Global Precipitation Extremes Associated with Diurnally Varying Low-Level Jets

2010 ◽  
Vol 23 (19) ◽  
pp. 5065-5084 ◽  
Author(s):  
Andrew J. Monaghan ◽  
Daran L. Rife ◽  
James O. Pinto ◽  
Christopher A. Davis ◽  
John R. Hannan
Keyword(s):  
Great Plains ◽  
Mesoscale Model ◽  
Extreme Rainfall ◽  
Companion Paper ◽  
Precipitation Extremes ◽  
Extreme Rainfall Events ◽  
Near Surface ◽  
Low Level ◽  
Rainfall Extremes ◽  
Low Level Jets

Abstract Extreme rainfall events have important societal impacts: for example, by causing flooding, replenishing reservoirs, and affecting agricultural yields. Previous literature has documented linkages between rainfall extremes and nocturnal low-level jets (NLLJs) over the Great Plains of North America and the La Plata River basin of South America. In this study, the authors utilize a 21-yr, hourly global 40-km reanalysis based on the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to examine whether NLLJ–rainfall linkages are common elsewhere on the earth. The reanalysis is uniquely suited for the task because of its comparatively high spatial and temporal resolution and because a companion paper demonstrated that it realistically simulates the vertical, horizontal, and diurnal structure of the winds in well-known NLLJ regions. The companion paper employed the reanalysis to identify and describe numerous NLLJs across the planet, including several previously unknown NLLJs. The authors demonstrate here that the reanalysis reasonably simulates the diurnal cycle, extremes, and spatial structure of rainfall globally compared to satellite-based precipitation datasets and therefore that it is suitable for examining NLLJ–rainfall linkages. A statistical approach is then introduced to categorize nocturnal precipitation extremes as a function of the NLLJ magnitude, wind direction, and wind frequency for January and July. Statistically significant relationships between NLLJs and nocturnal precipitation extremes exist in at least 10 widely disparate regions around the world, some of which are well known and others that have been undocumented until now. The regions include the U.S. Great Plains, Tibet, northwest China, India, Southeast Asia, southeast China, Argentina, Namibia, Botswana, and Ethiopia. Recent studies have recorded widespread changes in the amplitudes of near-surface diurnal heating cycles that in turn play key roles in driving NLLJs. It will thus be important for future work to address how rainfall extremes may be impacted if trends in diurnal cycles cause the position, magnitude, and frequency of NLLJs to change.

Teleconnections Governing the Interannual Variability of Great Plains Low-Level Jets in May

2021 ◽  
pp. 1-60
Author(s):  
Shubhi Agrawal ◽  
Craig R. Ferguson ◽  
Lance Bosart ◽  
D. Alex Burrows
Keyword(s):  
Interannual Variability ◽  
Great Plains ◽  
Large Scale ◽  
Warm Season ◽  
Near Surface ◽  
Low Level ◽  
South Central ◽  
Decadal Scale ◽  
Low Level Jets ◽  
Upper Level

AbstractA spectral analysis of Great Plains 850-hPa meridional winds (V850) from ECMWF’s coupled climate reanalysis of 1901-2010 (CERA-20C) reveals that their warm season (April-September) interannual variability peaks in May with 2-6 year periodicity, suggestive of an underlying teleconnection influence on low-level jets (LLJs). Using an objective, dynamical jet classification framework based on 500-hPa wave activity, we pursue a large scale teleconnection hypothesis separately for LLJs that are uncoupled (LLJUC) and coupled (LLJC) to the upper-level jet stream. Differentiating between jet types enables isolation of their respective sources of variability. In the South Central Plains (SCP), May LLJCs account for nearly 1.6 times more precipitation and 1.5 times greater V850 compared to LLJUCs. Composite analyses of May 250-hPa geopotential height (Z250) conditioned on LLJC and LLJUC frequencies highlight a distinct planetary-scale Rossby wave pattern with wavenumber-five, indicative of an underlying Circumglobal Teleconnection (CGT). An index of May CGT is found to be significantly correlated with both LLJC (r = 0.62) and LLJUC (r = −0.48) frequencies. Additionally, a significant correlation is found between May LLJUC frequency and NAO (r = 0.33). Further analyses expose decadal scale variations in the CGT-LLJC(LLJUC) teleconnection that are linked to the PDO. Dynamically, these large scale teleconnections impact LLJ class frequency and intensity via upper-level geopotential anomalies over the western U.S. that modulate near-surface geopotential and temperature gradients across the SCP.

scholarly journals Characteristics and Variations of Low-Level Jets and Environmental Factors Associated with Summer Precipitation Extremes over the Great Plains

2019 ◽  
Vol 32 (16) ◽  
pp. 5123-5144 ◽  
Author(s):  
Derek Hodges ◽  
Zhaoxia Pu
Keyword(s):  
Moisture Content ◽  
Great Plains ◽  
Large Scale ◽  
Summer Precipitation ◽  
Reanalysis Data ◽  
Precipitation Extremes ◽  
Structural Variations ◽  
Low Level ◽  
Low Level Jets ◽  
Regional Reanalysis

Abstract Low-level jets (LLJs) are associated with 10%–45% of the summer precipitation in the U.S. Great Plains region (GPR). This study uses the NCEP North American Regional Reanalysis data product (1979–2017) to characterize the association between LLJs and precipitation extremes (anomalously wet versus dry) during the summer months (June–August) over the GPR. It is found that the number, distribution, and direction of LLJs are not clearly associated with the precipitation anomalies. The characteristics and structural variations of the LLJs and their large-scale and mesoscale environment are then examined to identify the links between LLJs and precipitation extremes. Results show that dry and wet summers vary by synoptic anomaly patterns. During dry summers the anomalous ridging results in a warmer and drier environment, primarily through subsidence, which inhibits precipitation near LLJs. In contrast, during wet summers, a reduction in subsidence occurs, resulting in stronger lift and a cooler and moister environment, which leads to enhanced precipitation near LLJs. The LLJ speed, orientation, and spatial properties vary according to the synoptic anomaly patterns. LLJs do not drive precipitation extremes, but instead, they respond to them. Specifically, the LLJ exit region is characterized by stronger baroclinity and higher moisture content during the wet years. The higher moisture content allows for ascending air parcels to reach saturation more quickly, while the stronger baroclinity increases the warm advection associated with the LLJ. This, in turn, leads to faster rising motion and is therefore closely associated with the location and intensity of the LLJ associated precipitation.

scholarly journals Convection Initiation Caused by Heterogeneous Low-Level Jets over the Great Plains

2018 ◽  
Vol 146 (8) ◽  
pp. 2615-2637 ◽  
Author(s):  
Joshua G. Gebauer ◽  
Alan Shapiro ◽  
Evgeni Fedorovich ◽  
Petra Klein
Keyword(s):  
Great Plains ◽  
Three Dimensional ◽  
Warm Season ◽  
The United States ◽  
Dimensional Structure ◽  
Nonuniform Heating ◽  
Low Level ◽  
Low Level Jets ◽  
Convection Initiation ◽  
Capping Inversion

AbstractObservations from three nights of the Plains Elevated Convection at Night (PECAN) field campaign were used in conjunction with Rapid Refresh model forecasts to find the cause of north–south lines of convection, which initiated away from obvious surface boundaries. Such pristine convection initiation (CI) is relatively common during the warm season over the Great Plains of the United States. The observations and model forecasts revealed that all three nights had horizontally heterogeneous and veering-with-height low-level jets (LLJs) of nonuniform depth. The veering and heterogeneity were associated with convergence at the top-eastern edge of the LLJ, where moisture advection was also occurring. As time progressed, this upper region became saturated and, due to its placement above the capping inversion, formed moist absolutely unstable layers, from which the convergence helped initiate elevated convection. The structure of the LLJs on the CI nights was likely influenced by nonuniform heating across the sloped terrain, which led to the uneven LLJ depth and contributed toward the wind veering with height through the creation of horizontal buoyancy gradients. These three CI events highlight the importance of assessing the full three-dimensional structure of the LLJ when forecasting nocturnal convection over the Great Plains.

scholarly journals A Modified Framework for Quantifying Land–Atmosphere Covariability during Hydrometeorological and Soil Wetness Extremes in Oklahoma

2019 ◽  
Vol 58 (7) ◽  
pp. 1465-1483 ◽  
Author(s):  
Ryann A. Wakefield ◽  
Jeffrey B. Basara ◽  
Jason C. Furtado ◽  
Bradley G. Illston ◽  
Craig. R. Ferguson ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Great Plains ◽  
Hot Spot ◽  
Extreme Rainfall ◽  
Southern Great Plains ◽  
Low Level ◽  
Oklahoma Mesonet ◽  
Humidity Index ◽  
Grid Points ◽  
Regional Reanalysis

AbstractGlobal “hot spots” for land–atmosphere coupling have been identified through various modeling studies—both local and global in scope. One hot spot that is common to many of these analyses is the U.S. southern Great Plains (SGP). In this study, we perform a mesoscale analysis, enabled by the Oklahoma Mesonet, that bridges the spatial and temporal gaps between preceding local and global analyses of coupling. We focus primarily on east–west variations in seasonal coupling in the context of interannual variability over the period spanning 2000–15. Using North American Regional Reanalysis (NARR)-derived standardized anomalies of convective triggering potential (CTP) and the low-level humidity index (HI), we investigate changes in the covariance of soil moisture and the atmospheric low-level thermodynamic profile during seasonal hydrometeorological extremes. Daily CTP and HI z scores, dependent upon climatology at individual NARR grid points, were computed and compared to in situ soil moisture observations at the nearest mesonet station to provide nearly collocated annual composites over dry and wet soils. Extreme dry and wet year CTP and HI z-score distributions are shown to deviate significantly from climatology and therefore may constitute atmospheric precursors to extreme events. The most extreme rainfall years differ from climatology but also from one another, indicating variability in the strength of land–atmosphere coupling during these years. Overall, the covariance between soil moisture and CTP/HI is much greater during drought years, and coupling appears more consistent. For example, propagation of drought during 2011 occurred under antecedent CTP and HI conditions that were identified by this study as being conducive to positive dry feedbacks demonstrating potential utility of this framework in forecasting regional drought propagation.

scholarly journals LGM Summer Climate on the Southern Margin of the Laurentide Ice Sheet: Wet or Dry?*

2005 ◽  
Vol 18 (16) ◽  
pp. 3317-3338 ◽  
Author(s):  
David H. Bromwich ◽  
E. Richard Toracinta ◽  
Robert J. Oglesby ◽  
James L. Fastook ◽  
Terence J. Hughes
Keyword(s):  
Boundary Conditions ◽  
Great Plains ◽  
Land Surface ◽  
Thermal Gradient ◽  
Mesoscale Model ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Low Level ◽  
Summer Climate ◽  
Southern Margin

Abstract Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)–NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence.

scholarly journals Radar Refractivity Retrieval: Validation and Application to Short-Term Forecasting

2005 ◽  
Vol 44 (3) ◽  
pp. 285-300 ◽  
Author(s):  
Tammy M. Weckwerth ◽  
Crystalyne R. Pettet ◽  
Frédéric Fabry ◽  
Shin Ju Park ◽  
Margaret A. LeMone ◽  
...  
Keyword(s):  
Great Plains ◽  
Doppler Radar ◽  
Doppler Velocity ◽  
Good Representation ◽  
Short Term ◽  
Near Surface ◽  
National Network ◽  
Low Level ◽  
Short Term Forecasting ◽  
Convection Initiation

Abstract This study will validate the S-band dual-polarization Doppler radar (S-Pol) radar refractivity retrieval using measurements from the International H2O Project conducted in the southern Great Plains in May–June 2002. The range of refractivity measurements during this project extended out to 40–60 km from the radar. Comparisons between the radar refractivity field and fixed and mobile mesonet refractivity values within the S-Pol refractivity domain show a strong correlation. Comparisons between the radar refractivity field and low-flying aircraft also show high correlations. Thus, the radar refractivity retrieval provides a good representation of low-level atmospheric refractivity. Numerous instruments that profile the temperature and moisture are also compared with the refractivity field. Radiosonde measurements, Atmospheric Emitted Radiance Interferometers, and a vertical-pointing Raman lidar show good agreement, especially at low levels. Under most daytime summertime conditions, radar refractivity measurements are representative of an ∼250-m-deep layer. Analyses are also performed on the utility of refractivity for short-term forecasting applications. It is found that the refractivity field may detect low-level boundaries prior to the more traditional radar reflectivity and Doppler velocity fields showing their existence. Data from two days on which convection initiated within S-Pol refractivity range suggest that the refractivity field may exhibit some potential utility in forecasting convection initiation. This study suggests that unprecedented advances in mapping near-surface water vapor and subsequent improvements in predicting convective storms could result from implementing the radar refractivity retrieval on the national network of operational radars.

scholarly journals Heavy Rainfall Associated with Double Low-Level Jets over Southern China. Part II: Convection Initiation

2019 ◽  
Vol 147 (2) ◽  
pp. 543-565 ◽  
Author(s):  
Yu Du ◽  
Guixing Chen
Keyword(s):  
Heavy Rainfall ◽  
Mesoscale Model ◽  
Southern China ◽  
Northern South China Sea ◽  
Lower Troposphere ◽  
Small Scale ◽  
The South ◽  
Low Level ◽  
Low Level Jets ◽  
Convection Initiation

Abstract Heavy rainfall that occurred at the south coast of China on 10–11 May 2014 was associated with a synoptic-system-related low-level jet (SLLJ) and a boundary layer jet (BLJ). To clarify the role of the double low-level jets in convection initiation (CI), we perform convective-permitting simulations using a nonhydrostatic mesoscale model. The simulations reproduce the occurrence location and mesoscale evolution of new convective cells as well as their small-scale wavelike structures at the elevated layers, which are generally consistent with radar observations despite some differences in their orientation. The nighttime BLJ over the northern South China Sea strengthens the convergence at ~950 hPa near the coast where the BLJ’s northern terminus reaches the coastal terrain. Meanwhile, the SLLJ to the south of the inland cold front provides divergence at ~700 hPa near the SLLJ’s entrance region. Such low-level convergence and midlevel divergence collectively produce strong mesoscale lifting for CI at the coast. In addition to the enhanced mesoscale lifting, the double low-level jets also provide favorable conditions for the superimposed small-scale disturbances that can serve as effective moistening mechanisms of the lower troposphere during CI. In a sensitivity experiment with coastal terrain removed, CI still occurs near the coast but is delayed and weaker compared to the control run. This latter experiment suggests that double low-level jets and their coupling indeed exert key effects on CI, while the BLJ colliding with terrain may enhance coastal convergence for amplifying CI. These findings provide new insights into the occurrence of coastal heavy rainfall in the warm sector far ahead of the fronts.

Extreme rainfall events during and following heatwaves

2021 ◽  
Author(s):  
Christoph Sauter ◽  
Christopher White ◽  
Hayley Fowler ◽  
Seth Westra
Keyword(s):  
Natural Hazards ◽  
Rainfall Intensity ◽  
Daily Rainfall ◽  
Extreme Rainfall ◽  
Extreme Rainfall Events ◽  
Rainfall Events ◽  
Rainfall Extremes ◽  
Average Rainfall ◽  
Time Of Year ◽  
The Impact

<p>Heatwaves and extreme rainfall events are natural hazards that can have severe impacts on society. The relationship between temperature and extreme rainfall has received scientific attention with studies focussing on how single daily or sub-daily rainfall extremes are related to day-to-day temperature variability. However, the impact multi-day heatwaves have on sub-daily extreme rainfall events and how extreme rainfall properties change during different stages of a heatwave remains mostly unexplored.</p><p>In this study, we analyse sub-daily rainfall records across Australia, a country that experiences severe natural hazards on a frequent basis, and determine their extreme rainfall properties, such as rainfall intensity, duration and frequency during SH-summer heatwaves. These properties are then compared to extreme rainfall properties found outside heatwaves, but during the same time of year, to examine to what extent they differ from normal conditions. We also conduct a spatial analysis to investigate any spatial patterns that arise.</p><p>We find that rainfall breaking heatwaves is often more extreme than average rainfall during the same time of year. This is especially prominent on the eastern and south-eastern Australian coast, where frequency and intensity of sub-daily rainfall extremes show an increase during the last day or the day immediately after a heatwave. We also find that although during heatwaves the average rainfall amount and duration decreases, there is an increase in sub-daily rainfall intensity when compared to conditions outside heatwaves. This implies that even though Australian heatwaves are generally characterised by dry conditions, rainfall occurrences within heatwaves are more intense.</p><p>Both heatwaves and extreme rainfall events pose great challenges for many sectors such as agriculture, and especially if they occur together. Understanding how and to what degree these events co-occur could help mitigate the impacts caused by them.</p>

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