Orographic Precipitation

Mountainous regions occupy a significant fraction of the Earth’s continents and are characterized by specific meteorological phenomena operating on a wide range of scales. Being a home to large human populations, the impact of mountains on weather and hydrology has significant practical consequences. Mountains modulate the climate and create micro-climates, induce different types of thermally and dynamically driven circulations, generate atmospheric waves of various scales (known as mountain waves), and affect the boundary layer characteristics and the dispersion of pollutants.

At the local scale, strong downslope winds linked with mountain waves (such as the Foehn and Bora) can cause severe damage. Mountain wave breaking in the high atmosphere is a source of Clear Air Turbulence, and lee wave rotors are a major near-surface aviation hazard. Mountains also act to block strongly-stratified air layers, leading to the formation of valley cold-air pools (with implications for road safety, pollution, crop damage, etc.) and gap flows. Presently, neither the fine-scale structure of orographic precipitation nor the initiation of deep convection by mountainous terrain can be resolved adequately by regional-to global-scale models, requiring appropriate downscaling or parameterization. Additionally, the shortest mountain waves need to be parameterized in global weather and climate prediction models, because they exert a drag on the atmosphere. This drag not only decelerates the global atmospheric circulation, but also affects temperatures in the polar stratosphere, which control ozone depletion. It is likely that both mountain wave drag and orographic precipitation lead to non-trivial feedbacks in climate change scenarios.

Measurement campaigns such as MAP, T-REX, Materhorn, COLPEX and i-Box provided a wealth of mountain meteorology field data, which is only starting to be explored. Recent advances in computing power allow numerical simulations of unprecedented resolution, e.g. LES modelling of rotors, mountain wave turbulence, and boundary layers in mountainous regions. This will lead to important advances in understanding these phenomena, as well as mixing and pollutant dispersion over complex terrain, or the onset and breakdown of cold-air pools. On the other hand, recent analyses of global circulation biases point towards missing drag, especially in the southern hemisphere, which may be due to processes currently neglected in parameterizations.
A better understanding of flow over orography is also crucial for a better management of wind power and a more effective use of data assimilation over complex terrain.

This Research Topic includes contributions that aim to shed light on a number of these issues, using theory, numerical modelling, field measurements, and laboratory experiments.

Après un bref aperçu sur l'impact du la montagne sur la pluviométrie et sur la problématique du réchauffement climatique global, l'auteur expose les résultats aux tendances pluviométriques actuelles en régions montagneuses marocaines et cherche à détecter le début d'une éventuelle amorce d'u changement climatique

Atmospheric rivers (ARs) are narrow regions of enhanced water vapor transport, usually found on the
warm-sector side of the polar cold front in many midlatitude storms formed primarily over the oceans.
Nonbrightband (NBB) rain is a shallow orographic rainfall process driven by collision and coalescence that
has been observed in some of these storms. NBB rain accounts for about one-third, on average, of the total
winter season rainfall occurring at a coastal mountain site in Northern California. During the California
Energy Commission’s CalWater project, nearly the same fraction of NBB rain was observed at a northern
Sierra Nevada foothills site as compared to the coastal mountains, whereas less than half of the fractional
amount of NBB rain was observed at a southern Sierra Nevada foothills site. Both Sierra Nevada sites often
experience terrain-induced blocked flow, that is, Sierra barrier jet (SBJ) during landfalling winter storms.
However, the northern Sierra Nevada site often is oriented geographically downwind of a gap in the coastal
terrain near San Francisco duringARlandfall. This gap allows maritime air in theARto arrive at the northern
site and enhance the collision–coalescence process in orographic feeder clouds as compared with the southern
site. As a result, a greater amount and intensity of NBB rain and overall precipitation was produced at the
northern site. This study uses a variety of observations collected in the coastal and Sierra Nevada ranges from
the Hydrometeorology Testbed and CalWater field campaigns to document this behavior. A detailed case
study provides additional context on the interaction between AR flow, the SBJ, and precipitation processes.

Observed April 1 snowpack accumulations within and near the Gunnison River basin in southwestern Colorado are compared with simulations from the Rhea-orographic.precipitation model to determine if the model simulates reliable magnitudes and temporal and spatial variability in winter precipitation for the basin. Twenty simulations of the Rhea model were performed using 'optimal' parameter sets determined for 10-kilometer (km) grids (10-km by 10-km grid cells) through stochastic calibration. Comparisons of Rhea-model simulations of winter precipitation with April 1 snowpack accumulations at 32 snowcourse stations were performed for the years 1972-1990. For most stations and most years the Rhea model reliably simulates the temporal and spatial variability in April 1 snowpack accumulations. However, in general, the Rhea-model underestimates April 1 snowpack accumulations in the Gunnison River basin area, and the underestimation is greatest for locations that receive the largest amount of snow. A significant portion of the error in Rhea-model simulations is due to the calibration of the Rhea model using gauge-catch precipitation measurements which can be as much as 50 percent below actual snowfall accumulations. Additional error in the Rhea-model simulations is a result of the comparison of gridded precipitation values to observed values measured at points. (KEY TERMS: precipitation modeling; orographic precipitation; water balance; western United States.)

Mountainous regions occupy a significant fraction of the Earth's continents and are characterized by specific meteorological phenomena operating on a wide range of scales. Being a home to large human populations, the impact of mountains on weather and hydrology has significant practical implications. Presently, neither the fine-scale structure of orographic precipitation nor the initiation of convection by mountains can be resolved adequately by regional to global-scale models, requiring appropriate downscaling or parameterization. It is likely that orographic precipitation will be affected by non-trivial feedbacks in climate change scenarios. Mountains modulate the climate and generate atmospheric waves of various scales. Whilst the longest mountain waves are adequately resolved in weather prediction models, the shortest still need to be parameterized, as they exert a drag on the atmosphere. This drag not only decelerates the global atmospheric circulation, but also affects temperatures in the polar stratosphere, which control ozone depletion. Locally, strong downslope winds linked to mountain waves (such as the Föhn and Bora) can cause severe damage. Mountain wave breaking in the high troposphere is a source of Clear-Air Turbulence, and large amplitude lee waves, as well as hydraulic jumps, can induce rotors near the surface, which are a major aviation safety hazard. In this issue, Beluši´c et al. report a possible occurrence of horizontal roll vortices over the Adriatic Sea during a Bora wind event. Data measured by the National Center for Atmospheric Research Electra aircraft, which sampled the turbulence structure above the Adriatic Sea during the Mesoscale Alpine Program, indicate that horizontal roll vortices with a wavelength of about 1km were generated in the strong cold and dry air outbreak associated with the Bora downslope flow. Using model parameters based on observations taken in the Madeira archipelago, Sachsperger et al. compare several different linear models of two-dimensional interfacial waves developing on a temperature inversion (e.g., the top of the boundary layer), describing how stability in the free atmosphere limits the possible range of lee wavelengths, modulates the length of the stationary wave mode, and strongly constrains the validity of the shallow-water (or long-wave) approximation. Baines and Johnson theoretically address nonlinear, two-layer, non-Boussinesq, hydrostatic flow with a rigid lid over a long obstacle, where upstream hydraulic jumps can occur, using a new internal hydraulic jump model. Regime diagrams based on the Froude number and the obstacle height show some novel and surprising features, namely in supercritical flow there are situations where hysteresis may occur, with the flow having multiple possible states for the same input conditions. Mountains induce different types of thermally and dynamically driven circulations (e.g., katabatic and anabatic flows), and affect the boundary layer characteristics and the dispersion of pollutants. Mountains also act to block strongly stratified air layers, leading to the formation

Observed April 1 snowpack accumulations within and near the Gunnison River basin in southwestern Colorado are compared with simulations from the Rhea-orographic.precipitation model to determine if the model simulates reliable magnitudes and temporal and spatial variability in winter precipitation for the basin. Twenty simulations of the Rhea model were performed using 'optimal' parameter sets determined for 10-kilometer (km) grids (10-km by 10-km grid cells) through stochastic calibration. Comparisons of Rhea-model simulations of winter precipitation with April 1 snowpack accumulations at 32 snowcourse stations were performed for the years 1972-1990. For most stations and most years the Rhea model reliably simulates the temporal and spatial variability in April 1 snowpack accumulations. However, in general, the Rhea-model underestimates April 1 snowpack accumulations in the Gunnison River basin area, and the underestimation is greatest for locations that receive the largest amount of snow. A significant portion of the error in Rhea-model simulations is due to the calibration of the Rhea model using gauge-catch precipitation measurements which can be as much as 50 percent below actual snowfall accumulations. Additional error in the Rhea-model simulations is a result of the comparison of gridded precipitation values to observed values measured at points. (KEY TERMS: precipitation modeling; orographic precipitation; water balance; western United States.)

The relationship between the winter (DJFM) precipitation and the atmospheric circulation patterns is examined around Mount Olympus, Greece in order to assess the effects of orography and atmospheric dynamics over a small (less than 100 x 100 km) spatial domain. Winter accumulated rainfall datasets from 8 stations spread along the eastern (marine) and western (continental) sides of the Mount Olympus at elevations between 30 m and 1150 m are used during the period 1981 to 2000. Synoptic scale conditions of mean sea-level pressure and geopotential heights at 850 hPa and 500 hPa, were used to explain the multiyear rainfall variability. High pressure systems dominated over the central Mediterranean and most parts of central Europe during the late 1980's and early 1990's, are associated with minimum winter rainfall along both sides of Mount Olympus. The winter of 1996 was associated with peak in rainfall along the marine side of the mountain and was characterized by enhancement of upper level trough over the western Mediterranean and increased low tropospheric depressions over the southern Adriatic and the Ionian Seas. This atmospheric circulation pattern facilitated a southeasterly air flow that affected more (less) the marine (continental) sides of the mountain. In contrast, dominance of low pressure systems with cores over the Gulf of Genoa and the Central Mediterranean affect the study area mostly from west/southwest revealing

Detailed measurements of temporal variations in the stable isotopic composition of precipitation and lake water were conducted in the permafrost region near Yakutsk, eastern Siberia. The δ 18 O ranged from approximately-30 to-5‰ in precipitation and from-25 to-5‰ in lake water were observed from April to August, 2000. Temporal changes in δ 18 O of precipitation observed weekly at 12 sites all showed the same trend. The temporal variation of δ 18 O in lakes classified into two groups: isotopically steady-state lakes with less than 5‰ variation, and non-steady-state lakes with variation exceeding 10‰. In non-steadystate lakes, the water originated from snowmelt, and the δ 18 O of lake water gradually enriched as a result of evaporation during the summer. In steady-state lakes, the water originated predominantly from 18 Oenriched lake water that had evaporated in the previous summer. The temporal volumetric and isotopic variations in alas lakes are accurately depicted by an isotope mass-balance model using Rayleigh fractionation over daily time steps. The inflow of soil water (subsurface flow) was estimated to be constant (200 m 3 /day) from May to August, based on the difference between observed and simulated lake volumes. Taking the isotopic mass-balance into consideration, the soil water in lower part of the active layer is inferred as a major component of inflow which has a δ 18 O of about-23‰. lakes". Because stream networks are not present and runoff is minimal except for snowmelt, evaporation from lake surfaces is the dominant outflow in alas lakes (Ishii and Yabuki, 2001). In permafrost regions, alas lakes have an important role in the hydrological cycle. There have been some studies on the water cycle of thermokarst lakes in North America (e.g., Gibson, 2002), but little research has been done in Siberia. Although climatic conditions in Siberia are more amenable to thermokarst lake development than those in North America (French,

This paper presents analyses of data collected with three ground-based Doppler radars during the Mesoscale Alpine Programme (MAP) Intensive Observation Period (IOP) 2b from 1900 UTC 19 September until 1100 UTC 20 September 1999. During this period, the synoptic situation was characterized by the propagation of a deep upper-tropospheric trough towards the Alps, as often observed when heavy precipitation occurs on the southern slopes of the Alpine massif. A frontal cloud system with embedded convective cells passed over northern Italy in association with the trough moving rapidly eastwards. Ahead of the advancing cold front, a strong south to south-easterly low-level ow impinged on the mountains near Lago Maggiore and produced copious amounts of rain (>200 mm at several locations). An analysis of three-dimensional radar-derived wind and precipitation elds shows that the most intense precipitation occurred where and when the easterly component of the low-to mid-level ow was the strongest with, however, a preferential location on the southern slopes of the Alps. This phenomenon can be explained by enhanced con uence between the main southerly ow and an intensifying easterly ow, and/or more favourable orientation of the moist in ow perpendicular to the mountain slopes. In addition, the propagation of convective cells from the Po Valley toward the Alps led to enhanced precipitation over the mountainous area. Taking advantage of the approximately two-dimensional character of the event, the different terms of the water budget are calculated for a series of 25 successive vertical cross-sections to analyse the transformation of atmospheric moisture into surface rainfall.

This paper presents analyses of data collected with three ground-based Doppler radars during the Mesoscale Alpine Programme (MAP) Intensive Observation Period (IOP) 2b from 1900 UTC 19 September until 1100 UTC 20 September 1999. During this period, the synoptic situation was characterized by the propagation of a deep upper-tropospheric trough towards the Alps, as often observed when heavy precipitation occurs on the southern slopes of the Alpine massif. A frontal cloud system with embedded convective cells passed over northern Italy in association with the trough moving rapidly eastwards. Ahead of the advancing cold front, a strong south to south-easterly low-level ow impinged on the mountains near Lago Maggiore and produced copious amounts of rain (>200 mm at several locations). An analysis of three-dimensional radar-derived wind and precipitation elds shows that the most intense precipitation occurred where and when the easterly component of the low-to mid-level ow was the strongest with, however, a preferential location on the southern slopes of the Alps. This phenomenon can be explained by enhanced con uence between the main southerly ow and an intensifying easterly ow, and/or more favourable orientation of the moist in ow perpendicular to the mountain slopes. In addition, the propagation of convective cells from the Po Valley toward the Alps led to enhanced precipitation over the mountainous area. Taking advantage of the approximately two-dimensional character of the event, the different terms of the water budget are calculated for a series of 25 successive vertical cross-sections to analyse the transformation of atmospheric moisture into surface rainfall.

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.