) above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs) and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient "air pump" that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This enhances their ozone production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.

scholarly journals Multivariate statistical air mass classification for the high-alpine observatory at the Zugspitze Mountain, Germany

2019 ◽  
Vol 19 (19) ◽  
pp. 12477-12494 ◽  
Author(s):  
Armin Sigmund ◽  
Korbinian Freier ◽  
Till Rehm ◽  
Ludwig Ries ◽  
Christian Schunk ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Real Time ◽  
Statistical Approach ◽  
Free Troposphere ◽  
Time Data ◽  
Detection Scheme ◽  
Air Mass ◽  
Thermally Induced ◽  
Air Masses ◽  
Mechanistic Approach

Abstract. To assist atmospheric monitoring at high-alpine sites, a statistical approach for distinguishing between the dominant air masses was developed. This approach was based on a principal component analysis using five gas-phase and two meteorological variables. The analysis focused on the Schneefernerhaus site at Zugspitze Mountain, Germany. The investigated year was divided into 2-month periods, for which the analysis was repeated. Using the 33.3 % and 66.6 % percentiles of the first two principal components, nine air mass regimes were defined. These regimes were interpreted with respect to vertical transport and assigned to the BL (recent contact with the boundary layer), UFT/SIN (undisturbed free troposphere or stratospheric intrusion), and HYBRID (influences of both the boundary layer and the free troposphere or ambiguous) air mass classes. The input data were available for 78 % of the investigated year. BL accounted for 31 % of the cases with similar frequencies in all seasons. UFT/SIN comprised 14 % of the cases but was not found from April to July. HYBRID (55 %) mostly exhibited intermediate characteristics, whereby 17 % of the HYBRID class suggested an influence from the marine boundary layer or the lower free troposphere. The statistical approach was compared to a mechanistic approach using the ceilometer-based mixing layer height from a nearby valley site and a detection scheme for thermally induced mountain winds. Due to data gaps, only 25 % of the cases could be classified using the mechanistic approach. Both approaches agreed well, except in the rare cases of thermally induced uplift. The statistical approach is a promising step towards a real-time classification of air masses. Future work is necessary to assess the uncertainty arising from the standardization of real-time data.

scholarly journals Identifying source regions of air masses sampled at the tropical high-altitude site of Chacaltaya using WRF-FLEXPART and cluster analysis

2021 ◽  
Author(s):  
Diego Aliaga ◽  
Victoria A. Sinclair ◽  
Marcos Andrade ◽  
Paulo Artaxo ◽  
Samara Carbone ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Altitude ◽  
Long Range ◽  
Diurnal Cycle ◽  
Observation Time ◽  
Particle Nucleation ◽  
Free Troposphere ◽  
Air Mass ◽  
Air Masses ◽  
Source Regions

Abstract. Observations of aerosol and trace gases in the remote troposphere are vital to quantify background concentrations and identify long term trends in atmospheric composition on large spatial scales. Measurements made at high altitude are often used to study free tropospheric air however such high-altitude sites can be influenced by boundary layer air masses. Thus, accurate information on air mass origin and transport pathways to high altitude sites is required. Here we present a new method, based on the source-receptor relationship (SRR) obtained from backwards WRF-FLEXPART simulations and a k-means clustering approach, to identify source regions of air masses arriving at measurement sites. Our method is tailored to areas of complex terrain and to stations influenced by both local and long-range sources. We have applied this method to the Chacaltaya (CHC) GAW station (5240 m a.s.l.,16.35° S, 68.13° W) for the 6-month duration of the “Southern hemisphere high altitude experiment on particle nucleation and growth” (SALTENA) to identify where sampled air masses originate and to quantify the influence of the boundary layer and the free troposphere. A key aspect of our method is that it is probabilistic and for each observation time, more than one air mass (cluster) can influence the station and the percentage influence of each air mass can be quantified. This is in contrast to binary methods, which label each observation time as influenced either by boundary layer or free troposphere air masses. We find that on average, 9% of the air sampled at CHC, at any given observation time, has been in contact with the surface within 4 days prior to arriving at CHC, 24% of the air was located below 1.5 km above ground level and consequently, 76% of the measured air masses at CHC represent free tropospheric air. However, pure free-tropospheric influences are rare and often samples are concurrently influenced by both boundary-layer and free-tropospheric air masses. A clear diurnal cycle is present with very few air masses that have been in contact with the surface being detected at night. The 6-month analysis also shows that the most dominant air mass (cluster) originates in the Amazon and is responsible for 29% of the sampled air. Furthermore, short-range clusters (origins within 100 km of CHC) have high temporal frequency modulated by local meteorology driven by the diurnal cycle whereas the mid- and long-range clusters’ (>200 km) variability occurs on timescales governed by synoptic-scale dynamics. To verify the reliability of our method, in-situ sulfate observations from CHC are combined with the SRR clusters to correctly identify the (pre-known) source of the sulfate: the Sabancaya volcano located 400 km northwest from the station.

scholarly journals Identifying source regions of air masses sampled at the tropical high-altitude site of Chacaltaya using WRF-FLEXPART and cluster analysis

2021 ◽  
Vol 21 (21) ◽  
pp. 16453-16477
Author(s):  
Diego Aliaga ◽  
Victoria A. Sinclair ◽  
Marcos Andrade ◽  
Paulo Artaxo ◽  
Samara Carbone ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Altitude ◽  
Long Range ◽  
Diurnal Cycle ◽  
Observation Time ◽  
Particle Nucleation ◽  
Free Troposphere ◽  
Air Mass ◽  
Air Masses ◽  
Source Regions

Abstract. Observations of aerosol and trace gases in the remote troposphere are vital to quantify background concentrations and identify long-term trends in atmospheric composition on large spatial scales. Measurements made at high altitude are often used to study free-tropospheric air; however such high-altitude sites can be influenced by boundary layer air masses. Thus, accurate information on air mass origin and transport pathways to high-altitude sites is required. Here we present a new method, based on the source–receptor relationship (SRR) obtained from backwards WRF-FLEXPART simulations and a k-means clustering approach, to identify source regions of air masses arriving at measurement sites. Our method is tailored to areas of complex terrain and to stations influenced by both local and long-range sources. We have applied this method to the Chacaltaya (CHC) GAW station (5240 m a.s.l.; 16.35∘ S, 68.13∘ W) for the 6-month duration of the “Southern Hemisphere high-altitude experiment on particle nucleation and growth” (SALTENA) to identify where sampled air masses originate and to quantify the influence of the surface and the free troposphere. A key aspect of our method is that it is probabilistic, and for each observation time, more than one air mass (cluster) can influence the station, and the percentage influence of each air mass can be quantified. This is in contrast to binary methods, which label each observation time as influenced by either boundary layer or free-troposphere air masses. Air sampled at CHC is a mix of different provenance. We find that on average 9 % of the air, at any given observation time, has been in contact with the surface within 4 d prior to arriving at CHC. Furthermore, 24 % of the air has been located within the first 1.5 km above ground level (surface included). Consequently, 76 % of the air sampled at CHC originates from the free troposphere. However, pure free-tropospheric influences are rare, and often samples are concurrently influenced by both boundary layer and free-tropospheric air masses. A clear diurnal cycle is present, with very few air masses that have been in contact with the surface being detected at night. The 6-month analysis also shows that the most dominant air mass (cluster) originates in the Amazon and is responsible for 29 % of the sampled air. Furthermore, short-range clusters (origins within 100 km of CHC) have high temporal frequency modulated by local meteorology driven by the diurnal cycle, whereas the mid- and long-range clusters' (>200 km) variability occurs on timescales governed by synoptic-scale dynamics. To verify the reliability of our method, in situ sulfate observations from CHC are combined with the SRR clusters to correctly identify the (pre-known) source of the sulfate: the Sabancaya volcano located 400 km north-west from the station.

scholarly journals Climatology of Mountain Venting–Induced Elevated Moisture Layers in the Lee of the Alps

2005 ◽  
Vol 44 (5) ◽  
pp. 620-633 ◽  
Author(s):  
Stephan Henne ◽  
Markus Furger ◽  
AndréS. H. Prévôt
Keyword(s):  
Boundary Layer ◽  
Global Radiation ◽  
Atmospheric Stability ◽  
Relative Increase ◽  
Specific Humidity ◽  
Free Troposphere ◽  
Moist Convection ◽  
Fair Weather ◽  
Local Case ◽  
The Alps

Abstract Elevated moisture layers in the lower free troposphere (2000–6000 m MSL) in the lee of the Alps were investigated. Specific humidity was analyzed within a Lagrangian concept for fair-weather days during a 12-yr period at the windward and the leeward sides of the Alps for the sounding sites of Payerne, Switzerland, and Milan, Italy. During daytime fair-weather conditions (different criteria), specific humidity increased significantly in air masses that advected from Payerne to Milan in a layer ranging from ∼2500 to 4000 m MSL. The maximum relative increase of specific humidity in this layer was ∼0.3, meaning that ∼30% of the air in this layer originated from the Alpine atmospheric boundary layer. On average, ∼30% of the mass of the Alpine boundary layer was vented to altitudes higher than 2500 m MSL per hour during the daytime. The total precipitable water within a layer reaching from 2500 to 3500 m MSL increased by ∼1.3 mm. Similar elevated layers were observed for different selection methods of fair-weather days, and climatologically for the whole of June, July, and August. Average observations of the relative increase and boundary layer export rate agree with results from the local case studies. Daytime thermally driven flow systems seem to be the main source of additional water vapor in the observed elevated layers over the Alps. Subsequently, horizontal advection toward flat terrain where the average ABL top was well below the elevated layer bottom results in the export of ABL air to the free troposphere (mountain venting). Mountain venting was enhanced in situations with larger global radiation, lower atmospheric stability, and additional moist convection as was detected by lightning activity.

scholarly journals Atmospheric electric field in the Atlantic marine boundary layer: first results from the SAIL project

2020 ◽  
Author(s):  
Susana Barbosa ◽  
Mauricio Camilo ◽  
Carlos Almeida ◽  
José Almeida ◽  
Guilherme Amaral ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Electric Field ◽  
Marine Boundary Layer ◽  
Weather Conditions ◽  
Cape Verde ◽  
The Other ◽  
Atmospheric Electric Field ◽  
Fair Weather ◽  
Near Surface ◽  
First Results

<p><span>The study of the electrical properties of the atmospheric marine boundary layer is important as the effect of natural radioactivity in driving near surface ionisation is significantly reduced over the ocean, and the concentration of aerosols is also typically lower than over continental areas, allowing a clearer examination of space-atmosphere interactions. Furthermore, cloud cover over the ocean is dominated by low-level clouds and most of the atmospheric charge lies near the earth surface, at low altitude cloud tops. </span></p><p><span>The relevance of electric field observations in the marine boundary layer is enhanced by the the fact that the electrical conductivity of the ocean air is clearly linked to global atmospheric pollution and aerosol content. The increase in aerosol pollution since the original observations made in the early 20th century by the survey ship Carnegie is a pressing and timely motivation for modern measurements of the atmospheric electric field in the marine boundary layer. Project SAIL (Space-Atmosphere-Ocean Interactions in the marine boundary Layer) addresses this challenge by means of an unique monitoring campaign on board the ship-rigged sailing ship NRP Sagres during its 2020 circumnavigation expedition. </span></p><p><span>The Portuguese Navy ship NRP Sagres departed from Lisbon on January 5th in a journey around the globe that will take 371 days. Two identical field mill sensors (CS110, Campbell Scientific) are installed </span><span>o</span><span>n the mizzen mast, one at a height of 22 m, and the other at a height of 5 meters. </span><span>A visibility sensor (SWS050, Biral) was also set-up on the same mast in order to have measurements of the extinction coefficient of the atmosphere and assess fair-weather conditions.</span><span> Further observations include gamma radiation measured with a NaI(Tl) scintillator from 475 keV to 3 MeV, cosmic radiation up to 17 MeV, and atmospheric ionisation from a cluster ion counter (Airel). The</span><span> 1 Hz measurements of the atmospheric electric field</span><span> and from all the other sensors</span><span> are </span><span>linked to the same rigorous temporal reference frame and precise positioning through kinematic GNSS observations. </span></p><p><span>Here the first results of the SAIL project will be presented, focusing on fair-weather electric field over the Atlantic. The observations obtained in the first three sections of the circumnavigation journey, including Lisbon (Portugal) - Tenerife (Spain), from 5 to 10 January, Tenerife - Praia (Cape Verde) from 13 to 19 January, and across the Atlantic from Cape Verde to Rio de Janeiro (Brasil), from January 22nd to February 14th, will be presented and discussed.</span></p>

scholarly journals Simultaneous aerosol measurements of unusual aerosol enhancement in the troposphere over Syowa Station, Antarctica

2014 ◽  
Vol 14 (8) ◽  
pp. 4169-4183 ◽  
Author(s):  
K. Hara ◽  
M. Hayashi ◽  
M. Yabuki ◽  
M. Shiobara ◽  
C. Nishita-Hara
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Sea Salt ◽  
Free Troposphere ◽  
Syowa Station ◽  
Particle Counter ◽  
Air Masses ◽  
Optical Particle Counter ◽  
Strong Winds ◽  
Storm Condition

Abstract. Unusual aerosol enhancement is often observed at Syowa Station, Antarctica, during winter and spring. Simultaneous aerosol measurements near the surface and in the upper atmosphere were conducted twice using a ground-based optical particle counter, a balloon-borne optical particle counter, and micropulse lidar (MPL) in August and September 2012. During 13–15 August, aerosol enhancement occurred immediately after a storm condition. A high backscatter ratio and high aerosol concentrations were observed from the surface to ca. 2.5 km over Syowa Station. Clouds appeared occasionally at the top of the aerosol-enhanced layer during the episode. Aerosol enhancement was terminated on 15 August by strong winds from a cyclone's approach. In the second case, on 5–7 September, aerosol number concentrations in Dp > 0.3 μm near the surface reached > 104 L−1 at about 15:00 UT (Universal Time) on 5 September despite calm wind conditions, whereas MPL measurement exhibited aerosols were enhanced at about 04:00 UT at 1000–1500 m above Syowa Station. The aerosol enhancement occurred near the surface to ca. 4 km. In both cases, air masses with high aerosol enhancement below 2.5–3 km were transported mostly from the boundary layer over the sea-ice area. In addition, air masses at 3–4 km in the second case came from the boundary layer over the open-sea area. This air mass history strongly suggests that dispersion of sea-salt particles from the sea-ice surface contributes considerably to aerosol enhancement in the lower free troposphere (about 3 km) and that the release of sea-salt particles from the ocean surface engenders high aerosol concentrations in the free troposphere (3–4 km). Continuous MPL measurements indicate that high aerosol enhancement occurred mostly in surface–lower free troposphere (3 km) during the period July–September.

scholarly journals The lofting of Western Pacific regional aerosol by island thermodynamics as observed around Borneo

2012 ◽  
Vol 12 (13) ◽  
pp. 5963-5983 ◽  
Author(s):  
N. H. Robinson ◽  
J. D. Allan ◽  
J. A. Trembath ◽  
P. D. Rosenberg ◽  
G. Allen ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Breeze ◽  
Number Concentration ◽  
Western Pacific ◽  
Surface Mixed Layer ◽  
Free Troposphere ◽  
Mean Flow ◽  
Aerosol Composition ◽  
Air Mass ◽  
The Western Pacific

Abstract. Vertical profiles of aerosol chemical composition, number concentration and size were measured throughout the lower troposphere of Borneo, a large tropical island in the western Pacific Ocean. Aerosol composition, size and number concentration measurements (using an Aerodyne Aerosol Mass Spectrometer, Passive Cavity Aerosol Spectrometer Probe and Condensation Particle Counter, respectively) were made both upwind and downwind of Borneo, as well as over the island itself, on board the UK BAe-146 research aircraft as part of the OP3 project. Two meteorological regimes were identified – one dominated by isolated terrestrial convection (ITC) which peaked in the afternoon, and the other characterised by more regionally active mesoscale convective systems (MCS). Upwind profiles show aerosol to be confined to a shallow marine boundary layer below 930 ± 10 hPa (~760 m above sea level, a.s.l.). As this air mass advects over the island with the mean free troposphere synoptic flow during the ITC-dominated regime, it is convectively lofted above the terrestrial surface mixed layer to heights of between 945 ± 22 (~630 m a.s.l.) and 740 ± 44 hPa (~2740 m a.s.l.), consistent with a coupling between the synoptic steering level flow and island sea breeze circulations. Terrestrial aerosol was observed to be lofted into this higher layer through both moist convective uplift and transport through turbulent diurnal sea-breeze cells. At the peak of convective activity in the mid-afternoons, organic aerosol loadings in the lofted layer were observed to be substantially higher than in the morning (by a mean factor of three). This organic matter is dominated by secondary aerosol from processing of biogenic gas phase precursors. Aerosol number concentration profiles suggest formation of new particles aloft in the atmosphere. By the time the air mass reaches the west coast of the island, terrestrial aerosol is enhanced in the lofted layer. Such uplift of aerosol in Borneo is expected to increase aerosol lifetimes in the lower free troposphere downwind, as they are above the boundary layer and therefore less likely to be lost by wet or dry deposition. It is also likely to change the role they play in the semi-direct and direct aerosol effects. The long chain of islands extending from Malaysia to Australia may all similarly be expected to present an orographic barrier to low level mean flow. This would lead to significant transport of aerosol into the tropical free troposphere across the Western Pacific region.

scholarly journals Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea ice

2016 ◽  
Vol 16 (4) ◽  
pp. 2185-2206 ◽  
Author(s):  
R. S. Humphries ◽  
A. R. Klekociuk ◽  
R. Schofield ◽  
M. Keywood ◽  
J. Ward ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Southern Ocean ◽  
Sea Ice ◽  
Radiative Forcing ◽  
Free Troposphere ◽  
Radiative Balance ◽  
Air Masses ◽  
Antarctic Sea Ice ◽  
Ocean Region ◽  
The Antarctic

Abstract. Better characterisation of aerosol processes in pristine, natural environments, such as Antarctica, have recently been shown to lead to the largest reduction in uncertainties in our understanding of radiative forcing. Our understanding of aerosols in the Antarctic region is currently based on measurements that are often limited to boundary layer air masses at spatially sparse coastal and continental research stations, with only a handful of studies in the vast sea-ice region. In this paper, the first observational study of sub-micron aerosols in the East Antarctic sea ice region is presented. Measurements were conducted aboard the icebreaker Aurora Australis in spring 2012 and found that boundary layer condensation nuclei (CN3) concentrations exhibited a five-fold increase moving across the polar front, with mean polar cell concentrations of 1130 cm−3 – higher than any observed elsewhere in the Antarctic and Southern Ocean region. The absence of evidence for aerosol growth suggested that nucleation was unlikely to be local. Air parcel trajectories indicated significant influence from the free troposphere above the Antarctic continent, implicating this as the likely nucleation region for surface aerosol, a similar conclusion to previous Antarctic aerosol studies. The highest aerosol concentrations were found to correlate with low-pressure systems, suggesting that the passage of cyclones provided an accelerated pathway, delivering air masses quickly from the free troposphere to the surface. After descent from the Antarctic free troposphere, trajectories suggest that sea-ice boundary layer air masses travelled equatorward into the low-albedo Southern Ocean region, transporting with them emissions and these aerosol nuclei which, after growth, may potentially impact on the region's radiative balance. The high aerosol concentrations and their transport pathways described here, could help reduce the discrepancy currently present between simulations and observations of cloud and aerosol over the Southern Ocean.

scholarly journals Seasonal and Diurnal Variations in Aerosol Concentration on Whistler Mountain: Boundary Layer Influence and Synoptic-Scale Controls

2011 ◽  
Vol 50 (11) ◽  
pp. 2210-2222 ◽  
Author(s):  
John P. Gallagher ◽  
Ian G. McKendry ◽  
Anne Marie Macdonald ◽  
W. Richard Leaitch
Keyword(s):  
Boundary Layer ◽  
Meteorological Data ◽  
Warm Season ◽  
Weather Conditions ◽  
Careful Analysis ◽  
Median Number ◽  
Synoptic Scale ◽  
Mixing Processes ◽  
Air Masses ◽  
Seasonal And Diurnal Variations

AbstractA mountain air chemistry observatory has been operational on the summit of Whistler Mountain in British Columbia, Canada, since 2002. A 1-yr dataset of condensation nuclei (CN) concentration from this site has been analyzed along with corresponding meteorological data to assess the frequency and patterns of influence from the planetary boundary layer (PBL). Characterization of air masses sampled from the site as either PBL influenced or representative of the free troposphere (FT) is important to subsequent analysis of the chemistry data. Median CN concentrations and seasonal trends were found to be comparable to other midlatitude mountain sites. Monthly median number concentrations ranged from 120 cm−3 in January to 1601 cm−3 in July. Using well-defined diurnal cycles in CN concentration as an indicator of air arriving from nearby valleys, PBL influence was found to occur on a majority of days during spring and summer and less frequently in late autumn and winter. Days with PBL influence were usually associated with synoptic-scale weather patterns that were conducive to convective mixing processes. Although more common in the warm season, vertical mixing capable of transporting valley air to the mountaintop also occurred in February during a period of high pressure aloft. In contrast, an August case study indicated that the more stable character of marine air masses can at times keep the PBL below the summit on summer days. Considerable variability in the synoptic-scale weather conditions at Whistler means that careful analysis of available datasets must be made to discriminate FT from PBL periods at the observatory.

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