The temperature dependence of small-scale orographic precipitation enhancement

Abstract In this study, near-surface snow and graupel dynamics from formation to deposition are analyzed using WRF in a large-eddy configuration. The results reveal that a horizontal grid spacing of ≤50 m is required to resolve local orographic precipitation enhancement, leeside flow separation, and thereby preferential deposition. At this resolution, precipitation patterns across mountain ridges show a high temporal and spatial variability. Simulated and observed event-mean snow precipitation across three mountain ridges in the upper Dischma valley (Davos, Switzerland) for two precipitation events show distinct patterns, which are in agreement with theoretical concepts, such as small-scale orographic precipitation enhancement or preferential deposition. We found for our case study that overall terrain–flow–precipitation interactions increase snow accumulation on the leeward side of mountain ridges by approximately 26%–28% with respect to snow accumulation on the windward side of the ridge. Cloud dynamics and mean advection may locally increase precipitation on the leeward side of the ridge by up to about 20% with respect to event-mean precipitation across a mountain ridge. Analogously, near-surface particle–flow interactions, that is, preferential deposition, may locally enhance leeward snow precipitation on the order of 10%. We further found that overall effect and relative importance of terrain–flow–precipitation interactions are strongly dependent on atmospheric humidity and stability. Weak dynamic stability is important for graupel production, which is an essential component of solid winter precipitation. A comparison to smoothed measurements of snow depth change reveals a certain agreement with simulated precipitation across mountain ridges.

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Sensitivity Studies of the Role of Aerosols in Warm-Phase Orographic Precipitation in Different Dynamical Flow Regimes

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2492.1 ◽

2008 ◽

Vol 65 (8) ◽

pp. 2522-2542 ◽

Cited By ~ 42

Author(s):

Andreas Muhlbauer ◽

Ulrike Lohmann

Keyword(s):

Weather Prediction ◽

Flow Dynamics ◽

Leeward Side ◽

Small Scale ◽

Orographic Precipitation ◽

Precipitation Distribution ◽

Aerosol Cloud ◽

Orographic Clouds ◽

Aerosol Cloud Interactions ◽

Precipitation Enhancement

Abstract Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In the case of orographic clouds, it is suspected that aerosol–cloud interactions reduce the amount of precipitation on the upslope side of the mountain and enhance the precipitation on the downslope side when the number of aerosols is increased. The net effect may lead to a shift of the precipitation distribution toward the leeward side of mountain ranges, which affects the hydrological cycle on the local scale. In this study aerosol–cloud interactions in warm-phase clouds and the possible impact on the orographic precipitation distribution are investigated. Herein, simulations of moist orographic flow over topography are conducted and the influence of anthropogenic aerosols on the orographic precipitation formation is analyzed. The degree of aerosol pollution is prescribed by different aerosol spectra that are representative for central Switzerland. The simulations are performed with the Consortium for Small-Scale Modeling’s mesoscale nonhydrostatic limited-area weather prediction model (COSMO) with a horizontal grid spacing of 2 km and a fully coupled aerosol–cloud parameterization. It is found that an increase in the aerosol load leads to a downstream shift of the orographic precipitation distribution and to an increase in the spillover factor. A reduction of warm-phase orographic precipitation is observed at the upslope side of the mountain. The downslope precipitation enhancement depends critically on the width of the mountain and on the flow dynamics. In the case of orographic precipitation induced by stably stratified unblocked flow, the loss in upslope precipitation is not compensated by leeward precipitation enhancement. In contrast, flow blocking may lead to leeward precipitation enhancement and eventually to a compensation of the upslope precipitation loss. The simulations also indicate that latent heat effects induced by aerosol–cloud–precipitation interactions may considerably affect the orographic flow dynamics and consequently feed back on the orographic precipitation development.

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Synoptic Control over Orographic Precipitation Distributions during the Olympics Mountains Experiment (OLYMPEX)

Monthly Weather Review ◽

10.1175/mwr-d-17-0267.1 ◽

2018 ◽

Vol 146 (4) ◽

pp. 1023-1044 ◽

Cited By ~ 10

Author(s):

David J. Purnell ◽

Daniel J. Kirshbaum

Keyword(s):

Numerical Simulations ◽

Physical Interpretation ◽

Large Scale ◽

Local Development ◽

Moisture Flux ◽

Heavy Precipitation ◽

Static Stability ◽

Orographic Precipitation ◽

Areal Extent ◽

Precipitation Enhancement

The synoptic controls on orographic precipitation during the Olympics Mountains Experiment (OLYMPEX) are investigated using observations and numerical simulations. Observational precipitation retrievals for six warm-frontal (WF), six warm-sector (WS), and six postfrontal (PF) periods indicate that heavy precipitation occurred in both WF and WS periods, but the latter saw larger orographic enhancements. Such enhancements extended well upstream of the terrain in WF periods but were focused over the windward slopes in both PF and WS periods. Quasi-idealized simulations, constrained by OLYMPEX data, reproduce the key synoptic sensitivities of the OLYMPEX precipitation distributions and thus facilitate physical interpretation. These sensitivities are largely explained by three upstream parameters: the large-scale precipitation rate [Formula: see text], the impinging horizontal moisture flux I, and the low-level static stability. Both WF and WS events exhibit large [Formula: see text] and I, and thus, heavy orographic precipitation, which is greatly enhanced in amplitude and areal extent by the seeder–feeder process. However, the stronger stability of the WF periods, particularly within the frontal inversion (even when it lies above crest level), causes their precipitation enhancement to weaken and shift upstream. In contrast, the small [Formula: see text] and I, larger static stability, and absence of stratiform feeder clouds in the nominally unsaturated and convective PF events yield much lighter time- and area-averaged precipitation. Modest enhancements still occur over the windward slopes due to the local development and invigoration of shallow convective showers.

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Characteristics of Large-Scale Orographic Precipitation in a Linear Perspective

Journal of Hydrometeorology ◽

10.1175/2010jhm1231.1 ◽

2011 ◽

Vol 12 (1) ◽

pp. 27-44 ◽

Cited By ~ 8

Author(s):

Michael Kunz

Keyword(s):

Time Scales ◽

Characteristic Time ◽

Large Scale ◽

Linear Perspective ◽

Horizontal Wind ◽

Small Scale ◽

Orographic Precipitation ◽

Observation Data ◽

Horizontal Wind Speed ◽

Precipitation Patterns

Abstract Simulations of orographic precipitation over the low mountain ranges of southwestern Germany and eastern France with two different physics-based linear precipitation models are presented. Both models are based on 3D airflow dynamics from linear theory and consider advection of condensed water and leeside drying. Sensitivity studies for idealized conditions and a real case study show that the amount and spatial distribution of orographic precipitation is strongly controlled by characteristic time scales for cloud and hydrometeor advection and background precipitation due to large-scale lifting. These parameters are estimated by adjusting the model results on a 2.5-km grid to observed precipitation patterns for a sample of 40 representative orography-dominated stratiform events (24 h) during a calibration period (1971–80). In general, the best results in terms of lowest rmse and bias are obtained for characteristic time scales of 1600 s and background precipitation of 0.4 mm h−1. Model simulations of a sample of 84 events during an application period (1981–2000) with fixed parameters demonstrate that both models are able to reproduce quantitatively precipitation patterns obtained from observations and reanalyses from a numerical model [Consortium for Small-scale Modeling (COSMO)]. Combining model results with observation data shows that heavy precipitations over mountains are restricted to situations with strong atmospheric forcings in terms of synoptic-scale lifting, horizontal wind speed, and moisture content.

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Strong Cross-Barrier Flow under Stable Conditions Producing Intense Winter Orographic Precipitation: A Case Study over the Subtropical Central Andes

Weather and Forecasting ◽

10.1175/2009waf2222168.1 ◽

2009 ◽

Vol 24 (4) ◽

pp. 1009-1031 ◽

Cited By ~ 22

Author(s):

Maximiliano Viale ◽

Federico A. Norte

Keyword(s):

Heavy Precipitation ◽

Stable Stratification ◽

Central Andes ◽

Precipitation Event ◽

Windward Side ◽

Orographic Precipitation ◽

Near Surface ◽

Low Level ◽

Precipitation Enhancement ◽

Subtropical Central Andes

Abstract The most intense orographic precipitation event over the subtropical central Andes (36°–30°S) during winter 2005 was examined using observational data and a regional model simulation. The Eta-Programa Regional de Meteorología (PRM) model forecast was evaluated and used to explore the airflow structure that generated this heavy precipitation event, with a focus on orographic influences. Even though the model did not realistically reproduce any near-surface variables, nor the precipitation shadow in the leeside lowlands, its reliable forecast of heavy precipitation over the windward side and the wind fields suggests that it can be used as a valuable forecasting tool for such events in the region. The synoptic flow of the 26–29 August 2005 storm responded to a well-defined dipole from low to upper levels with anomalous low (high) geopotential heights at midlatitudes (subtropical) latitudes located off the southeast Pacific coast, resulting in a large meridional geopotential height gradient that drove a strong anomalous cross-barrier flow. Precipitation enhancement in the Andes was observed during the entire event; however, the highest rates were in the prefrontal sector under the low-level stable stratification and cross-barrier winds exceeding 2.5 standard deviations (σ) from the climatological monthly mean. The combination of strong cross-mountain winds with the stable stratification in the air mass of a frontal system, impinging on the high Andes range, appears to be the major factor in determining the flow structure that produced the pattern of precipitation enhancement, with uplift maximized near mountaintops and low-level blocking upwindleading to the formation of a low-level along-barrier jet. Additionally, only the upstream wind anomalies for the 15 heaviest events over a 10-yr (1967–76) period were investigated. They exhibited strong anomalous northwesterly winds for 14 of the 15 events, whereas for the remaining event there were no available observations to evaluate. Thus, these anomalies may also be exploited for forecasting capabilities.

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Intensity-dependent parameterization of elevation effects in precipitation analysis

Advances in Geosciences ◽

10.5194/adgeo-20-33-2009 ◽

2009 ◽

Vol 20 ◽

pp. 33-38 ◽

Cited By ~ 35

Author(s):

T. Haiden ◽

G. Pistotnik

Keyword(s):

Elevation Gradient ◽

Precipitation Rate ◽

Precipitation Process ◽

Orographic Precipitation ◽

Elevation Gradients ◽

Feeder Mechanism ◽

Annual Total ◽

The Cost ◽

Precipitation Enhancement

Abstract. Elevation effects in long-term (monthly to inter-annual) precipitation data have been widely studied and are taken into account in the regionalization of point-like precipitation amounts by using methods like external drift kriging and cokriging. On the daily or hourly time scale, precipitation-elevation gradients are more variable, and difficult to parameterize. For example, application of the annual relative precipitation-elevation gradient to each 12-h sub-period reproduces the annual total, but at the cost of a large root-mean-square error. If the precipitation-elevation gradient is parameterized as a function of precipitation rate, the error can be substantially reduced. It is shown that the form of the parameterization suggested by the observations conforms to what one would expect based on the physics of the orographic precipitation process (the seeder-feeder mechanism). At low precipitation rates, orographic precipitation is "conversion-limited", thus increasing roughly linearly with precipitation rate. At higher rates, orographic precipitation becomes "condensation-limited" thus leading to an additive rather than multiplicative orographic precipitation enhancement. Also it is found that for large elevation differences it becomes increasingly important to take into account those events where the mountain station receives precipitation but the valley station remains dry.

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Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-6681-2021 ◽

2021 ◽

Vol 21 (9) ◽

pp. 6681-6706

Author(s):

Fabiola Ramelli ◽

Jan Henneberger ◽

Robert O. David ◽

Johannes Bühl ◽

Martin Radenz ◽

...

Keyword(s):

Mixed Phase ◽

Swiss Alps ◽

Ice Formation ◽

Small Scale ◽

Orographic Precipitation ◽

Local Flow ◽

Low Level ◽

Cloud Radar ◽

Ice Production

Abstract. The seeder–feeder mechanism has been observed to enhance orographic precipitation in previous studies. However, the microphysical processes active in the seeder and feeder region are still being understood. In this paper, we investigate the seeder and feeder region of a mixed-phase cloud passing over the Swiss Alps, focusing on (1) fallstreaks of enhanced radar reflectivity originating from cloud top generating cells (seeder region) and (2) a persistent low-level feeder cloud produced by the boundary layer circulation (feeder region). Observations were obtained from a multi-dimensional set of instruments including ground-based remote sensing instrumentation (Ka-band polarimetric cloud radar, microwave radiometer, wind profiler), in situ instrumentation on a tethered balloon system, and ground-based aerosol and precipitation measurements. The cloud radar observations suggest that ice formation and growth were enhanced within cloud top generating cells, which is consistent with previous observational studies. However, uncertainties exist regarding the dominant ice formation mechanism within these cells. Here we propose different mechanisms that potentially enhance ice nucleation and growth in cloud top generating cells (convective overshooting, radiative cooling, droplet shattering) and attempt to estimate their potential contribution from an ice nucleating particle perspective. Once ice formation and growth within the seeder region exceeded a threshold value, the mixed-phase cloud became fully glaciated. Local flow effects on the lee side of the mountain barrier induced the formation of a persistent low-level feeder cloud over a small-scale topographic feature in the inner-Alpine valley. In situ measurements within the low-level feeder cloud observed the production of secondary ice particles likely due to the Hallett–Mossop process and ice particle fragmentation upon ice–ice collisions. Therefore, secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountaintop observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.

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Intercomparison of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-8173-2010 ◽

2010 ◽

Vol 10 (17) ◽

pp. 8173-8196 ◽

Cited By ~ 39

Author(s):

A. Muhlbauer ◽

T. Hashino ◽

L. Xue ◽

A. Teller ◽

U. Lohmann ◽

...

Keyword(s):

Cloud Droplet ◽

Cloud Microphysics ◽

Mixed Phase ◽

Small Scale ◽

Orographic Precipitation ◽

Model Intercomparison ◽

Droplet Coalescence ◽

Aerosol Cloud ◽

Aerosol Effect ◽

Mixed Phase Clouds

Abstract. Anthropogenic aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is hypothesized to retard the cloud droplet coalescence and the riming in mixed-phase clouds, thereby decreasing orographic precipitation. This study presents results from a model intercomparison of 2-D simulations of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds. The sensitivity of orographic precipitation to changes in the aerosol number concentrations is analysed and compared for various dynamical and thermodynamical situations. Furthermore, the sensitivities of microphysical processes such as coalescence, aggregation, riming and diffusional growth to changes in the aerosol number concentrations are evaluated and compared. The participating numerical models are the model from the Consortium for Small-Scale Modeling (COSMO) with bulk microphysics, the Weather Research and Forecasting (WRF) model with bin microphysics and the University of Wisconsin modeling system (UWNMS) with a spectral ice habit prediction microphysics scheme. All models are operated on a cloud-resolving scale with 2 km horizontal grid spacing. The results of the model intercomparison suggest that the sensitivity of orographic precipitation to aerosol modifications varies greatly from case to case and from model to model. Neither a precipitation decrease nor a precipitation increase is found robustly in all simulations. Qualitative robust results can only be found for a subset of the simulations but even then quantitative agreement is scarce. Estimates of the aerosol effect on orographic precipitation are found to range from −19% to 0% depending on the simulated case and the model. Similarly, riming is shown to decrease in some cases and models whereas it increases in others, which implies that a decrease in riming with increasing aerosol load is not a robust result. Furthermore, it is found that neither a decrease in cloud droplet coalescence nor a decrease in riming necessarily implies a decrease in precipitation due to compensation effects by other microphysical pathways. The simulations suggest that mixed-phase conditions play an important role in buffering the effect of aerosol perturbations on cloud microphysics and reducing the overall susceptibility of clouds and precipitation to changes in the aerosol number concentrations. As a consequence the aerosol effect on precipitation is suggested to be less pronounced or even inverted in regions with high terrain (e.g., the Alps or Rocky Mountains) or in regions where mixed-phase microphysics is important for the climatology of orographic precipitation.

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