Southerly Low-level Jet Research Articles

Doppler Lidar Measurements of Wind Variability and LLJ Properties in Central Oklahoma during the August 2017 Land–Atmosphere Feedback Experiment

Abstract Low-level jets (LLJs) are an important nocturnal source of wind energy in the U.S. Great Plains. An August 2017 lidar-based field-measurement campaign [the Land–Atmosphere Feedback Experiment (LAFE)] studied LLJs over the central SGP site in Oklahoma and found nearly equal occurrences of the usual southerly jets and postfrontal northeasterly jets—typically rare during this season—for an opportunity to compare the two types of LLJs during this month. Southerly winds were stronger than the northeasterlies by more than 4 m s−1 on average, reflecting a significantly higher frequency of winds stronger than 12 m s−1. The analysis of this dataset has been expanded to other SGP Doppler-lidar sites to quantify the variability of winds and LLJ properties between sites of different land use. Geographic variations of winds over the study area were noted: on southerly wind nights, the winds blew stronger at the highest, westernmost sites by 2 m s−1, whereas on the northeasterly flow nights, the easternmost sites had the strongest wind speeds. Lidar measurements at 5 sites during August 2017, contrasted to the 2016–21 summertime data, revealed unusual wind and LLJ conditions. Temporal hodographs using hourly averaged winds at multiple heights revealed unorganized behavior in the turbulent stable boundary layer (SBL) below the jet nose. Above the nose, some nights showed veering qualitatively similar to inertial oscillation (IO) behavior, but at amplitudes much smaller than expected for an IO, whereas other nights showed little veering. Vertical hodographs had a linear shape in the SBL, indicating little directional shear there, and veering above, resulting in a hook-shaped hodograph with height. Significance Statement Doppler-lidar measurements at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) were used to quantify the variability of winds and low-level jet (LLJ) properties between five sites of different land use and wind regimes across this area. Knowledge of wind and LLJ structure and dynamics is important for many applications, and strong southerly LLJ winds at night are an important resource for wind energy. The analysis of multiyear (2016–21) summertime LLJ parameters provided insight into the LLJ climatology in this part of the Great Plains.

On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10\u201318 August 2020

A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.

Synoptic-Scale Precursors, Characteristics and Typing of Nocturnal Mesoscale Convective Complexes in the Great Plains

Mesoscale convective complexes (MCCs) occur frequently during the warm season in the central U.S. and can produce flooding rains, hail and tornadoes. Previous work has found that the synoptic-scale environment can greatly affect, and be affected by, the development and maintenance of MCCs. Ninety-two MCC cases from 2006-2011 are manually identified using infrared satellite imagery and partitioned into three types (upstream trough, zonal and ridge) using a unique manual synoptic typing based on 500-hPa height patterns. Upstream trough cases feature an amplified longwave 500-hPa trough upstream of the MCC genesis region (GR), while the 500-hPa flow is relatively flat in zonal cases, and a strong 500-hPa ridge is present over the Rockies in ridge cases. Individual case and storm-relative composite analyses of a subset of 28 cases show that of the three types, upstream trough cases feature both the strongest quasigeostrophic forcing for ascent and lower-tropospheric frontogenesis, the latter of which enhances ascent and is associated with a strong southerly low-level jet (LLJ). Zonal and ridge cases feature smaller magnitudes (in descending order) of all ascent-forcing parameters. Ridge cases, in particular, are characterized by weak Q-vector convergence, but easterly upslope flow likely acts as a compensating ascent mechanism. A thermodynamic analysis shows that high-θe air is advected into the GR in all three MCC types, and serves as fuel for development and maintenance. However, while the southerly LLJ advects high-θe air from the Gulf of Mexico in the upstream trough and zonal cases, such air is already pooled in the High Plains in the ridge cases and advected into the GR by easterly flow. In accordance with the synoptic-dynamic analysis, upstream trough cases have the longest duration and largest impact on the synoptic-scale environment, while ridge cases are the shortest-lived. The various underlying precipitation structures of each group are also explored; zonal cases, for example, appear to preferentially be associated with bow echoes.

Multi-model convection-resolving simulations of the October 2018 Vaia storm over Northeastern Italy

The aim of this study is to identify the main mesoscale features and mechanisms responsible for the generation of a very intense precipitation and wind storm event, named “Vaia”, that affected the eastern Italian Alps on 27–29 October 2018. The event was characterized by extreme accumulated precipitation (up to 850 mm in three days) and exceptionally strong winds, causing severe and widespread impacts, such as floods, landslides, and extensive damages to forests and growing stock. The synoptic situation was characterized by a trough, which deepened over the eastern Atlantic, extending to France and Spain, driving a strong moist flow towards the Alpine region. At the surface, a wide cyclonic area developed over the western Mediterranean, east of the trough axis, and moved, deepening, towards northwestern Italy. The storm is investigated using a comprehensive dataset composed of both observations and numerical simulations by means of two models, namely WRF and MOLOCH, at convection-permitting resolution. The analysis highlights that the storm was characterized by two consecutive phases with strong precipitations, both fed by an intense moist southerly flow. In particular, the second phase was also marked by strong wind gusts in the Alpine area, exceeding 50 m s−1 at some weather stations. It is found that these extreme wind gusts were connected to the presence of an intense southerly low-level jet immediately ahead of a cold front, displaying an average wind speed of 35 m s−1 at 1500 m MSL. The comparison between observations and numerical results shows that the main characteristics of the storm are well simulated by both models, confirming the high predictability of this kind of events, typically associated with well-defined large-scale forcing. Also local scale features are reasonably captured by the simulations, despite the high complexity of the Alpine orography. However, WRF significantly underestimates total precipitation amounts over the most affected areas, while wind speed is overestimated by both models in the inner Alpine sectors.

Observations and Simulation of Elevated Nocturnal Convection Initiation on 24 June 2015 during PECAN

Abstract The environment of elevated nocturnal deep convection initiation (CI) on 24 June 2015 is investigated using radiosonde data from the Plains Elevated Convection at Night (PECAN) field experiment and a convection-allowing simulation. Elevated CI occurs around midnight in ascending westerly flow above the northeastern terminus of the nocturnal low-level jet (LLJ) several hundred kilometers poleward of the leading edge of a surface warm front. This CI originates from within preexisting banded altocumulus clouds that are supported by persistent large-scale ascent within the entrance region of a midtropospheric jet streak. Model trajectories calculated backward from convective updraft cores during CI indicate abrupt lifting at the leading edge of the surface front during the late afternoon to altitudes above that of the subsequent southerly LLJ. This air remains significantly subsaturated during northward movement until after several hours of weaker but persistent ascent within the highly elevated westerly airstream during the evening. Unlike in many previous studies of frontal overrunning by the LLJ, strong local drying occurs within the LLJ core. Nevertheless, vertical displacements from persistent mesoscale ascent were sufficient for trajectory air parcels to reach their LFC and sustain deep convection. The mesoscale upward displacement along trajectories is well explained by isentropic upglide associated with frontal overrunning at horizontal distances greater than 100 km from the CI and subsequent mature convection. However, the significant additional mesoscale vertical displacements needed for deep CI to occur in the westerlies above the horizontally convergent ~100-km-wide LLJ terminus region, were associated with local cooling and are not accounted for by steady isentropic upglide.

Contrasting Characteristics and Evolution of Southerly Low-Level Jets During Different Boundary-Layer Regimes

Nocturnal low-level jets (LLJs) have been documented at many places around the world, particularly across the Great Plains of the United States. Nocturnal LLJs significantly affect the regional weather and climate, as well as several industrial activities. While the LLJ has been extensively studied, little attention has been paid to the relationship between the evolution of the LLJ, its properties, and the boundary-layer structure. Here, observations collected in northern Oklahoma, USA are used to analyze the characteristics and evolution of the LLJ during conditions of weak and strong turbulence, respectively, coinciding with strongly and weakly stable nocturnal boundary layers. Additionally, the relationship between the LLJ and the temperature profile under these different conditions is examined. Three case studies are used during conditions of weak and strong turbulence, followed by statistics computed from a larger dataset of LLJs. When turbulence is weak, the LLJ is shown to reach peak intensity later near sunrise, while a maximum in the intensity is reached about six hours after sunset in the highly turbulent boundary layer. Furthermore, the LLJ tends to stay constant with height when turbulence is maintained throughout the night, while the LLJ tends to follow isentropic surfaces when vertical mixing is suppressed. When the LLJ follows isentropic surfaces, changes in the wind speed are related to the stability modification.

An Overview of Low‐Level Jet Winds and Corresponding Mixed Layer Depths During PECAN

AbstractThirty southerly low‐level jet (LLJ) events were observed during the Plains Elevated Convection at Night (PECAN) field campaign in the Great Plains region of the United States during summer 2015. Here we present Doppler lidar wind data from three PECAN instrumentation sites to explore characteristics of LLJs and the boundary layer as well as some of the heterogeneities possible within the wind field of a LLJ. Southerly LLJs were observed on 66% of nights at the southwestern site (Greensburg, KS) but only 56% and 53% of nights at the eastern and northern sites, respectively (Hesston and Ellis, KS). The northernmost site had a relative abundance of weaker jets or nonjet conditions due to fronts or convective systems that only affected part of the observation domain. Plotting mean wind fields of each LLJ type reveals that the strongest LLJs tend to develop under very similar conditions but begin to show variability in wind profile evolution after several hours. A robust mixed layer height retrieval algorithm is used to investigate the interplay between the jets and the turbulent convective boundary layer, showing that stronger LLJs are preceded by deeper afternoon mixed layers and often have a later decoupling of mixing between the upper convective mixed layer and the near‐surface layer. Only the strongest LLJs generated a shallow mixing layer overnight. Comparing jet strength and direction to pristine nocturnal convection initiation shows that the strongest southerly LLJs yielded the most pristine nocturnal convection initiation events per night, and the pristine nocturnal convection initiation occurred farther north.

The Moisture Sources and Transport Processes for a Sudden Rainstorm Associated with Double Low-Level Jets in the Northeast Sichuan Basin of China

A sudden rainstorm that occurred in the northeast Sichuan Basin of China in early May 2017 was associated with a southwest low-level jet (SWLJ) and a mountainous low-level jet (MLLJ). This study investigates the impact of the double low-level jets (LLJs) on rainfall diurnal variation by using the data from ERA5 reanalysis, and explores the characteristics of water vapor transport, including the main paths and sources of moisture, by using the HYSPLIT-driven data of the ERA—interim, GDAS (Global Data Assimilation System), and NCEP/NCAR reanalysis data. The analysis shows that the sudden rainstorm in the mountain terrain was located at the left side of the large-scale SWLJ at 700 hPa, and at the exit region of the meso-scale MLLJ at 850 hPa. The double LLJs provide favorable moisture conditions, and the enhancement (weakening) of the LLJs is ahead of the start (end) of the rainstorm. The capacity of the LLJ at 850 hPa with respect to moisture convergence is superior to that at 700 hPa, especially when the MLLJ and the southerly LLJ at 850 hPa appear at the same time. The HYSPLIT backward trajectory model based on Lagrangian methods has favorable applicability in the event of sudden rainstorms in mountainous terrain, and there is no special path of moisture transport in this precipitation event. The main moisture sources of this process are the East China Sea–South China Sea, the Arabian Sea–Indian Peninsula, the Bay of Bengal, and the Middle East, accounting for 38%, 34%, 17% and 11% of the total moisture transport, respectively. Among them, the moisture transport in the Bay of Bengal and the South China Sea–East China Sea is mainly located in the lower troposphere, which is below 900 hPa, while the moisture transport in the Arabian Sea–Indian Peninsula and the Middle East is mainly in the middle and upper layers of the troposphere. The moisture changes of the transport trajectories are affected by the topography, especially the high mountains around the Sichuan Basin.

The Potential Role of Atmospheric Bores and Gravity Waves in the Initiation and Maintenance of Nocturnal Convection over the Southern Great Plains

Abstract This investigation explores the relationship among bores, gravity waves, and convection within the nocturnal environment through the utilization of measurements taken during the International H2O Project (IHOP_2002) over the Southern Great Plains. The most favorable conditions for deep convection were found to occur within the boundary layer during the late afternoon and early evening hours in association with the diurnal cycle of solar insolation. At night, the layers most favorable for deep convection occur at and above the height of the nocturnal southerly low-level jet in association with distinct maxima in both the southerly and westerly components of the wind. Observations taken during the passage of 13 nocturnal wave disturbances over a comprehensive profiling site show the average maximum and net upward displacements with these waves were estimated to be ~900 and ~660 m, respectively. The lifting was not limited to the stable boundary layer, but reached into the conditionally unstable layers aloft. Since the net upward displacements persisted for many hours as the disturbances propagated away from the convection, areas well in excess of 10 000 km2 are likely impacted by this ascent. This lifting can directly maintain existing convection and aid in the initiation of new convection by reducing the convective inhibition in the vicinity of the active convection. In agreement with past studies, strong ascent in the lowest ~1.5 km was generally consistent with the passage of a bore. However, separate wave responses also occurred well above the bores, and low-frequency gravity waves may explain such disturbances.

A New Look at the Identification of Low-Level Jets in South America

Abstract Criteria currently employed in algorithms that identify low-level jets (LLJs) in South America utilizing rawinsonde and gridded model data fail to detect an important number of LLJ events. This study discusses shortcomings in the existing approaches for LLJ identification in South America and proposes modifications to the criteria regarding layer depth for LLJ identification and wind direction. Episodes of southerly LLJs, which have received less attention in the La Plata basin, are also included in the investigation. A sensitivity analysis of LLJ detection in South America upon the choice of the criteria applied to a sample period of 15 years (1996–2010) of gridded numerical data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and to a 20-yr dataset (1996–2015) of actual rawinsondes for the La Plata basin, reveals the benefits of revising the criteria. The modified criteria allow for the characterization of a wider spectrum of LLJs over key regions of South America, such as over the Bolivian–Paraguayan border, Sierras de Córdoba in Argentina, and southern-southeastern Brazil. This wider range of events includes elevated LLJs, mostly with strong zonal components, that account for approximately 20% of the full sample of LLJs identified in the rawinsonde dataset investigated here. The revised criteria have the advantage of retaining the identification of episodes that meet the consecrated definition of the South American LLJ, while at the same time providing an augmented sample of such wind systems. Our results provide further insights into the forcing mechanisms of distinct types of LLJs in South America, ranging from topographic to baroclinic effects.

The meso-<x content-type="symbol">β</x><sub><x content-type="symbol"> </x></sub>scale characteristics during the “0866” extremely heavy rainfall in South China by using SACDM

The research on heavy rainfall depended greatly on high-resolution data, which could be obtained by using the intensive observation, model simulation, radar detection, and satellite detection. However, it was difficult to discover the fine features of the heavy rainfall system because the limitation of the method of analyzing the big data mentioned above. Therefore, the paper aimed at introducing a new method (synthetical analysis and classifying diagnosis method, SACDM) for effectively analyzing various kinds of data. SACDM possessed a suit of scheme for the data processing and diagnosing. Firstly, the secondary processed data could be obtained by using various key techniques such as numerical simulation technology, numerical calculation technology, radar mosaic technology, and 3D wind field retrieval technology. Secondly, based on the idea of classifying diagnosis, different data were effectively used to analyze the corresponding features on heavy rainfall. Based on various high-resolution data including the intensive observation data from SCHeREX (South China Heavy Rainfall Experiment), the radar mosaic data, the radar retrieved data, the meso-scale model data, the satellite data, the convectional observation data, the automatic station data, and the NCEP (National Centers for Environmental Prediction) reanalysis data, the paper studied the fine features of the South China extremely heavy rainfall occurred at 5–7 June 2008 by using SACDM, and the following conclusions could be obtained. The unique coupling between the southerly upper level jet and low level jet (LLJ) was favorable for the South China rainfall event. An obvious low-latitude vortex was discovered. Different from most middle-high latitude vortexes, the vortex was greatly influenced by the southwesterly monsoon surges. Dynamical elements played more important roles than thermal elements during the formation period of vortex. During the mature period of vortex, thermal elements played more important roles than dynamical elements. Divergent wind produced positive kinetic energy crossing isobars, which was favorable of the vortex rainfall. The monsoon surges from the Bay of Bengal penetrated into a Key Zone (the southeast of vortex) and enhanced the southwesterly wind there. The monsoon surges also took advantageous physical quantities including water vapor, vorticity, divergence, and vertical velocity to the Key Zone. With the influence of favorable physical quantities, meso-scale convective system (MCS) frequently emerged at the southeast of vortex. MCS-E was the strongest convection in the heavy rainfall event. It possessed a southwest-northeast echo belt and a strong TBB (temperature of blackbody brightness) less than 200 K. A meso- β scale LLJ was also discovered to be parallel to MCS-E, and it indeed triggered and fed the linear convection according to high-resolution analysis. The accumulation and release of CAPE (convective available potential energy) was a key reason for the evolution of MCS-E. Active meso-γ scale convective cells prolonged the lifetime of MCS-E, and the convective cells directly produced the precipitation peaks in the rainfall center according to the 6-min rainfall and radar echo. As a result, all the aforementioned features were reduced to a suit of conceptual model to interpret the possible reasons of the heavy rainfall. SACDM could play an important role in studying the fine features of heavy rainfall according to this paper.

More frequent intense and long-lived storms dominate the springtime trend in central US rainfall.

The changes in extreme rainfall associated with a warming climate have drawn significant attention in recent years. Mounting evidence shows that sub-daily convective rainfall extremes are increasing faster than the rate of change in the atmospheric precipitable water capacity with a warming climate. However, the response of extreme precipitation depends on the type of storm supported by the meteorological environment. Here using long-term satellite, surface radar and rain-gauge network data and atmospheric reanalyses, we show that the observed increases in springtime total and extreme rainfall in the central United States are dominated by mesoscale convective systems (MCSs), the largest type of convective storm, with increased frequency and intensity of long-lasting MCSs. A strengthening of the southerly low-level jet and its associated moisture transport in the Central/Northern Great Plains, in the overall climatology and particularly on days with long-lasting MCSs, accounts for the changes in the precipitation produced by these storms.

WRF Forecasts of Great Plains Nocturnal Low-Level Jet-Driven MCSs. Part II: Differences between Strongly and Weakly Forced Low-Level Jet Environments

Abstract The classic Great Plains southerly low-level jet (LLJ) is a primary factor in sustaining nocturnal convection. This study compares convection-allowing WRF forecasts of LLJ events associated with MCSs in strongly and weakly forced synoptic environments. The depth of the LLJs and magnitude, altitude, and times of the LLJ peak wind were evaluated in observations and WRF forecasts for 31 cases as well as for case subsets of strongly and weakly forced synoptic regimes. LLJs in strongly forced regimes were stronger, deeper, and peaked at higher altitudes and at earlier times compared to weakly forced cases. Mean error MCS-centered composites of WRF forecasts versus RUC analyses were derived at MCS initiation time for the LLJ atmospheric water vapor mixing ratio, LLJ total wind magnitude, convergence, most unstable convective available potential energy (MUCAPE), and most unstable convective inhibition (MUCIN). In most configurations, simulated MCSs in strongly and weakly forced regimes initiated to the north and east of observations, generally in a region where LLJ moisture, MUCAPE, and MUCIN fields were forecast well, with larger errors outside this region. However, WSM6 simulations for strongly forced cases showed a southward displacement in MCS initiation, where a combination of ambient environmental factors and microphysics impacts may simultaneously play a role in the location of forecast MCS initiation. Strongly forced observed and simulated MCSs initiated west of the LLJ axis and moved eastward into the LLJ, while observed and simulated MCSs in weakly forced environments traversed the termini of the LLJ. A northward bias existed for simulated MCS initiation and LLJ termini for weakly forced regimes.

The inter‐annual variability of southerly low‐level jets in North America

The inter-annual variability of southerly low-level jets (SLLJs) over North America during the warm (April–September) and cool (October–March) seasons is investigated. SLLJ occurrences over a 31-year period (1979–2009) were identified from the North American Regional Reanalysis (NARR) vertical wind profiles. The first empirical orthogonal function (EOF) modes of the SLLJ frequency during the warm and cool seasons account for about 30 and 20% of the total variance, respectively. Both modes can be interpreted as a strengthening or weakening of the core area of SLLJ anomalies. The principal component (PC) time series display significant positive trends, suggesting an increase in SLLJ activity during both seasons on inter-decadal time scales and are significantly correlated to the summertime Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) for the warm season and the wintertime PDO, AMO and El Nino Modoki for the cool season. The second modes account for about 20 and 15% of the total variance for the warm and cool seasons, respectively, and are interpreted as primarily a subseasonal latitudinal shift in SLLJ activity between the central Great Plains and the western Gulf of Mexico and southern Texas during the warm season and a longitudinal shift between the western Gulf of Mexico and the Caribbean during the cool season. The second mode appears to be significantly correlated to El Nino Modoki for the warm season and to Nino 3.4 for the cool season.

Evaluation of the southerly low‐level jet climatology for the central United States as simulated by NARCCAP regional climate models

An ensemble of simulations from four regional climate models (RCMs) driven by a global reanalysis was obtained from the North American Regional Climate Change Assessment Program (NARCCAP) and used to evaluate the ability of the RCMs to simulate the long-term (1979–2000) climatology of southerly low-level jets (S-LLJs) in the central United States. The RCM-derived S-LLJ climatologies were evaluated against rawinsonde observations for the same period. The use of a small ensemble of RCM simulations helped to identify model differences and assisted with interpretation. The RCMs generally reproduced the broad spatial patterns and temporal variations of jet frequency and average jet height and speed. No model consistently outperformed the others in all aspects of the evaluation, although differences existed between models in the placement, migration and relative strength of ‘hotspots’ of more frequent jet activity. In particular, three of the four models placed the centre of greatest nocturnal S-LLJ activity during the warm season in northern and central Texas, whereas for the other model the greatest jet activity was located in the south-central plains (Kansas/Oklahoma). The magnitude of a S-LLJ frequency maximum over south Texas also varied between models, with simulated frequencies exceeding observed frequencies for some models but substantially underestimating for others. The evaluation presented here highlights the potential applications of RCMs in S-LLJ research for future climate and other assessment studies that require three-dimensional data with relatively high spatial and temporal resolutions. The overall performance of the models in reproducing the long-term S-LLJ climatology supports the use of NARCCAP RCM simulations in climate assessments for the central United States where S-LLJs are an important contributor to the regional climatology.

Thermodynamic and Turbulence Characteristics of the Southern Great Plains Nocturnal Boundary Layer Under Differing Turbulent Regimes

The nocturnal stable boundary layer (SBL) can generally be classified into the weakly stable boundary layer (wSBL) and very stable boundary layer (vSBL). Within the wSBL, turbulence is relatively continuous, whereas in the vSBL, turbulence is intermittent and not well characterized. Differentiating characteristics of each type of SBL are still unknown. Herein, thermodynamic and kinematic data collected by a suite of instruments in north central Oklahoma in autumn 2012 are analyzed to better understand both SBL regimes and their differentiating characteristics. Many low-level jets were observed during the experiment, as it took place near a climatological maximum. A threshold wind speed, above which bulk shear-generated turbulence develops, is found to exist up to 300 m. The threshold wind speed must also be exceeded at lower heights (down to the surface) in order for strong turbulence to develop. Composite profiles, which are normalized using low-level jet scaling, of potential temperature, wind speed, vertical velocity variance, and the third-order moment of vertical velocity ( $$\overline{w'^3}$$ ) are produced for weak and moderate/strong turbulence regimes, which exhibit features of the vSBL and wSBL, respectively. Within the wSBL, turbulence is generated at the surface and transported upward. In the vSBL, values of vertical velocity variance are small throughout the entire boundary layer, likely due to the fact that a strong surface inversion typically forms after sunset. The temperature profile tends to be approximately isothermal in the lowest portions of the wSBL, and it did not substantially change over the night. Within both types of SBL, stability in the residual layer tends to increase as the night progresses. It is thought that this stability increase is due to differential warm air advection, which frequently occurs in the southern Great Plains when southerly low-level jets and a typical north–south temperature gradient are present. Differential radiative flux divergence also contributes to this increase in stability.

Mechanisms Governing the Persistence and Diurnal Cycle of a Heavy Rainfall Corridor

Abstract Observations and convection-permitting simulations are used to study a 12-day warm-season heavy precipitation corridor over the central U.S. plains and Mississippi River valley regions. Such precipitation corridors, defined by narrow latitudinal widths (~3°–4°) and only modest north–south drifts of their centroids (<2° day−1), often yield extreme total precipitation (100–250 mm), resulting in both short-term and seasonal impacts on the regional hydrologic cycle. The corridor precipitation is predominately nocturnal and located several hundred kilometers north of a quasi-stationary surface front. There, hot, dry air from the daytime boundary layer located underneath a persistent upper-level anticyclone requires large vertical displacements along the axis of the southerly low-level jet (LLJ) above the front to eliminate convection inhibition (CIN). Composites reveal ~500 J kg−1 of average convective available potential energy (CAPE) when this air reaches the southern edge of the precipitation corridor. Despite the relatively modest CAPE, convection is favored by the large reductions in CIN along the vertical displacements and by high ambient midtropospheric relative humidity located above, which is influenced by persistent nightly convection in the region. Though internal feedbacks resulting from the large nightly spatial coherence of convection (including enhanced midtropospheric relative humidities and frontogenetic daytime sensible heat flux gradients owing to residual cloudiness) are favorable for maintaining the corridor, its persistence is most sensitive to large-scale external factors. Here, changes to the intensity and position of the large-synoptic upper-tropospheric anticyclone are associated with changes in the frequency of strong LLJs and the surface frontal position, dramatically affecting the intensity and stationarity of the precipitation corridor.

Forcing mechanisms of a heavy precipitation event in the southeastern Adriatic area

The aim of this study was to identify the main mesoscale features and mechanisms responsible for the generation of an extreme precipitation event as a contribution to improving the modelling of processes that produce HPEs. The event occurred during the morning hours on 22 November 2010 over the Dubrovnik coast in Croatia and the hinterland mountain range of the southern Dinaric Alps and caused severe flash floods and landslides and consequent interruption of traffic and electricity supply as well as other infrastructural damage. The analysis is geographically focused on the southern portion of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events that occur mostly in autumn. This area is one of the rainiest in Europe with expected annual amounts of precipitation greater than 5,000 mm in the mountainous hinterland. The mechanisms responsible for the formation of convection were analysed using synop measurements, satellite data and numerical experiments performed with the WRF model, which was set up at the convection-permitting resolution in the innermost domain. Satellite data were used to identify the precipitation systems and to estimate the intensity of the precipitation during the period of interest. The development of the precipitation system was connected to a strong large-scale ascent over the southern Italy and southern Adriatic due to the advection of warm air and cyclonic vorticity advection, which increases with height. The numerical simulations highlighted the essential role of a southerly low-level jet stream in the transport of warm and moist air towards the affected area. The convergence of two branches of low-level marine air favoured convection triggered over the coast and sea. Furthermore, numerical sensitivity experiments suggested that the orography of the Dinaric Alps plays an essential role in the precipitation maximum over the mountainous hinterland, but also that the orography was not the crucial factor in the heavy precipitation near Dubrovnik. This study highlights the need for a dense network of observations, especially radar measurements, to validate the simulated mechanisms and improve numerical forecasts via data assimilation.

Tornadoes in the New York Metropolitan Region: Climatology and Multiscale Analysis of Two Events

Abstract This paper describes the climatology of tornadoes around New York City (NYC) and Long Island (LI), New York, and the structural evolution of two tornadic events that affected NYC on 8 August 2007 and 16 September 2010. Nearly half (18 of 34 events from 1950 to 2010) of NYC–LI tornadoes developed between 0500 and 1300 EDT, and August is the peak tornado month as compared to July for most of the northeast United States. A spatial composite highlights the approaching midlevel trough, moderate most unstable convective available potential energy (MUCAPE), and frontogenesis along a low-level baroclinic zone. Shortly before the early morning tornadoes on 8 August 2007, a mesoscale convective system intensified in the lee of the Appalachians in a region of low-level frontogenesis and moderate MUCAPE (~1500 J kg−1). Warm advection at low levels and evaporative cooling within an elevated mixed layer (EML) ahead of the mesoscale convective system (MCS) helped steepen the low-level lapse rates. Meanwhile, a surface mesolow along a quasi-stationary frontal zone enhanced the warm advection and low-level shear. The late afternoon event on 16 September 2010 was characterized by a quasi-linear convective system (QLCS) that also featured an EML aloft, a surface mesolow just west of NYC, low-level frontogenesis, and a southerly low-level jet ahead of an approaching midlevel trough. The QLCS intensified approaching NYC and generated mesovortices as the QLCS bowed outward. These cases illustrate the benefit of high-density surface observations, terminal Doppler radars, and sounding profiles from commercial aircraft for nowcasting these events.

Physical Processes Associated with Heavy Flooding Rainfall in Nashville, Tennessee, and Vicinity during 1–2 May 2010: The Role of an Atmospheric River and Mesoscale Convective Systems

A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supported the successive development of two quasi-stationary mesoscale convective systems (MCSs) on 1 and 2 May, respectively. These MCSs were both linearly organized and exhibited back-building and echo-training, processes that afforded the repeated movement of convective cells over the same area of western and central Tennessee and Kentucky, resulting in a narrow band of rainfall totals of 200–400 mm. Mesoscale analyses reveal that the MCSs developed on the warm side of a slow-moving cold front and that the interaction between the southerly LLJ and convectively generated outflow boundaries was fundamental for generating convection.