Special Issue "Atmospheric Processes over Complex Terrain"

A special issue of Atmosphere (ISSN 2073-4433). This special issue belongs to the section "Climatology and Meteorology".

Deadline for manuscript submissions: closed (31 January 2018)

Special Issue Editors

Guest Editor
Prof. Dr. Mathias Rotach

University of Innsbruck, Institute of Atmospheric and Cryospheric Sciences, Innsbruck, Austria
Website | E-Mail
Interests: boundary layer dynamics; turbulence and exchange processes; atmospheric dynamics; high-resolution numerical modeling; mountain meteorology; air pollution modeling; complex topography; complex surfaces
Guest Editor
Prof. Dr. Dino Zardi

University of Trento, Department of Civil, Environmental and Mechanical Engineering, Trento, Italy
Website | E-Mail
Interests: atmospheric boundary layer processes; turbulence measurements and analysis; earth-atmosphere exchange processes; mountain meteorology; air pollution measurement and modelling

Special Issue Information

Dear Colleagues,

The Earth’s surface has a profound impact on atmospheric flows, i.e., it constitutes their lower boundary condition and determines the occurrence and efficiency of exchange processes of energy, mass and momentum from and to the free atmosphere. While over flat terrain, this exchange is largely determined by the state of the Atmospheric Boundary Layer (ABL), over complex terrain this exchange is additionally modified by meso-scale flows and, in particular, the interaction between the latter and the (complex) ABL. ABLs over complex topography are intrinsically inhomogeneous and thus violate the basic assumptions, on which our current understanding of ABL turbulence and exchange efficiency is based. Out of necessity, numerical models of all spatial resolutions, except for the very highest (i.e., below the so-called grey zone of turbulence) use ABL parameterizations that are based on previous knowledge from flat and horizontally homogeneous terrain. On the other hand, many aspects of the involved meso-scale flows, such as thermally forced valley winds, are quite well known—but their role in the earth-atmosphere exchange and in particular the interaction with turbulent exchange (efficiency) has received much less attention.

The increase in computing power over the last decades has led to the use of ‘high-resolution (km-scale) numerical modeling’ even in operational settings (numerical weather forecast and climate scenarios). Validation and verification efforts, however, even lack the basic knowledge about the processes those models should be able to reproduce. A number of recent field campaigns have started to address these questions; therefore, it seems timely to establish the current state of affairs in this Special Issue on Atmospheric Processes over Complex Terrain.

Papers are welcome on all aspects of exchange processes over complex terrain, including, but not restricted to:

  • Atmospheric boundary layer characteristics and processes over complex terrain

  • Wet and dry processes—and their interaction

  • Convective initiation in complex terrain

  • Gravity waves and their role in atmospheric transport

  • Numerical methods over steep and complex terrain

  • Physics parameterizations

  • Verification strategies in intrinsically inhomogeneous environments

  • Experimental methods and data processing

  • Exchange efficiency over complex topography

  • Theoretical advancements

Prof. Dr. Mathias Rotach
Prof. Dr. Dino Zardi
Guest Editors

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Published Papers (9 papers)

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Research

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Open AccessArticle Challenges and Opportunities for Data Assimilation in Mountainous Environments
Atmosphere 2018, 9(4), 127; doi:10.3390/atmos9040127
Received: 15 February 2018 / Revised: 16 March 2018 / Accepted: 21 March 2018 / Published: 27 March 2018
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Abstract
This contribution aims to summarize the current state of data assimilation research as applied to land and atmosphere simulation and prediction in mountainous environments. It identifies and explains critical challenges, and offers opportunities for productive research based on both models and observations. Though
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This contribution aims to summarize the current state of data assimilation research as applied to land and atmosphere simulation and prediction in mountainous environments. It identifies and explains critical challenges, and offers opportunities for productive research based on both models and observations. Though many of the challenges to optimal data assimilation in the mountains are also challenges in flatter terrain, the complex land–atmosphere interactions and increased surface heterogeneity in the mountains violate key assumptions and methods in the data assimilation algorithms and the underlying models. The effects of model inadequacy are particularly acute in complex terrain. Recent research related to some of the key challenges suggest opportunities to make gains in both land and atmospheric data assimilation in the mountains. Research directions are suggested, focusing on model improvement in a data assimilation context, and design of field programs aimed at data assimilation. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
Atmosphere 2018, 9(4), 118; doi:10.3390/atmos9040118
Received: 25 January 2018 / Revised: 8 March 2018 / Accepted: 16 March 2018 / Published: 21 March 2018
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Abstract
Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial
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Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial distribution and temporal evolution of pollutant concentrations. They can be very complex and difficult to represent in numerical weather prediction models, particularly under stable conditions. Better knowledge of these local winds from observations is also a prerequisite to improving air quality prediction capability. This paper analyses local winds during the Passy-2015 field experiment that took place in a section of the Arve river valley, near Chamonix–Mont-Blanc. This location is one of the worst places in France regarding air quality. The wind analysis, which is mainly based on scanning Doppler lidar data sampling a persistent temperature inversion episode, reveals features consistent with the higher pollutant concentrations observed in this section of the valley as well as their spatial heterogeneities. In particular, an elevated down-valley jet is observed at night in the northern half of the valley, which, combined with a weak daytime up-valley wind, leads to very poor ventilation of the lowest layers. A northeast–southwest gradient in ventilation is observed on a daily-average, and is consistent with the PM10 heterogeneities observed within the valley. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle Estimating Hourly Beam and Diffuse Solar Radiation in an Alpine Valley: A Critical Assessment of Decomposition Models
Atmosphere 2018, 9(4), 117; doi:10.3390/atmos9040117
Received: 5 March 2018 / Revised: 19 March 2018 / Accepted: 20 March 2018 / Published: 21 March 2018
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Abstract
Accurate solar radiation estimates in Alpine areas represent a challenging task, because of the strong variability arising from orographic effects and mountain weather phenomena. These factors, together with the scarcity of observations in elevated areas, often cause large modelling uncertainties. In the present
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Accurate solar radiation estimates in Alpine areas represent a challenging task, because of the strong variability arising from orographic effects and mountain weather phenomena. These factors, together with the scarcity of observations in elevated areas, often cause large modelling uncertainties. In the present paper, estimates of hourly mean diffuse fraction values from global radiation data, provided by a number (13) of decomposition models (chosen among the most widely tested in the literature), are evaluated and compared with observations collected near the city of Bolzano, in the Adige Valley (Italian Alps). In addition, the physical factors influencing diffuse fraction values in such a complex orographic context are explored. The average accuracy of the models were found to be around 27% and 14% for diffuse and beam radiation respectively, the largest errors being observed under clear sky and partly cloudy conditions, respectively. The best performances were provided by the more complex models, i.e., those including a predictor specifically explaining the radiation components’ variability associated with scattered clouds. Yet, these models return non-negligible biases. In contrast, the local calibration of a single-equation logistical model with five predictors allows perfectly unbiased estimates, as accurate as those of the best-performing models (20% and 12% for diffuse and beam radiation, respectively), but at much smaller computational costs. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle Micro-Scale Properties of Different Bora Types
Atmosphere 2018, 9(4), 116; doi:10.3390/atmos9040116
Received: 24 January 2018 / Revised: 15 March 2018 / Accepted: 16 March 2018 / Published: 21 March 2018
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Abstract
In this paper we use 20 Hz wind measurements on three levels (2, 5, and 10 m) to investigate the differences in micro-scale properties of different bora types, i.e., deep and shallow bora with further subdivision to cyclonic and anticyclonic bora cases. Using
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In this paper we use 20 Hz wind measurements on three levels (2, 5, and 10 m) to investigate the differences in micro-scale properties of different bora types, i.e., deep and shallow bora with further subdivision to cyclonic and anticyclonic bora cases. Using Fourier spectral analysis, we investigate a suitable turbulence averaging scale and bora gust pulsations. The obtained data set is further used to test the Monin–Obukhov similarity theory, the surface layer stratification, the behavior of the terms in the prognostic turbulence kinetic energy equation, and the wind profiles. One of our main goals is to identify possible micro-scale differences between shallow and deep bora types because of the possible different mountain wave dynamics in those flows. We found that a turbulence averaging scale of 30 min is suitable for this location and is in agreement with previous bora studies. The wind speed power spectral densities of all selected bora episodes showed pulsations with periods of 2–8 min. This suggests that mountain wave breaking was present in all cases, regardless of flow depth and synoptic type. The stability parameter analysis confirmed the near-neutral thermal stratification of bora; a consequence of intensive mechanical mixing. No significant differences related to bora type were observed in other micro-scale parameters. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle Observations and Predictability of Gap Winds in the Salmon River Canyon of Central Idaho, USA
Atmosphere 2018, 9(2), 45; doi:10.3390/atmos9020045
Received: 30 November 2017 / Revised: 23 January 2018 / Accepted: 29 January 2018 / Published: 31 January 2018
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Abstract
This work investigates gap winds in a steep, deep river canyon prone to wildland fire. The driving mechanisms and the potential for forecasting the gap winds are investigated. The onset and strength of the gap winds are found to be correlated to the
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This work investigates gap winds in a steep, deep river canyon prone to wildland fire. The driving mechanisms and the potential for forecasting the gap winds are investigated. The onset and strength of the gap winds are found to be correlated to the formation of an along-gap pressure gradient linked to periodic development of a thermal trough in the Pacific Northwest, USA. Numerical simulations are performed using a reanalysis dataset to investigate the ability of numerical weather prediction (NWP) to simulate the observed gap wind events, including the timing and flow characteristics within the canyon. The effects of model horizontal grid spacing and terrain representation are considered. The reanalysis simulations suggest that horizontal grid spacings used in operational NWP could be sufficient for simulating the gap flow events given the regional-scale depression in which the Salmon River Canyon is situated. The strength of the events, however, is under-predicted due, at least in part, to terrain smoothing in the model. Routine NWP, however, is found to have mixed results in terms of forecasting the gap wind events, primarily due to problems in simulating the regional sea level pressure system correctly. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle The Peñalara Mountain Meteorological Network (1999–2014): Description, Preliminary Results and Lessons Learned
Atmosphere 2017, 8(10), 203; doi:10.3390/atmos8100203
Received: 7 July 2017 / Revised: 31 August 2017 / Accepted: 12 October 2017 / Published: 17 October 2017
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Abstract
This work describes a mountain meteorological network that was in operation from 1999 to 2014 in a mountain range with elevations ranging from 1104 to 2428 m in Central Spain. Additionally, some technical details of the network are described, as well as variables
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This work describes a mountain meteorological network that was in operation from 1999 to 2014 in a mountain range with elevations ranging from 1104 to 2428 m in Central Spain. Additionally, some technical details of the network are described, as well as variables measured and some meta information presented, which is expected to be useful for future users of the observational database. A strong emphasis is made on showing the observational methods and protocols evolution, as it will help researchers to understand the sources of errors, data gaps and the final stage of the network. This paper summarizes mostly the common sources of errors when designing and operating a small network of this kind, so it can be useful for individual researchers and small size groups that undertake a similar task on their own. Strengths and weaknesses of some of the variables measured are discussed and some basic calculations are made in order to show the otential of the database and to anticipate future deeper climatological analyses over the area. Finally, the configuration of an automatic mountain meteorology station is suggested as a result of the lessons learned and the the common state of the art automatic measuring techniques Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessArticle Evaluation of Surface Fluxes in the WRF Model: Case Study for Farmland in Rolling Terrain
Atmosphere 2017, 8(10), 197; doi:10.3390/atmos8100197
Received: 11 July 2017 / Revised: 27 September 2017 / Accepted: 2 October 2017 / Published: 8 October 2017
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Abstract
The partitioning of available energy into surface sensible and latent heat fluxes impacts the accuracy of simulated near surface temperature and humidity in numerical weather prediction models. This case study evaluates the performance of the Weather Research and Forecasting (WRF) model on the
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The partitioning of available energy into surface sensible and latent heat fluxes impacts the accuracy of simulated near surface temperature and humidity in numerical weather prediction models. This case study evaluates the performance of the Weather Research and Forecasting (WRF) model on the simulation of surface heat fluxes using field observations collected from a surface flux tower in Oregon, USA. Further, WRF-modeled heat flux sensitivities to North American Mesoscale (NAM) and North American Regional Reanalysis (NARR) large-scale input forcing datasets; U.S. Geological Survey (USGS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) land use datasets; Pleim-Xiu (PX) and Noah land surface models (LSM); Yonsei University (YSU) and Mellor-Yamada-Janjic (MYJ) planetary boundary layer (PBL) schemes using the Noah LSM; and Asymmetric Convective Model version 2 (ACM2) PBL scheme using PX LSM are investigated. The errors for simulating 2-m temperature, 2-m humidity, and 10-m wind speed were reduced on average when using NAM compared with NARR. Simulated friction velocity had a positive bias on average, with the YSU PBL scheme producing the largest overestimation in the innermost domain (0.5 km horizontal grid resolution). The simulated surface sensible heat flux had a similar temporal behavior as the observations but with a larger magnitude. The PX LSM produced lower and more reliable sensible heat fluxes compared with the Noah LSM. However, Noah latent heat fluxes were improved with a lower RMSE compared to PX, when NARR forcing data was used. Overall, these results suggest that there is not one WRF configuration that performs best for all the simulated variables (surface heat fluxes and meteorological variables) and situations (day and night). Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Review

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Open AccessReview Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain
Atmosphere 2018, 9(3), 102; doi:10.3390/atmos9030102
Received: 29 January 2018 / Revised: 17 February 2018 / Accepted: 19 February 2018 / Published: 12 March 2018
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Abstract
The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and
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The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave–turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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Open AccessReview Moist Orographic Convection: Physical Mechanisms and Links to Surface-Exchange Processes
Atmosphere 2018, 9(3), 80; doi:10.3390/atmos9030080
Received: 11 January 2018 / Revised: 15 February 2018 / Accepted: 21 February 2018 / Published: 25 February 2018
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Abstract
This paper reviews the current understanding of moist orographic convection and its regulation by surface-exchange processes. Such convection tends to develop when and where moist instability coincides with sufficient terrain-induced ascent to locally overcome convective inhibition. The terrain-induced ascent can be owing to
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This paper reviews the current understanding of moist orographic convection and its regulation by surface-exchange processes. Such convection tends to develop when and where moist instability coincides with sufficient terrain-induced ascent to locally overcome convective inhibition. The terrain-induced ascent can be owing to mechanical (airflow over or around an obstacle) and/or thermal (differential heating over sloping terrain) forcing. For the former, the location of convective initiation depends on the dynamical flow regime. In “unblocked” flows that ascend the barrier, the convection tends to initiate over the windward slopes, while in “blocked” flows that detour around the barrier, the convection tends to initiate upstream and/or downstream of the high terrain where impinging flows split and rejoin, respectively. Processes that destabilize the upstream flow for mechanically forced moist convection include large-scale moistening and ascent, positive surface sensible and latent heat fluxes, and differential advection in baroclinic zones. For thermally forced flows, convective initiation is driven by thermally direct circulations with sharp updrafts over or downwind of the mountain crest (daytime) or foot (nighttime). Along with the larger-scale background flow, local evapotranspiration and transport of moisture, as well as thermodynamic heterogeneities over the complex terrain, regulate moist instability in such events. Longstanding limitations in the quantitative understanding of related processes, including both convective preconditioning and initiation, must be overcome to improve the prediction of this convection, and its collective effects, in weather and climate models. Full article
(This article belongs to the Special Issue Atmospheric Processes over Complex Terrain)
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