Slope Winds Research Articles

Experimental investigation of the stable water isotope distribution in an Alpine lake environment (L-WAIVE)

Abstract. In order to gain understanding on the vertical structure of atmospheric water vapour above mountain lakes and to assess its link with the isotopic composition of the lake water and with small-scale dynamics (i.e. valley winds, thermal convection above complex terrain), the L-WAIVE (Lacustrine-Water vApor Isotope inVentory Experiment) field campaign was conducted in the Annecy valley in the French Alps during 10 d in June 2019. This field campaign was based on an original experimental synergy between a suite of ground-based, boat-borne, and two ultra-light aircraft (ULA) measuring platforms implemented to characterize the thermodynamic and isotopic composition above and in the lake. A cavity ring-down spectrometer and an in-cloud liquid water collector were deployed aboard one of the ULA to characterize the vertical distribution of the main stable water isotopes (H216O, H218O and H2H16O) both in the air and in shallow cumulus clouds. The temporal evolution of the meteorological structures of the low troposphere was derived from an airborne Rayleigh–Mie lidar (embarked on a second ULA), a ground-based Raman lidar, and a wind lidar. ULA flight patterns were repeated several times per day to capture the diurnal evolution as well as the variability associated with the different weather events encountered during the field campaign, which influenced the humidity field, cloud conditions, and slope wind regimes in the valley. In parallel, throughout the campaign, liquid water samples of rain, at the air–lake water interface, and at 2 m depth in the lake were taken. A significant variability of the isotopic composition was observed along time, depending on weather conditions, linked to the transition from the valley boundary layer towards the free troposphere, the valley wind intensity, and the vertical thermal stability. Thus, significant gradients of isotopic content have been revealed at the transition to the free troposphere, at altitudes between 2.5 and 3.5 km. The influence of the lake on the atmosphere isotopic composition is difficult to isolate from other contributions, especially in the presence of thermal instabilities and valley winds. Nevertheless, such an effect appears to be detectable in a layer of about 300 m thickness above the lake in light wind conditions. We also noted similar isotopic compositions in cloud drops and rainwater.

Martian boundary layer wind profiles during the landings of Viking and InSight

Interaction between mesoscale, synoptic and planetary scale circulations on Mars can combine to produce locally complex winds that make interpretation of lander wind data a challenge. We investigate the structure of the boundary layer winds by obtaining new wind speed and directions from trajectory analysis of the Viking 1, Viking 2 and InSight landings. We use the new wind data for InSight together with a column model to investigate the near-surface winds at the InSight landing site.We use our Trajectory Model and Analysis Tool (TMAT) to verify wind data obtained by Viking lander 1 (VL-1) as it passed through a region of strong winds. This is important as the winds may be evidence of Western Boundary Currents. We constrain the vertical extent of this wind layer. We also verify the wind data for Viking lander 2 (VL-2). For VL-2 the winds remain constant from parachute deployment to lander release below which there is a large change in wind direction. For InSight our analysis indicates the wind speeds remain approximately constant with altitude. We found the wind blows from the west in the upper part of InSight's descent and from the north northwest closer to the surface.We use our new TMAT inferred wind data for InSight to initialise the winds of a column model (University of Helsinki/Finnish Meteorological Institute). We investigate the influence of slope winds on the local winds at the InSight landing site. Our results demonstrate that Martian slopes and large-scale circulations can interact in a complex way to produce the near-surface wind patterns as observed by InSight.We have been able to obtain new wind velocity data at several altitudes and locations on Mars by utilizing atmospheric modelling and the known locations of landed hardware from Mars surface missions. We have applied these new velocity data to investigate the interactions of large-scale winds and the Martian topography.

The Effect of Upstream Orography on the Onset of Sundowner Winds in Coastal Santa Barbara, CA

AbstractThe impact of upstream terrain on the diurnal variability of downslope windstorms on the south‐facing slopes of the Santa Ynez Mountains (SYM) is investigated using numerical simulations. These windstorms, called Sundowners due to their typical onset around sunset, have intensified all major wildfires in the area. This study investigates the role of the orography upstream of the SYM in the diurnal behavior of Sundowners. Two types of Sundowners are examined: western sundowners (winds with dominant northwesterly direction) and eastern Sundowners (winds with dominant northeasterly direction). By using semi‐idealized simulations, in which we progressively reduce the upstream terrain, we show that the onset of the lee slope jet occurs in the late afternoon only when the flow approaches the SYM from the northeast, after interacting with a considerably higher mountain barrier. We demonstrate that during the eastern regime, the progressive reduction of the upstream terrain results in strong lee slope winds throughout the day. Conversely, the diurnal cycle of downslope winds during the western regime is less sensitive to the reduction of the upstream terrain. The Sundowner diurnal cycle during the eastern regime can be explained by boundary‐layer processes in the valley and the blocking effect of high mountains upstream of the SYM. These results contribute to a better understanding of the influence of upstream orography in the cycle and intensity of downslope windstorms in coastal mountains.

Measuring Ripple and Dune Migration in Coprates Chasma, Valles Marineris: A Source to Sink Aeolian System on Mars?

AbstractActive aeolian systems are present across the martian surface, with active dune fields commonly found in depressions such as craters and valleys. There are many dune fields within the equatorial region of Mars, including Valles Marineris, but the effects of topography on wind regimes and consequently dune migration in Valles Marineris is poorly understood. We investigated both the ripple and dune migration in a dune field in Coprates Chasma using High Resolution Science Experiment Images (HiRISE) and Context Camera (CTX) images. Migration rates of dune brinks and ripples were measured over varying time scales, between Earth years 2007–2014. The dunes here are some of the tallest on Mars, with heights of up to 182 m. The dune brinks are migrating eastwards at a rate of 0.1–0.3 m/EY through the valley due to topographical influences on the local winds. Potential sediment sources for the dune field were identified and investigated by studying thermal inertia and mineralogy. The topographic slope‐related katabatic winds travel down the valley walls and converge with the dominant winds traveling through the center of the valley, causing overall eastwards dune migration. Topography is likely the dominant control on the local wind regime; slope winds travel down the sides of the valley walls and are funneled through the center of the valley. These local winds subsequently facilitate the migration of the large dunes in Coprates Chasma, thus expanding our understanding of local winds in the Martian environment.

An engineering model of Titan surface winds for Dragonfly landed operations

Winds near the ground on Titan for the Dragonfly landing site (near Selk crater, 10°N) for the mid-2030s (Titan late southern summer, Ls ~ 310°) are estimated for mission design purposes. Prevailing winds due to the global circulation are typically 0.5 m/s, and do not exceed 1 m/s. Local terrain-induced flows such as slope winds appear to be similarly capped at 1 m/s. At various landing sites and times, these two contributions will vectorially combine to yield steady winds (for part of a Titan day, Tsol) of up to 2.0 m/s, but typically less – the slope wind component will be small in the mid-morning. In early afternoon, as on Earth and Mars, solar-driven convection in the planetary boundary layer will cause wind fluctuations of the order of 0.1 m/s, varying with a typical timescale of ~1000 s. Occasionally this convection organizes into coherent ‘dust devil’ vortices: detectable vortices with speeds of 1 m/s are predicted about once per Titan day. We have introduced the convective velocity scale combined with the advection time of PBL cells as a metric to derive the frequency of occurrence of gusts associated with convective vortices (‘dust devils’). Maximum possible vortex winds on Titan of 2.8 m/s may be expected only once per 40 Tsols, and define the maximum wind (4.8 m/s at 10 m height) that Dragonfly must tolerate without damage. The applicability of different wind combinations, scaled to the height of relevant Dragonfly components above the ground (e.g. the maximum corresponds to 3.9 m/s at 1.3 m height) by a logarithmic wind profile, to Dragonfly design and operations are discussed.

Temperature and humidity monitoring to identify ideal periods for liquefaction on Earth and Mars – data from the High Andes

During an almost two week-long field campaign in the Atacama Desert high altitude region of Ojos del Salado volcano, temperature (T) and relative humidity (RH) values were monitored on the surface and <1–5 cm sized rocks, focusing on the night-time values. The aim was to identify and evaluate potential temporal characteristics of daily T and RH changes, searching for ideal periods for deliquescence that has recently been proposed for Mars. Although the atmospheric pressure on Mars is much lower than on Earth, and the atmosphere is drier in general, the huge daily temperature fluctuation there could produce elevated humidity values at night-time; this aspect has thus been analysed on Earth at a desert location, where because of the high elevation night-time cooling is very strong, just like on Mars. Different nearby surface locations showed the same temporal T/RH characteristics, but evident variations were observed between different days. Strong fluctuations could be observed on 10–20 minute long temporal scales, that might influence the deliquescence process, and should be accounted for in future missions aiming to analyse this process on Mars. Night-time periods were favourable for deliquescence. Among the modelled Mars-relevant salts [CaCl2, Ca(ClO4)2, Mg(ClO4)2, NaCl] the longest durations of possible deliquescence were for CaCl2, Ca(ClO4)2 and Mg(ClO4)2, ~7–12 hours for one day. The duration for deliquescence showed some increase along with the rising elevation, due to the decreasing night-time temperature. Thus despite the low humidity on Mars, the cold nights may cause elevated RH towards deliquescence. The Atacama Desert locations analysed are a useful analogue of the deliquescence process on Mars. Fluctuation in RH was observed in night-time, suggesting that similar variability might be present on Mars, and that should be considered in the future, including in evaluating how fast the microscopic liquid formation progresses. Night-time slope winds expected on Mars might have a strong impact on the local T/RH conditions. A more detailed analysis in the future should focus on identifying and separating regions with and without much of the expected night-time fluctuation

Earth’s Polar Night Boundary Layer as an Analog for Dark Side Inversions on Synchronously Rotating Terrestrial Exoplanets

Abstract A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free atmosphere. Here we analyze data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogs on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called “mesoscale slope winds,” which have horizontal scales of 10–100 s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models.

The Land-Sea Breeze of the Red Sea: Observations, Simulations, and Relationships to Regional Moisture Transport.

Unique in situ observations of atmospheric conditions over the Red Sea and the coastal Arabian Peninsula are examined to study the dynamics and regional impacts of the local land‐sea breeze cycle (LSBC). During a 26‐month data record spanning 2008–2011, observed LSBC events occurred year‐round, frequently exhibiting cross‐shore wind velocities in excess of 8 m/s. Observed onshore and offshore features of both the land‐ and sea‐breeze phases of the cycle are presented, and their seasonal modulation is considered. Weather Research and Forecasting climate downscaling simulations and satellite measurements are used to extend the analysis. In the model, the amplitude of the LSBC is significantly larger in the vicinity of the steeper terrain elements encircling the basin, suggesting an enhancement by the associated slope winds. Observed and simulated conditions also reflected distinct gravity‐current characteristics of the intrinsic moist marine air mass during both phases of the LSBC. Specifically, the advance and retreat of marine air mass was directly tied to the development of internal boundary layers onshore and offshore throughout the period of study. Convergence in the lateral moisture flux resulting from this air mass ascending the coastal topography (sea‐breeze phase) as well as colliding with air masses from the opposing coastline (land‐breeze phase) further resulted in cumulous cloud formation and precipitation.

A convection-permitting model for the Lake Victoria Basin: evaluation and insight into the mesoscale versus synoptic atmospheric dynamics

Aiming for an improved understanding of the different factors that determine the regional climate of the Lake Victoria Basin, the COSMO-CLM regional climate model is set up in a tropical, convection-permitting configuration and is directly nested in a recent reanalysis product (ERA5). The convection-permitting simulation outperforms state-of-the-art climate integrations that rely on convection parametrisations. Yet the domain-averaged model precipitation is larger than in the multi-observational ensemble, but the latter shows large spread. Overestimations of outgoing TOA shortwave and longwave radiation are much reduced compared to the COSMO-CLM CORDEX-Africa integration, but still suggest a general underestimation of the cloud fraction or frequency. Comparing the control with a no-lake simulation, the presence of Lake Victoria implies strong intensification of over-lake rainfall, but it only slightly increases the total domain-averaged precipitation. In addition, the easterly trade winds are shown to largely affect the mesoscale circulation and precipitation patterns. During daytime, fast trades and anabatic slope winds trigger convection at the lee-wind slopes, and subsidence over the basin. Slow trades allow the stationary air to produce spontaneous convective cells and to develop anabatic winds that result in orographic precipitation. During night-time, trade winds curl around the eastern branch of the East African Rift, generating a southerly and northerly evening convergence front entering the Lake Victoria Basin. Thus, our results highlight the key importance of the easterly trade winds and the complex orography in determining the total accumulation and location of precipitation in the Lake Victoria region.

Mixing layer height and slope wind oscillation: Factors that control ambient air SO2 in a tropical mountain city

Distinct times of sulfur dioxide concentration (SO2) were observed in the main square of a tropical Andean city, revealing strong meteorological control of SO2. Concentrations were measured by UV fluorescence at 5-minute intervals, and compared to local meteorological forcing. A consistent morning peak of SO2 (4.32 μg∙m⁻³ at 7:40 AM) was concurrent with peak traffic, and occurred 10 minutes into an 80-minute period of calm (wind velocities ˂ 0.3 m s⁻1). Instead of accumulating, SO2 decreased 54%, due an expanding mixing layer. Peak evening SO2 (5.2 μg∙m⁻³ at 8:15 PM) was observed 2:15 hours after flow reversal, and consistent with travel-times of an air parcel descending from industrial facilities. The consistent anthropogenic emissions formed a pattern of daily SO2 concentration that aided in identifying more random volcanic SO2. Manizales (pop. 400000) is located on the western slope of the Cordillera Central in proximity to the Nevado del Ruiz volcano. The complex trajectory necessary for volcanic SO2 to arrive in the main square was simulated for the highest observed concentration (114 μg•m⁻³), using Hysplit and Aura-OMI information. This type of analysis can be applied to urban planning - from locating industrial zones to managing transportation - particularly in tropical montane cities.

Modeling of Aeration of Buildings and Facilities Erected in a Mountain Valley

The article considers an acute ecological and social situation of mountainous regions determining the complex study of “Mountain-city-man”. When analyzing urban planning on a complex terrain, it becomes clear that there is a certain gap in a field of horticultural research in special extreme hot-calm climatic conditions. Orographic features of the relief cause the presence of a local winds system, from which the most pronounced are slopes and mountain-valleys. The compiled mathematical model of slope winds allows predicting aerodynamic characteristics of natural ventilation of buildings and facilities erected in the mountain-hollow space. The proposed calculation technique can be widely used in construction practice and allow to determine the aerodynamic characteristics of the building, aerodynamic coefficients, wind loads, air exchange between the room and the external environment, heat input through the infiltration of air through the enclosing structures.

Gale surface wind characterization based on the Mars Science Laboratory REMS dataset. Part II: Wind probability distributions

The characterization of Martian surface wind speed as a function of time of day and season at one location can increase our knowledge of Mars surface conditions and assist in planning for future unmanned and manned missions, since the probability of the wind speed exceeding a given value is often required for both engineering and geophysical applications. Wind speeds are also useful for assessing the aeolian impact of the circulation. The Rover Environmental Monitoring Station (REMS) instrument on the Mars Science Laboratory (MSL) rover has been measuring Martian winds since 2012, thus has provided more than two Mars years of wind data in the first Martian landing site with significant topography. Unfortunately, dust debris during the MSL landing stage damaged the sensor, making difficult to extract useful wind data. This paper complements the characterization performed in the companion paper, by producing a wind speed characterization based on probability distribution models. Significant diurnal and seasonal wind speed variability is found, due to complex interactions between the synoptic flow, the regional and local slope winds and microscale flow around MSL. The highest wind speed probabilities are found in general during the midday period, particularly around the equinoxes. In addition, the REMS data suggest strong flows during the summer nighttime, which could be related to the increased aeolian activity detected then by MSL.

Meteorological pressure at Gale crater from a comparison of REMS/MSL data and MCD modelling: Effect of dust storms

We examine the record of atmospheric pressure in Gale crater measured in-situ by the Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) on the Mars Science Laboratory (MSL) rover over two Martian years. We compare the data with pressure predictions from the Mars Climate Database (MCD) (Forget et al., 1999; Millour et al., 2015) version 5.2, which is a climatological database derived from numerical simulations of the Martian atmosphere produced by a General Circulation Model run over several Martian years. Seasonal and daily trends in pressure data from REMS are well reproduced by the standard climatology of the MCD using its high resolution mode. This high-resolution mode extrapolates pressure values from the nominal model into the altitude of each location using a high-resolution topography model and a fine tuning of the vertical scale height that was chosen to mimic effects of slope winds not directly accounted for in the General Circulation Model on which the MCD is based. Differences between the synthetic MCD pressure data and the REMS measurements are produced by meteorological features that are identified on particular groups of sols and quantified in intensity. We show that regional dust storms outside Gale crater and dust abundance at the crater are important factors in the behaviour of the pressure exciting larger amplitudes on the daily pressure variations and causing most of the largest REMS-MCD differences. We compare the pressure signals with published data of the dust optical depth obtained by the REMS ultraviolet photodiodes and the Mastcam instrument on MSL, and with orbital images of the planet acquired by the MARCI instrument on the Mars Reconnaissance Orbiter (MRO). We show that in some cases regional dust storms induce a characteristic signature in the surface pressure measured by REMS several sols before the dust arrives to Gale crater. We explore the capability of daily pressure measurements to serve as a fast detector of the development of dust storms in the context of the MSL, Insight and Mars 2020 missions.

Planetary boundary layer and slope winds on Venus

Few constraints are available to characterize the deep atmosphere of Venus, though this region is crucial to understand the interactions between surface and atmosphere on Venus. Based on simulations performed with the IPSL Venus Global Climate Model, the possible structure and characteristics of Venus’ planetary boundary layer (PBL) are investigated. The vertical profile of the potential temperature in the deepest 10 km above the surface and its diurnal variations are controlled by radiative and dynamical processes. The model predicts a diurnal cycle for the PBL activity, with a stable nocturnal PBL while convective activity develops during daytime. The diurnal convective PBL is strongly correlated with surface solar flux and is maximum around noon and in low latitude regions. It typically reaches less than 2 km above the surface, but its vertical extension is much higher over high elevations, and more precisely over the western flanks of elevated terrains. This correlation is explained by the impact of surface winds, which undergo a diurnal cycle with downward katabatic winds at night and upward anabatic winds during the day along the slopes of high-elevation terrains. The convergence of these daytime anabatic winds induces upward vertical winds, that are responsible for the correlation between height of the convective boundary layer and topography.

Can a Simple Dynamical System Describe the Interplay between Drag and Buoyancy in Terrain-Induced Canopy Flows?

Abstract Under nonneutral stratification and in the presence of topography, the dynamics of turbulent flow within a canopy is not yet completely understood. This has, among other consequences, serious implications for the measurement of surface–atmosphere exchange by means of eddy covariance: for example, the measurement of carbon dioxide fluxes is strongly influenced if drainage flows occur during night, when the flow within the canopy decouples from the flow aloft. An improved physical understanding of the behavior of scalars under canopy turbulence in complex terrain is urgently needed. In the present work, the authors investigate the dynamics of turbulent flow within sloped canopies, focusing on the slope wind and potential temperature. The authors concentrate on the presence of oscillatory behavior in the flow variables in terms of switching of flow regimes by conducting linear stability analysis. The authors revisit and correct the simplified theory that exists in the literature, which is based on the interplay between the drag force and the buoyancy. The authors find that the simplified description of this dynamical system cannot exhibit the observed richness of the dynamics. To augment the simplified dynamical system’s analysis, the authors make use of large-eddy simulation of a three-dimensional hill covered by a homogeneous forest and analyze the phase synchronization behavior of the buoyancy and drag forces in the momentum budget to explore the turbulent dynamics in more detail.

Crater Mound Formation by Wind Erosion on Mars

AbstractMost of Mars' ancient sedimentary rocks by volume are in wind‐eroded sedimentary mounds within impact craters and canyons, but the connections between mound form and wind erosion are unclear. We perform mesoscale simulations of different crater and mound morphologies to understand the formation of sedimentary mounds. As crater depth increases, slope winds produce increased erosion near the base of the crater wall, forming mounds. Peak erosion rates occur when the crater depth is ∼2 km. Mound evolution depends on the size of the host crater. In smaller craters mounds preferentially erode at the top, becoming more squat, while in larger craters mounds become steeper sided. This agrees with observations where smaller craters tend to have proportionally shorter mounds and larger craters have mounds encircled by moats. If a large‐scale sedimentary layer blankets a crater, then as the layer recedes across the crater it will erode more toward the edges of the crater, resulting in a crescent‐shaped moat. When a 160 km diameter mound‐hosting crater is subject to a prevailing wind, the surface wind stress is stronger on the leeward side than on the windward side. This results in the center of the mound appearing to “march upwind” over time and forming a “bat‐wing” shape, as is observed for Mount Sharp in Gale crater.

New column simulations for the Viking landers: Winds, fog, frost, adsorption?

Boundary layer simulations are shown for the first sols of the two Viking landers (VL1, VL2) on Mars. The column model (with cloud/radiation interaction and Prandtl slope wind terms), used successfully for Phoenix and Curiosity, is equipped here with an adsorption-diffusion scheme for water vapor transport in porous regolith. The model's 1.6 m temperatures and winds are quite close to those observed by the two landers; in particular the weak summer slope winds of VL2 are excellently reproduced.The model predicts for both sites diffusion and adsorption of water into regolith in the evening, very thin ground frost deposition from about midnight, and an early morning fog with an inflection in T1.6 m slightly weaker than observed. At the moister VL2 site fog increases optical depth as observed. Fogs and frosts sublimate away after sunrise, allowing desorption and diffusion of water off the sun-heated regolith. For porosity of 22% column water is approximately conserved from sol to sol at both sites with only little diurnal variation, as the depleted layer of air moisture is quite shallow.In simulations without adsorption frost forms early and it grows thick. At VL2 fog now forms earlier and the jump in optical depth is larger than observed. At the drier VL1 fog still forms nearly as with adsorption, so observations could also be explained without adsorption. On the other hand VL1 certainly landed onto porous regolith and frost was not observed. Hence it is suggested that adsorption is likely at both VL sites in summer.

A New Wind Speed Retrieval Method for an Ocean Surface Using the Waveform Width of a Laser Altimeter

ABSTRACTThe current retrieval method of sea wind speeds using a space-borne laser device is based on the received energies, which are significantly influenced by the absorption and scatter effect in the atmosphere. A new method is derived to retrieve the wind speed by the path of waveform width—root mean square (RMS) surface height—wind speed, which differs from the classical path of received energy—mean sea slope—wind speed. First, by deriving a more rigorous expression of the waveform model of an ocean surface for a laser altimeter, the theoretical relationship between the surface height RMS and waveform width is established. Second, the surface height RMS is fitted to a new empirical formula that is related to the wind speed using the geoscience laser altimeter system (GLAS) waveform data and the National Centers for Environmental Prediction wind speeds. Then, the JONSWAP spectra with swell effect and the waveform simulator are used to generate sea surface profiles and echo waveforms to verify the new retrieval method. Finally, in a Pacific region by using GLAS waveform data, the retrieval results indicate that the bias was approximately 0.2 m/s, the RMSE was approximately 1.2 m/s, and the ratio of valid data exceeded 20%.

Spectral gap characteristics in a daytime valley boundary layer

A correct estimation of turbulent variances and covariances in the atmospheric boundary layer relies on the determination of turbulent perturbations of wind speed components and scalar quantities, which requires the presence of a so‐called spectral gap. The goal of this work is to determine the range of gap scales necessary to define turbulent perturbations in a daytime valley boundary layer. To accomplish this, we analyze data from a large number of propeller‐vane and sonic anemometers using the fast Fourier transformation and the multiresolution flux decomposition. Daytime gap scales are found to range from 17 to 29 min and show large spatial variability across the valley floor and the adjacent slopes. Synoptically driven conditions that favour the occurrence of mesoscale phenomena, such as rotors and mountain waves, shift daytime gap scales toward longer periods. The low‐frequency end of the gap is also affected by the presence of slope winds that are characterized by a periodicity ranging from 80 to 200 min. Finally, we present a conceptual model of the daytime valley spectral gap which summarizes the findings of this study.

Numerical study of the interplay between thermo-topographic slope flow and synoptic flow on canopy transport processes

Canopy flow resulting from interaction between thermo-topographic slope flow and large-scale synoptic flow is very complicated and has been poorly understood. We apply a Reynolds-averaged Navier-Stokes (RANS) turbulence model to investigate how the interactions between local flow and synoptic winds affect CO2 movement in the canopy layer at the Renon site in the Italian Alps. Since the RANS simulations are compared to the data measured by multiple-tower experiments conducted during CarboEurope-IP advection campaigns (ADVEX) at Renon, our study can be viewed as a case study of a relatively common wooded slope. The thermal condition in the canopy is directly related to the canopy morphology: the dense canopy at our site causes stronger cooling but limits vertical exchange of heat flux, resulting in weak temperature inversion in the deep canopy. Under conditions with no synoptic wind, local flow leads to CO2 build-up mainly at downslope locations and no recirculation is formed. Recirculation that holds high CO2 mole fraction in the canopy is developed only under the condition that local slope wind is enhanced by northerly synoptic winds. No recirculation forms when southerly synoptic wind direction is opposite to the local wind direction, in which case CO2 is quite well mixed. This numerical study approach brings to light a better understanding of the CO2 closure problem: the measured net ecosystem exchange of CO2 is more likely to be underestimated in local non-synoptic slope flow and local synoptic-enhanced slope flow regimes at Renon. However, small-scale heterogeneity in canopy structure, variability in the CO2 source from soil and higher-resolution and larger-scale topography still challenge the application of this numerical approach in the FLUXNET community.