Upstream Orographic Enhancement of a Narrow Cold-Frontal Rainband Approaching the Andes

2013 ◽  
Vol 141 (5) ◽  
pp. 1708-1730 ◽  
Author(s):  
Maximiliano Viale ◽  
Robert A. Houze ◽  
Kristen L. Rasmussen
Keyword(s):  
Wrf Model ◽  
Tropical Rainfall Measuring Mission ◽  
Mountain Ranges ◽  
Low Level ◽  
Atmospheric River ◽  
The Andes ◽  
The Pacific ◽  
Stratiform Clouds ◽  
Orographic Enhancement ◽  
Trmm Precipitation

Abstract Upstream orographic enhancement of the rainfall from an extratropical cyclone approaching the Andes from the Pacific is investigated using the Weather Research and Forecasting (WRF) Model and the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar. The main precipitation from the cyclone over central and coastal Chile fell when a narrow cold-frontal rainband (NCFR) interacted with a midlevel layer cloud deck formed from the orographically induced ascent of the prefrontal “atmospheric river” upstream of the Andes. Model output indicates that low-level convergence enhanced the NCFR where it met low-level blocked flow near the mountains. The NCFR had stronger updrafts with decreasing distance from the mountains, and the NCFR produced larger rain accumulations over the land region upstream of the Andes than over the open ocean. A sensitivity simulation with a 50% reduction in the Andes topography, for comparison to various west coast mountain ranges of North America, demonstrates that the extreme height of the real mountain barrier strengthens frontogenesis and upstream blocking, which produces stronger frontal lifting and a slower progression of the frontal system. The model and the satellite data suggest that the larger precipitation rates upstream of the Andes resulted from a seeder–feeder effect connected with the orographically invigorated NCFR updrafts, when they penetrated the orographically enhanced midlevel stratiform clouds forming as a result of the upstream orographic ascent of the atmospheric river. The supercooled water of the NCFR updrafts formed a feeder zone for the snow particles in the midlevel stratiform cloud just upstream of the Andes.

scholarly journals Evaluation of the Moisture Sources in two Extreme Landfalling Atmospheric River Events using an Eulerian WRF-Tracers tool

2017 ◽  
Author(s):  
Jorge Eiras-Barca ◽  
Francina Dominguez ◽  
Huancui Hu ◽  
A. Daniel Garaboa-Paz ◽  
Gonzalo Miguez-Macho
Keyword(s):  
Cross Sections ◽  
North Atlantic ◽  
Cold Front ◽  
Wrf Model ◽  
Atmospheric River ◽  
The North ◽  
Northern Latitudes ◽  
The Pacific ◽  
Low Level Jet ◽  
Tropical Moisture

Abstract. A new 3D Tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme Atmospheric River (AR) events: the so-called Great Coast Gale of 2007 in the Pacific Basin, and the Great Storm of 1987 in the North Atlantic. Results show that between 80 % and 90 % of the moisture advected by the ARs, as well as between 70 % and 80 % of the associated precipitation have a tropical or subtropical origin. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives to land. Vertical cross sections of the moisture suggest that the maximum in humidity does not necessarily coincide with the Low-Level Jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes, and can be located above, below or ahead the LLJ in northern latitudes in both analyzed cases.

Evaluation and Comparison of Microphysical Algorithms in ARW-WRF Model Simulations of Atmospheric River Events Affecting the California Coast

2009 ◽  
Vol 10 (4) ◽  
pp. 847-870 ◽  
Author(s):  
Isidora Jankov ◽  
Jian-Wen Bao ◽  
Paul J. Neiman ◽  
Paul J. Schultz ◽  
Huiling Yuan ◽  
...  
Keyword(s):  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Test Bed ◽  
Cool Season ◽  
Model Simulations ◽  
Surface Precipitation ◽  
Low Level ◽  
Atmospheric River ◽  
Microphysical Parameterization ◽  
Precipitation Events

Abstract Numerical prediction of precipitation associated with five cool-season atmospheric river events in northern California was analyzed and compared to observations. The model simulations were performed by using the Advanced Research Weather Research and Forecasting Model (ARW-WRF) with four different microphysical parameterizations. This was done as a part of the 2005–06 field phase of the Hydrometeorological Test Bed project, for which special profilers, soundings, and surface observations were implemented. Using these unique datasets, the meteorology of atmospheric river events was described in terms of dynamical processes and the microphysical structure of the cloud systems that produced most of the surface precipitation. Events were categorized as “bright band” (BB) or “nonbright band” (NBB), the differences being the presence of significant amounts of ice aloft (or lack thereof) and a signature of higher reflectivity collocated with the melting layer produced by frozen precipitating particles descending through the 0°C isotherm. The model was reasonably successful at predicting the timing of surface fronts, the development and evolution of low-level jets associated with latent heating processes and terrain interaction, and wind flow signatures consistent with deep-layer thermal advection. However, the model showed the tendency to overestimate the duration and intensity of the impinging low-level winds. In general, all model configurations overestimated precipitation, especially in the case of BB events. Nonetheless, large differences in precipitation distribution and cloud structure among model runs using various microphysical parameterization schemes were noted.

scholarly journals Terrain Trapped Airflows and Precipitation Variability during an Atmospheric River Event

2020 ◽  
Vol 21 (2) ◽  
pp. 355-375 ◽  
Author(s):  
Ju-Mee Ryoo ◽  
Sen Chiao ◽  
J. Ryan Spackman ◽  
Laura T. Iraci ◽  
F. Martin Ralph ◽  
...  
Keyword(s):  
Wrf Model ◽  
Wind Fields ◽  
Low Level ◽  
Atmospheric River ◽  
Gap Flow ◽  
Period 2 ◽  
Modeling Systems ◽  
In Situ Observations ◽  
Aircraft Data

AbstractWe examine thermodynamic and kinematic structures of terrain trapped airflows (TTAs) during an atmospheric river (AR) event impacting Northern California 10–11 March 2016 using Alpha Jet Atmospheric eXperiment (AJAX) aircraft data, in situ observations, and Weather and Research Forecasting (WRF) Model simulations. TTAs are identified by locally intensified low-level winds flowing parallel to the coastal ranges and having maxima over the near-coastal waters. Multiple mechanisms can produce TTAs, including terrain blocking and gap flows. The changes in winds can significantly alter the distribution, timing, and intensity of precipitation. We show here how different mechanisms producing TTAs evolve during this event and influence local precipitation variations. Three different periods are identified from the time-varying wind fields. During period 1 (P1), a TTA develops during synoptic-scale onshore flow that backs to southerly flow near the coast. This TTA occurs when the Froude number (Fr) is less than 1, suggesting low-level terrain blocking is the primary mechanism. During period 2 (P2), a Petaluma offshore gap flow develops, with flows turning parallel to the coast offshore and with Fr > 1. Periods P1 and P2 are associated with slightly more coastal than mountain precipitation. In period 3 (P3), the gap flow initiated during P2 merges with a pre-cold-frontal low-level jet (LLJ) and enhanced precipitation shifts to higher mountain regions. Dynamical mixing also becomes more important as the TTA becomes confluent with the approaching LLJ. The different mechanisms producing TTAs and their effects on precipitation pose challenges to observational and modeling systems needed to improve forecasts and early warnings of AR events.

scholarly journals Numerical simulation of a rare winter hailstorm event over Delhi, India on 17 January 2013

2014 ◽  
Vol 14 (12) ◽  
pp. 3331-3344 ◽  
Author(s):  
A. Chevuturi ◽  
A. P. Dimri ◽  
U. B. Gunturu
Keyword(s):  
Numerical Simulation ◽  
Deep Convection ◽  
Baroclinic Instability ◽  
Wrf Model ◽  
Tropical Rainfall Measuring Mission ◽  
Cloud Formation ◽  
New Delhi ◽  
National Capital Region ◽  
Microphysics Scheme ◽  
Low Level

Abstract. This study analyzes the cause of the rare occurrence of a winter hailstorm over New Delhi/NCR (National Capital Region), India. The absence of increased surface temperature or low level of moisture incursion during winter cannot generate the deep convection required for sustaining a hailstorm. Consequently, NCR shows very few cases of hailstorms in the months of December-January-February, making the winter hail formation a question of interest. For this study, a recent winter hailstorm event on 17 January 2013 (16:00–18:00 UTC) occurring over NCR is investigated. The storm is simulated using the Weather Research and Forecasting (WRF) model with the Goddard Cumulus Ensemble (GCE) microphysics scheme with two different options: hail and graupel. The aim of the study is to understand and describe the cause of hailstorm event during over NCR with a comparative analysis of the two options of GCE microphysics. Upon evaluating the model simulations, it is observed that the hail option shows a more similar precipitation intensity with the Tropical Rainfall Measuring Mission (TRMM) observation than the graupel option does, and it is able to simulate hail precipitation. Using the model-simulated output with the hail option; detailed investigation on understanding the dynamics of hailstorm is performed. The analysis based on a numerical simulation suggests that the deep instability in the atmospheric column led to the formation of hailstones as the cloud formation reached up to the glaciated zone promoting ice nucleation. In winters, such instability conditions rarely form due to low level available potential energy and moisture incursion along with upper level baroclinic instability due to the presence of a western disturbance (WD). Such rare positioning is found to be lowering the tropopause with increased temperature gradient, leading to winter hailstorm formation.

Orogenic Convection in Subtropical South America as Seen by the TRMM Satellite

2011 ◽  
Vol 139 (8) ◽  
pp. 2399-2420 ◽  
Author(s):  
Kristen L. Rasmussen ◽  
Robert A. Houze
Keyword(s):  
South America ◽  
Tropical Rainfall Measuring Mission ◽  
The United States ◽  
South American ◽  
Convective Storms ◽  
Mountain Ranges ◽  
Low Level ◽  
Global Understanding ◽  
Low Level Jet ◽  
Downslope Flow

AbstractExtreme orogenic convective storms in southeastern South America are divided into three categories: storms with deep convective cores, storms with wide convective cores, and storms containing broad stratiform regions. Data from the Tropical Rainfall Measuring Mission satellite’s Precipitation Radar show that storms with wide convective cores are the most frequent, tending to originate near the Sierra de Cordoba range. Downslope flow at upper levels caps a nocturnally enhanced low-level jet, thus preventing convection from breaking out until the jet hits a steep slope of terrain, such as the Sierra de Cordoba Mountains or Andean foothills, so that the moist low-level air is lifted enough to release the instability and overcome the cap. This capping and triggering is similar to the way intense convection is released near the northwestern Himalayas. However, the intense storms with wide convective cores over southeastern South America are unlike their Himalayan counterparts in that they exhibit leading-line/trailing-stratiform organization and are influenced by baroclinic troughs more similar to storms east of the Rocky Mountains in the United States. Comparison of South American storms containing wide convective cores with storms in other parts of the world contributes to a global understanding of how major mountain ranges influence precipitating cloud systems.

Characteristics of Precipitating Convective Systems in the Premonsoon Season of South Asia

2011 ◽  
Vol 12 (2) ◽  
pp. 157-180 ◽  
Author(s):  
Ulrike Romatschke ◽  
Robert A. Houze
Keyword(s):  
South Asia ◽  
Tropical Rainfall Measuring Mission ◽  
Maximum Size ◽  
Early Morning ◽  
Mountain Ranges ◽  
Convective Systems ◽  
Diurnal Cycles ◽  
Synoptic Forcing ◽  
Trmm Precipitation ◽  
Downslope Flow

Abstract Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data obtained over South Asia during eight premonsoon seasons (March–May) show that the precipitation is more convective in nature and more sensitive to synoptic forcing than during the monsoon. Over land areas, most rain falls from medium-sized systems (8500–35 000 km2 in horizontal area). In continental regions near the Himalayas, these medium-sized systems are favored by 500-mb trough conditions and are of two main types: 1) systems triggered by daytime heating over high terrain and growing to reach maximum size a few hours later and 2) systems triggered at night, as moist upstream flow is lifted over cold downslope flow from the mountains, and reaching maximum development upstream of the central and eastern Himalayas in the early morning hours. Systems triggered by similar mechanisms also account for the precipitation maxima in mountainous coastal regions, where the diurnal cycles are dominated by systems triggered in daytime over the higher coastal terrain. Medium-sized nocturnal systems are also found upstream of coastal mountain ranges. West-coastal precipitation systems over India and Myanmar are favored when low pressure systems occur over the upstream oceans, whereas Indian east-coastal systems occur when high pressure dominates over Bangladesh. Over the Bay of Bengal, the dominant systems are larger (>35 000 km2), and have large stratiform components. They occur in connection with depressions over the Bay and exhibit a weaker diurnal cycle.

scholarly journals Detection of Spatial Rainfall Variation over the Andean Region Demonstrated by Satellite-Based Observations

Atmosphere ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1204
Author(s):  
Dibas Shrestha ◽  
Shankar Sharma ◽  
Rocky Talchabhadel ◽  
Rashila Deshar ◽  
Kalpana Hamal ◽  
...  
Keyword(s):  
Global Climate ◽  
Deep Convection ◽  
Austral Summer ◽  
Tropical Rainfall Measuring Mission ◽  
Andean Region ◽  
Southern Andes ◽  
Low Level ◽  
Rainfall Variation ◽  
The Andes ◽  
Study Characteristics

Topography has an important role in shaping regional and global climate systems, as it acts as a mechanical barrier to the low-level moisture flow. Thus, a complex spatial pattern of rainfall can exist over the mountainous region. Moreover, it is critical to advance our understanding of the relationship between rainfall and topography in terms of rainfall timing, frequency, and magnitude. In this study, characteristics of austral summer (December–February) precipitation are analyzed using 17-year (1998–2014) high-spatial-resolution (0.05° × 0.05°) data obtained from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) over the Andean region focusing on topographic impact. We observe an interaction between precipitation patterns and topography, with clear precipitation–elevation relationships in the Andes regions. The rainfall maxima zone was observed over the higher terrain of the central and southern Andes, and the zone is attributed to frequency and intensity of rainfall, respectively. In the foothills of the central Andes, we find there was a persistent rain system when a moist, low-level flow was lifted due to topography. In contrast, steep mountain slopes and a relatively dry atmosphere modulate deep convection in the foothills of southern Andes.

scholarly journals Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers tool

2017 ◽  
Vol 8 (4) ◽  
pp. 1247-1261 ◽  
Author(s):  
Jorge Eiras-Barca ◽  
Francina Dominguez ◽  
Huancui Hu ◽  
Daniel Garaboa-Paz ◽  
Gonzalo Miguez-Macho
Keyword(s):  
Cross Sections ◽  
North Atlantic ◽  
Wrf Model ◽  
Atmospheric River ◽  
The North ◽  
Northern Latitudes ◽  
The Pacific ◽  
The Pacific Ocean ◽  
Low Level Jet ◽  
Tropical Moisture

Abstract. A new 3-D tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme atmospheric river (AR) events: the so-called Great Coastal Gale of 2007 in the Pacific Ocean and the Great Storm of 1987 in the North Atlantic. Results show that between 80 and 90 % of moisture advected by the ARs, and a high percentage of the total precipitation produced by the systems have a tropical origin. The tropical contribution to precipitation is in general above 50 % and largely exceeds this value in the most affected areas. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives on land. Vertical cross sections of the moisture content suggest that the maximum in tropical humidity does not necessarily coincide with the low-level jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes and can be located above, below or ahead of the LLJ in northern latitudes in both analyzed cases.

scholarly journals Variations in Precipitating Convective Feature Populations with ITCZ Width in the Pacific Ocean

2020 ◽  
Vol 33 (10) ◽  
pp. 4391-4401 ◽  
Author(s):  
Kyle R. Wodzicki ◽  
Anita D. Rapp
Keyword(s):  
Pacific Ocean ◽  
Southern Oscillation ◽  
Walker Circulation ◽  
Tropical Rainfall Measuring Mission ◽  
Strong Positive Correlation ◽  
Stratiform Rain ◽  
The Pacific ◽  
Columnar Water Vapor ◽  
Trmm Precipitation ◽  
The Pacific Ocean

AbstractMany recent studies have aimed to better understand changes in the characteristics of the intertropical convergence zone (ITCZ), including ITCZ location, width, and precipitation intensity. However, very few studies have looked at the relationship between characteristics of convection within the ITCZ and ITCZ width. The present work uses information from an ITCZ identification database and Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database to quantify variations in convective characteristics across the ITCZ in the Pacific Ocean. Data are partitioned into wide and narrow ITCZ regimes to quantify differences in convection between different ITCZ regimes. Under the wide regime, convection deeper than 5 km, with areas greater than 100 km2, or stratiform rain fractions greater than 0.5 is, on average, 24%, 23%, and 12% more frequent, respectively. In the narrow regime, the signal is reversed, with average increases in the frequency of convection with heights below 5 km, areas less than 100 km2, or stratiform rain fractions less than 0.5 of 15%, 4%, and 6%, respectively. Positive and negative anomalies in columnar water vapor (CWV) and sea surface temperature (SST) across the ITCZ are observed in the wide and narrow regimes, respectively. There is also a strong positive correlation between an El Niño–Southern Oscillation (ENSO) index and ITCZ width anomalies, with wide (narrow) ITCZs occurring during warm (cold) phases of ENSO. This implies that the strengthening and weakening of the Walker circulation associated with ENSO may play a role in modulating the convective populations that contribute to the Pacific ITCZ width variations.

scholarly journals The Globally Coherent Pattern of Autumn Monsoon Precipitation

2021 ◽  
pp. 1-56
Author(s):  
Nandini Ramesh ◽  
Quentin Nicolas ◽  
William R. Boos
Keyword(s):  
Seasonal Cycle ◽  
Large Scale ◽  
Atmospheric Stability ◽  
Monsoon Circulation ◽  
Moist Convection ◽  
Mountain Ranges ◽  
Low Level ◽  
Annual Peak ◽  
The Pacific ◽  
The Tropics

AbstractOver most tropical land areas, the annual peak in precipitation occurs during summer, associated with the local monsoon circulation. However, in some coastal regions in the tropics the bulk of annual precipitation occurs in autumn, after the low-level summer monsoon westerlies have abated. Examples include the Nordeste region of Brazil, southeastern India and Sri Lanka, and coastal Tanzania. Unlike equatorial regions, they receive little rainfall during local spring. Such regions are present along the eastern coasts of nearly all continents, suggesting that they comprise a coherent yet previously unrecognized global phenomenon.In this study, we identify eight tropical locations that experience an “autumn monsoon” and show that this unusual seasonal cycle is generated by similar mechanisms in all of these. When these regions receive their peak rainfall, they lie poleward of the ITCZ in easterly low-level winds. The spatial structure of precipitation in these regions can be explained by their placement to the east of mountain ranges that organize moist convection on their windward sides. However, orographic forcing alone cannot explain their unique seasonal cycle: despite similarities in wind direction, surface humidity, and sea surface temperatures (SSTs) between autumn and spring, these regions receive significantly more rainfall in autumn than in spring. We show that this is due to differences in the large-scale atmospheric stability between the equinoctial seasons, which can be captured by a relative SST metric and is influenced by SSTs in the remote eastern upwelling zones of the Pacific and Atlantic Oceans.

Sign in / Sign up

Export Citation Format

Share Document