Climatology of Winter Orographic Precipitation over the Subtropical Central Andes and Associated Synoptic and Regional Characteristics

Abstract Winter orographic precipitation over the Andes between 30° and 37°S is examined using precipitation gauges in the mountains and adjacent lowlands. Because of the limited number of precipitation gauges, this paper focuses on the large-scale variation in cross-barrier precipitation and does not take into account the fine ridge–valley scale. The maximum amount of precipitation was observed on the windward slope of the mountain range below the crest, which was twice that observed on the low-windward side between 32.5° and 34°S. Toward the east of the crest, precipitation amounts drop sharply, generating a strong cross-barrier gradient. The rain shadow effect is greater in the north (32°–34.5°S) than in the south (35°–36.5°S) of the low-lee side, which is probably due to more baroclinic activity in southernmost latitudes and a southward decrease in the height of the Andes enabling more spillover precipitation. The effect of the Andes on winter precipitation is so marked that it modifies the precipitation regimes in the adjacent windward and leeward lowlands north of 35°S. Based on the fact that ~75% of the wintertime precipitation accumulated in the fourth quartile, through four or five heavy events on average, the synoptic-scale patterns of the heavy (into fourth quartile) orographic precipitation events were identified. Heavy events are strongly related to strong water vapor transport from the Pacific Ocean in the pre-cold-front environment of extratropical cyclones, which would have the form of atmospheric rivers as depicted in the reanalysis and rawinsonde data. The composite fields revealed a marked difference between two subgroups of heavy precipitation events. The extreme (100th–95th percentiles) events are associated with deeper cyclones than those for intense (95th–75th percentiles) events. These deeper cyclones lead to much stronger plumes of water vapor content and cross-barrier moisture flux against the high Andes, resulting in heavier orographic precipitation for extreme events. In addition, regional airflow characteristics suggest that the low-level flow is typically blocked and diverted poleward in the form of an along-barrier jet. On the lee side, downslope flow dominates during heavy events, producing prominent rain shadow effects as denoted by the domain of downslope winds extending to low-leeward side (i.e., zonda wind).

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Orographic Precipitation and Water Vapor Fractionation over the Southern Andes

Journal of Hydrometeorology ◽

10.1175/jhm555.1 ◽

2007 ◽

Vol 8 (1) ◽

pp. 3-19 ◽

Cited By ~ 81

Author(s):

Ronald B. Smith ◽

Jason P. Evans

Keyword(s):

Water Vapor ◽

Small Scale ◽

Orographic Precipitation ◽

Mountain Range ◽

Southern Andes ◽

Sharp Drop ◽

Convective Clouds ◽

Isotope Data ◽

Moderate Resolution Imaging Spectroradiometer ◽

Precipitation Events

Abstract The climatological nature of orographic precipitation in the southern Andes between 40° and 48°S is investigated primarily using stable isotope data from streamwater. In addition, four precipitation events are examined using balloon soundings and satellite images. The Moderate Resolution Imaging Spectroradiometer (MODIS) images taken during precipitation events reveal complex patterns of upstream open-cell convection over the ocean, stratus and/or convective clouds over the mountains, and sharp leeside clearing and roll convection over the steppe. Using the water vapor bands on MODIS reveals a sharp drop in column water vapor from about 1.4 to 0.7 cm across the mountain range. Seventy-one water samples from streams across the southern Andes provide deuterium and oxygen-18 isotope data to determine the drying ratio (DR) of airstreams crossing the mountain range and to constrain free parameters in a mathematical model of orographic precipitation. From the strong isotope fractionation associated with orographic precipitation, it is estimated that DR is ∼50%, the highest value yet found for a mountain range. The cloud delay parameters in a high-resolution linear precipitation model were optimized to fit the streamwater isotope data. The model agrees well with the data when the cloud delay time (i.e., elapsed time from condensation to precipitation) is about 1700 s. The tuned model is used to discuss the small-scale spatial pattern of precipitation. The isotope data from streams are also compared with data from sapwater. The good agreement suggests that future isotope mapping could be done using trees.

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Estimating the snow-rain transition zone in a semi-arid Andean catchment

10.5194/egusphere-egu21-2477 ◽

2021 ◽

Author(s):

Simone Schauwecker ◽

Gabriel Palma ◽

Shelley MacDonell ◽

Katerina Goubanova

Keyword(s):

Transition Zone ◽

Early Warning Systems ◽

Mountain Range ◽

Arid Environments ◽

Local Conditions ◽

The Andes ◽

Snow Line ◽

Snow Cover Extent ◽

Semi Arid ◽

Precipitation Events

The height of the snow-rain transition during infrequent but high impact precipitation events, closely related to the 0&#8304;C-isotherm, is a crucial variable for snow cover extent, high discharge flows and flash floods in semi-arid northern Chile. Estimations of the snow-rain transition zone and its past and future changes are therefore fundamental for adaptation strategies and might eventually serve to develop early warning systems in this region. However, there are important challenges that hinder the assessment of the snow-rain transition zone in semi-arid environments and little is known about past and future changes under different global warming scenarios. For example, there are few radiosonde observations along the Andes and most weather stations are located in valley bottoms, influenced by local conditions and the assumption of free-air temperature lapse rates contributes to the uncertainty. We combine different data sets to estimate the past snow-rain transition zone of our study site, the semi-arid Elqui river catchment. Pictures of the snow line after precipitation events - available from social networks - are used to visually estimate the snow line elevation. These values are in high agreement with vertically extrapolated temperature from meteorological stations. Furthermore, we identified considerable biases between the extrapolated 0&#8304;C-isotherm from meteorological stations and ERA5 reanalysis data. These large biases are probably due to the lowering of the freezing level over complex terrain and need further analysis. Our results contribute to an improved understanding of the snow-rain transition in this region, but also serve to derive a climatology of this key variable along the Andes mountain range, needed for future projections.

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The Impact of the Sierra Nevada on Low-Level Winds and Water Vapor Transport

Journal of Hydrometeorology ◽

10.1175/jhm599.1 ◽

2007 ◽

Vol 8 (4) ◽

pp. 790-804 ◽

Cited By ~ 18

Author(s):

Jinwon Kim ◽

Hyun-Suk Kang

Keyword(s):

Water Vapor ◽

Sierra Nevada ◽

Climate Model ◽

Northern Region ◽

Vapor Transport ◽

Water Vapor Transport ◽

Western Slope ◽

Mountain Range ◽

Low Level ◽

The Impact

Abstract To understand the influence of the Sierra Nevada on the water cycle in California the authors have analyzed low-level winds and water vapor fluxes upstream of the mountain range in regional climate model simulations. In a low Froude number (Fr) regime, the upstream low-level wind disturbances are characterized by the rapid weakening of the crosswinds and the appearance of a stagnation point over the southwestern foothills. The weakening of the low-level inflow is accompanied by the development of along-ridge winds that take the form of a barrier jet over the western slope of the mountain range. Such upstream wind disturbances are either weak or nonexistent in a high-Fr case. A critical Fr (Frc) of 0.35 inferred in this study is within the range of those suggested in previous observational and numerical studies. The depth of the blocked layer estimated from the along-ridge wind profile upstream of the northern Sierra Nevada corresponds to Frc between 0.3 and 0.45 as well. Associated with these low-level wind disturbances are significant low-level southerly moisture fluxes over the western slope and foothills of the Sierra Nevada in the low-Fr case, which result in significant exports of moisture from the southern Sierra Nevada to the northern region. This along-ridge low-level water vapor transport by blocking-induced barrier jets in a low-Fr condition may result in a strong north–south precipitation gradient over the Sierra Nevada.

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Quantitative Precipitation Forecasting of Wintertime Storms in the Sierra Nevada: Sensitivity to the Microphysical Parameterization and Horizontal Resolution

Monthly Weather Review ◽

10.1175/mwr3004.1 ◽

2005 ◽

Vol 133 (10) ◽

pp. 2834-2859 ◽

Cited By ~ 51

Author(s):

Vanda Grubišić ◽

Ramesh K. Vellore ◽

Arlen W. Huggins

Keyword(s):

Sierra Nevada ◽

Mesoscale Model ◽

Horizontal Resolution ◽

Orographic Precipitation ◽

Mountain Range ◽

Filamentary Structure ◽

Precipitation Forecasting ◽

Skill Scores ◽

Microphysical Parameterization ◽

Precipitation Events

Abstract The skill of a mesoscale model in predicting orographic precipitation during high-impact precipitation events in the Sierra Nevada, and the sensitivity of that skill to the choice of the microphysical parameterization and horizontal resolution, are examined. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and four bulk microphysical parameterization schemes examined are the Dudhia ice scheme, and the Schultz, GSFC, and Reisner2 mixed-phase schemes. The verification dataset consists of ground precipitation measurements from a selected number of wintertime heavy precipitation events documented during the Sierra Cooperative Pilot Project in the 1980s. At high horizontal resolutions, the predicted spatial precipitation patterns on the upwind Sierra Nevada slopes were found to have filamentary structure, with precipitation amounts over the transverse upwind ridges exceeding severalfold those over the nearby deep river valleys. The verification results show that all four tested bulk microphysical schemes in MM5 produce overprediction of precipitation on both the windward and lee slopes of the Sierra Nevada. The examined accuracy measures indicate that the Reisner2 scheme displays the best overall performance on both sides of the mountain range. The examined statistical skill scores on the other hand reveal that, regardless of the microphysical scheme used, the skill of the MM5 model in predicting the observed spatial distribution of the Sierra Nevada orographic precipitation is fairly low, that this skill is not improved by increasing the horizontal resolution of the model simulations, and that on average the quantitative precipitation forecasting (QPF) skill is better on the windward than on the lee side. Furthermore, a significance test shows that differences in skill scores obtained with the four microphysical schemes are not statistically significant.

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The Importance of Near-Surface Winter Precipitation Processes in Complex Alpine Terrain

Journal of Hydrometeorology ◽

10.1175/jhm-d-18-0055.1 ◽

2019 ◽

Vol 20 (2) ◽

pp. 177-196 ◽

Cited By ~ 7

Author(s):

Franziska Gerber ◽

Rebecca Mott ◽

Michael Lehning

Keyword(s):

Winter Precipitation ◽

Snow Accumulation ◽

Windward Side ◽

Leeward Side ◽

Small Scale ◽

Orographic Precipitation ◽

Near Surface ◽

Simulated Precipitation ◽

Snow Precipitation ◽

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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Effect of the Andes Cordillera on Precipitation from a Midlatitude Cold Front

Monthly Weather Review ◽

10.1175/2009mwr2881.1 ◽

2009 ◽

Vol 137 (9) ◽

pp. 3092-3109 ◽

Cited By ~ 53

Author(s):

Bradford S. Barrett ◽

RenéD. Garreaud ◽

Mark Falvey

Keyword(s):

Cold Front ◽

Orographic Precipitation ◽

Mountain Range ◽

Frontal Surface ◽

Central Chile ◽

The Andes ◽

Southeast Pacific ◽

Upper Level ◽

High Topography ◽

Andes Cordillera

Abstract The effects of the Andes Cordillera, the major mountain range in South America, on precipitation patterns of baroclinic systems approaching from the southeast Pacific remain largely unstudied. This study focuses on a case in late May 2008 when an upper-level trough and surface cold front produced widespread precipitation in central Chile. The primary goal was to analyze the physical mechanisms responsible for the structure and evolution of the precipitation. Weather Research and Forecasting (WRF) model simulations indicate that as an upper-level trough approached central Chile, midtropospheric flow below 700 hPa was blocked by the high topography and deflected poleward in the form of a barrier jet. This northerly jet had wind maxima in excess of 15 m s−1, was centered around 925 hPa, and extended westward 200 km from the mountains. It intersected the cold front, which approached from the south near the coast, thereby increasing convergence along the frontal surface, slowing its equatorward progress, and enhancing rainfall over central Chile. Another separate region of heavy precipitation formed over the upwind slopes of the cordillera. A trajectory analysis confirmed that the barrier jet moved low-level parcels from their origin in the moist southeast Pacific boundary layer to the coast. When model topography was reduced to twenty percent of its original height, the cold front advanced more rapidly to the northeast, generated less precipitation in central Chile between 33° and 36°S, and produced minimal orographic precipitation on the upwind Andean slopes. Based on these findings, the high topography appears responsible for not only orographic precipitation but also for substantially increasing precipitation totals over the central coast and valley.

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Impact of Tibetan Plateau Surface Heating on Persistent Extreme Precipitation Events in Southeastern China

Monthly Weather Review ◽

10.1175/mwr-d-17-0061.1 ◽

2017 ◽

Vol 145 (9) ◽

pp. 3485-3505 ◽

Cited By ~ 14

Author(s):

Bingcheng Wan ◽

Zhiqiu Gao ◽

Fei Chen ◽

Chungu Lu

Keyword(s):

High Pressure ◽

Water Vapor ◽

Southern China ◽

Vapor Transport ◽

Surface Heating ◽

Water Vapor Transport ◽

Precipitation Event ◽

Southeastern China ◽

Pressure Anomaly ◽

Precipitation Events

This paper combines observations, climatic analysis, and numerical modeling to investigate the Tibetan Plateau’s (TP) surface heating conditions’ influence on extreme persistent precipitation events (PEPEs) in southeastern China. Observations indicated an increase of TP surface air temperature 3–4 days prior to extreme persistent precipitation events in southeastern China. NCEP reanalysis data revealed a significant low pressure anomaly in southern China and a high pressure anomaly in northern China during extreme persistent precipitation event periods. Using correlation analysis and random resampling nonparametric statistics, a typical PEPE event from 17 to 25 June 2010 was selected for numerical simulation. The Weather Research and Forecasting (WRF) Model was used to investigate the impact of the TP’s surface heating on the evolution of this event. Three contrasting WRF experiments were conducted with different surface heating strengths by changing initial soil moisture over the TP. Different soil conditions generate different intensities of surface sensible heat fluxes and boundary layer structures over the TP resulting in two main effects on downstream convective rainfall: modulating large-scale atmospheric circulations and modifying the water vapor transport at southern China. Increased surface heating in the TP strengthens a high pressure system over the Yangtze Plain, thereby blocking the northward movement of precipitation. It also enhances the water vapor transport from the South China Sea to southern China. The combined effects substantially increase precipitation over most of the southeastern China region.

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Water Vapor Transport and the Production of Precipitation in the Eastern Fertile Crescent

Journal of Hydrometeorology ◽

10.1175/jhm550.1 ◽

2006 ◽

Vol 7 (6) ◽

pp. 1295-1307 ◽

Cited By ~ 30

Author(s):

J. P. Evans ◽

R. B. Smith

Keyword(s):

Water Vapor ◽

Land Surface ◽

Clustering Algorithm ◽

Mesoscale Model ◽

Water Vapor Transport ◽

Surface Model ◽

Precipitation Pattern ◽

Total Precipitation ◽

Fertile Crescent ◽

Precipitation Events

Abstract The study presented here attempts to quantify the significance of southerly water vapor fluxes on precipitation occurring in the eastern Fertile Crescent region. The water vapor fluxes were investigated at high temporal and spatial resolution by using a Regional Climate Model [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5)–Noah land surface model] to downscale the NCEP–NCAR reanalysis. Using the Iterative Self-Organizing Data Analysis Techniques (ISODATA) clustering algorithm, the 200 largest precipitation events, occurring from 1990 through 1994, were grouped into classes based on the similarity of their water vapor fluxes. Results indicate that, while southerly fluxes were dominant in 24% of tested events, these events produced 43% of the total precipitation produced by the 200 largest events. Thus, while the majority of precipitation events occurring in the Fertile Crescent involve significant water vapor advected from the west, those events that included southerly fluxes produced much larger precipitation totals. This suggests that changes that affect these southerly fluxes more than the westerly fluxes (e.g., changes in the Indian monsoon, movement of the head of the Persian Gulf, etc.) may have a relatively strong affect on the total precipitation falling in the Fertile Crescent even though they affect relatively few precipitation events. To obtain a clearer view of the precipitation mechanisms, the authors used a linear model, along with the estimated water vapor fluxes, to downscale from 25 to 1 km. The result shows a spectrum of mountain scales not seen in the regional model, exerting tight control on the precipitation pattern.

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Intensified Moisture Sources of Heavy Precipitation Events Contributed to Interannual Trend in Precipitation Over the Three-Rivers-Headwater Region in China

Frontiers in Earth Science ◽

10.3389/feart.2021.674037 ◽

2021 ◽

Vol 9 ◽

Author(s):

Ruiyu Zhao ◽

Bin Chen ◽

Xiangde Xu

Keyword(s):

Water Vapor ◽

Heavy Precipitation ◽

Water Vapor Transport ◽

Boreal Summer ◽

Precipitation Trend ◽

Moisture Source ◽

Moisture Sources ◽

Increasing Trend ◽

Three Rivers ◽

Precipitation Events

Evidence has indicated an overall wetting trend over the Three-Rivers Headwater Region (TRHR) in the recent decades, whereas the possible mechanisms for this change remain unclear. Detecting the main moisture source regions of the water vapor and its increasing trend over this region could help understand the long-term precipitation change. Based on the gauge-based precipitation observation analysis, we find that the heavy precipitation events act as the main contributor to the interannual increasing trend of summer precipitation over the TRHR. A Lagrangian moisture tracking methodology is then utilized to identify the main moisture source of water vapor over the target region for the boreal summer period of 1980–2017, with focus particularly on exploring its change associated with the interannual trend of precipitation. On an average, the moisture sources for the target regions cover vast regions, including the west and northwest of the Tibetan Plateau by the westerlies, the southwest by the Indian summer monsoon, and the adjacent regions associated with the local recycling. However, the increased interannual precipitation trend over the TRHR could be largely attributed to the enhanced moisture sources from the neighboring northeastern areas of the targeted region, particularly associated with the heavy precipitation events. The increased water vapor transport from the neighboring areas of the TRHR potentially related to the enhanced local hydrological recycling over these regions plays a first leading role in the recent precipitation increase over the TRHR.

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Mineralogical and chemical variability of soils across a tropical ocean island climate gradient, San Cristóbal, Galápagos

10.5194/egusphere-egu2020-4266 ◽

2020 ◽

Author(s):

Madelyn Percy ◽

Larry Benninger

Keyword(s):

Mass Balance ◽

Chemical Weathering ◽

Amorphous Material ◽

Windward Side ◽

Leeward Side ◽

Tropical Ocean ◽

Balance Approach ◽

Precipitation Regimes ◽

Weathering Intensity ◽

Mass Balance Approach

Little data currently exist on the chemistry of soils on the island of San Crist&#243;bal, Gal&#225;pagos, despite the importance of this data in understanding how the island has weathered through time. We sought to resolve this lack of data by surveying soils from different elevations and in different climate zones across the island. We collected soil samples from transects and sites across a precipitation-gradient in order to describe the mineralogy and chemistry of the soils, and to understand how soils have weathered in different precipitation regimes across the island. We used a mass balance approach, coupled with chemical weathering indices, to understand profile-scale to site-scale differences in weathering.Climate-dependent shifts in soil characteristics are apparent: at the wettest sites, the soils have the lowest pH, the highest percentage of amorphous material, and the highest loss on ignition values. We compared the saprolite, the basal material from the soil pits in which the basalt bedrock&#8217;s texture was still apparent but the material was extremely friable, and previously reported unweathered bedrock data, showing that the saprolite was highly weathered relative to the unweathered bedrock. Using the mass balance approach, we show that while base cations have been lost from soils relative to the parent material underlying the profiles, aluminum and iron concentrations have remained the same or have increased.&#160;We used chemical indices of weathering as evidence for the relationship between weathering intensity and precipitation, with greater weathering intensity observed in the very humid highlands compared to the less intense weathering that has occurred in the arid lowlands. The windward side of the island shows higher intensities of weathering than the leeward side. Our findings conform with other soil chemistry studies on the islands of Santa Cruz and Isabela, also in the Gal&#225;pagos archipelago, showing that more intense weathering, accompanied by a greater loss of mobile elements, is observed at wetter sites.

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