International Mountain Conference

Abstract ID 414 | Date: 2022-09-12 13:40 – 13:50 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Ahrens, Bodo; Halbig, Alexander; Singh, Prashant

The interplay of the Indian Monsoon and the Himalayas is vital to many climatological aspects of the Himalayan foothill and foreland regions. A unique climate feature in the Himalayan foothill and foreland regions is a bi-modal diurnal cycle of precipitation with high rainfall amounts in the afternoon and around midnight. The reason for this night-time precipitation maximum is not yet fully understood, and current climate models do not well represent the regions' diurnal cycle of precipitation. Nevertheless, estimation of realistic spatiotemporal precipitation patterns is crucial for the climate community (e.g., for impact modeling). This study reviews discussions in literature, available observational findings (ERA5, CMORPH, and IMDAA), and simulation results with the regional climate model (RCM) and convection-permitting models (from the CORDEX FPS CPTP initiative). Our COSMO-CLM simulations indicate that the model is not able to recover the nighttime's precipitation behavior with currently typical horizontal RCM grid-spacings (e.g., 20 or 50 km), but it can do so with convection-permitting grid-spacing (~3 km) which sufficiently resolves the relevant orographic thermal wind together with the moist monsoonal flow characteristics in the area.

Abstract ID 787 | Date: 2022-09-12 13:50 – 14:00 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Singh, Shweta; Schmidli, Juerg

The Atmospheric Boundary Layer (ABL) affects the transport and storage of passive tracers, like greenhouse gases (GHGs), through advection, turbulence, and vertical mixing and therefore impacts the vertical distribution of these trace gases especially on a regional scale. ICON hindcast simulations (model grid spacing of 1 km) using a standard TKE-based turbulence closure are performed (hereafter ICON-NWP), followed by a detailed evaluation against surface stations, profile observations, and tall tower measurements over two sites on the Swiss Plateau namely Payerne and Beromüsnter. The target cases include a strong diurnal cycle of the GHGs under different weather regimes, initially in particular clear-sky fair-weather situations. For the stable boundary layer (SBL), ICON-NWP tends to produce a cooler and more humid surface layer compared to observations. During daytime, ICON-NWP exhibits a shallower and warmer convective boundary layer compared to observations. These differences indicate that the current boundary layer scheme needs further tuning. Different tuning parameters like surface exchange coefficient, etc. are tested. Furthermore, the newly developed two-energies turbulence scheme (Ďurán et al. 2018) is tested and evaluated. Apart from different turbulence closures, an evaluation at finer model grid spacings will be discussed.

Abstract ID 457 | Date: 2022-09-12 14:00 – 14:10 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Reynolds, Dylan (1); Kruyt, Bert (1); Gutmann, Ethan (2); Jonas, Tobias (1); Lehning, Michael (1,3); Mott, Rebecca (1)

Snow models rely on accurate meteorological input data at the spatial scales at which they operate. However, even the highest resolution operational atmospheric models often run at horizontal resolutions at least an order of magnitude coarser than most snow models. Different downscaling techniques can be employed to bridge this scale gap, typically being sorted into either statistical or dynamical techniques. Recent efforts have been made to optimize dynamic downscaling techniques, reducing computational demand while maintaining physical accuracy of predicted variables as well as the interdependency of downscaled variables such as winds and precipitation. The Intermediate Complexity Atmospheric Research (ICAR) model recently demonstrated an ability to match precipitation patterns from WRF, but with computational costs at least two orders of magnitude lower. While promising, these results from a 4km comparison did not translate to finer spatial resolutions often needed as input to snow models.

Thus, we introduce the High-resolution Intermediate Complexity Atmospheric Research Model (HICAR), a new variant of the ICAR model developed for spatial resolutions as high as 50m. Relative to a traditional atmospheric model like WRF, HICAR maintains the orders-of-magnitude reduction in computational demand which ICAR displayed, while resolving terrain-induced effects on the wind field not seen in ICAR. This is achieved through a novel combination of adjustments to a background wind field based on terrain descriptors with a wind solver. The solver enforces a mass-conservation constraint on the 3D wind field. These modifications successfully mimic dynamic effects such as flow blocking, ridge-crest speed up, and lee-side recirculation to be captured in the resulting wind field. These features are of particular importance for resolving snow deposition patterns, where the snow particles are particularly susceptible to advection by the near-surface flow field. We validate the accuracy of HICAR's flow features using a wind LiDAR deployed in complex terrain and show a comparison between flow fields from HICAR and WRF at a horizontal resolution of 50 m. These comparisons demonstrate HICAR's ability to resolve terrain-induced modifications to the flow field which result in increased heterogeneity of ridge-scale snowfall patterns. To this point, preliminary comparisons of snow deposition patterns in complex terrain between the HICAR model and snow accumulation during the 2021/2022 winter are presented. With this new model, physically-based downscaling of precipitation and other atmospheric variables which preserves their interdependencies is made available for high-resolutions (100m) and large-spatial extents (10,000 km2) which are often demanded by operational land-surface models.

Abstract ID 358 | Date: 2022-09-12 14:10 – 14:20 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Rostkier-Edelstein, Dorita (1); Kunin, Pavel (2); Alpert, Pinhas (2)

The Mediterranean Sea breeze (MSB) is very pronounced during the spring and the summer in the Levant. It flows up the Judean hills, then, it drops into the Dead Sea (DS) valley in the early evening hours. Hence, this flow descends about 1200 m from the Judean Mountains to the DS at about -430 m below MSL. We used WRF model to analyze and better understand the complex processes in and around the DS area, e.g., the MSB structure and its propagation into the DS Valley. A pre-requisite for relying on model simulations to analyze atmospheric processes is to verify their skill with respect to observations. WRF was configured with four nested domain with 30, 10, 3.3 and 1.1 km grid spacing. The model sensitivity was checked for: (1) landuse/vegetation databases, (2) PBL schemes, (3) number of vertical levels, (4) atmospheric and soil initial and lateral boundary conditions from different global models (IC/BC), (5) initial conditions for the finest domain, (7) initialization time of finest domain. Model simulations were verified against unique very high-resolution observations first conducted in the DS valley as part of the Virtual Institute DEad SEa Research Venue (DESERVE) project using the KITcube instruments (a ground-based microwave radiometer, two wind lidars and radiosoundings) along with Energy Balance Stations. These provided horizontal and vertical winds, temperature, humidity, pressure, radiation, and visibility data. Evaluation of model simulations against observations was made for seven parameters that were used to analyze the atmospheric dynamics of the MSB penetration into the DS valley: surface specific humidity, temperature and wind; vertically integrated water vapor; time evolution of horizontal and vertical-wind and profile; and time of MSB arrival to the DS valley. Our sensitivity study shows that the surface variables and the temporal evolution of the horizontal and vertical wind were most sensitive to the PBL scheme and IC/BC from global models. Vertically integrated water vapor was most sensitive to the PBL scheme. The time of arrival of the MSB to the DS valley was most sensitive to IC/BC from global models. The best simulation includes landuse/vegetation from MODIS 15-seconds resolution dataset, the Mellor-YamadaJanjic PBL scheme, 40 vertical levels, IC/BC from NCEP/GFS global model for all nested domains, and later initialization time of the of the finest domain with respect to the coarser ones by 24 hours. In our presentation, we will discuss physical and numerical factors responsible for the most favorable outcomes.

Abstract ID 453 | Date: 2022-09-12 14:20 – 14:30 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Sugimoto, Shiori (1); Ueno, Kenichi (2); Fujinami, Hatsuki (3); Nasuno, Tomoe (1); Sato, Tomonori (4); Takahashi, Hiroshi G. (5)

A numerical experiment with a 2-km horizontal resolution was conducted using the Weather Research and Forecasting (WRF) model to understand the physical processes controlling nocturnal precipitation occurrence over the Himalayas during the mature monsoon season from 2003 to 2010. The model experiments simulated a diurnal variation in the precipitation over the Himalayan slopes and foothills, i.e., the two daily peaks in the afternoon and in the midnight over the slopes and the single nocturnal peak over the foothills. When the nocturnal precipitation dominated over the Himalayan slopes, moisture supply associated with westward propagation of low pressure systems was found in the synoptic-scale. Meanwhile, moisture was directly transported by the monsoon westerlies when nocturnal precipitation occurred over the foothills. In the model experiment, afternoon precipitation was simulated on the mountain ridges in the Himalayas where the altitude was approximately 2000–2500 m, and precipitation area expanded to the lower-elevation regions during the night. In the midnight, downslope wind was formed by radiative cooling at the surface and was intensified by evaporative cooling by hydrometeors in the near-surface layer. This downslope wind converged with the synoptic-scale moisture flow, and then nocturnal precipitation was promoted over the Himalayan slopes and foothills. Importance of topographic resolution for the nocturnal precipitation simulation was confirmed in the sensitivity experiment as well as a number of vertical layers in the model. This work was published as Sugimoto et al. (2021; Journal of Hydrometeorology).

Abstract ID 815 | Date: 2022-09-12 16:00 – 16:10 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Steger, Christian R. (1); Steger, Benjamin (2); Schär, Christoph (1)

In regions with complex topography, incoming short- and longwave surface radiation is strongly influenced by local and surrounding terrain. Direct shortwave radiation depends both on local slope as well as on neighbouring terrain, which can induce topographic shading. Incoming diffuse shortwave radiation can be enhanced by terrain reflection – particularly under snow covered conditions, which feature a relatively high surface albedo. Finally, incoming longwave radiation can also be modulated by radiative exchange between facing slopes. All these effects can influence the energy balance of snow-covered, snow-free and glaciated surfaces in mountains significantly. Such influences can feedback to the atmosphere through e.g. changes in near-surface air temperatures and the development of meso-scale wind systems.

In many atmospheric and climate models, topographic effects on surface radiation are not considered at all. Radiation exchange is typically modelled in the vertical direction using the column approximation, which does not allow to consider the above-mentioned effects explicitly. However, parameterisations, which are based on pure geometrical considerations and/or derived from offline three-dimensional ray-tracing simulations, were developed and implemented in some models. We focus on the former approach with the aim of developing an improved parameterisation, which can be applied in weather and climate simulations – particularly at high spatial resolutions (~1 km and below). To consider topographic effects on surface radiation, a typical required parameter is the so-called Sky View Factor (SVF). Computing this parameter, especially from a sub-grid scale Digital Elevation Model, is computationally expensive. In a first step, we developed a faster method for this task, which is based on a high-performance ray tracing library. Beside its higher performance, the new algorithm is also less prone to artefacts in the SVF caused by terrain representation. Ongoing work addresses various subsequent challenges like representing terrain-reflected shortwave radiation more accurately and reducing the computational overhead of the scheme to make it affordable for online simulations.

Abstract ID 333 | Date: 2022-09-12 16:10 – 16:20 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Lin, Changgui (1,2); Yang, Kun (3,4); Chen, Deliang (1); Guyennon, Nicolas (5); Balestrini, Raffaella (5); Yang, Xiaoxin (4,6); Acharya, Sunil (7); Ou, Tinghai (1); Yao, Tandong (4,6); Tartari, Gianni (5); Salerno, Franco (5)

Little is known about the effects of glacier-air interactions on the Himalayan glacier mass balance. Until this knowledge gap is filled, a reliable projection of the future changes in the Himalayan glaciers is hardly possible. Here, we describe the drying effect of the katabatic winds on the up-valley summer monsoon flows by creating favorable conditions for local convergence-induced precipitation to occur near the glacier fronts. We postulate that this retarding effect on the up-valley monsoon flows results in a negative feedback mechanism mediated by glacier-air interactions, in which glacial retreat pushes precipitation upwards as the down-valley katabatic winds weaken, resulting in greater local precipitation and enhanced snow accumulation across the upper parts of the Himalayan glaciers. Our analyses are based on the exclusive data recorded in the Khumbu valley and the Langtang valley in the Nepalese Himalayas. These data revealed higher afternoon precipitation in summer associated with surface wind convergence near the glacier fronts and a sharp decrease in the temperature lapse rate over the glacier surfaces. The principle of the observed phenomena was proven by our high-resolution modeling sensitive experiment, which involved two simulations, one with the present glaciers and the other without. This numerical experiment also supports the proposed negative feedback. Furthermore, we report a low deuterium excess near the glacier fronts, indicating below-cloud re-evaporation facilitated by the local convergence induced by the dry katabatic winds. Our study suggests that current models may overestimate the retreat of Himalayan glaciers because they have completely ignored the glacier-air interactions.

Abstract ID 354 | Date: 2022-09-12 16:20 – 16:30 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Jansing, Lukas; Papritz, Lukas; Sprenger, Michael

South Foehn winds are associated with a characteristic warming as the air parcels descend from the Alpine crest or pass levels downslope into the northern lee-side valleys. Both upstream latent heating in clouds on the Alpine south side, and adiabatic descent and compression (isentropic drawdown) on the Alpine north side, have been invoked as key mechanisms for this warming. We use a mesoscale model simulation at 1.1 km grid spacing, coupled to an online trajectory module and combined with a Lagrangian heat budget, to illustrate that both adiabatic and diabatic warming can contribute substantially to the overall heat budget. To this end, a hindcast of an intense and long-lasting Alpine South Foehn event in November 2016 is evaluated focusing on Swiss and Austrian Foehn valleys. It is revealed that, while adiabatic warming constitutes the overall most important process for most of the air parcels arriving in the northern Foehn valleys (57%), upstream diabatic heating on the Alpine south side is dominant for a considerable fraction of the trajectories (35%). The analysis demonstrates that there is a clear east-west gradient in terms of the different heating contributions, with diabatic heating being more important for valleys in the western Alps ('Swiss Foehn') and adiabatic heating driving the warming in the eastern Alps ('Austrian Foehn'). Further, we identify a distinct temporal evolution of the respective warming mechanisms. Hereby, our results underline the need for a nuanced view on Foehn air warming, as both mechanisms can co-occur with varying relative importance, depending on the valley and the time period of a Foehn event.

It is well-known that the Alpine South Foehn can develop not only to the east of an upper-level trough and the associated southwesterly flow impinging on the Alps (Deep Foehn), but a rich palette of different Foehn types exists based on varying synoptic and mesoscale situations (e.g., Shallow Foehn, Dry Foehn, Dimmer Foehn, Gegenstrom Foehn). The heat budget analysis will thus be expanded to a multitude of model simulations representing these different Foehn flavors. Furthermore, a preliminary analysis of the main penetration pathways of Foehn air into several northern Foehn valleys will be presented. Finally, the different Foehn types will be set into a climatological perspective using five years of km-scale operational analyses from the Swiss national weather service.

Abstract ID 935 | Date: 2022-09-12 16:30 – 16:40 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Westerhuis, Stephanie

Most numerical models employ terrain-following vertical coordinates. Close to the ground, the vertical coordinate surfaces run parallel to the underlying orography. With increasing altitude, the orographic signal gradually decays until the levels become truly flat. While it is desirable to have flat vertical coordinate surfaces at low altitudes, the design also has to incorporate a number of constraints: E.g. levels need a minimal thickness in order that the model remains numerically stable, at the same time they may not become too thick to be able to represent steep vertical gradients. This contribution discusses ongoing work about finding an optimal vertical coordinate formulation for the ICON model employing a 1 km horizontal mesh over the Alps.

Abstract ID 695 | Date: 2022-09-12 16:40 – 16:50 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Weinkaemmerer, Jan; Bastak Duran, Ivan; Schmidli, Jürg

Local thermal circulations developing over heated valley slopes strongly influence the convective boundary layer over mountainous terrain. In this study, large-eddy simulations (LES) are carried out over an idealized alpine valley. The flow is decomposed into a turbulent part, a local mean circulation capturing the slope winds, and a large-scale (upper-level) wind. This allows a detailed budget analysis for heat and moisture. The temperature distribution is horizontally fairly uniform inside the valley due to the homogenizing effect of the thermally-induced circulations. In contrast to that, the slope winds contribute strongly to the export of moisture out of the valley. The entrainment of dry air by the recirculation leads to a horizontally non-uniform moisture distribution. Consequently, a large-scale, upper-level wind hardly affects the horizontally homogeneous temperature distribution while it can considerably reduce the vertical moisture transport: a horizontal wind mixes the moisture from the slope-wind layer into the dryer regions of the valley.

          Focusing on the plume structures at the small-scale end of the coherent motions in the upslope flow, a conditional sampling method is applied in order to identify and characterize the thermals using a passive tracer emitted at the surface. It is found that mixed-layer plumes are moving upslope with the slope wind. In order to quantify the contribution of these plumes to the vertical heat and moisture flux, the turbulent part of the flow is further decomposed into organized turbulence and local turbulence in and outside of the plumes. In general, the plumes turn out to dominate the vertical fluxes inside the valley. Furthermore, the joint probability density functions of the vertical turbulent fluxes of heat and moisture at different locations and heights are calculated from the LES data and decomposed into a coherent and a local part. The contribution of the plumes to the turbulent heat and moisture flux over flat and mountainous terrain are compared using a quadrant analysis.

Abstract ID 324 | Date: 2022-09-12 16:50 – 17:00 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Thomas, Marius Levin (1); Wirth, Volkmar (1); Piotrowski, Zbigniew (2)

Banner clouds are clouds that appear to be attached on the leeward side of a steep mountain or ridge on otherwise cloud-free days. The current work considers fundamental questions associated with the formation of this type of clouds using large-eddy simulations. Previous work was based on an idalized model configuration with pyramid-shaped orography; there, it was shown that the shear of the oncoming flow plays a key role for the geometry of the lee-side vortex and, hence, for the shape of the banner cloud.
In the current work, the scope is extended from an idealized pyramid to the realistic orography of Mt Matterhorn. The simulations show that the wind shear of the oncoming flow is less essential than before, because the underlying rough orography creates "its own" flow profile by the time the flow reaches the windward side of the mountain. By contrast, the wind speed turns out to be quite relevant, because large windspeed is associated with strong turbulence, turbulence reduces stratification, and reduced stratification promotes the formation of uplift regions at Mt Matterhorn. However, the flow field for realistic Matterhorn orography makes it much harder to identify a coherent lee vortex to be associated with the banner cloud.

Abstract ID 964 | Date: 2022-09-12 17:00 – 17:10 | Type: Oral Presentation | Place: SOWI – Seminar room U1 |

Account, Normal; Lamprecht, Christian; Gurgiser, Wolfgang

My only purpose is to allow some tests of our websystem.

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Abstract ID 348 | Date: 2022-09-12 17:45 – 17:47 | Type: Poster Presentation | Place: SOWI – Garden |

Serafin, Stefano (1); Potter, Emily (1,2)

The climatological frequency of deep moist convection is enhanced in the vicinity of mountainous regions, causing storms. Most studies to date focus on convection initiation (CI) on the windward side of mountains, but uplift and CI can also occur on their lee side. The factors controlling this phenomenon are only partially understood, but it is frequently hypothesized that a lee-side hydraulic jump may provide the uplift required to initiate lee-side convection.

Here we argue that lee-side CI is best understood as a consequence of low-level convergence along an orographic dryline. The dryline marks the boundary between relatively dry air descending from the mountains and conditionally unstable air over an adjacent plain. The stronger the convergence along the dryline, the more likely is CI to occur.

We initially focus on the atmospheric conditions leading to strong lee-side convergence and uplift in an unsaturated atmosphere. A variety of scenarios are investigated using an idealised modelling setup, ranging in linearity and hydrostaticity of the cross-mountain flow, and with varying surface fluxes. By computing a convergence budget, we determine the dominant forcings affecting lee-side convergence and how these vary across flow regimes. A relationship is determined between the level of hydrostaticity and linearity of the flow, the strength of lee side convergence, and the corresponding boundary-layer uplift.

We then turn to considering flows with a conditionally unstable leeside environment. We replicate the scenarios in which strong lee-side convergence and low-level uplift are expected, and we determine whether the uplift is actually sufficient to initiate deep moist convection.

Abstract ID 855 | Date: 2022-09-12 17:47 – 17:49 | Type: Poster Presentation | Place: SOWI – Garden |

Le Bouëdec, Enzo (1); Staquet, Chantal (1); Chemel, Charles (2)

The Grenoble metropolitan area is located in a basin surrounded by the alpine massifs of Vercors, Chartreuse and Belledonne. This setting makes the Grenoble basin particularly subject to air pollution. The basin is found to present a characteristic local atmospheric circulation for a large-scale flow regime associated with wintertime anticyclonic blocking. A set of high-resolution numerical simulations of atmospheric dynamics in the basin for episodes representative of this regime is analysed to characterise the dispersion of tracers with emissions taken as those of fine particulate matter. Air pollution hot spots are identified and their locations are discussed in light of the underlying local atmospheric dynamics.

Abstract ID 943 | Date: 2022-09-12 17:49 – 17:51 | Type: Poster Presentation | Place: SOWI – Garden |

Yi, Chuixiang (1,2); Kutter, Eric (1); Sabot, Manon (3); Wohlfahrt, Georg (4); Rotach, Mathias (5); Krakauer, Nir (6,2); Hendrey, George (1,2)

During the past few decades, the Alps have warmed twice as fast as the global average. At present we lack the knowledge and data for assessing what likely future climate change might hold for Alpine ecosystems and how these changes will affect their current role in the cycling of water and carbon and in providing ecosystem services to people. Plant stomata opening/closing dynamics control the two most abundant greenhouse gases (GHGs) H₂O and CO₂, and are sensitive to droughts and heatwaves. The atmospheric boundary layer (ABL) mixes GHGs and controls their exchange with the free troposphere. The feedback between plant stomata opening and diurnal ABL growth is a key to understand ongoing climate change and its impact on ecosystems and GHG fluxes, but remains unknown. We propose a quantitative framework for exploring following questions: (1) Can stomatal opening/closing change ABL depth? (2) Does ABL growth influence stomatal opening/closing? (3) Does feedback between ABL growth and stomatal opening/closing play a significant role in optimizing stomatal conductance? We plan to apply the Yi, Davis, Berger, and Bakwin (YDBB) model to estimate ABL depth from eddy-covariance flux measurements, and use the unified stomatal optimization model to calculate plant stomatal conductance. We plan to use the i-Box measurements of turbulence and exchange processes conducted by a cluster of eddy-flux towers in the Inn Valley, Austria to evaluate model parameters in the Alpine setting.

Abstract ID 345 | Date: 2022-09-12 17:51 – 17:53 | Type: Poster Presentation | Place: SOWI – Garden |

Dixit, Ankur (1); Sahany, Sandeep (2); Mishra, Saroj Kanta (1); Mesquita, Michel (3)

We set up WRF with three nested domains using initial and lateral boundary conditions from ERA-Interim (ERA-I) data. Model was initialized with the conditions of 01 October 2002 and run until 01 January 2004. The lateral boundary conditions were updated every 6 hours. We performed six experiments using three microphysics (MP3, MP8, and WSM6) and two cumulus (KF, and BMJ) schemes. During DJF, MP8_KF, MP3_BMJ, and MP8_BMJ showed relatively lesser precipitation, however, WSM6_KF (~6.4 mm day-1) and WSM6_BMJ (~6.2 mm day-1) were found to have maximum precipitation. MP8_KF (4.6 mm day-1) had the average value closer to the observation (1.75 mm day-1) than the rest of the experiments, though overestimated by a large amount. MP8_KF was found to have the least normalized standard deviation, along with a higher skill score than most of the experiments. Overall, MP8_KF could be considered reasonable because of its lesser deviation and better skill score

Afterwards, we performed the WRF-Hydro calibration using the WRF downscaled meteorological forcing (MP8KF, and WSM6_BMJ). The model was calibrated for the year 2003 and validated for 2004-2005. The station observed discharge at the basin outlet was used to perform the calibration and validation. We found JJAS discharge was underestimated for MP8KF, possibly due to underestimation in the JJAS precipitation in MP8KF simulations. WSM6BMJ did reasonably well for the JJAS, but it showed some erroneous high peaks for the non-JJAS. We also designed two simulations using the JJAS observation instead of the complete annual cycle, forced by the WSM6_BMJ meteorological forcing.

Abstract ID 518 | Date: 2022-09-12 17:53 – 17:55 | Type: Poster Presentation | Place: SOWI – Garden |

Farina, Sofia (1,2); Bisignano, Andrea (3); Zardi, Dino (1,2)

An Eulerian model for the dispersion of a passive tracer over a simplified slope driven by a thermally driven circulation is presented here. The source of the tracer is point-like and the emission continuous, the local circulation is a pure anabatic flow modeled following Prandtl's (1942) steady state model. The eddy diffusivity is considered, at first, constant along the vertical direction, and later specific parameterizations are tested. The incapability of a classical gaussian model to forecast the concentration field is shown through a comparison between the results of the Gaussian and Eulerian models. A study of the sensitivity of the concentration field to the position of the source and to the maximum values of the wind field is proposed. Moreover, a mathematical relationship between the position and the intensity of the ground concentration field, together with its dependence to the environmental parameters is found.
Prandtl L. 1942. Führer durch die Strömungslehre, Chapter 5. Vieweg und Sohn: Braunschweig, Germany. [English translation: Prandtl L. 1952. Mountain and valley winds in stratified air, in Essentials of Fluid Dynamics: 422–425. Hafner Publishing Company: New York, NY]


Abstract ID 662 | Date: 2022-09-12 17:55 – 17:57 | Type: Poster Presentation | Place: SOWI – Garden |

Pritchard, Hamish Daniel (1); Golding, Charlotte (2); Potter, Emily (3)

Snowfall is notoriously difficult to measure and existing preciptiation gauges are very poorly suited to constraining mountain precipitation in weather and climate models. They often suffer from large undercatch biases, are sparse and poorly distributed, and they represent only a tiny point in the landscape. We present an entirely new approach to observing snowfall from time series of lake water pressure. We demonstrate our method through a winter in wet and stormy coastal East Greenland using a 3 km wide, 17 km² lake as our precipitation gauge. Our sub-daily, unbiased, large-area measurements of precipitating snow water content are uniquely well-suited to evaluating high-resolution weather models, and we use them to test the output of regional climate model RACMO2. Because the seasonally frozen lakes that our new method uses are widely distributed at high latitudes and are particularly common in mountain ranges, it is particularly well suited to the widespread, autonomous monitoring of snowfall in remote areas that are largely unmonitored today. This is potentially transformative in reducing uncertainty in regional precipitation and runoff in seasonally cold climates.

Abstract ID 546 | Date: 2022-09-12 17:57 – 17:59 | Type: Poster Presentation | Place: SOWI – Garden |

Šácha, Petr; Procházková, Zuzana

Internal gravity waves (GWs) play important roles in atmospheric dynamics and transport especially above mountaineous regions. The effects on dynamics are better understood and parameterized in global climate or numerical weather prediction models. Also, it is widely understood that GWs can influence atmospheric composition and transport, either directly via turbulent mixing during their breaking or via so-called non-dissipative effects connected with GW propagation and fluctuating trajectories inside the GWs. However, such GW effects (either dissipative or non-dissipative) are not parameterized in current generation CCMs. This presents a great motivation for GW resolving modeling.

In our research, we test several methods for GW analysis (from traditional filtering to Lagrangian based methods) in high-resolution regional model simulations to assess the uncertainty in GW effects connected with different GW detection methods.

Abstract ID 854 | Date: 2022-09-12 17:59 – 18:01 | Type: Poster Presentation | Place: SOWI – Garden |

Kumar, Manish (1); Krishnaswamy, Jagdish (2); Sen, Sumit (3)

Moisture recycling by vegetation is a crucial ecohydrological process in tropical forest hydrology. In steep mountains, vegetation-driven evapotranspiration can have significant impact on land surface heat and water fluxes through the increased fraction of transpiration to evapotranspiration. In turn, the increased atmospheric moisture and localised circulation air in narrow mountains valleys can create complex precipitation patterns at diurnal and seasonal scales. However, the cumulative effect of vegetation on precipitation is usually reported at the scale of continents, such as the biotic pump theory on the role of inland vegetation in driving precipitation in the tropics. Yet, sporadic evidence from pan-tropical Himalayan mountains reports a significant contribution of localised moisture recycling to diurnal and annual precipitation. Currently, such observations are constrained by the lack of ground validated high-resolution precipitation datasets. This paper investigates the relative influence of vegetation-drive evapotranspiration and climate on the diurnal patterns in precipitation in Sikkim Himalaya. We use the gridded precipitation dataset from the high-resolution Global Precipitation Mission precipitation and employ multi-variate spatial regression analysis to understand spatial linkages between diurnal patterns of precipitation as the response variable and geo-environmental variables like temperature, elevation and vegetation indices (NDVI, LAI, and fPAR) derived from MODIS and SRTM DEM. Based on visual inspection of the semi-variograms, We fitted the Generalised least squares (GLS) model with exponential auto-correlated error structure to evaluate the relative influence of geo-environment-vegetation variables on the timing of precipitation in a day while accounting for spatial autocorrelation in the variables. NDVI was the best predictor of late-night precipitation in monsoon and summer. Morning precipitation in monsoon was well explained by combination model of NDVI and Temperature or only NDVI, whereas afternoon-evening precipitation pattern in summer was equally well explained by NDVI or Elevation models, but without significant p-value. NDVI was very significant for late-night precipitation in winters, whereas LAI was significant for afternoon-evening precipitation. Overall, vegetation indices, in particular NDVI, significantly explained late-night precipitation timing in monsoon and winters while taking into account the spatial autocorrelation in the variables. This supports our hypothesis that vegetation plays a significant role in influencing the timing of precipitation in Sikkim, especially nocturnal precipitation, most likely through active recycling of evapotranspiration. In the absence of isotopic studies, the results provide indirect evidence of moisture recycling in the mountainous regions of Sikkim Himalaya.