Orographic Precipitation

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This paper constitutes a step towards the understanding of some characteristics associated with high rainfall amounts and flooding on Madeira Island. The high precipitation events that occurred during the winter of 2009/2010 have been considered with three main goals: to analyze the main atmospheric characteristics associated with the events; to expand the understanding of the interaction between the island and the atmospheric circulations, mainly the effects of the island on the generation or intensification of orographic precipitation; and to evaluate the performance of high resolution numerical modeling in simulating and forecasting heavy precipitation events over the island. The MESO-NH model with a horizontal resolution of 1 km is used, as well as rain gauge data, synoptic charts and measurements of precipitable water obtained from the Atmospheric InfraRed Sounder (AIRS). The results confirm the influence of the orographic effects on precipitation over Madeira as well as the tropical–extratropical interaction, since atmospheric rivers were detected in six out of the seven cases analyzed, acting as a low level moisture supplier, which together with the orographic lifting induced the high rainfall amounts. Only in one of the cases the presence of a low pressure system was identified over the archipelago.

There are many uncertainties associated with aerosol-precipitation interactions, particularly in mountain regions where a variety of processes at different spatial scales influence precipitation patterns. Statistical relationships between aerosols and precipitation were examined in the southern Appalachian Mountains to determine the seasonal and synoptic influences on these relationships, as well as the influence of air mass source region. Precipitation events were identified based on regional precipitation data and classified using a synoptic classification scheme developed for this study and published in a separate manuscript (Kelly et al. 2012). Hourly aerosol data were collected at the Appalachian Atmospheric Interdisciplinary Research (AppalAIR) facility at Appalachian State University in Boone, NC (1110 m asl, 36.215•,-81.680•). Backward air trajectories provided information on upstream atmospheric characteristics and source regions. During the warm season (June-September), greater aerosol loading dominated by larger particles was observed, whereas cool season (November-April) precipitation events exhibited overall lower aerosol loading with an apparent influence from biomass burning particles. A significant relationship between aerosol optical properties and precipitation intensity was observed, which may be indicative of aerosol-induced precipitation enhancement in each season, particularly during warm season non-frontal precipitation.

California has deployed key land-based sensors An Atmospheric River-focused long-term observing network is being installed in CA as part of a 5-year project between CA Dept. of Water Resources (DWR), NOAA and Scripps Inst. Of Oceanography-Installed 2008-2014->100 field sites ¼-scale 449-MHz wind profiler with RASS

Recent studies reported that precipitation and mountain waves induced low tropospheric level circulations may be decoupled or masked by greater spatial scale variability despite generally there is a connection between microphysical processes of precipitation and mountain driven air flows. In this paper we analyse two periods of a winter storm in the Eastern Pyrenees mountain range (NE Spain) with different mountain wave induced circulations and low-level turbulence as revealed by Micro Rain Radar (MRR), microwave radiometer and Parsivel disdrometer data during the Cerdanya-2017 field campaign. We find that during the event studied mountain wave wind circulations and low-level turbulence do not affect neither the snow crystal riming or aggregation along the vertical column nor the surface particle size distribution of the snow. This study illustrates that precipitation profiles and mountain induced circulations may be decoupled which can be very relevant for either ground-based or sp...

The quasi-steady-state limit of the diurnal valley wind system is investigated over idealized three-dimensional topography. Although this limit is rarely attained in reality due to ever-changing forcings, the investigation of this limit can provide valuable insight, in particular on the mass and heat fluxes associated with the along-valley wind. We derive a scaling relation for the quasi-steady-state along-valley mass flux as a function of valley geometry, valley size, atmospheric stratification, and surface sensible heat flux forcing. The scaling relation is tested by comparison with the mass flux diagnosed from numerical simulations of the valley wind system. Good agreement is found. The results also provide insight into the relation between surface friction and the strength of the along-valley pressure gradient.

The purpose of this article is to introduce a new diagnostic measure of the time-integrated diabatic (thermal) forcing of a valley–plain system. This measure can be used to synchronize the evolution of thermally induced valley winds with respect to their forcing. Differences among numerical models or model configurations originating from diabatic forcing versus those originating from the model dynamics (e.g., turbulence scheme, dynamical core, etc.) can then be distinguished.

In a recent study, the authors investigated the mechanisms leading to the formation of diurnal along-valley winds in a valley formed by two isolated mountain ridges on a horizontal plain. The main focus was on the relation between the valley heat budget and the valley–plain pressure difference. The present work investigates the influence of the valley surroundings on the evolution of the valley winds. Three valley–plain configurations with identical valley volumes are studied: a periodic valley, an isolated valley on a plain (the former case), and an isolated valley entrenched in an elevated plateau. According to the valley volume argument (topographic amplification factor), these three cases should develop identical temperature perturbations and thus similar along-valley winds. However, substantial differences are found between the three cases, in particular a much stronger daytime up-valley wind and nighttime down-valley wind for the plateau configuration. The analysis demonstrate...

Although the qualitative influence of mountains over the atmosphere has been known for a long time, numerous deficiencies, linked to orography, are still noted, either in forecasts by regional models, or in the long-term behavior of climate models. This is why the French and Spanish weather services are undertaking an important field campaign to document the dynamic modifications to the atmospheric flow generated by the Pyrenean range during a 2-month period (October and November 1990) with six intensive observation periods (lOPs) of 2 to 3 days. The experimental strategy is based largely on mesoscale numerical-model results and will help to validate these models. The main focus is on the documentation of clear-air turbulence generated either by breaking mountain waves, by surface roughness, or by the wind shear induced by the lateral-flow deviation around the mountain. Experimental means include several networks of surface stations, radio soundings, constant-level balloons, four wind profilers, and several research aircraft.

The island of Madeira is well known for giving rise to atmospheric wakes. Strong and unsteady atmospheric wakes, resembling a von Kármán vortex street, are frequently observed in satellite images leeward of Madeira, especially during summer months, when conditions favoring the formation of atmospheric wakes occur frequently under the influence of the Azores high. Reported here is the analysis of the first airborne measurements of Madeira's wake collected during the 2010 Island-induced Wake (I-WAKE) campaign. High-resolution in situ and remote sensing data were collected in the I-WAKE by a research aircraft. The measurements reveal distinctive wake signatures, including strong lateral wind shear zones and warm and dry eddies downwind of the island. A strong anticorrelation of the horizontal wind speed and sea surface temperature (SST) was found within the wake. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to study the dynamics of the wake generation and its temporal evolution. The comparison of the model results and observations reveals a remarkable fidelity of the simulated wake features within the marine boundary layer (MBL). Strong potential vorticity (PV) anomalies were found in the simulated MBL wake, emanating from the flanks of the island. The response of the wake formation within the MBL to surface friction and enhanced thermal forcing is explored through the model sensitivity analyses.

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The BAST Editorial Board invites papers that investigate relationships between the initiation, spread, state, effects, or consequences of SARS-CoV-2/COVID-19 (COVID-19 hereafter for brevity) and atmospheric science and technology. Long-term exposure to coarse and fine particulate matter (PM10 and PM 2.5) and nitrogen compounds has been recognized as an important risk factor for a variety of pulmonary pathologies and viral respiratory infections. Important links between atmospheric phenomena-from dispersion to chemical reactions to seasonal climatology-and COVID-19 have just started to emerge. The following topics are especially welcome: & Impacts of COVID-19 on indoor and outdoor air quality at regional scale or larger, including country-or region-specific analyses & Impacts of air quality on COVID-19, such as the effects associated with enhanced levels of PM 2.5 /PM 10 particles & Numerical simulations (e.g., predictions of the virus spread or air quality changes based on traffic patterns and stay-at-home orders)

The influence of climate on mountain denudation has been the topic of an intense debate for two decades. This debate partly arises from the covariation of rainfall and topography during the growth of mountain ranges, both of which influence denudation. However, the denudational response of this co‐evolution is poorly understood. Here, we use a landscape evolution model where the rainfall evolves according to a prescribed rainfall–elevation relationship. This relationship is a bell curve defined by a rainfall base level, a rainfall maximum and a width around the rainfall peak elevation. This is a first‐order model that fits a large range of orographic rainfall data at the ca. 1‐km spatial scale. We carried out simulations of an uplifting block with an alluvial apron, starting from an initially horizontal surface, and testing different rainfall–elevation relationships. We find that the denudation history is different from that with constant rainfall models. The results essentially dep...

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower atmosphere. On 25 September 2017 during a field campaign, a persistent easterly LLJ and gravity waves are observed simultaneously in a coastal area in the north of France. In the present study, an attempt is to explore the variability of the wind speed, turbulent eddies, and turbulence kinetic energy in the time-frequency and space domain using an ultrasonic anemometer and a scanning wind lidar. The results reveal a significant enhancement of the turbulence kinetic energy dissipation (by ~50%) due to gravity waves in the LLJ shear layer (below the jet core) during the period of wave propagation. Large values of zonal and vertical components of the shear stress (~|0.4| and ~|1.5| m 2 s-2 respectively) are found during that period. Large size eddies (~110 to 280 m), matching high-speed wind regime, are found to propagate the momentum downward. This enhances the downward mass transport from the LLJ shear layer to the roughness layer. Furthermore, these large-scale eddies are associated with the crests while comparatively small-scale eddies are associated with the troughs of the gravity wave.

The mountain gap flow characteristics in and around the Chisapani mountain gap of Karnali River basin has been numerically simulated using the Weather Research and Forecasting (WRF) model to understand its spatial and temporal characteristics and its implications. The model was initialized with NCEP initial and boundary conditions to carry continuous integrations 168 long hours to identify the general flow field during early springtime. The river valley accumulates channels and flushes a narrow jet stream of about 10 ms-1 in speed and 500 m in its depth into the vast southern plain via Chisapani mountain gap during night and morning time whereas in the afternoon, the mountain feeds up-valley wind of about 4 ms-1 from the southern plain up into the Karnali River basin.

A major freezing rain storm causing catastrophic losses occurred in early January 1998 over eastern Canada and the northeastern United States. The various types of precipitation and associated flow and precipitation features of this storm are discussed with a particular emphasis on the effects due to precipitation phase changes in the Montréal area. Hourly surface observations, Doppler radar data and model reanalysis information are used to describe the hydrometeor phase and production. There were two periods of significant ice accumulation at the ground. They were characterized by more organized precipitation compared to other times but there were also distinct differences between the two periods in terms of wind fields. This hydrometeor development was linked in part to the flow fields in the St. Lawrence River Valley and its topographic features. The results from a time dependent microphysical model suggest that there was considerable horizontal temperature advection that balanced the warming/cooling of the atmosphere due to precipitation phase changes aloft and at the surface. Overall, local-scale factors significantly affected the nature of this major event.

This study is devoted to a better comprehension of the pollutant transport and diffusion mechanisms in cities located in valleys. A code is developed on the basis of ARPS (University of Oklahoma), a 3D non hydrostatic model with fully compressible equations. Tests are carried out on a 2D isolated hill for different atmospherical stratifications. Then, we have simulated the effects of an alpin valley topography on the flow generated by radiative cooling/ warming of the slopes. The transport of the non reactive pollutant emitted at the center of the valley.

Inland-penetrating atmospheric rivers (ARs) can affect the southwestern United States and significantly contribute to cool season (November–March) precipitation. In this work, a climatological characterization of AR events that have led to cool season extreme precipitation in the Verde River basin (VRB) in Arizona for the period 1979/80–2010/11 is presented. A “bottom up” approach is used by first evaluating extreme daily precipitation in the basin associated with AR occurrence, then identifying the two dominant AR patterns (referred to as Type 1 and Type 2, respectively) using a combined EOF statistical analysis. The results suggest that AR events in the Southwest do not form and develop in the same regions. Water vapor content in Type 1 ARs is obtained from the tropics near Hawaii (central Pacific) and enhanced in the midlatitudes, with maximum moisture transport over the ocean at low levels of the troposphere. On the other hand, moisture in Type 2 ARs has a more direct tropical o...

In this essay, we highlight some challenges the atmospheric community is facing concerning adequate treatment of flows over mountains and their implications for numerical weather prediction (NWP), climate simulations, and impact modeling. With recent increases in computing power (and hence model resolution) numerical models start to face new limitations (such as numerical instability over steep terrain). At the same time there is a growing need for sufficiently reliable NWP model output to drive various impact models (for hydrology, air pollution, agriculture, etc.). The input information for these impact models is largely produced by the boundary layer (BL) parameterizations of NWP models. All known BL parameterizations assume flat and horizontally homogeneous surface conditions, and their performance and interaction with resolved flows is massively understudied over mountains—hence their output may be accidentally acceptable at best. We therefore advocate the systematic investigat...

On 20 October 2016, aircraft observations documented a significant train of lee waves above and downstream of the Snæfellsnes Peninsula on the west coast of Iceland. Simulations of this event with the Weather Research and Forecasting (WRF) Model provide an excellent representation of the observed structure of these mountain waves. The orographic features producing these waves are characterized by the isolated Snæfellsjökull volcano near the tip of the peninsula and a fairly uniform ridge along its spine. Sensitivity simulations with the WRF Model document that the observed wave train consists of a superposition of the waves produced individually by these two dominant orographic features. This behavior is consistent with idealized simulations of a flow over an isolated 3D mountain and over a 2D ridge, which reproduce the essential behavior of the observed waves and those captured in the WRF simulations. Linear analytic analysis confirms the importance of a strong inversion at the top...

We present Sval_Imp, a high-resolution gridded dataset designed for forcing models of terrestrial surface processes on Svalbard. The dataset is defined on a 1 km grid covering the archipelago of Svalbard, located in the Norwegian Arctic (74-82 • N). Using a hybrid methodology, combining multidimensional interpolation with simple dynamical modeling, the atmospheric reanalyses ERA-40 and ERA-Interim by the European Centre for Medium-Range Weather Forecasting have been downscaled to cover the period 1957-2017 at steps of 6 h. The dataset is publicly available from a data repository. In this paper, we describe the methodology used to construct the dataset, present the organization of the data in the repository and discuss the performance of the downscaling procedure. In doing so, the dataset is compared to a wealth of data available from operational and project-based measurements. The quality of the downscaled dataset is found to vary in space and time, but it generally represents an improvement compared to unscaled values, especially for precipitation. Whereas operational records are biased to low elevations around the fringes, we stress the hitherto underused potential of project-based measurements at higher elevation and in the interior of the archipelago for evaluating atmospheric models. For instance, records of snow accumulation on large ice masses may represent measures of seasonally integrated precipitation in regions sensitive to the downscaling procedure and thus providing added value. Sval_Imp (Schuler, 2018) is publicly available from the Norwegian Research Data Archive NIRD, a data repository (

We present Sval_Imp, a high-resolution gridded dataset designed for forcing models of terrestrial surface processes on Svalbard. The dataset is defined on a 1 km grid covering the archipelago of Svalbard, located in the Norwegian Arctic (74-82 • N). Using a hybrid methodology, combining multidimensional interpolation with simple dynamical modeling, the atmospheric reanalyses ERA-40 and ERA-Interim by the European Centre for Medium-Range Weather Forecasting have been downscaled to cover the period 1957-2017 at steps of 6 h. The dataset is publicly available from a data repository. In this paper, we describe the methodology used to construct the dataset, present the organization of the data in the repository and discuss the performance of the downscaling procedure. In doing so, the dataset is compared to a wealth of data available from operational and project-based measurements. The quality of the downscaled dataset is found to vary in space and time, but it generally represents an improvement compared to unscaled values, especially for precipitation. Whereas operational records are biased to low elevations around the fringes, we stress the hitherto underused potential of project-based measurements at higher elevation and in the interior of the archipelago for evaluating atmospheric models. For instance, records of snow accumulation on large ice masses may represent measures of seasonally integrated precipitation in regions sensitive to the downscaling procedure and thus providing added value. Sval_Imp (Schuler, 2018) is publicly available from the Norwegian Research Data Archive NIRD, a data repository (

We apply a linear model of orographic precipitation (LT model) to estimate snow accumulation on the western Svartisen ice cap (220 km 2) in northern Norway. This model combines 3D airflow dynamics with simple parameterizations of cloud physics. The model is forced by large-scale atmospheric input variables taken from the ECMWF Re-analysis (ERA-40) of the European Center for Medium Range Weather Forecast (ECMWF), and the model parameters are kept constant for the entire simulation period, after optimization. The domain covers a 120 ð 125 km area surrounding the ice cap. The model is run using a 1-km resolution digital elevation model, and 6-h time steps over the period from 1958 to 2002. Precipitation data from surrounding meteorological stations and winter glacier mass balance measurements on several glaciers within the region are used to evaluate the model results. Precipitation obtained from the LT model agrees well with observations from precipitation gauges and there is also fair agreement between model results and specific winter mass balance observations on the ice cap if these are corrected for winter rain. The LT model reproduces well the spatial pattern of winter accumulation across the ice cap as well as the area-averaged winter mass balances of several other glaciers in the region. This indicates that it is a useful tool in providing high-resolution, deterministic estimates of precipitation in complex terrain as required for distributed hydrological and/or glacier mass balance modelling of unmeasured areas.

A dense network of raingauges is operated across a 700 m high mountain ridge in SW-Iceland in the autumn of 2002. The mountain ridge is oriented perpendicular to the main wind direction during precipitation. The maximum precipitation is found immediately in the lee of the mountain crest and it amounts to about 350% of the precipitation in the lowlands upstream of the mountains and 600% of the precipitation in the lowlands in the lee of the mountains. The stronger the winds are, the more difference there is between precipitation in the mountains and on the lowlands. This is particularly clear for the upstream side.

The Yarnell Hill Fire, triggered by dry lightning on 28 June 2013, was initiated by hot and dry westerly winds, which rapidly shifted to north-northeast by convective-induced outflows. This sudden wind shift led to the demise of 19 firefighters. This study focuses on the environment and its predictive potential in terms of erratically changing the fire spread. Three numerical sensitivity tests are performed investigating the evolving synoptic-meso-β scale environmental wind flow: (1) deactivating the evaporative cooling, (2) deactivating surface-driven diurnal heating/cooling, and (3) removing the mountain. Results show the strong north-northeasterly wind induced by the density current(s) and the diurnal surface sensible heating played the most significant roles in enhancing the mesoscale environment conducive to the rapid change in the fire spread direction. While the mountain played a less significant role in weakening the magnitude of the airflow affecting the fire, it still had ...

Orographic precipitation is a result of very complex processes and its study using numerical models is of utmost importance since it can open an important avenue to the improvement of precipitation forecasts, especially during the warm season. Mainland Portugal is characterised by a very variable terrain between the north and south regions, the latter being much smoother, with sparse mountains that barely reach 1000 m. Conversely, several mountain ranges are distributed over Spain with heights often exceeding 1500 m altitude. A mesoscale non-hydrostatic atmospheric model (MesoNH) is used to study the orographic precipitation during a limited period in spring of 2002 over the Iberian Peninsula. In order to assess the effects of the mountains, case study simulations are done, with and without the orography. MesoNH is initialized and forced by the ECMWF analyses. The effects of orography on precipitation over neighbouring regions are also analyzed. Simulations show that orography is a key factor in determining the spatial distribution of precipitation over the Iberian Peninsula, with enhancements in the regions with mountain ranges and diminution or suppression over certain valleys. The simulated precipitation fields were visually compared with radar observations in central Portugal and quantitatively compared with rain gauge data all over Portugal in order to assess the model performance in the analyzed cases.

A simple model of temperature inversion breakup in mountain valleys is developed. This model is an extension of the conceptual model of D. Whiteman (1981), and its main features are: (1) The topography is represented by a digital elevation model, thus leading to a spatially-distributed model. (2) A simplified, yet realistic, representation of the surface energy balance for each atmospheric column is explicitly introduced. (3) The spatial variation of solar radiation, as a result of the journey of the sun over the Valley, is considered. Preliminary results show the evolution of the convective boundary layer and the top of the inversion layer. Overall, these results are in good agreement with measurements, and the model performance appears to be reasonably good, since it represents expected physical behaviors of the involved phenomena, and captures sentitivities to physical parameters.

Urban valleys can experience serious air pollution problems as a combined result of their limited ventilation and the high emission of pollutants from the urban areas. Idealized simulations were analyzed to elucidate the breakup of an inversion layer in urban valleys subject to a strong low-level temperature inversion and topographic effects on surface heating such as topographic shading, as well as the associated air pollution transport mechanisms. The results indicate that the presence and evolution in time of the inversion layer and its interplay with an urban heat island within the valley strongly influence the venting of pollutants out of urban valleys. Three mechanisms of air pollution transport were identified. These are transport by upslope winds, transport by an urban heat island–induced circulation, and transport within a closed slope-flow circulation below an inversion layer.

Exploring novel renewable energy resources such as thermal winds in mountainous areas and valleys is critical to reduce the energy production from fossil fuels and thus mitigating climate change. These winds occur due to thermal gradients and related buoyancy effects. Basically, the latter are mainly associated with the diurnal heating-cooling cycles of the lower layers of the atmosphere. Thermal winds usually develop by convection over complex terrains of different scales, and they invert their direction twice a day, driven by the emergence and dissipation of temperature inversions. Namely, these winds will blow up-valley (anabatic winds), or from the plain to the mountain massif during day-time. Contrary, during night-time, these winds will blow down-valley (katabatic winds), or from the mountain massif to the plain. Former investigations have shown that thermal winds can reach comparably high speeds [1], which could be interesting for wind energy applications. Moreover, in compar...

The technique of large-eddy simulations has been used to investigate thermally driven local circulations in deep valleys for a complete diurnal cycle. A soil model simulates the thermal forcing at the ground, which depends on the season, the soil characteristics, the valley orientation, and the atmospheric variables. The scales of interest are characteristic of an urban site located in a mountainous area, and the research focuses on low wind conditions without the influence of large-scale pressure gradients. This study highlights the influence of the season on the mechanisms responsible for the formation and the destruction of the thermal inversion layer. The spatial distribution of the convective boundary layer (CBL) within the valley is directly influenced by the season because of the variation of the solar warming. In summer, the altitude of the top of the CBL remains approximately constant across the valley, whereas in winter, this altitude varies with its location within the valley.

Upstream track deflection of a propagating cyclonic vortex past an isolated mountain range is investigated by using idealized simulations with both boundary layer turbulent mixing and cloud effects. The westbound vortex past a shorter mountain range may experience an earlier northward deflection prior to landfall. The vortex then takes a sudden southward turn as it gets closer to the mountain range, in response to the effects of the stronger northerly wind over the mountain due to the effects of channeling flow. The vortex may deflect southward when approaching a longer mountain range and then rebound northward upstream of the mountain ridge. The southward deflection is primarily induced by the convergence (stretching) effect due to the combination of the speedy core at the southwestern flank of the vortex and a northerly jet between the vortex and the mountain. The vortex then rebounds northward to pass over the mountain as the speedy core rotates counterclockwise to the eastern fl...

The Antarctic peninsula and Patagonia region (the south of South America) have recently been identified as the regions with the highest gravity-wave activity in the world. In this work, the generation and propagation of gravity waves in the Patagonia region in an event of strong wave activity from 30 October 1995-1 November 1995 is examined by means of radiosonde measurements and simulations with the Weather Research and Forecasting (WRF) model. The waves are generated by strong surface winds found near the Andes mountains at a latitude of 49-51 • S. The strong low-level winds are related to an extratropical cyclone that propagates southeastward in the South Pacific ocean and approaches the western coast of the continent. The waves propagate southeast toward Tierra del Fuego and they continue their propagation over the Drake Passage. They are found to propagate long meridional (lateral) distances due to the shear background conditions. This fact is corroborated with WRF simulations and a novel technique that combines wavelet analysis and backward ray-tracing. Therefore, this work provides further evidence that high gravity-wave activity found by several studies over Drake Passage may have an orographic origin. During the orographic wave event, which lasts about 72 hours, the horizontal wavelength is unexpectedly found to change day-today. The analysis shows that changes in the near-surface meteorological conditions produced by the cyclone passage may trigger different components of the forcing orography. The orographic waves propagate toward their critical levels, which are found at 25 km and above. The radiosonde measurements show that the wave is breaking continuously along a wide altitude range; this finding from measurements supports the picture of continuous wave erosion along the ray path instead of abrupt wave-breaking for the examined wave event.

In this study, we examined the sensitivity of tropical cyclone (TC) characteristics and precipitation to the Cordillera Mountain Range (CMR) in Luzon, Philippines. Using the Weather Research and Forecasting (WRF) model, we simulated eight TCs with four different CMR orographic elevations: Control, Flat, Reduced, and Enhanced. We found that at significance level α=0.05, TC intensity significantly weakened as early as 21 h prior to landfall in the Enhanced experiment relative to the Control, whereas there was little change in the Flat and Reduced experiments. However, throughout the period when the TC crossed Luzon, we found no significant differences for TC movement speed and position in the different orographic elevations. When a TC made landfall, associated precipitation over the CMR increased as the mountain height increased. We further investigated the underpinning processes relevant to the effect of the CMR on precipitation by examining the effects of mountain slope, incoming pe...

An orographic precipitation model has been used to estimate the spatial distribution of precipitation in Iceland with a horizontal resolution of 1 km and time scales ranging from a day over the period 1958 to 2006. This model combines airflow dynamics with a simple parameterization of cloud physics (cloud water formation time, hydrometeor fallout time, condensed water advection and leeside evaporation). The model is forced with large-scale atmospheric variables taken from the European Centre for Medium Range ...

One year of observations from a network of five 915-MHz boundary-layer radar wind profilers equipped with radio acoustic sounding systems located in California's Central Valley are used to investigate the annual variability of convective boundary-layer depth and its correlation to meteorological parameters and conditions. Results from the analysis show that at four of the sites, the boundary-layer height reaches its maximum in the late-spring months then surprisingly decreases during the summer months, with mean July depths almost identical to those for December. The temporal decrease in boundary-layer depth, as well as its spatial variation, is found to be consistent with the nocturnal low-level lapse rate observed at each site. Multiple forcing mechanisms that could explain the unexpected seasonal behaviour of boundary-layer depth are investigated, including solar radiation, precipitation, boundary-layer mesoscale convergence, low-level cold-air advection, local surface characteristics and irrigation patterns and synoptic-scale subsidence. Variations in solar radiation, precipitation and synoptic-scale subsidence do not explain the shallow summertime convective boundary-layer depths observed. Topographically forced cold-air advection and local land-use characteristics can help explain the shallow CBL depths at the four sites, while topographically forced low-level convergence helps maintain larger CBL depths at the fifth site near the southern end of the valley.

How do the feedbacks between tectonics, sediment transport and climate work to shape the topographic evolution of the Earth? This question has been widely addressed via numerical models constrained with thermochronological and geomorphological data at scales ranging from local to orogenic. Here we present a novel numerical model that aims at reproducing the interaction between these processes at the continental scale. For this purpose, we combine in a single computer program: 1) a thin-sheet viscous model of continental deformation; 2) a stream-power surface-transport approach; 3) flexural isostasy allowing for the formation of large sedimentary foreland basins; and 4) an orographic precipitation model that reproduces basic climatic effects such as continentality and rain shadow. We quantify the feedbacks between these processes in a synthetic scenario inspired by the India-Asia collision and the growth of the Tibetan Plateau. We identify a feedback between erosion and crustal thickening leading locally to a <50% increase in deformation rates in places where orographic precipitation is concentrated. This climatically-enhanced deformation takes place preferentially at the upwind flank of the growing plateau, specially at the corners of the indenter (syntaxes). We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurisms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-tectonic interactions make the development of steady-state topography at the continental scale unlikely.

A simple model of temperature inversion breakup in mountain valleys is developed. This model is an extension of the conceptual model of D. Whiteman (1981), and its main features are: (1) The topography is represented by a digital elevation model, thus leading to a spatially-distributed model. (2) A simplified, yet realistic, representation of the surface energy balance for each atmospheric column is explicitly introduced. (3) The spatial variation of solar radiation, as a result of the journey of the sun over the Valley, is considered. Preliminary results show the evolution of the convective boundary layer and the top of the inversion layer. Overall, these results are in good agreement with measurements, and the model performance appears to be reasonably good, since it represents expected physical behaviors of the involved phenomena, and captures sentitivities to physical parameters.

Many cities located in valleys with limited ventilation experience serious air pollution problems. The ventilation of an urban valley can be limited not only by orographic barriers, but also by urban heat island–induced circulations and/or the capping effect of temperature inversions. Furthermore, land-use/-cover changes caused by urbanization alter the dynamics of temperature inversions and urban heat islands, thereby affecting air quality in an urban valley. By means of idealized numerical simulations, it is shown that in a mountain valley subject to temperature inversions urbanization can have an important influence on air quality through effects on the inversion breakup. Depending on the urban area fraction in the simulations, the breakup time changes, the cross-valley wind system can evolve from a confined to an open system during the daytime, the slope winds can be reversed by the interplay between the urban heat island and the temperature inversion, and the breakup pattern ca...

Urban valleys can experience serious air pollution problems as a combined result of their limited ventilation and the high emission of pollutants from the urban areas. Idealized simulations were analyzed to elucidate the breakup of an inversion layer in urban valleys subject to a strong low-level temperature inversion and topographic effects on surface heating such as topographic shading, as well as the associated air pollution transport mechanisms. The results indicate that the presence and evolution in time of the inversion layer and its interplay with an urban heat island within the valley strongly influence the venting of pollutants out of urban valleys. Three mechanisms of air pollution transport were identified. These are transport by upslope winds, transport by an urban heat island–induced circulation, and transport within a closed slope-flow circulation below an inversion layer.

The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions. To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemis...

Two methods were used to identify the paths of moisture transport that reach the U.S. Intermountain West (IMW) during heavy precipitation events in winter. In the first, the top 150 precipitation events at stations located within six regions in the IMW were identified, and then back trajectories were initiated at 6-h intervals on those days at the four Climate Forecast System Reanalysis grid points nearest the stations. The second method identified the leading patterns of integrated water vapor transport (IVT) using the three leading empirical orthogonal functions of IVT over land that were first normalized by the local standard deviation. The top 1% of the associated 6-hourly time series was used to construct composites of IVT, atmospheric circulation, and precipitation. The results from both methods indicate that moisture originating from the Pacific that leads to extreme precipitation in the IMW during winter takes distinct pathways and is influenced by gaps in the Cascades (Oreg...

Exploring novel renewable energy resources such as thermal winds in mountainous areas and valleys is critical to reduce the energy production from fossil fuels and thus mitigating climate change. These winds occur due to thermal gradients and related buoyancy effects. Basically, the latter are mainly associated with the diurnal heating-cooling cycles of the lower layers of the atmosphere. Thermal winds usually develop by convection over complex terrains of different scales, and they invert their direction twice a day, driven by the emergence and dissipation of temperature inversions. Namely, these winds will blow up-valley (anabatic winds), or from the plain to the mountain massif during day-time. Contrary, during night-time, these winds will blow down-valley (katabatic winds), or from the mountain massif to the plain. Former investigations have shown that thermal winds can reach comparably high speeds [1], which could be interesting for wind energy applications. Moreover, in compar...

Using rain-gauge station records for the statistical characterization and simulation modeling of spatio-temporal precipitation field involves many issues and simplistic assumptions. One major issue is related to dealing with uncertainty at-site sample statistical inference, because of the limited length of records. Regional frequency analysis uses the idea of substituting space for time in order to reduce uncertainty. It assumes equal shapes of the precipitation statistical distributions in a region. However, this assumption limits the area of the analyzed region where this assumption is valid. The extension is dependent on terrain complexity. This work presents a new approach for the statistical regionalization of a large precipitation field, replacing the shape constancy assumption for the hypothesis of smooth spatial variation. The approach accounts for every uncertainty on site information, using an L-moment method for inference analysis. Additionally, the orographic effect is introduced in the regionalization, which substantially improves the interpolation performance and estimation of areal precipitation. The approach is used for modeling the monthly precipitation field in the Júcar River Basin Authority Demarcation (Spain), incorporating its stochastic structure, and spatial dependency coming from a geostatistical analysis. Issues related to the estimation of regional precipitation, and mean areal precipitation are discussed in the exposition.

Exploring novel renewable energy resources such as thermal winds in mountainous areas and valleys is critical to reduce the energy production from fossil fuels and thus mitigating climate change. These winds occur due to thermal gradients and related buoyancy effects. Basically, the latter are mainly associated with the diurnal heating-cooling cycles of the lower layers of the atmosphere. Thermal winds usually develop by convection over complex terrains of different scales, and they invert their direction twice a day, driven by the emergence and dissipation of temperature inversions. Namely, these winds will blow up-valley (anabatic winds), or from the plain to the mountain massif during day-time. Contrary, during night-time, these winds will blow down-valley (katabatic winds), or from the mountain massif to the plain. Former investigations have shown that thermal winds can reach comparably high speeds [1], which could be interesting for wind energy applications. Moreover, in compar...