scholarly journals Current Challenges in Understanding and Predicting Transport and Exchange in the Atmosphere over Mountainous Terrain

Atmosphere ◽  
2018 ◽  
Vol 9 (7) ◽  
pp. 276 ◽  
Author(s):  
Manuela Lehner ◽  
Mathias Rotach
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Transport Processes ◽  
Mountainous Terrain ◽  
Moist Convection ◽  
Free Atmosphere ◽  
Spatial And Temporal Scales ◽  
Exchange Processes ◽  
Thermally Driven ◽  
Definition Of

Coupling of the earth’s surface with the atmosphere is achieved through an exchange of momentum, energy, and mass in the atmospheric boundary layer. In mountainous terrain, this exchange results from a combination of multiple transport processes, which act and interact on different spatial and temporal scales, including, for example, orographic gravity waves, thermally driven circulations, moist convection, and turbulent motions. Incorporating these exchange processes and previous studies, a new definition of the atmospheric boundary layer in mountainous terrain, a mountain boundary layer (MBL), is defined. This paper summarizes some of the major current challenges in measuring, understanding, and eventually parameterizing the relevant transport processes and the overall exchange between the MBL and the free atmosphere. Further details on many aspects of the exchange in the MBL are discussed in several other papers in this issue.

scholarly journals Influence of along-valley terrain heterogeneity on exchange processes over idealized valleys

2015 ◽  
Vol 15 (12) ◽  
pp. 6589-6603 ◽  
Author(s):  
J. S. Wagner ◽  
A. Gohm ◽  
M. W. Rotach
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Trajectory Analysis ◽  
Layer Structure ◽  
Volume Effect ◽  
Transport Processes ◽  
Valley Floor ◽  
Mass Fluxes ◽  
Exchange Processes ◽  
Thermally Driven

Abstract. Idealized numerical simulations of thermally driven flows over various valley–plain topographies are performed under daytime conditions. Valley floor inclination and narrowing valley cross sections are systematically varied to study the influence of along-valley terrain heterogeneity on the developing boundary layer structure, as well as horizontal and vertical transport processes. Valley topographies with inclined valley floors of 0.86° increase upvalley winds by a factor of about 1.9 due to smaller valley volumes (volume effect) and by a factor of about 1.6 due to additional upslope buoyancy forces. Narrowing the valley cross section by 20 km per 100 km along-valley distance increases upvalley winds by a factor of about 2.6. Vertical mass fluxes out of the valley are strongly increased by a factor between 1.8 and 2.8 by narrowing the valley cross sections and by a factor of 1.2 by inclining the valley floor. Trajectory analysis shows intensified horizontal transport of parcels from the foreland into the valley within the boundary layer in cases with inclined floors and narrowing cross sections due to increased upvalley winds.

scholarly journals Influence of along-valley terrain heterogeneity on exchange processes over idealized valleys

2015 ◽  
Vol 15 (1) ◽  
pp. 415-451 ◽  
Author(s):  
J. S. Wagner ◽  
A. Gohm ◽  
M. W. Rotach
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Trajectory Analysis ◽  
Layer Structure ◽  
Volume Effect ◽  
Transport Processes ◽  
Valley Floor ◽  
Mass Fluxes ◽  
Exchange Processes ◽  
Thermally Driven

Abstract. Idealized numerical simulations of thermally driven flows over various valley-plain topographies are performed under daytime conditions. Valley floor inclination and narrowing valley cross sections are systematically varied to study the influence of along-valley terrain heterogeneity on the developing boundary layer structure, as well as horizontal and vertical transport processes. Valley topographies with inclined valley floors of 0.86° increase upvalley winds by about 100% due to smaller valley volumes (volume effect) and by about 62% due to additional upslope buoyancy forces. Narrowing the valley cross section by 20 km per 100 km along-valley distance increases upvalley winds by about 75%. Vertical mass fluxes out of the valley are strongly increased by about 57 to 84% by narrowing the valley cross sections and by 22 to 32% by reducing the valley volume (e.g., by inclining the valley floor). Trajectory analysis shows intensified horizontal transport of parcels from the foreland into the valley within the boundary layer in cases with inclined floors and narrowing cross sections due to increased upvalley winds.

scholarly journals The topography contribution to the influence of the atmospheric boundary layer at high altitude stations

2017 ◽  
Author(s):  
Martine Collaud Coen ◽  
Elisabeth Andrews ◽  
Diego Aliaga ◽  
Marcos Andrade ◽  
Hristo Angelov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
High Altitude ◽  
Geometric Mean ◽  
Convective Transport ◽  
Transport Processes ◽  
The Tibetan Plateau ◽  
Diurnal Cycles ◽  
Air Convection ◽  
Aerosol Parameters

Abstract. High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude is not a sufficient parameter to characterize the ABL influence. Topography data from the global digital elevation model GTopo30 were used to calculate 5 parameters for 46 high altitude stations situated in five continents. The geometric mean of these 5 parameters determines a topography based index called ABL-TopoIndex which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or plateaus, if the slopes around the station are steep, and finally if the drainage basin for air convection is small. All stations on volcanic islands exhibit a low ABL-TopoIndex whereas stations in the Himalaya and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (s.s.) correlations are found between the 5 and 50 percentiles of all aerosol parameters and the ABL-TopoIndex whereas no s.s. correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a s.s. correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex. Finally, the main flow paths for air convection were calculated for various ABL heights.

scholarly journals Revisiting the Definition of the Drag Coefficient in the Marine Atmospheric Boundary Layer

2010 ◽  
Vol 40 (10) ◽  
pp. 2325-2332 ◽  
Author(s):  
Richard J. Foreman ◽  
Stefan Emeis
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Atmospheric Boundary Layer ◽  
Drag Coefficient ◽  
Friction Velocity ◽  
Marine Boundary Layer ◽  
Marine Atmospheric Boundary Layer ◽  
Wind Speeds ◽  
Traditional Definition ◽  
Definition Of

Abstract A new functional form of the neutral drag coefficient for moderate to high wind speeds in the marine atmospheric boundary layer for a range of field measurements as reported in the literature is proposed. This new form is found to describe a wide variety of measurements recorded in the open ocean, coast, fetch-limited seas, and lakes, with almost one and the same set of parameters. This is the result of a reanalysis of the definition of the drag coefficient in the marine boundary layer, which finds that a constant is missing from the traditional definition of the drag coefficient. The constant arises because the neutral friction velocity over water surfaces is not directly proportional to the 10-m wind speed, a consequence of the transition to rough flow at low wind speeds. Within the rough flow regime, the neutral friction velocity is linearly dependent on the 10-m wind speed; consequently, within this rough regime, the new definition of the drag coefficient is not a function of the wind speed. The magnitude of the new definition of the neutral drag coefficient represents an upper limit to the magnitude of the traditional definition.

scholarly journals A theory for surface-layer scaling of thermally-driven slope flows

2021 ◽  
Author(s):  
Dino Zardi
Keyword(s):  
Boundary Layer ◽  
Surface Layer ◽  
Sensible Heat Flux ◽  
Viscous Sublayer ◽  
Transport Processes ◽  
Heat Fluxes ◽  
Night Time ◽  
Slope Winds ◽  
Thermally Driven ◽  
Momentum And Heat Fluxes

<p>Sloping terrains of any inclination favour the development, under the daily cycle of day time surface heating and night time cooling, of thermally-driven organised flows, displaying peculiar boundary layer structures, and eventually triggering the development of atmospheric convection.</p><p>The ubiquitous occurrence over the Earth of variously tilted surfaces - from gently sloping plains to steep cliffs, or valley and basin sidewalls – makes the understanding of such flows of utmost importance in view of the appropriate forecasting of the associated boundary layer transport processes. Also, they display a highly conceptual relevance, as they represent a prototypal situations for many other thermally driven-flows over complex terrain.   </p><p>An appropriate surface-layer scaling for slope wind is derived extending the classical analysis for flat horizontal terrain situations to the cover inclines. In the former, momentum and heat fluxes at the surface are two independent quantities, and vertical profiles of velocity and temperature can only be connected to them by means  of similiarity relationships, as fluxes are nearly invariant with height.</p><p>Instead, equations governing slope winds show that the mean wind and temperature profiles are closely connected to the flux structure normal to the slope, as this is not constant. Also, surface values of momentum flux and sensible heat flux are shown to be proportional to each other.</p><p>Based on the above relationships, suitable expressions are derived for the slope-normal profiles of velocity and temperature, both in the viscous sublayer and in the fully turbulent surface layer, as well as for the appropriate scaling factors in the two regions.</p>

scholarly journals Antarctic atmospheric boundary layer observations with the Small Unmanned Meteorological Observer (SUMO)

2021 ◽  
Vol 13 (3) ◽  
pp. 969-982
Author(s):  
John J. Cassano ◽  
Melissa A. Nigro ◽  
Mark W. Seefeldt ◽  
Marwan Katurji ◽  
Kelly Guinn ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Ross Sea ◽  
Ground Level ◽  
The United States ◽  
Free Atmosphere ◽  
Stable Conditions ◽  
Antarctic Continent ◽  
Entire Depth ◽  
The Antarctic

Abstract. Between January 2012 and June 2017 a small unmanned aerial system (sUAS), known as the Small Unmanned Meteorological Observer (SUMO), was used to observe the state of the atmospheric boundary layer in the Antarctic. During six Antarctic field campaigns, 116 SUMO flights were completed. These flights took place during all seasons over both permanent ice and ice-free locations on the Antarctic continent and over sea ice in the western Ross Sea. Sampling was completed during spiral ascent and descent flight paths that observed the temperature, humidity, pressure and wind up to 1000 m above ground level and sampled the entire depth of the atmospheric boundary layer, as well as portions of the free atmosphere above the boundary layer. A wide variety of boundary layer states were observed, including very shallow, strongly stable conditions during the Antarctic winter and deep, convective conditions over ice-free locations in the summer. The Antarctic atmospheric boundary layer data collected by the SUMO sUAS, described in this paper, can be retrieved from the United States Antarctic Program Data Center (https://www.usap-dc.org, last access: 8 March 2021). The data for all flights conducted on the continent are available at https://doi.org/10.15784/601054 (Cassano, 2017), and data from the Ross Sea flights are available at https://doi.org/10.15784/601191 (Cassano, 2019).

scholarly journals On Offshore Propagating Diurnal Waves

2012 ◽  
Vol 69 (5) ◽  
pp. 1562-1581 ◽  
Author(s):  
Qingfang Jiang
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Linear Theory ◽  
Wave Amplitude ◽  
Trapped Waves ◽  
Stable Layer ◽  
Free Atmosphere ◽  
Heating Source ◽  
Differential Heating ◽  
Heating Profiles

Abstract Characteristics and dynamics of offshore diurnal waves induced by land–sea differential heating are examined using linear theory. Two types of heating profiles are investigated, namely a shallow heating source confined within an atmospheric boundary layer (BL) and a deep heating source located above the boundary layer. It is demonstrated that a boundary layer top inversion or a more stable layer aloft tends to partially trap diurnal waves in the BL and consequently extend perturbations well offshore. The wave amplitude decays with offshore distance due to BL friction and leakage of energy into the free atmosphere. The dependence of trapped waves on the inversion height and strength, atmosphere stratification, latitude, BL friction, and background winds is investigated. Diurnal waves generated by a deep heating source extending well above the BL are characterized by longer wavelengths, faster propagation, and substantially longer e-folding decay distances than waves induced by a BL source. For the latter, BL friction has little impact on the e-folding decay distance, as waves are mostly located in the free atmosphere rather than in a frictional BL.

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