a preliminary investigation of boundary layer effects on daytime ... · systematic investigation of...

Acta Geophysica vol. 57, no. 4, pp. 904-922

DOI: 10.2478/s11600-009-0033-6

________________________________________________ © 2009 Institute of Geophysics, Polish Academy of Sciences

A Preliminary Investigation of Boundary Layer Effects on Daytime Atmospheric CO2 Concentrations

at a Mountaintop Location in the Rocky Mountains

Stephan F.J. DE WEKKER1, Alex AMEEN1, Guan SONG1, Britton B. STEPHENS2, Anna G. HALLAR3, and Ian B. McCUBBIN3

1Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA; e-mails: [email protected] (corresponding author),

[email protected], [email protected] 2National Center for Atmospheric Research, Boulder, CO, USA

e-mail: [email protected] 3Storm Peak Laboratory, Desert Research Institute, Steamboat Springs, CO, USA

e-mails: [email protected], [email protected]

A b s t r a c t

Observations of CO2 concentration at a mountaintop in the Colorado Rockies in summer show a large diurnal variability with minimum CO2 concentrations found between 10:00 and 18:00 MST. Simulations are per-formed with a mesoscale model to examine the effects of atmospheric structure and large-scale flows on the diurnal variability. In the simulations initialized without large-scale winds, the CO2 minimum occurs earlier compared to the observations. Upslope flows play an important role in the presence of this early (pre-noon) minimum while the timing and magnitude of the minimum depend only weakly on the temperature structure. An in-crease in large-scale flow has a noticeable impact on the diurnal variability with a more gradual decrease in daytime CO2 concentration, similar to summer-averaged observations. From the idealized simulations and a case study, it is concluded that multi-scale flows and their interactions have a large influence on the observed diurnal variability.

Key words: mesoscale circulations, atmospheric boundary layer, com-plex terrain, CO2 concentration, mountaintop observatory.

BOUNDARY LAYER EFFECTS ON CO2 CONCENTRATIONS AT MOUNTAINTOP

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1. INTRODUCTION The diurnal variability of CO2 concentration on a flat homogeneous surface is determined by the surface CO2 flux, CO2 advection, and the volume of air in which the CO2 is mixed. The CO2 concentration in a well-mixed daytime atmospheric boundary layer (ABL), characterized by small vertical and hori-zontal gradients in trace gas concentration, is representative for a horizontal scale on the order of tens to hundreds of kilometers (Bakwin et al. 1998, Gloor et al. 2001). These concentrations, typically measured on tall towers, can be used to determine regional-scale CO2 fluxes (Bakwin et al. 2004, Hel-liker et al. 2004). Measurements on mountaintop locations may be used simi-larly but a detailed understanding of the diurnal variability of CO2 and the factors affecting this variability at these locations is required (Law et al. 2008).

Mountaintop locations have gained popularity by their ability to measure trace gas concentrations in the free atmosphere, less affected by ABL influ-ences than locations at low elevation. These background CO2 concentrations have been measured at Mauna Loa, Hawaii, a renowned mountaintop loca-tion, for more than fifty years and have provided important information on the global carbon cycle. Measurements at Mauna Loa are filtered to exclude ABL and other local effects on the CO2 concentration. Pales and Keeling (1965) and Keeling et al. (1976) recognized the importance of these local ef-fects for Mauna Loa. For example, they found that the CO2 concentrations exhibited a pronounced afternoon minimum which they explained by the transport of CO2 depleted air by upslope flows. It is now common practice at mountaintop locations to apply filters to the measured concentrations to ex-clude ABL and other effects (e.g., Thoning et al. 1989). Knowledge of these effects often does not come from CO2 concentrations alone but also from concurrent measurements of other trace gases and aerosols. Several studies have been performed to investigate the transport of aerosol to mountaintop locations such as the Jungfraujoch (Lugauer et al. 1998, Seibert et al. 1998) and Schauinsland (Fiedler et al. 2000). Many of these studies have specu-lated about the important role of upslope flows and ABL mixing. However, a systematic investigation of the role of these processes and other factors af-fecting the diurnal variability of CO2 and other trace gases has not been car-ried out. The current study is a first step towards such an investigation using the Storm Peak Laboratory in Colorado, USA, as an example. Using sum-mertime data from the Storm Peak Lab, the temporal CO2 variability is in-vestigated along with the aerosol and water vapor variability. Mesoscale model and particle model simulations are performed in an attempt to retrieve information about the relative contribution of CO2 advection and ABL struc-ture on the temporal CO2 variability. The focus of this study is on diurnal va-riability in the summertime at Storm Peak Lab. Results from measurements

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and modeling at other mountaintops in the Rocky Mountains and during other times of the year will be the topic of future papers.

2. METHODS

2.1 Measurements

CO2, aerosol, and meteorological measurements are taken from the Storm Peak Laboratory (SPL), a mountaintop research facility in the Colorado Rockies. CO2 concentrations are monitored at SPL as part of the Regional Atmospheric Continuous CO2 Network in the Rocky Mountains (Rocky RACCOON). The network, installed in fall of 2005 and spring of 2006, includes two other mountaintop sites: Niwot Ridge, near Ward, Colorado, and Hidden Peak, near Snowbird, Utah, as well as several valley sites. The network uses the NCAR-developed Autonomous Inexpensive Robust CO2 Analyzer (AIRCOA, Stephens et al. 2006). These units measure CO2 con-centrations at three levels on a tower, producing individual measurements every 2.5 min precise to 0.1 ppm CO2 and closely tied to the WMO CO2 scale. Aerosol number concentrations are measured using a stand-alone TSI model 3010 Concentration Particle Counter for particles with diameters larg-er than 10 nm. SPL is located at 3210 m above sea level at the top of Steam-boat Ski Area near Steamboat Springs, Colorado (40.455N, 106.744W), in the upper Yampa Valley (Fig. 1).

Fig. 1. Topography of the area surrounding SPL. Contours are drawn every 200 m. Light-grey and dark-grey shading represent elevations between 3000 and 3200 m MSL and above 3200 m MSL, respectively.

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Fig. 2. View from Storm Peak Laboratory looking west into the Yampa Valley (pho-to by B.B. Stephens). Colour version of this figure is available in electronic edition only.

The Storm Peak Lab is situated on a 70 km north-south mountain range perpendicular to the prevailing westerly winds. Sparse trees are present around the site (Fig. 2). The site has been used in cloud and aerosol studies for several decades (Hindman et al. 1994) and considerable knowledge has been acquired on aerosol patterns and the interaction of aerosols with clouds (Borys and Wetzel 1997, Hindman et al. 1994, Lowenthal et al. 2002). The record of CO2 at SPL begins in August 2005. Both the CO2 and the meteoro-logical data are available to the public for free in near real-time through web-sites (http://raccoon.ucar.edu and http://www.wrcc.dri.edu/weather/strm.html, respectively).

2.2 Mesoscale model simulations

The mesoscale numerical model used is the Regional Atmospheric Modeling System (RAMS) (Pielke et al. 1992, Cotton et al. 2003) version 4.3 in which land-surface processes are represented by the Land Ecosystem Atmosphere Feedback Model, version 2 (LEAF-2) (Walko et al. 2000). Turbulent ex-change at the surface is determined with the Louis scheme which is based on the Monin–Obukhov similarity theory. The computed surface water and energy fluxes serve as the lower boundary for the sub-grid diffusion scheme for the atmosphere. For more detailed descriptions of the treatment of phys-ics in RAMS, see Pielke et al. (1992) and Cotton et al. (2000).

The model domain consists of two grids which are both centered at SPL. The outer grid covers a large area (500×500 km) of the Colorado Rockies with a horizontal grid spacing of 5 km. The inner grid has a horizontal grid


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spacing of 1 km and covers a 97×97 km area (Fig. 1). The grid has 38 vertic-al levels with a grid spacing from 70 m near the surface that gradually in-creases to 1000 m near the model top at about 16 km. Due to vertical grid staggering, the first model level for all variables except for vertical velocity is at about 35 m. Thirteen soil nodes are used to a depth of 0.9 m below the surface. The simulations start at 06:00 UTC (23:00 MST) and last for 24 hours. The simulations are performed for 26 June 2007. Sunrise and sunset times on this day are at approximately 05:00 and 19:00 MST, respectively. The topography was derived from 30 arcsecond (~1 km) resolution data from the United States Geological Survey (USGS) data set. To reduce the complexity of the land-atmosphere interactions, vegetation, soil type, and soil moisture was set to short grass, sandy loam, and 0.20 m3 m–3, respective-ly, in the two domains. Surface CO2 fluxes are the result of a complex inte-raction between processes in the soil, vegetation, and atmosphere and can be highly variable in space and time (e.g., Monson et al. 2002). To standardize the simulations and isolating meteorological effects on atmospheric CO2 concentration, the model is forced by a spatially homogeneous sinusoidal CO2 flux at the surface with amplitude of 10 μmol CO2 m–2 s–1. This idea-lized diurnal pattern is based on the averaged surface CO2 flux measured in a subalpine forest ecosystem (Monson et al. 2002). The CO2 flux is 180o out of phase with the incoming global radiation curve with CO2 uptake between sunrise and sunset. Background CO2 concentration in the model is set at 380 ppm. Summertime observations during the ACME’04 field study (Sun et al. 2009) show that vertical gradients of CO2 are small (0-2 ppm) in the at-mosphere over the Rocky Mountains implying that entrainment effects on CO2 variability at SPL will be minimal. To investigate the effect of atmos-pheric structure on CO2 variability at SPL, a set of idealized simulations is performed in which the initial temperature profile and winds are varied. In the ‘standard’ simulation, the vertical potential temperature profile is based on the morning (12:00 UTC or 05:00 MST) sounding at Grand Junction, CO, averaged for June 2006. Grand Junction is located about 170 km southwest of SPL and is the radiosonde station closest to SPL in the National Weather Service radiosonde network. The June-averaged potential temperature profile has a constant lapse rate of 1.5 K/km up to 5500 m MSL and 4 K/km above. This vertical temperature structure was changed to investigate the influence on stability in the layer up to 6000 m MSL (0.75; 1.5; 4 K/km) and on the height of a strong temperature inversion (2500 and 3000 m). The tempera-ture structure of this set of idealized simulations is shown in Fig. 3 and represents the range of observed vertical temperature profiles. Additional idealized simulations were performed to investigate the effect of ambient (synoptic) wind speed, and surface sensible heat flux. The complete set of idealized simulations is listed in Table 1.


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Fig. 3. Various stability scenarios as listed in Table 1.

Table 1 Idealized simulations performed in the current study and the corresponding figure

that shows the results Panels of Figure 8 Characteristics of idealized simulations and abbreviations

Fig. 8a Flat terrain Fig. 8b Stability up to 6000 m MSL standard: 1.5 K/km stab_0.75: 0.75 K/km stab_4: 4 K/km Fig. 8c Strong inversion (13K/km) below or at mountain top height standard: no inversion inv2500: 2500-3000 m MSL inv3000: 3000-3500 m MSL Fig. 8d Reduced sensible heat flux standard: max. sensible heat flux 300 W/m2 soilm: 30% increase in soil moisture max. sensible heat flux 150 W/m2 sep (September simulation): max. sensible heat flux 200 W/m2 Fig. 8e Ambient U wind standard: 0 m/s U2: 2 m/s westerly wind U2_neg: 2 m/s easterly wind Fig. 8f Ambient V wind standard: 0 m/s V2: 2 m/s southerly wind V2_neg: 2 m/s northerly wind


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In addition to the idealized simulation, a case study is performed for 26 June 2007. In this simulation, the five outermost lateral boundary points in the domain were nudged toward NCEP EDAS objective analysis fields at 3-hour intervals to allow changes in large-scale conditions to influence the model simulations. Nudging towards objective analysis fields was only ap-plied to the outermost grid; no interior nudging was applied. To simulate the transport of aerosols to SPL in the case study, a Lagrangian particle disper-sion model (LPDM), the Hybrid Particle And Concentration Transport (HYPACT) was run using the RAMS model output. HYPACT simulates the motion of atmospheric tracers under the influence of winds and turbulence. Its Lagrangian component enables representation of sources of any size and the maintenance of concentrated, narrow plumes until atmospheric disper-sion dictates that they should broaden. Recent applications of HYPACT, as described by Walko et al. 2001, can be found in De Wekker et al. (2004) and Lagouvardos et al. (1996).

3. RESULTS

3.1 Observations

Daily averaged CO2 concentrations were subtracted from hourly values and averaged hourly to get an averaged diurnal variation for the summer months (June, July, and August) for 2006, 2007, and 2008 (Fig. 4).

CO2 concentrations decrease after sunrise and typically reach a minimum in the late afternoon. The averaged daily amplitude of the CO2 concentration is about 4 ppm and does not change significantly from June to August and from 2006 to 2008 (Fig. 4). Even though the minimum CO2 concentration typically occurs during the afternoon, there are a significant number of occa-sions where the minimum occurs earlier in the day and even before noon. This is shown for 2007 in Fig. 5. In a number of these cases, the CO2 con-centration exhibits a maximum in the late afternoon. These afternoon max-ima, however, are never larger than the maxima that occur at night (Fig. 5).

The decrease in CO2 concentration can be explained by the start of pho-tosynthetic activity after sunrise by the vegetation in the area around SPL. Also, any CO2 accumulation occurring at night in a stable surface layer will be mixed in a larger volume of air due to ABL growth. Simultaneous with photosynthesis, evapotranspiration occurs and causes an increase in the wa-ter vapor content (Fig.6, top row). Part of the decrease in CO2 concentration and increase in water vapor mixing ratio can also be explained by the trans-port of moist and CO2 depleted air from upper parts of the valley sidewall by local upslope flows. In fact, such a diurnal pattern of water vapor concen-trations is typical of high-elevation research stations such as Jungfraujoch


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Fig. 4. Monthly averaged diurnal variability of CO2 concentration for June, July, and August for the years 2006 (top row: a, b, c), 2007 (middle row: d, e, f), and 2008 (bottom row: g, h, i). Error bars denote = +/– 1 standard deviation, but include synoptic and monthly variability and thus overestimate the variability in the diurnal shape. Colour version of this figure available in electronic edition only.

Fig. 5. Frequency distribution of maximum (dashed line) and minimum (solid line) daily CO2 concentration for June, July, and August 2007.

(Baltensperger et al. 1997), Sonnblick Mountain Observatory (Seibert et al. 1998), and Mt. Bachelor Observatory (Weiss-Penzias et al. 2006) and is sometimes used as a tracer for ABL/free atmosphere transitions (Obrist et al. 2008). Obrist et al. (2008) calculated linear regressions of diurnal patterns between different pollutants and water vapor for wintertime and concluded that upward mixing and slope flow processes are the main causes leading to enhanced levels of pollutants at SPL during daytime hours. In another study,

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Fig. 6. Monthly averaged diurnal variability for June, July, and August 2007 for the water vapor mixing ratio (top row: a, b, c), and aerosol concentration (bottom row, d, e, f). Error bars denote = +/– 1 standard deviation. Colour version of this figure available in electronic edition only.

Watson et al. (1996) showed that emissions from motor vehicles and resi-dential burning accumulated at night and during the morning in the Yampa Valley and are mixed upward later in the morning. Interestingly, the onset of increase in aerosol concentration typically lags the onset of CO2 decrease and humidity increase by a couple of hours (Fig. 6). This could indicate that the initial decrease in CO2 concentration and increase in humidity is not caused by upslope flow transport originating at the valley floor but rather by an upslope flow originating above the valley temperature inversion and by local ABL and vegetation processes. The time between sunrise and the brea-kup of a nighttime inversion in the Yampa Valley can take about 3 to 4 hours as shown by Whiteman (1982) during which time the moist and CO2 enriched air is confined within the valley. Some simulations were performed in an effort to understand the meteorological processes affecting the ob-served diurnal CO2 variability. These simulations are described next.

3.2 Idealized simulations

In the “standard” idealized simulations, the mesoscale model is initialized without ambient (large-scale or synoptic) winds and the flows that develop in the domain are thermally-driven. Figure 7 shows the vertical cross sec-tions of potential temperature and CO2 concentration and horizontal maps of the surface wind in the inner model domain for 10:00 and 13:00 MST. Sur-face-based temperature inversions that develop in the valleys during the night, break-up between 09:00 and 10:00 MST, 3 to 4 hours after sunrise in agreement with the observations by Whiteman (1982). A transition occurs at that time from downvalley to upvalley flows in the Yampa valley.

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Fig. 7. Vertical cross section of potential temperature (K) at 10:00 MST (a) and 13:00 MST (b), CO2 mixing ratio (ppm) at 10:00 MST (c) and 13:00 MST (d), and horizontal maps of surface winds at 10:00 MST (e) and 13:00 MST (f) for the inner grid in the ‘standard’ idealized simulation. The location of SPL is indicated in (a)-(d) by an arrow. The location of the vertical cross section and of SPL is indicated in (e) and (f) by the black solid line and the ‘X’, respectively. Darker shading indicates larger potential temperature in (a) and (b), larger CO2 concentration in (c) and (d) and higher elevation in (e) and (f). Horizontal wind vectors in (e) and (f) are shown every 5 grid points for clarity.

Also, westerly upslope flows in a shallow layer transport CO2 enriched air to SPL in the simulation. This causes the CO2 concentration to increase at SPL between 10:00 and 12:00 MST (Fig. 8a) in contrast to the observations discussed in Section 3.1. Before 10:00 MST, a decrease in CO2 concentra-tion at SPL is simulated, in agreement with the observations. The decrease

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occurs shortly before sunrise before the onset of photosynthesis. The simula-tion indicates that weak subsiding motions just above the mountaintop occur before sunrise, transporting CO2 from aloft to mountaintop level. In the first couple of hours after sunrise, ABL growth on the ridge and CO2 uptake are more important than upslope flows in the simulation. After 12:00 MST, the simulated ABL extends well above mountain height and increasing westerly winds and convection efficiently mix the CO2 in the boundary layer. Due to the continued uptake of CO2 at the surface, the CO2 concentration gradually decreases at SPL in the afternoon (Fig. 8a). In a simulation in which the to-pography was removed and replaced by a flat surface at 20:00 m MSL, the nighttime increase in CO2 concentration in the stable boundary layer is much more pronounced but no pre-noon CO2 minimum occurs (Fig. 8a). This sug-gests that this minimum is the result of upslope flows.

Initial stability below 6000 m MSL does not have a large impact on the diurnal variability of CO2 (Fig. 8b). The minimum in CO2 concentration oc-curs before noon in all cases. In the most stable case, the ABL growth is smaller but westerly upslope flows stronger, resulting in a small difference in the CO2 concentration between the different cases. The presence of an ini-tial strong elevated temperature inversion at mid-slope or mountain-top ele-vation also does not affect the diurnal CO2 variability much (Fig. 8c). In the cases above, the maximum ABL heights vary by at least a few hundred me-ters but in all cases reach well above mountaintop height despite differences in initial stability. Therefore, the simulations suggest that as long as the ABL

Fig. 8. Diurnal variation of CO2 concentration for the various idealized simulations listed in Table 1. Note that the range in CO2 concentration in subfigure (a) is differ-ent from the range in the other subfigures.

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heights are well above mountaintop height, ABL height and growth are not well correlated with CO2 concentration on a mountain ridge. This situation is valid for summertime conditions in the Rocky Mountains where it has been shown that the ABL typically exceeds 3000 m AGL (Holzworth 1964).

Reducing the sensible heat flux by increasing the soil moisture or by re-ducing the incoming radiation (model initialization on 26 September instead of 26 June) decreases the intensity of the upslope flows and delays the trans-port of CO2 rich air from the valley floor (Fig. 8d). The delay can be a couple of hours.

Including an ambient flow in the initial conditions leads to a weakening or disappearance of the pre-noon minimum in CO2 concentration that is so characteristic of the idealized simulations without ambient flow. Even a weak wind (2 m s–1) allows for more efficient mixing in the ABL and trans-port of CO2 towards SPL (Fig. 8e and 8f). The variability between the differ-ent cases in the morning hours depends on how much CO2 was accumulated upwind of SPL at night. Advection of CO2 from the valleys south and west from SPL causes an increase in CO2 concentration until 08:00 MST. Nor-therly and easterly winds result in a decrease of CO2 concentration in the morning. In the case of northerly flow (V2_neg), the CO2 concentration min-imum is still pronounced as in the standard case. Terrain is on average higher north and east of SPL than in other directions and CO2 does not get a chance to accumulate there. Therefore, no significant CO2 advection can occur.

Fig. 9. Diurnal variability of the hourly averaged wind speed on days in June, July, and August of 2006, 2007, and 2008 with a minimum CO2 concentration before 10:00 MST (solid line) and after 12:00 MST (dashed line).


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Important differences between the simulated and observed diurnal varia-bility include the timing of the minimum in the CO2 concentration and the magnitude of the diurnal cycle. In the simulations, a minimum in CO2 con-centration occurs later during the day (more similar to the observations) if an ambient flow is included in the initial conditions. Disaggregation of the days where a CO2 minimum occurred before 10:00 MST and after 12:00 MST in the observations showed that wind speeds were significantly lower on days where a CO2 minimum occurred before 10:00 MST (Fig. 9). Furthermore, fair weather days showed a larger frequency of a CO2 minimum in the morn-ing than overcast days. The simulated diurnal range of CO2 is about twice as large as the observed diurnal range. By decreasing the amplitude of the idea-lized diurnal surface CO2 flux in the simulations and/or making the CO2 flux spatially heterogeneous, we would be able to reduce the simulated diurnal CO2 range to compare better with the observed range. However, the goal of this paper is not to obtain good quantitative agreement between observed and simulated CO2 concentrations but rather to explain the characteristics of the diurnal CO2 variability.

3.3 Case study

Idealized simulations provide a convenient way to investigate the processes underlying some general characteristics of the diurnal CO2 variability. How-ever, there is a large day-to-day variability in the CO2 concentration that can offer additional insights into important processes on specific days. Therefore, in addition to the idealized simulations, a simulation was performed for 26 June 2007 covering 24 hours starting at 06:00 UTC (23:00 MST on 25 June 2007). This day was characterized by clear skies and weak ambient flows.

Observations for this day (Fig. 10) show generally decreasing CO2 con-centrations until around noon, followed by an increase that lasts for about four hours. Aerosol numbers also increase during that time while water va-por decreases. Wind direction changes from easterly to westerly at around 09:00 MST. These easterly flows at SPL occur frequently during the evening and nighttime in summer when synoptic forcing is weaker than in winter. The model output agrees well with the observations for wind speed, wind di-rection, and water vapor mixing ratio (Fig. 10b-d), demonstrating the ability of RAMS to simulate atmospheric processes well for the area around SPL. However, the simulated and observed CO2 concentration do not agree well in the afternoon (Fig. 10a). The increase in CO2 is not simulated and the simul-taneous increase in observed aerosol number suggests that anthropogenic sources may play an important role. Obviously, anthropogenic CO2 fluxes are not accounted for in the model setup. The observed and simulated de-crease in water vapor suggests that upslope flows do not play a role in the


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Fig. 10. Observations (solid line) and model output (dashed line) of CO2 concentra-tion (a), wind speed (b), water vapor mixing ratio (c), and wind direction (d) for SPL for 26 June 2007. In (a), the observed aerosol number is also shown (line connected by crosses).

transport of air enriched in CO2 and aerosols to SPL. To explain the transport processes responsible for the observed increase in CO2 concentration and aerosol number, we now use the LPDM. The LPDM is set up to release 500 particles every 90 seconds from 08:00 to 10:00 MST. These particles can be considered aerosols and CO2 (both respired and anthropogenic). Particles in-itially accumulate in the Yampa Valley but are soon advected northward along the valley floor by downvalley winds (Fig. 11a, c).

The location to which the particles are advected is not very sensitive to the exact location of the emissions. After the break-up of the surface based inversion in the Yampa Valley, particles are mixed upward partly by convec-tion and partly by upslope flows. Once residing in the upper parts of the ABL or above, particles are advected to SPL horizontally from the northwest (Fig. 11b, d). This horizontal advection is a more important process in the transport of particles to SPL than upslope flows in this case. The relative contribution of the advection by ambient flows and by local upslope flows will vary from case to case and partly explains the large day-to-day variabili-ty in CO2. This example demonstrates the power of combining mesoscale model output with a LPDM to explain the variability of CO2 at a mountain top and motivates us to use this approach in future studies.

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Fig. 11. Location of particles at 10:00 MST (a, c) and 14:00 MST (b, d) in a vertical cross section (a, b) and horizontal cross section (c, d). Darker shading indicates higher elevation in (c) and (d). In the LPDM simulation, particles were released in the morning in the Yampa Valley between 08:00 and 10:00 MST. In (b) only those particles are plotted that are located in a rectangle that is 10 km in the north south direction and 97 km (the domain size) in the east west direction, centered on SPL. The location of SPL is shown with the white solid circle and the black horizontal line denoted the location of the vertical cross sections. Arrows in (c) and (d) indicate the direction of the flows responsible for the advection of the particles.

4. CONCLUSIONS AND OUTLOOK A mesoscale model and a LPDM were used to investigate the effect of tem-perature structure and ambient winds on the variability of CO2 concentration at a mountaintop location in the Rocky Mountains. A set of idealized simula-tions show that initial stability does not affect the variability much. In the summer months, the surface based inversion in the valley breaks up after a couple of hours and ABL convection reaches to well above the mountaintop height. As soon as this happens, CO2 rich air is transported aloft either though upslope flows or through free convection. This causes the CO2 con-centration to increase at the mountaintop before noon. If ambient flows are included, the effect of the upslope flows diminishes and the CO2 concentra-tion decreases more gradually during the day. The resulting CO2 concentra-tions agree better with the summer-averaged observations than the concentra-

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tions without ambient flow. From a case study, it was found that much of the observed day-to-day difference in the temporal CO2 variability is likely caused by differences in magnitude and direction of thermally-driven and synoptic flows, by the interaction of these flows, and by the spatial variability of CO2.

This study shows that mesoscale models are a useful tool in understand-ing the variability of CO2 at mountaintop locations. On the other hand, moun-taintop observations can be used to evaluate the performance of mesoscale models in their ability to simulate mesoscale circulations and ABL growth in complex terrain, two important factors affecting the CO2 concentration.

In a future paper, data from the two other mountain stations in Rocky RACCOON will be used as well as data from other months. Differences in observed CO2 variability resulting from meteorological factors (wind speed, wind direction, cloudiness) will be investigated in more detail than in the current study. The sensitivity of the simulations to the specification of the CO2 flux, including using more realistic surface fluxes, and to the horizontal and vertical resolution will also be addressed. As more mountaintop loca-tions are used to constrain regional CO2 fluxes, it will become increasingly important to investigate the questions what measurements represent free-tropospheric, regional, and local atmospheric conditions, and how well mod-els represent the observations. Addressing these important questions requires knowledge of the relative contribution of multi-scale flows in complex ter-rain. This paper was a first step towards acquiring this knowledge for the Storm Peak Laboratory.

Acknowledgments . This work was partly supported by The Institute for Integrative and Multidisciplinary Earth Studies TIMES, at the National Center for Atmospheric Research. The National Center for Atmospheric Re-search is sponsored by the National Science Foundation. The Desert Research Institute is an equal opportunity service provider and employer and is a permit-tee of the Medicine-Bow Routt National Forests. We would like to thank the reviewers and Ken Davis for providing comments that improved the paper.

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Received 13 March 2009 Accepted 4 August 2009

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