scholarly journals An Observational Study of Mesoscale Convective Systems with Heavy Rainfall over the Korean Peninsula

2006 ◽  
Vol 21 (2) ◽  
pp. 125-148 ◽  
Author(s):  
Hyung Woo Kim ◽  
Dong Kyou Lee
Keyword(s):  
Observational Study ◽  
Korean Peninsula ◽  
Wind Shear ◽  
Heavy Rainfall ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract A heavy rainfall event induced by mesoscale convective systems (MCSs) occurred over the middle Korean Peninsula from 25 to 27 July 1996. This heavy rainfall caused a large loss of life and property damage as a result of flash floods and landslides. An observational study was conducted using Weather Surveillance Radar-1988 Doppler (WSR-88D) data from 0930 UTC 26 July to 0303 UTC 27 July 1996. Dominant synoptic features in this case had many similarities to those in previous studies, such as the presence of a quasi-stationary frontal system, a weak upper-level trough, sufficient moisture transportation by a low-level jet from a tropical storm landfall, strong potential and convective instability, and strong vertical wind shear. The thermodynamic characteristics and wind shear presented favorable conditions for a heavy rainfall occurrence. The early convective cells in the MCSs initiated over the coastal area, facilitated by the mesoscale boundaries of the land–sea contrast, rain–no rain regions, saturated–unsaturated soils, and steep horizontal pressure and thermal gradients. Two MCSs passed through the heavy rainfall regions during the investigation period. The first MCS initiated at 1000 UTC 26 July and had the characteristics of a supercell storm with small amounts of precipitation, the appearance of a mesocyclone with tilting storm, a rear-inflow jet at the midlevel of the storm, and fast forward propagation. The second MCS initiated over the upstream area of the first MCS at 1800 UTC 26 July and had the characteristics of a multicell storm, such as a broken areal-type squall line, slow or quasi-stationary backward propagation, heavy rainfall in a concentrated area due to the merging of the convective storms, and a stagnated cluster system. These systems merged and stagnated because their movement was blocked by the Taebaek Mountain Range, and they continued to develop because of the vertical wind shear resulting from a low-level easterly inflow.

scholarly journals Observational Analysis of the Predictability of Mesoscale Convective Systems

2007 ◽  
Vol 22 (4) ◽  
pp. 813-838 ◽  
Author(s):  
Israel L. Jirak ◽  
William R. Cotton
Keyword(s):  
United States ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
The United States ◽  
Vertical Wind ◽  
Mesoscale Convective Systems ◽  
Convective Initiation ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract Mesoscale convective systems (MCSs) have a large influence on the weather over the central United States during the warm season by generating essential rainfall and severe weather. To gain insight into the predictability of these systems, the precursor environments of several hundred MCSs across the United States were reviewed during the warm seasons of 1996–98. Surface analyses were used to identify initiating mechanisms for each system, and North American Regional Reanalysis (NARR) data were used to examine the environment prior to MCS development. Similarly, environments unable to support organized convective systems were also investigated for comparison with MCS precursor environments. Significant differences were found between environments that support MCS development and those that do not support convective organization. MCSs were most commonly initiated by frontal boundaries; however, features that enhance convective initiation are often not sufficient for MCS development, as the environment needs also to be supportive for the development and organization of long-lived convective systems. Low-level warm air advection, low-level vertical wind shear, and convective instability were found to be the most important parameters in determining whether concentrated convection would undergo upscale growth into an MCS. Based on these results, an index was developed for use in forecasting MCSs. The MCS index assigns a likelihood of MCS development based on three terms: 700-hPa temperature advection, 0–3-km vertical wind shear, and the lifted index. An evaluation of the MCS index revealed that it exhibits features consistent with common MCS characteristics and is reasonably accurate in forecasting MCSs, especially given that convective initiation has occurred, offering the possibility of usefulness in operational forecasting.

scholarly journals Seasonality and Trends of Drivers of Mesoscale Convective Systems in Southern West Africa

2021 ◽  
Vol 34 (1) ◽  
pp. 71-87
Author(s):  
Cornelia Klein ◽  
Francis Nkrumah ◽  
Christopher M. Taylor ◽  
Elijah A. Adefisan
Keyword(s):  
West Africa ◽  
Wind Shear ◽  
Dominant Role ◽  
West African Monsoon ◽  
Extreme Rainfall ◽  
Temperature Gradients ◽  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective

AbstractMesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

scholarly journals A Numerical Simulation Study of the Effects of Anvil Shading on Quasi-Linear Convective Systems

2013 ◽  
Vol 70 (3) ◽  
pp. 767-793 ◽  
Author(s):  
Andrew J. Oberthaler ◽  
Paul M. Markowski
Keyword(s):  
Solar Radiation ◽  
Wind Shear ◽  
Wind Profile ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Convective System ◽  
Near Surface ◽  
Low Level ◽  
Convective Systems ◽  
Gust Front

Abstract Numerical simulations are used to investigate how the attenuation of solar radiation by the intervening cumulonimbus cloud, particularly its large anvil, affects the structure, intensity, and evolution of quasi-linear convective systems and the sensitivity of the effects of this “anvil shading” to the ambient wind profile. Shading of the pre-gust-front inflow environment (as opposed to shading of the cold pool) has the most important impact on the convective systems. The magnitude of the low-level cooling, associated baroclinicity, and stabilization of the pre-gust-front environment due to anvil shading generally increases as the duration of the shading increases. Thus, for a given leading anvil length, a slow-moving convective system tends to be affected more by anvil shading than does a fast-moving convective system. Differences in the forward speeds of the convective systems simulated in this study are largely attributable to differences in the mean environmental wind speed over the depth of the troposphere. Anvil shading reduces the buoyancy realized by the air parcels that ascend through the updrafts. As a result, anvil shading contributes to weaker updrafts relative to control simulations in which clouds are transparent to solar radiation. Anvil shading also affects the convective systems by modifying the low-level (nominally 0–2.5 km AGL) vertical wind shear in the pre-gust-front environment. The shear modifications affect the slope of the updraft region and system-relative rear-to-front flow, and the sign of the modifications is sensitive to the ground-relative vertical wind profile in the far-field environment. The vertical wind shear changes are brought about by baroclinic vorticity generation associated with the horizontal buoyancy gradient that develops in the shaded boundary layer (which makes the pre-gust-front, low-level vertical wind shear less westerly) and by a reduction of the vertical mixing of momentum due to the near-surface (nominally 0–300 m AGL) stabilization that accompanies the shading-induced cooling. The reduced mixing makes the pre-gust-front, low-level vertical shear more (less) westerly if the ambient, near-surface wind and wind shear are westerly (easterly).

scholarly journals Environmental Factors in the Upscale Growth and Longevity of MCSs Derived from Rapid Update Cycle Analyses

2010 ◽  
Vol 138 (9) ◽  
pp. 3514-3539 ◽  
Author(s):  
Michael C. Coniglio ◽  
Jason Y. Hwang ◽  
David J. Stensrud
Keyword(s):  
Potential Vorticity ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Cold Pool ◽  
Inertial Oscillation ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Rapid Transition ◽  
Upper Level

Abstract Composite environments of mesoscale convective systems (MCSs) are produced from Rapid Update Cycle (RUC) analyses to explore the differences between rapidly and slowly developing MCSs as well as the differences ahead of long- and short-lived MCSs. The composite analyses capture the synoptic-scale features known to be associated with MCSs and depict the inertial oscillation of the nocturnal low-level jet (LLJ), which remains strong but tends to veer away from decaying MCSs. The composite first storms environment for the rapidly developing MCSs contains a stronger LLJ located closer to the first storms region, much more conditional instability, potential instability, and energy available for downdrafts, smaller 3–10-km vertical wind shear, and smaller geostrophic potential vorticity in the upper troposphere, when compared to the environment for the slowly developing MCSs. The weaker shear above 3 km for the rapidly developing MCSs is consistent with supercell or discrete cell modes being less likely in weaker deep-layer shear and the greater potential for a cold pool to trigger convection when the shear is confined to lower levels. Furthermore, these results suggest that low values of upper-level potential vorticity may signal a rapid transition to an MCS. The composite environment ahead of the genesis of long-lived MCSs contains a broader LLJ, a better-defined frontal zone, stronger low-level frontogenesis, deeper moisture, and stronger wind shear above 2 km, when compared to short-lived MCSs. The larger shear above 2 km for the long-lived MCSs is consistent with the importance of shear elevated above the ground to help organize and maintain convection that feeds on the elevated unstable parcels after dark and is indicative of the enhanced baroclinicity ahead of the MCSs.

Assessing the influence of complex terrain on severe convective environments in northeastern Alabama

2021 ◽  
Author(s):  
Branden Katona ◽  
Paul Markowski
Keyword(s):  
Boundary Layer ◽  
Complex Terrain ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Spatial Distributions ◽  
Convective Storm ◽  
Self Organizing Maps ◽  
Low Level ◽  
Wind Regimes

AbstractStorms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective stormenvironments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

scholarly journals A numerical study of convection in rainbands of Typhoon Morakot (2009) with extreme rainfall: roles of pressure perturbations with low-level wind maxima

2015 ◽  
Vol 15 (6) ◽  
pp. 8479-8523
Author(s):  
C.-C. Wang ◽  
H.-C. Kuo ◽  
R. H. Johnson ◽  
C.-Y. Lee ◽  
S.-Y. Huang ◽  
...  
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Extreme Rainfall ◽  
Vertical Wind ◽  
Cold Pool ◽  
Rear Side ◽  
Gradient Force ◽  
Typhoon Morakot ◽  
Low Level ◽  
Pressure Perturbations

Abstract. This paper investigates the formation and evolution of deep convection inside the east–west oriented rainbands associated with a low-level jet (LLJ) in Typhoon Morakot (2009). With typhoon center to the northwest of Taiwan, the westerly LLJ was resulted from the interaction of typhoon circulation with the southwest monsoon flow, which supplied the water vapor for the extreme rainfall (of ~1000 mm) over southwestern Taiwan. The Cloud-Resolving Storm Simulator with 1 km grid spacing was used to simulate the event, and it successfully reproduced the slow-moving rainbands, the embedded cells, and the dynamics of merger and back-building (BB) on 8 August as observed. Our model results suggest that the intense convection interacted strongly with the westerly LLJ that provided reversed vertical wind shear below and above the jet core. Inside mature cells, significant dynamical pressure perturbations (pd') are induced with positive (negative) pd' at the western (eastern) flank of the updraft near the surface and a reversed pattern aloft (>2 km). This configuration produced an upward directed pressure gradient force (PGF) to the rear side and favors new development to the west, which further leads to cell merger as the mature cells slowdown in eastward propagation. The strong updrafts also acted to elevate the jet and enhance the local vertical wind shear at the rear flank. Additional analysis reveals that the upward PGF there is resulted mainly by the shearing effect but also by the extension of upward acceleration at low levels. In the horizontal, the upstream-directed PGF induced by the rear-side positive pd' near the surface is much smaller, but can provide additional convergence for BB development upstream. Finally, the cold-pool mechanism for BB appears to be not important in the Morakot case, as the conditions for strong evaporation in downdrafts do not exist.

How does vertical wind shear influence entrainment in squall lines?

2021 ◽  
Author(s):  
Jake P. Mulholland ◽  
John M. Peters ◽  
Hugh Morrison
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Leading Edge ◽  
Vertical Wind ◽  
Squall Line ◽  
Cold Pool ◽  
Low Level ◽  
Boundary Shear ◽  
Squall Lines ◽  
Outflow Boundary

AbstractThe influence of vertical wind shear on updraft entrainment in squall lines is not well understood. To address this knowledge gap, a suite of high-resolution idealized numerical model simulations of squall lines were run in various vertical wind shear (hereafter “shear”) environments to study the effects of shear on entrainment in deep convective updrafts. Low-level horizontal mass flux into the leading edge of the cold pool was strongest in the simulations with the strongest low-level shear. These simulations consequently displayed wider updrafts, less entrainment-driven dilution, and larger buoyancy than the simulations with comparatively weak low-level shear. An analysis of vertical accelerations along trajectories that passed through updrafts showed larger net accelerations from buoyancy in the simulations with stronger low-level shear, which demonstrates how less entrainment-driven dilution equated to stronger updrafts. The effects of upper-level shear on entrainment and updraft vertical velocities were generally less pronounced than the effects of low-level shear. We argue that in addition to the outflow boundary-shear interactions and their effect on updraft tilt established by previous authors, decreased entrainment-driven dilution is yet another beneficial effect of strong low-level shear on squall line updraft intensity.

scholarly journals Mesoscale Convective Systems: A Case Scenario of the ‘Heavy Rainfall’ Event of 15–20 January 2013 over Southern Africa

Climate ◽  
2019 ◽  
Vol 7 (6) ◽  
pp. 73
Author(s):  
Modise Wiston ◽  
Kgakgamatso Marvel Mphale
Keyword(s):  
Atlantic Ocean ◽  
Southern Africa ◽  
Heavy Rainfall ◽  
Rainfall Event ◽  
Mesoscale Convective Systems ◽  
Heavy Rainfall Event ◽  
Convergence Zone ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Upper Level

Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa.

scholarly journals Diagnosing Convective Dependencies on Near-Storm Environments Using Ensemble Sensitivity Analyses

2019 ◽  
Vol 147 (2) ◽  
pp. 495-517 ◽  
Author(s):  
Christopher A. Kerr ◽  
David J. Stensrud ◽  
Xuguang Wang
Keyword(s):  
Wind Shear ◽  
Initial Conditions ◽  
Wrf Model ◽  
Vertical Wind Shear ◽  
Mesoscale Convective System ◽  
Vertical Wind ◽  
Sensitivity Analyses ◽  
Convective System ◽  
Mesoscale Convective ◽  
Storm Evolution

AbstractConvection intensity and longevity is highly dependent on the surrounding environment. Ensemble sensitivity analysis (ESA), which quantitatively and qualitatively interprets impacts of initial conditions on forecasts, is applied to very short-term (1–2 h) convective-scale forecasts for three cases during the Mesoscale Predictability Experiment (MPEX) in 2013. The ESA technique reveals several dependencies of individual convective storm evolution on their nearby environments. The three MPEX cases are simulated using a previously verified 36-member convection-allowing model (Δx = 3 km) ensemble created via the Weather Research and Forecasting (WRF) Model. Radar and other conventional observations are assimilated using an ensemble adjustment Kalman filter. The three cases include a mesoscale convective system (MCS) and both nontornadic and tornadic supercells. Of the many ESAs applied in this study, one of the most notable is the positive sensitivity of supercell updraft helicity to increases in both storm inflow region deep and shallow vertical wind shear. This result suggests that larger values of vertical wind shear within the storm inflow yield higher values of storm updraft helicity. Results further show that the supercell storms quickly enhance the environmental vertical wind shear within the storm inflow region. Application of ESA shows that these storm-induced perturbations then affect further storm evolution, suggesting the presence of storm–environment feedback cycles where perturbations affect future mesocyclone strength. Overall, ESA can provide insight into convection dependencies on the near-storm environment.

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