Modeling large dust deposition events to alpine snow and their impacts: the role of model resolution

<div>Mineral dust and black carbon (BC) constitute the most important aerosols present in the atmosphere and cryosphere and have well known potential effects on regional and global climate. Upon their deposition they can impact snow albedo, snowpack evolution and timing of snow-melt. However, capturing BC and dust deposition events in mountain regions is currently a challenge due to the complexity of aerosol-cloud interactions and the specifics of mountain meteorological systems, which are difficult to represent in large scale models. Here, we use a case study of dust deposition, between 30 March and 5 April 2018, when a significant dust deposition event was observed within the seasonal snowpack at the Col du Lautaret in the French Alps. This comes in addition to the background BC deposition that occurred during the same period. Specifically, we investigate the role of model resolution in capturing both mountain meteorology, precipitation, and the resulting model predicted dust and BC deposition. For this, the meteorological-chemical model WRF-Chem is used with three nested domains including the primary dust emissions region in Africa (low resolution domain), a second domain that includes Europe, and a third high resolution domain over the Alps. We compare WRF-Chem predicted aerosol and meteorological properties (at different model resolution) with in-situ, remote sensing, and reanalysis products to validate the model and quantify the added value of high resolution modelling within the Alps. We conclude that predicted mountain meteorology including precipitation is significantly better when using the high resolution configuration (3 x 3 km horizontal resolution domain). Additionally, this improved meteorology predicted by the model has significant impacts on predicted dust deposition and BC. The better representation of the mountain meteorology when the resolution becomes finer leads to improved model predicted dust and BC deposition to alpine snow. Implications for this, including improved resolution within models that consider the full aerosol lifecycle in the atmosphere and in snow covered mountain regions is discussed.</div>

The Alaiz experiment: untangling multi-scale stratified flows over complex terrain

We present novel measurements from a field campaign that aims to characterize multi-scale flow patterns, ranging from 0.1 to 10 km in a time-resolved manner, in a mountainous region in northwestern Spain with a mountain–valley–ridge configuration. We select two flow cases where topographic-flow interactions were measured by five synchronized scanning Doppler wind lidars along a 10 km transect line that includes a cross section of the valley. We observed a hydraulic jump in the lee side of the mountain. For this case, the Froude number transition from supercritical (>1) at the mountain to subcritical (<1) at the valley is in agreement with previous experiments at a smaller scale. For a 1-year period, the measurements show such a transition about 10 % of the time, indicating a possible high occurrence of hydraulic jumps. The second flow case presents valley winds that are decoupled from the northerly flow aloft and show a stratified layered pattern, which is well captured by the lidar scans and complementary ground-based observations. These measurements can aid the evaluation of multi-scale numerical models as well as improve our knowledge with regards to mountain meteorology.

Eulerian and Lagrangian perspectives on a Foehn event in the Alpine region

&lt;p&gt;Foehn-related research has a long-standing tradition in mountain meteorology. In this context, the reason for Foehn air warming and the factors determining the descent of the air into the valleys have gained particular interest. Here, we readdress those research questions by combining a COSMO model hindcast at 1 km horizontal and 10 min temporal resolution with air parcel trajectories for a South Foehn case study in November 2016. The sub-synoptic situation in the model is studied using horizonal cross sections at different levels. Vertical cross sections in the Po valley and along the axes of major Foehn valleys complement the Eulerian analysis.&lt;/p&gt;&lt;p&gt;The selected event is characterized by its long duration, a far northern extent and exceptionally strong gusts. A low-level jet is discernible west of the Alps and a pronounced north-south pressure gradient develops. A striking feature is the strong tilt of the isentropes downstream of the Alpine crest. Trajectories reveal the versatile pathways of air parcels over major Alpine passes before they descend into the Foehn valleys. Differences with respect to upstream ascent and descent are observed for the different valleys. By tracing meteorological variables along the trajectories, the relative importance of adiabatic and diabatic processes for the Foehn air warming is quantified. The properties of air parcels that descend into the valleys or stay at higher levels are contrasted in order to identify factors that determine the descent.&lt;/p&gt;&lt;p&gt;The case study will set the scene for a forthcoming detailed analysis of Foehn flows based on online trajectories that make use of the wind fields at every model time step. The analysis will be extended to a number of cases representing the different South Foehn varieties. We will trace the temperature tendencies due to all diabatic processes (cloud processes, radiation, turbulence) along the trajectories in order to quantify their respective importance for Foehn air warming. First results in this extended framework will be presented.&lt;/p&gt;

An Overview of the Integrated Meteorological Observations in Complex Terrain Region at Dali National Climate Observatory, China

Systematically observing components of the climate system as well as their processes and interactions are crucial to understand the weather, climate, climate change, etc. In order to launch long-term, continuous, stereoscopic, and integrated meteorological observations for key regions of the climate system in southwestern China where it is sensitive to interactions among multiple layers and exchanges of mass and energy, the Dali National Climate Observatory (DNCO) was established in May 2006. To date, the DNCO has gradually performed an integrated meteorological observation network in a complex terrain region over the southeastern Tibetan Plateau including the conventional observations of weather and climate, and the special observations of radiation, lightning, soil moisture, wind profile, water vapor, water quality, water level, water temperature profile, turbulent fluxes of momentum, sensible heat, latent heat, carbon dioxide, and methane, etc. Furthermore, the DNCO mainly focuses on the field observation experiments and scientific research activities for mountain meteorology. This paper presents an overview of the DNCO including its location, climatology, scientific objectives, research tasks, and existing observation projects. The progresses in observation and associated research including data quality controls and assessments, recent observation results, and regional numerical model tests are summarized. Future works are also discussed.

Mountain Meteorology

Meteorology is the science that examines the configuration of fundamental weather elements in Earth’s atmosphere: pressure, temperature, humidity, wind, precipitation, and clouds. Weather defines the arrangement of these elements over short periods (minutes to days) and climate defines the average weather at a location over longer periods (months to centuries). Mountains cover roughly 25% of the Earth’s land surface and are home to about 12% of the world’s population. With innumerable topographic configurations, mountains disrupt airflow in the troposphere from the valley scale in the mountains themselves to hundreds of kilometers beyond their foothills. Atmospheric motion interacts with mountains via thermal and mechanical forces. Thermally, diurnal heating and cooling differences in complex terrain create circulating motions that reverse direction, day and night. Examples include fine-scale mountain and valley winds and more broad-scale anabatic and katabatic winds. Mechanically, disturbances and waves in atmospheric motion are forced upstream, downstream, above, and adjacent to mountains. Examples include blocking, upslope flow, downslope winds, gravity waves, boundary-layer turbulence, and lee cyclogenesis. Given the right atmospheric conditions, orographic processes generate extreme, damaging, and costly weather events such as strong Chinook/Föhn winds, heavy upslope snow events, steep terrain forcing in monsoon or atmospheric river flows, long-duration frigid valley temperature inversions, invisible lee rotors, and local zones of convergence that initiate strong convective thunderstorms. Finally, mountains create distinctive and often photographed clouds that include caps, lenticulars, and corrugations of standing waves. While the focus of this bibliography is weather, some atmosphere/landscape interactions, such as windward vs. leeward orographic effects, fall largely within climate science literature. Other more fine-scale interactions between the atmosphere and mountains, such as local shade effects and valley temperature inversions, are captured in planetary surface boundary layer topoclimate/microclimate literature. These near-surface processes influence the weather-making troposphere, and as such, are seamlessly tied to mountain meteorology. To build a robust context into this bibliography, select multiscalar climate-themed sources are identified. To glaciologists, atmospheric scientists (including forecasters), climatologists, ecologists, agricultural scientists, hydrologists, and others whose specializations operate in and adjacent to mountainous terrain, understanding the unique influences of mountains is essential. This article identifies key literature and other relevant sources that explore how mountains influence and modify weather, and to a lesser degree, climate. Together, these resources provide a rich set of teaching materials and a platform for more in-depth studies into mountain meteorology set in the context of geography.

100 Years of Progress on Mountain Meteorology Research

ABSTRACT Mountains significantly influence weather and climate on Earth, including disturbed surface winds; altered distribution of precipitation; gravity waves reaching the upper atmosphere; and modified global patterns of storms, fronts, jet streams, and climate. All of these impacts arise because Earth’s mountains penetrate deeply into the atmosphere. This penetration can be quantified by comparing mountain heights to several atmospheric reference heights such as density scale height, water vapor scale height, airflow blocking height, and the height of natural atmospheric layers. The geometry of Earth’s terrain can be analyzed quantitatively using statistical, matrix, and spectral methods. In this review, we summarize how our understanding of orographic effects has progressed over 100 years using the equations for atmospheric dynamics and thermodynamics, numerical modeling, and many clever in situ and remote sensing methods. We explore how mountains disturb the surface winds on our planet, including mountaintop winds, severe downslope winds, barrier jets, gap jets, wakes, thermally generated winds, and cold pools. We consider the variety of physical mechanisms by which mountains modify precipitation patterns in different climate zones. We discuss the vertical propagation of mountain waves through the troposphere into the stratosphere, mesosphere, and thermosphere. Finally, we look at how mountains distort the global-scale westerly winds that circle the poles and how varying ice sheets and mountain uplift and erosion over geologic time may have contributed to climate change.

The Community Foehn Classification Experiment

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called foehn in this paper although different names exist depending on the region, the sign of the temperature change at onset, and the depth of the overflowing layer. These winds affect the local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition, nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility, and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of master’s degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 h were classified for foehn–no foehn conditions at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine the reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a test bed for automatic classification algorithms, which—if successful—eliminate the drawbacks of manual classifications: lack of scalability and reproducibility.