scholarly journals Dynamics Governing a Simulated Mesoscale Convective System with a Training Convective Line

2016 ◽  
Vol 73 (7) ◽  
pp. 2643-2664 ◽  
Author(s):  
John M. Peters ◽  
Russ S. Schumacher
Keyword(s):  
Large Scale ◽  
Mesoscale Convective System ◽  
Cold Pool ◽  
Convective System ◽  
Low Level ◽  
Lateral Boundary Conditions ◽  
Mesoscale Convective ◽  
Western Side ◽  
Outflow Boundary ◽  
Stationary Behavior

Abstract This research investigates the dynamics of a simulated training line/adjoining stratiform (TL/AS) mesoscale convective system (MCS), with composite atmospheric fields used as initial and lateral boundary conditions for the simulation. An initial forward-propagating MCS developed within a region of elevated convective instability and low-level lifting associated with warm-air advection along the terminus of the low-level jet. The environmental conditions external to the MCS continued to provide lift, moisture, and instability to the western side of the forward-propagating MCS, and these conditions were initially responsible for backbuilding on the system’s western side. Most parcels that encountered the southwestern outflow boundary were lifted insufficiently far to reach their levels of free convection (LFCs), and their LFC heights were increased by latent heating above them. These parcels continued northeastward beyond the surface outflow boundary (OFB), were gradually lifted, and initiated convection 80–100 km beyond encountering the OFB. Eventually the surface cold pool became sufficiently deep so that gradual ascent of parcels with moisture and instability over the OFB began initiating new convection close to the OFB—this drove backbuilding during the later portion of the MCS lifetime. These results disentangle the relative contributions of large-scale environmental factors and storm-scale processes on the quasi-stationary behavior of the MCS and show that both contributed to upstream backbulding at different times during the MCS life cycle.

scholarly journals The Impact of Low-Level Moisture Errors on Model Forecasts of an MCS Observed during PECAN

2017 ◽  
Vol 145 (9) ◽  
pp. 3599-3624 ◽  
Author(s):  
John M. Peters ◽  
Erik R. Nielsen ◽  
Matthew D. Parker ◽  
Stacey M. Hitchcock ◽  
Russ S. Schumacher
Keyword(s):  
Convective Available Potential Energy ◽  
Mesoscale Convective System ◽  
Squall Line ◽  
Convective System ◽  
Low Level ◽  
Eastern Flank ◽  
Western Flank ◽  
Mesoscale Convective ◽  
Outflow Boundary ◽  
The Impact

This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved southeastward along this OFB as a squall line. The western flank of the MCS remained quasi stationary approximately 100 km north of the system’s OFB and produced localized flooding. A total of 16 radiosondes were launched near the MCS’s eastern flank and 4 were launched near the MCS’s western flank. Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 h prior to the arrival of the squall line. In contrast, the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to their underrepresentation of moisture. Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed. MCS displacement error was strongly correlated with models’ underrepresentation of low-level moisture and their associated overrepresentation of the vertical distance between a parcel’s initial height and its level of free convection ([Formula: see text], which is correlated with CIN). The overpredicted [Formula: see text] in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso- α-scale lift before these parcels initiated convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.

scholarly journals Dominant processes of extreme rainfall-producing mesoscale convective system over southeastern Korea: 7 July 2009 case

2015 ◽  
Vol 3 (10) ◽  
pp. 6459-6489
Author(s):  
J.-H. Jeong ◽  
D.-I. Lee ◽  
C.-C. Wang ◽  
I.-S. Han
Keyword(s):  
Metropolitan Area ◽  
Heavy Rainfall ◽  
Deep Convection ◽  
Mesoscale Convective System ◽  
Extreme Rainfall ◽  
Cold Pool ◽  
Convective System ◽  
Frontal Surface ◽  
Mesoscale Convective ◽  
Outflow Boundary

Abstract. An extreme rainfall-producing mesoscale convective system (MCS) associated with the Changma front in southeastern Korea was investigated using observational data. This event recorded historic rainfall and led to devastating flash floods and landslides in the Busan metropolitan area on 7 July 2009. The aim of the present study is to analyze and better understand the synoptic and mesoscale environment, and the behavior of quasi-stationary MCS causing extreme rainfall. Synoptic and mesoscale analyses indicate that the MCS and heavy rainfall occurred association with a stationary front which resembled a warm front in structure. A strong southwesterly low-level jet (LLJ) transported warm and humid air and supplied the moisture toward the front, and the air rose upwards above the frontal surface. As the moist air was conditionally unstable, repeated upstream initiation of deep convection by back-building occurred at the coastline, while old cells moved downstream parallel to the convective line with training effect. Because the motion of convective cells nearly opposed the backward propagation, the system as a whole moved slowly. The back-building behavior was linked to the convectively produced cold pool and its outflow boundary, which played an essential role in the propagation and maintenance of the rainfall system. As a result, the quasi-stationary MCS caused a prolonged duration of heavy rainfall, leading to extreme rainfall over the Busan metropolitan area.

Mechanisms for Organization and Echo Training in a Flash-Flood-Producing Mesoscale Convective System

2015 ◽  
Vol 143 (4) ◽  
pp. 1058-1085 ◽  
Author(s):  
John M. Peters ◽  
Russ S. Schumacher
Keyword(s):  
Pressure Gradient ◽  
Mesoscale Convective System ◽  
Cold Pool ◽  
Gradient Force ◽  
Convective System ◽  
Pressure Gradient Force ◽  
Unstable Flow ◽  
Mesoscale Convective ◽  
Upward Transport ◽  
Outflow Boundary

Abstract In this research, a numerical simulation of an observed training line/adjoining stratiform (TL/AS)-type mesoscale convective system (MCS) was used to investigate processes leading to upwind propagation of convection and quasi-stationary behavior. The studied event produced damaging flash flooding near Dubuque, Iowa, on the morning of 28 July 2011. The simulated convective system well emulated characteristics of the observed system and produced comparable rainfall totals. In the simulation, there were two cold pool–driven convective surges that exited the region where heavy rainfall was produced. Low-level unstable flow, which was initially convectively inhibited, overrode the surface cold pool subsequent to these convective surges, was gradually lifted to the point of saturation, and reignited deep convection. Mechanisms for upstream lifting included persistent large-scale warm air advection, displacement of parcels over the surface cold pool, and an upstream mesolow that formed between 0500 and 1000 UTC. Convection tended to propagate with the movement of the southeast portion of the outflow boundary, but did not propagate with the southwest outflow boundary. This was explained by the vertical wind shear profile over the depth of the cold pool being favorable (unfavorable) for initiation of new convection along the southeast (southwest) cold pool flank. A combination of a southward-oriented pressure gradient force in the cold pool and upward transport of opposing southerly flow away from the boundary layer moved the outflow boundary southward. Upward transport of southerly momentum by convection along the southward-moving outflow boundary, along with convectively induced southward pressure gradient forces cut off southerly inflow to the MCS, which temporarily disrupted backbuilding.

scholarly journals Structure of Highly Sheared Tropical Storm Chantal during CAMEX-4

2006 ◽  
Vol 63 (1) ◽  
pp. 268-287 ◽  
Author(s):  
G. M. Heymsfield ◽  
Joanne Simpson ◽  
J. Halverson ◽  
L. Tian ◽  
E. Ritchie ◽  
...  
Keyword(s):  
Large Scale ◽  
Mesoscale Convective System ◽  
Tropical Storm ◽  
Vertical Shear ◽  
Convective System ◽  
Low Level ◽  
Convective Scale ◽  
Mesoscale Convective ◽  
Upper Level ◽  
The Individual

Abstract Tropical Storm Chantal during August 2001 was a storm that failed to intensify over the few days prior to making landfall on the Yucatan Peninsula. An observational study of Tropical Storm Chantal is presented using a diverse dataset including remote and in situ measurements from the NASA ER-2 and DC-8 and the NOAA WP-3D N42RF aircraft and satellite. The authors discuss the storm structure from the larger-scale environment down to the convective scale. Large vertical shear (850–200-hPa shear magnitude range 8–15 m s−1) plays a very important role in preventing Chantal from intensifying. The storm had a poorly defined vortex that only extended up to 5–6-km altitude, and an adjacent intense convective region that comprised a mesoscale convective system (MCS). The entire low-level circulation center was in the rain-free western side of the storm, about 80 km to the west-southwest of the MCS. The MCS appears to have been primarily the result of intense convergence between large-scale, low-level easterly flow with embedded downdrafts, and the cyclonic vortex flow. The individual cells in the MCS such as cell 2 during the period of the observations were extremely intense, with reflectivity core diameters of 10 km and peak updrafts exceeding 20 m s−1. Associated with this MCS were two broad subsidence (warm) regions, both of which had portions over the vortex. The first layer near 700 hPa was directly above the vortex and covered most of it. The second layer near 500 hPa was along the forward and right flanks of cell 2 and undercut the anvil divergence region above. There was not much resemblance of these subsidence layers to typical upper-level warm cores in hurricanes that are necessary to support strong surface winds and a low central pressure. The observations are compared to previous studies of weakly sheared storms and modeling studies of shear effects and intensification. The configuration of the convective updrafts, low-level circulation, and lack of vertical coherence between the upper- and lower-level warming regions likely inhibited intensification of Chantal. This configuration is consistent with modeled vortices in sheared environments, which suggest the strongest convection and rain in the downshear left quadrant of the storm, and subsidence in the upshear right quadrant. The vertical shear profile is, however, different from what was assumed in previous modeling in that the winds are strongest in the lowest levels and the deep tropospheric vertical shear is on the order of 10–12 m s−1.

scholarly journals Multiscale Processes Enabling the Longevity and Daytime Persistence of a Nocturnal Mesoscale Convective System

2019 ◽  
Vol 147 (2) ◽  
pp. 733-761 ◽  
Author(s):  
Manda B. Chasteen ◽  
Steven E. Koch ◽  
David B. Parsons
Keyword(s):  
Great Plains ◽  
Vertical Wind Shear ◽  
Mesoscale Convective System ◽  
Cold Pool ◽  
Convective System ◽  
Near Surface ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Unstable Environment

Abstract Nocturnal mesoscale convective systems (MCSs) frequently develop over the Great Plains in the presence of a nocturnal low-level jet (LLJ), which contributes to convective maintenance by providing a source of instability, convergence, and low-level vertical wind shear. Although these nocturnal MCSs often dissipate during the morning, many persist into the following afternoon despite the cessation of the LLJ with the onset of solar heating. The environmental factors enabling the postsunrise persistence of nocturnal convection are currently not well understood. A thorough investigation into the processes supporting the longevity and daytime persistence of an MCS was conducted using routine observations, RAP analyses, and a WRF-ARW simulation. Elevated nocturnal convection developed in response to enhanced frontogenesis, which quickly grew upscale into a severe quasi-linear convective system (QLCS). The western portion of this QLCS reorganized into a bow echo with a pronounced cold pool and ultimately an organized leading-line, trailing-stratiform MCS as it moved into an increasingly unstable environment. Differential advection resulting from the interaction of the nocturnal LLJ with the topography of west Texas established considerable heterogeneity in moisture, CAPE, and CIN, which influenced the structure and evolution of the MCS. An inland-advected moisture plume significantly increased near-surface CAPE during the nighttime over central Texas, while the environment over southeastern Texas abruptly destabilized following the commencement of surface heating and downward moisture transport. The unique topography of the southern plains and the close proximity to the Gulf of Mexico provided an environment conducive to the postsunrise persistence of the organized MCS.

Mesoscale Processes Contributing to Extreme Rainfall in a Midlatitude Warm-Season Flash Flood

2008 ◽  
Vol 136 (10) ◽  
pp. 3964-3986 ◽  
Author(s):  
Russ S. Schumacher ◽  
Richard H. Johnson
Keyword(s):  
Gravity Wave ◽  
Flash Flood ◽  
Warm Season ◽  
Mesoscale Convective System ◽  
Extreme Rainfall ◽  
Cold Pool ◽  
Convective System ◽  
Low Level ◽  
Mesoscale Convective ◽  
Moist Environment

Observations and numerical simulations are used to investigate the atmospheric processes that led to extreme rainfall and resultant destructive flash flooding in eastern Missouri on 6–7 May 2000. In this event, a quasi-stationary mesoscale convective system (MCS) developed near a preexisting mesoscale convective vortex (MCV) in a very moist environment that included a strong low-level jet (LLJ). This nocturnal MCS produced in excess of 300 mm of rain in a small area to the southwest of St. Louis, Missouri. Operational model forecasts and simulations using a convective parameterization scheme failed to produce the observed rainfall totals for this event. However, convection-permitting simulations using the Weather Research and Forecasting Model were successful in reproducing the quasi-stationary organization and evolution of this MCS. In both observations and simulations, scattered elevated convective cells were repeatedly initiated 50–75 km upstream before merging into the mature MCS and contributing to the heavy rainfall. Lifting provided by the interaction between the LLJ and the MCV assisted in initiating and maintaining the convection. Simulations indicate that the MCS was long lived despite the lack of a convectively generated cold pool at the surface. Instead, a nearly stationary low-level gravity wave helped to organize the convection into a quasi-linear system that was conducive to extreme local rainfall amounts. Idealized simulations of convection in a similar environment show that such a low-level gravity wave is a response to diabatic heating and that the vertical wind profile featuring a strong reversal of the wind shear with height is responsible for keeping the wave nearly stationary. In addition, the convective system acted to reintensify the midlevel MCV and also caused a distinct surface low pressure center to develop in both the observed and simulated system.

scholarly journals Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

2014 ◽  
Vol 142 (8) ◽  
pp. 2838-2859 ◽  
Author(s):  
Buo-Fu Chen ◽  
Russell L. Elsberry ◽  
Cheng-Shang Lee
Keyword(s):  
Inner Core ◽  
Outer Region ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Cold Pool ◽  
Convective System ◽  
Rain Area ◽  
Characteristic Structure ◽  
Low Level ◽  
Mesoscale Convective

Abstract Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the inner-core rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting Model simulations. Quick Scatterometer (QuikSCAT) observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north–south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer, and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and midlevel northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (Δθ < −3 K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS.

The Analysis and Prediction of Microphysical States and Polarimetric Radar Variables in a Mesoscale Convective System Using Double-Moment Microphysics, Multinetwork Radar Data, and the Ensemble Kalman Filter

2014 ◽  
Vol 142 (1) ◽  
pp. 141-162 ◽  
Author(s):  
Bryan J. Putnam ◽  
Ming Xue ◽  
Youngsun Jung ◽  
Nathan Snook ◽  
Guifu Zhang
Keyword(s):  
Kalman Filter ◽  
Ensemble Kalman Filter ◽  
Mesoscale Convective System ◽  
Radar Data ◽  
Cold Pool ◽  
Convective System ◽  
Polarimetric Radar ◽  
Microphysics Scheme ◽  
Mesoscale Convective ◽  
Double Moment

Abstract Doppler radar data are assimilated with an ensemble Kalman Filter (EnKF) in combination with a double-moment (DM) microphysics scheme in order to improve the analysis and forecast of microphysical states and precipitation structures within a mesoscale convective system (MCS) that passed over western Oklahoma on 8–9 May 2007. Reflectivity and radial velocity data from five operational Weather Surveillance Radar-1988 Doppler (WSR-88D) S-band radars as well as four experimental Collaborative and Adaptive Sensing of the Atmosphere (CASA) X-band radars are assimilated over a 1-h period using either single-moment (SM) or DM microphysics schemes within the forecast ensemble. Three-hour deterministic forecasts are initialized from the final ensemble mean analyses using a SM or DM scheme, respectively. Polarimetric radar variables are simulated from the analyses and compared with polarimetric WSR-88D observations for verification. EnKF assimilation of radar data using a multimoment microphysics scheme for an MCS case has not previously been documented in the literature. The use of DM microphysics during data assimilation improves simulated polarimetric variables through differentiation of particle size distributions (PSDs) within the stratiform and convective regions. The DM forecast initiated from the DM analysis shows significant qualitative improvement over the assimilation and forecast using SM microphysics in terms of the location and structure of the MCS precipitation. Quantitative precipitation forecasting skills are also improved in the DM forecast. Better handling of the PSDs by the DM scheme is believed to be responsible for the improved prediction of the surface cold pool, a stronger leading convective line, and improved areal extent of stratiform precipitation.

scholarly journals Synoptic-scale conditions and convection-resolving hindcast experiments of a cold-season derecho on 3 January 2014 in western Europe

2019 ◽  
Vol 19 (5) ◽  
pp. 1023-1040 ◽  
Author(s):  
Luca Mathias ◽  
Patrick Ludwig ◽  
Joaquim G. Pinto
Keyword(s):  
Boundary Conditions ◽  
Surface Pressure ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Mesoscale Convective System ◽  
Cold Season ◽  
Limited Area ◽  
Convective System ◽  
Lateral Boundary ◽  
Lateral Boundary Conditions

Abstract. A major linear mesoscale convective system caused severe weather over northern France, Belgium, the Netherlands and northwestern Germany on 3 January 2014. The storm was classified as a cold-season derecho with widespread wind gusts exceeding 25 m s−1. While such derechos occasionally develop along cold fronts of extratropical cyclones, this system formed in a postfrontal air mass along a baroclinic surface pressure trough and was favoured by a strong large-scale air ascent induced by an intense mid-level jet. The lower-tropospheric environment was characterised by weak latent instability and strong vertical wind shear. Given the poor operational forecast of the storm, we analyse the role of initial and lateral boundary conditions to the storm's development by performing convection-resolving limited-area simulations with operational analysis and reanalysis datasets. The storm is best represented in simulations with high temporally and spatially resolved initial and lateral boundary conditions derived from ERA5, which provide the most realistic development of the essential surface pressure trough. Moreover, simulations at convection-resolving resolution enable a better representation of the observed derecho intensity. This case study is testimony to the usefulness of ensembles of convection-resolving simulations in overcoming the current shortcomings of forecasting cold-season convective storms, particularly for cases not associated with a cold front.

scholarly journals The Vertical Structure of Mesoscale Convective Vortices

2009 ◽  
Vol 66 (3) ◽  
pp. 686-704 ◽  
Author(s):  
Christopher A. Davis ◽  
Thomas J. Galarneau
Keyword(s):  
Vertical Structure ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Mesoscale Convective System ◽  
Secondary Phase ◽  
Radical Change ◽  
Convective System ◽  
Mesoscale Convective ◽  
Outflow Boundary ◽  
Convective Vortices

Abstract Simulations of two cases of developing mesoscale convective vortices (MCVs) are examined to determine the dynamics governing the origin and vertical structure of these features. Although one case evolves in strong vertical wind shear and the other evolves in modest shear, the evolutions are remarkably similar. In addition to the well-known mesoscale convergence that spins up vorticity in the midtroposphere, the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system (MCS). Lateral transport of this vorticity from the convective region back beneath the midtropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity. However, this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower-tropospheric convergence. Following the decay of the nocturnal MCS, subsequent convection occurs in a condition of greater relative humidity through the lower troposphere and small conditional instability. Vorticity and potential vorticity are efficiently produced near the top of the boundary layer and a cyclonic circulation appears at the surface.

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