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 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 Surface and Atmospheric Controls on the Onset of Moist Convection over Land

2013 ◽  
Vol 14 (5) ◽  
pp. 1443-1462 ◽  
Author(s):  
Pierre Gentine ◽  
Albert A. M. Holtslag ◽  
Fabio D'Andrea ◽  
Michael Ek
Keyword(s):  
Boundary Layer ◽  
Relative Humidity ◽  
Water Vapor Pressure ◽  
Bowen Ratio ◽  
Specific Humidity ◽  
Tropospheric Temperature ◽  
Free Troposphere ◽  
Moist Convection ◽  
Conceptual Approach ◽  
Evaporative Fraction

Abstract The onset of moist convection over land is investigated using a conceptual approach with a slab boundary layer model. The authors determine the essential factors for the onset of boundary layer clouds over land and study their relative importance. They are 1) the ratio of the temperature to the moisture lapse rates of the free troposphere, that is, the inversion Bowen ratio; 2) the mean daily surface temperature; 3) the relative humidity of the free troposphere; and 4) the surface evaporative fraction. A clear transition is observed between two regimes of moistening of the boundary layer as assessed by the relative humidity at the boundary layer top. In the first so-called wet soil advantage regime, the moistening results from the increase of the mixed-layer specific humidity, which linearly depends on the surface evaporative fraction and inversion Bowen ratio through a dynamic boundary layer factor. In the second so-called dry soil advantage regime, the relative humidity tendency at the boundary layer top is controlled by the thermodynamics and changes in the moist adiabatic induced by the decreased temperature at the boundary layer top and consequent reduction in saturation water vapor pressure. This regime pertains to very deep boundary layers under weakly stratified free troposphere over hot surface conditions. In the context of the conceptual model, a rise in free-tropospheric temperature (global warming) increases the occurrence of deep convection and reduces the cloud cover over moist surfaces. This study provides new intuition and predictive capacity on the mechanism controlling the occurrence of moist convection over land.

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 Disentangling different moisture transport pathways over the eastern subtropical North Atlantic using multi-platform isotope observations and high-resolution numerical modelling

2021 ◽  
Vol 21 (21) ◽  
pp. 16319-16347
Author(s):  
Fabienne Dahinden ◽  
Franziska Aemisegger ◽  
Heini Wernli ◽  
Matthias Schneider ◽  
Christopher J. Diekmann ◽  
...  
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Isotopic Composition ◽  
Canary Islands ◽  
North Atlantic ◽  
Moisture Transport ◽  
Specific Humidity ◽  
Free Troposphere ◽  
Physical Processes ◽  
Transport Pathways

Abstract. Due to its dryness, the subtropical free troposphere plays a critical role in the radiative balance of the Earth's climate system. But the complex interactions of the dynamical and physical processes controlling the variability in the moisture budget of this sensitive region of the subtropical atmosphere are still not fully understood. Stable water isotopes can provide important information about several of the latter processes, namely subsidence drying, turbulent mixing, and dry and moist convective moistening. In this study, we use high-resolution simulations of the isotope-enabled version of the regional weather and climate prediction model of the Consortium for Small-Scale Modelling (COSMOiso) to investigate predominant moisture transport pathways in the Canary Islands region in the eastern subtropical North Atlantic. Comparison of the simulated isotope signals with multi-platform isotope observations (aircraft, ground- and space-based remote sensing) from a field campaign in summer 2013 shows that COSMOiso can reproduce the observed variability of stable water vapour isotopes on timescales of hours to days, thus allowing us to study the mechanisms that control the subtropical free-tropospheric humidity. Changes in isotopic signals along backward trajectories from the Canary Islands region reveal the physical processes behind the synoptic-scale isotope variability. We identify four predominant moisture transport pathways of mid-tropospheric air, each with distinct isotopic signatures: air parcels originating from the convective boundary layer of the Saharan heat low (SHL) – these are characterised by a homogeneous isotopic composition with a particularly high δD (median mid-tropospheric δD=-122‰), which results from dry convective mixing of low-level moisture of diverse origin advected into the SHL; air parcels originating from the free troposphere above the SHL – although experiencing the largest changes in humidity and δD during their subsidence over West Africa, these air parcels typically have lower δD values (median δD=-148‰) than air parcels originating from the boundary layer of the SHL; air parcels originating from outside the SHL region, typically descending from tropical upper levels south of the SHL, which are often affected by moist convective injections from mesoscale convective systems in the Sahel – their isotopic composition is much less enriched in heavy isotopes (median δD=-175‰) than those from the SHL region; air parcels subsiding from the upper-level extratropical North Atlantic – this pathway leads to the driest and most depleted conditions (median δD=-255‰) in the middle troposphere near the Canary Islands. The alternation of these transport pathways explains the observed high variability in humidity and δD on synoptic timescales to a large degree. We further show that the four different transport pathways are related to specific large-scale flow conditions. In particular, distinct differences in the location of the North African mid-level anticyclone and of extratropical Rossby wave patterns occur between the four transport pathways. Overall, this study demonstrates that the adopted Lagrangian isotope perspective enhances our understanding of air mass transport and mixing and offers a sound interpretation of the free-tropospheric variability of specific humidity and isotope composition on timescales of hours to days in contrasting atmospheric conditions over the eastern subtropical North Atlantic.

Airborne water vapor isotope measurements over the Iceland Sea in winter conditions

2021 ◽  
Author(s):  
Alexandra Touzeau ◽  
Hans-Christian Steen-Larsen ◽  
Ian Renfrew ◽  
Þorsteinn Jónsson ◽  
Andrew Elvidge ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Stable Isotope ◽  
Sea Ice ◽  
Water Vapour ◽  
Cloud Cover ◽  
Isotope Composition ◽  
Specific Humidity ◽  
Free Troposphere ◽  
Cold Air ◽  
Isotope Measurements

<p>Improved understanding of evaporation and condensation processes is critical to improve the representation of the water cycle in atmospheric models. Thereby, in-situ measurements along the entire moisture transport pathway, covering evaporation, mixing between different air masses in the atmospheric boundary layer and the free troposphere, and resulting precipitation are highly valuable to obtain new insight. In particular, coherent measurements of the stable isotope composition in atmospheric vapour can provide additional constraints on phase change processes of water vapour from source to sink, enabling direct comparison within isotope-enabled models.</p><p>Here we present stable isotope measurements from the Iceland Greenland Seas Project field campaign that took place in February-March 2018. This unique dataset includes simultaneous measurements from a land-station in Husavik, Iceland, a ship and an air plane in the subpolar region. Alternation between cold-air outbreaks and mid-latitude airmasses characterized the measurement period. Here we focus on the stable water isotope composition in water vapour obtained from 10 research flights, covering a large geographic range (64 °N to 72 °N). Careful data treatment was applied to ensure the quality of isotope measurements in the predominant cold, dry conditions with large gradients in isotope composition and humidity.</p><p>From an intercomparison flight over the Husavik station, we find good agreement between ground and airborne measurements. Out of 7 flights dedicated to the study of atmosphere-ocean-ice interactions, with both low-levels legs and vertical sections in predominant Cold Air Outbreak (CAO) conditions, we focus on the marginal ice zone and regions covered by shallow cumulus clouds. For open water flights, we find the horizontal and vertical distribution of δ<sup>18</sup>O in the marine boundary layer to covary with cloud cover. Thereby, downdrafts bring dry and <sup>18</sup>O-depleted air from the free troposphere towards the surface, corresponding to openings in cloud cover. For flights passing over sea ice edge, both δ<sup>18</sup>O and specific humidity show a clear east-west gradient, with increasing values towards the open sea reflecting ocean moisture availability. Additionally, open leads in the sea ice also have a visible impact on isotope values. Lastly, relatively low d-excess values are observed over the sea-ice, which could either be caused by local processes or advection.</p>

scholarly journals The Impact of Warm and Moist Airmass Perturbations on Arctic Mixed-Phase Stratocumulus

2020 ◽  
Vol 33 (22) ◽  
pp. 9615-9628
Author(s):  
Gesa K. Eirund ◽  
Anna Possner ◽  
Ulrike Lohmann
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Lower Troposphere ◽  
Temperature Inversion ◽  
Open Ocean ◽  
Mixed Phase ◽  
Specific Humidity ◽  
The Arctic ◽  
Free Troposphere ◽  
The Impact

AbstractThe Arctic is known to be particularly sensitive to climate change. This Arctic amplification has partially been attributed to poleward atmospheric heat transport in the form of airmass intrusions. Locally, such airmass intrusions can introduce moisture and temperature perturbations. The effect of airmass perturbations on boundary layer and cloud changes and their impact on the surface radiative balance has received increased attention, especially over sea ice with regard to sea ice melt. Utilizing cloud-resolving model simulations, this study addresses the impact of airmass perturbations occurring at different altitudes on stratocumulus clouds for open-ocean conditions. It is shown that warm and moist airmass perturbations substantially affect the boundary layer and cloud properties, even for the relatively moist environmental conditions over the open ocean. The cloud response is driven by temperature inversion adjustments and strongly depends on the perturbation height. Boundary layer perturbations weaken and raise the inversion, which destabilizes the lower troposphere and involves a transition from stratocumulus to cumulus clouds. In contrast, perturbations occurring in the lower free troposphere lead to a lowering but strengthening of the temperature inversion, with no impact on cloud fraction. In simulations where free-tropospheric specific humidity is further increased, multilayer mixed-phase clouds form. Regarding energy balance changes, substantial surface longwave cooling arises out of the stratocumulus break-up simulated for boundary layer perturbations. Meanwhile, the net surface longwave warming increases resulting from thicker clouds for airmass perturbations occurring in the lower free troposphere.

scholarly journals Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx

2010 ◽  
Vol 10 (21) ◽  
pp. 10639-10654 ◽  
Author(s):  
C. S. Bretherton ◽  
R. Wood ◽  
R. C. George ◽  
D. Leon ◽  
G. Allen ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Liquid Water ◽  
Vertical Structure ◽  
Layer Structure ◽  
Cloud Droplet ◽  
Free Troposphere ◽  
Liquid Water Path ◽  
Typical Structure ◽  
Southeast Pacific ◽  
Over The Top

Abstract. Multiplatform airborne, ship-based, and land-based observations from 16 October–15 November 2008 during the VOCALS Regional Experiment (REx) are used to document the typical structure of the Southeast Pacific stratocumulus-topped boundary layer and lower free troposphere on a~transect along 20° S between the coast of Northern Chile and a buoy 1500 km offshore. Strong systematic gradients in clouds, precipitation and vertical structure are modulated by synoptically and diurnally-driven variability. The boundary layer is generally capped by a strong (10–12 K), sharp inversion. In the coastal zone, the boundary layer is typically 1 km deep, fairly well mixed, and topped by thin, nondrizzling stratocumulus with accumulation-mode aerosol and cloud droplet concentrations exceeding 200 cm−3. Far offshore, the boundary layer depth is typically deeper (1600 m) and more variable, and the vertical structure is usually decoupled. The offshore stratocumulus typically have strong mesoscale organization, much higher peak liquid water paths, extensive drizzle, and cloud droplet concentrations below 100 cm−3, sometimes with embedded pockets of open cells with lower droplet concentrations. The lack of drizzle near the coast is not just a microphysical response to high droplet concentrations; smaller cloud depth and liquid water path than further offshore appear comparably important. Moist boundary layer air is heated and mixed up along the Andean slopes, then advected out over the top of the boundary layer above adjacent coastal ocean regions. Well offshore, the lower free troposphere is typically much drier. This promotes strong cloud-top radiative cooling and stronger turbulence in the clouds offshore. In conjunction with a slightly cooler free troposphere, this may promote stronger entrainment that maintains the deeper boundary layer seen offshore. Winds from ECMWF and NCEP operational analyses have an rms difference of only 1 m s−1 from collocated airborne leg-mean observations in the boundary layer and 2 m s−1 above the boundary layer. This supports the use of trajectory analysis for interpreting REx observations. Two-day back-trajectories from the 20° S transect suggest that eastward of 75° W, boundary layer (and often free-tropospheric) air has usually been exposed to South American coastal aerosol sources, while at 85° W, neither boundary-layer or free-tropospheric air has typically had such contact.

Wind Stress Over Waves: Effects of Sea Roughness and Atmospheric Stability

2002 ◽  
Vol 124 (3) ◽  
pp. 169-172 ◽  
Author(s):  
Dag Myrhaug ◽  
Olav H. Slaattelid
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
Local Equilibrium ◽  
Surface Wind ◽  
Atmospheric Stability ◽  
Sea Surface ◽  
Stability Function ◽  
Sea Surface Wind ◽  
Surface Wind Stress ◽  
Sea Surface Roughness

The paper considers the effects of sea roughness and atmospheric stability on the sea surface wind stress over waves, which are in local equilibrium with the wind, by using the logarithmic boundary layer profile including a stability function, as well as adopting some commonly used sea surface roughness formulations. The engineering relevance of the results is also discussed.

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