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 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.

Simulation of Mesoscale Convective Systems near the Tropical Andes: Insights from Convection-Permitting Simulations of Two Events

2021 ◽  
Author(s):  
J. Alejandro Martinez ◽  
Juan Carlos Camacho ◽  
Daniel Vasquez ◽  
Daniel Espinosa ◽  
Paola A. Arias
Keyword(s):  
Extreme Rainfall ◽  
Sea Breeze ◽  
Mesoscale Convective Systems ◽  
Tropical Andes ◽  
Low Level ◽  
Convective Systems ◽  
The Andes ◽  
Simulated Precipitation ◽  
Synoptic Conditions ◽  
Mesoscale Convective

<p>Mesoscale Convective Systems (MCSs) are associated with an important fraction of total precipitation in the vicinity of the Tropical Andes, and are related to high impact weather events and extreme rainfall.  Important ingredients include input of moisture and synoptic conditions particular of each location, depending on the regional scale circulation and the local topography.  Convection-Permitting (CP) simulations can help to better describe events with MCSs, including details of surface processes, low-level moisture transport and mountain-related circulations. Here we present a description of two MCSs in the vicinity of the Tropical Andes based on gridded observation-based data (ERA5 and GPM), in situ measurements and CP simulations with the Weather Research and Forecasting (WRF) model.  One of the events took place near the Andes-Amazon transition region (Mocoa-Colombia), with, reportedly, more than 100mm of precipitation accumulated in 3 hours in one location, accompanied with strong low-level transport of moisture by the (nocturnal) Orinoco Low-Level Jet (OLLJ) and strong mid-tropospheric easterly winds towards the Andes, favorable for orographic enhancenment of precipitation.  The other event took place over the low-lands of the Magdalena-Cauca basin (Cordoba-Colombia), with an approximate size of 71304 km<sup>2 </sup>, according to its cloud top temperature pattern.  In this region a sea-breeze provides moisture from oceanic origin, and the nearby Andes might help to enhance low-level convergence via orographic blocking and other mountain-related effects.  Based on kilometer-scale CP simulations we describe details of the initiation and life cycle of these two MCSs as simulated by WRF, including a description of the low-level input of moisture provided by the sea-breeze and the nocturnal jet during the initiation and mature stages, the corresponding mesoscale circulations in the vicinity of the Andes, and the intensity of the simulated precipitation.  Preliminary 3-km simulations of the Mocoa event show the low-level flow blocking by the Andes, the enhanced orographic precipitation, and an underestimation of the maximum intensity of rainfall. This study might help on understanding the skill and limitations of CP simulations for representing weather systems associated to extreme rainfall in the Tropical Andes. </p>

Tracking Mesoscale Convective Systems that are Potential Candidates for Tropical Cyclogenesis

2020 ◽  
Vol 148 (2) ◽  
pp. 655-669 ◽  
Author(s):  
Kelly M. Núñez Ocasio ◽  
Jenni L. Evans ◽  
George S. Young
Keyword(s):  
Propagation Speed ◽  
Extreme Rainfall ◽  
Life Cycles ◽  
Mesoscale Convective Systems ◽  
Tropical Cyclogenesis ◽  
Convective System ◽  
Background Flow ◽  
Extreme Rainfall Events ◽  
Convective Systems ◽  
Mesoscale Convective

Abstract This study introduces the development of the Tracking Algorithm for Mesoscale Convective Systems (TAMS), an algorithm that allows for the identifying, tracking, classifying, and assigning of rainfall to mesoscale convective systems (MCSs). TAMS combines area-overlapping and projected-cloud-edge tracking techniques to maximize the probability of detecting the progression of a convective system through time, accounting for splits and mergers. The combination of projection on area overlapping is equivalent to setting the background flow in which MCSs are moving on. Sensitivity tests show that area-overlapping technique with no projection (thus, no background flow) underestimates the real propagation speed of MCSs over Africa. The MCS life cycles and propagation derived using TAMS are consistent with climatology. The rainfall assignment is also more reliable than with previous methods as it utilizes a combination of regridding through linear interpolation with high temporal and spatial resolution data. This makes possible the identification of extreme rainfall events associated with intense MCSs more effectively. TAMS will be utilized in future work to build an AEW–MCS dataset to study tropical cyclogenesis.

scholarly journals Three MCS Cases Occurring in Different Synoptic Environments in the Sub-Sahelian Wet Zone during the 2002 West African Monsoon

2006 ◽  
Vol 63 (9) ◽  
pp. 2369-2382 ◽  
Author(s):  
Jon M. Schrage ◽  
Andreas H. Fink ◽  
Volker Ermert ◽  
Epiphane D. Ahlonsou
Keyword(s):  
West Africa ◽  
West African Monsoon ◽  
Lower Troposphere ◽  
Integrated Approach ◽  
West African ◽  
Easterly Waves ◽  
Convective Systems ◽  
Efficient Management ◽  
Mesoscale Convective ◽  
Wet Zone

Abstract Three mesoscale convective systems (MCSs) occurring in the sub-Sahelian wet zone of West Africa are examined using observations from the 2002 Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) field campaign, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, and Meteosat infrared imagery. These datasets enable the analysis of the synoptic-scale environment in which the MCSs were embedded, along with a high-resolution monitoring of surface parameters during the systems’ passages. The available data imply that cases I and II were of a squall-type nature. Case I propagated into a moderately sheared and rather moist lower and middle troposphere over the Upper Ouémé Valley (UOV). In contrast, case II was associated with a well-sheared and dry lower troposphere and a large, moist instability. In either case, behind the convective cluster a westward-propagating cyclonic vorticity maximum that was likely captured by the ECMWF analysis as a result of the special upper-air station at Parakou (Benin). In case I, the fast-moving vorticity signal slowed down over the Guinean Highlands where convection dissipated. Farther downstream, it might have played a role in the consolidation of an African easterly waves (AEW) trough over the West African coast and the eastern Atlantic. Case III proved to be a more stationary pattern of convection associated with a vortex in the monsoon flow. It also exhibited a moist and low shear environment.

scholarly journals Near-Surface Thermodynamic Sensitivities in Simulated Extreme-Rain-Producing Mesoscale Convective Systems

2017 ◽  
Vol 145 (6) ◽  
pp. 2177-2200 ◽  
Author(s):  
Russ S. Schumacher ◽  
John M. Peters
Keyword(s):  
Water Vapor ◽  
Initial Conditions ◽  
Weather Prediction ◽  
Mesoscale Convective Systems ◽  
Cold Pool ◽  
Near Surface ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Outflow Boundary

Abstract This study investigates the influences of low-level atmospheric water vapor on the precipitation produced by simulated warm-season midlatitude mesoscale convective systems (MCSs). In a series of semi-idealized numerical model experiments using initial conditions gleaned from composite environments from observed cases, small increases in moisture were applied to the model initial conditions over a layer either 600 m or 1 km deep. The precipitation produced by the MCS increased with larger moisture perturbations as expected, but the rainfall changes were disproportionate to the magnitude of the moisture perturbations. The experiment with the largest perturbation had a water vapor mixing ratio increase of approximately 2 g kg−1 over the lowest 1 km, corresponding to a 3.4% increase in vertically integrated water vapor, and the area-integrated MCS precipitation in this experiment increased by nearly 60% over the control. The locations of the heaviest rainfall also changed in response to differences in the strength and depth of the convectively generated cold pool. The MCSs in environments with larger initial moisture perturbations developed stronger cold pools, and the convection remained close to the outflow boundary, whereas the convective line was displaced farther behind the outflow boundary in the control and the simulations with smaller moisture perturbations. The high sensitivity of both the amount and location of MCS rainfall to small changes in low-level moisture demonstrates how small moisture errors in numerical weather prediction models may lead to large errors in their forecasts of MCS placement and behavior.

scholarly journals Influence of Tropical Waves on the Lifecycle of Mesoscale Convective Systems over West Africa

2021 ◽  
Author(s):  
Marlon Maranan ◽  
Andreas Schlueter ◽  
Andreas H. Fink ◽  
Peter Knippertz
Keyword(s):  
West Africa ◽  
High Frequency ◽  
Rainfall Variability ◽  
Weather Prediction ◽  
Mesoscale Convective Systems ◽  
Convection Current ◽  
Maturity Stage ◽  
Convective Systems ◽  
Tropical Waves ◽  
Mesoscale Convective

<p>Rainfall variability over West Africa remains a major challenge for numerical weather prediction (NWP). Due to the largely stochastic and sub-grid nature of tropical convection, current NWP models still fail to provide reliable precipitation forecasts – even for a 1-day leadtime – and are barely more skillful than climatology-based forecasts. Thus, several recent studies have investigated the presumably more predictable influence of tropical waves on environmental conditions for convection and found distinct and coherent (thermo-)dynamical patterns depending on the type and phase of the wave. Of particular interest in this context is the interaction of the wave with the lifecycle of usually westward propagating mesoscale convective systems (MCSs), which are the major providers of rain in the region and can occasionally even lead to flooding. The exact mechanisms and strength of this interaction are still not entirely known.</p><p>This study combines two recent datasets in a novel way in order to systematically investigate the influence of tropical waves on MCS characteristics and lifecycle. First, MCSs are tracked within northern tropical Africa (20°W-30°E / 2°-15°N) over an 11-year period during the West African rainy season (April-October) using infrared brightness temperature fields provided by the Spinning enhanced visible and infrared imager (SEVIRI). Second, tropical waves are isolated by applying a filtering method in the wave-frequency domain to precipitation data of the Tropical Rainfall Measuring Mission (TRMM) within the 5°-15°N latitude band for the same target period. By combining the two datasets in space and time, the magnitude and phase of each wave is known at every timestep of the MCS tracks, which enables a systematic investigation of MCS characteristics as a function of wave properties.</p><p>Preliminary results suggest that long-lived MCSs (lifetime ≥ 12h) frequently couple with the “wet” phase of high-frequency tropical waves, in particular Kelvin, eastward inertia-gravity (EIG), and African easterly waves (AEW). Showing an enhanced occurrence frequency of MCS initiation, the wet phase of AEWs appears to have strong modulation capabilities during the genesis stage and further accompanies these long-lived MCSs during their entire lifetime. In the case of Kelvin waves and EIGs, the wet phase overlaps only with the intensification and maturity stage of these MCSs as a consequence of opposite directions of movement. Similar coupling patterns also exist for mixed Rossby gravity waves (MRGs), although to a weaker extent. Furthermore, no consistent coupling tendencies with long-lived MCSs are evident for low-frequency waves (Madden-Julian Oscillation (MJO), equatorial Rossby wave (ER)), arguably since they act on larger spatio-temporal scales. For short-lived MCSs (lifetime < 6h), the coupling with high-frequency waves is substantially weaker.</p><p>In the future we will also address potential influences of wave-wave interactions on MCSs as well as potential differences in coupling mechanisms between the Guinea Coast region and the Sahel farther north. With increasing efforts in the prediction of tropical waves, this study has the potential to aid the short-term forecasting of MCS development and its lifecycle. This can be of particular importance for the anticipation of extreme rainfall events and subsequent risk assessment in West Africa.</p>

Organizational Modes of Mesoscale Convective Systems over Central East China

2013 ◽  
Vol 28 (5) ◽  
pp. 1081-1098 ◽  
Author(s):  
Linlin Zheng ◽  
Jianhua Sun ◽  
Xiaoling Zhang ◽  
Changhai Liu
Keyword(s):  
Severe Weather ◽  
Mesoscale Convective Systems ◽  
East China ◽  
Short Term ◽  
Low Level ◽  
Convective Systems ◽  
Intense Precipitation ◽  
Stratiform Precipitation ◽  
Mesoscale Convective ◽  
High Winds

Abstract Composite reflectivity Doppler radar data from June to September of 2007–2010 were used to classify mesoscale convective systems (MCSs) over central east China into seven morphologies. The morphologies included one nonlinear mode (NL) and six linear modes: convective lines with no stratiform precipitation (NS), trailing stratiform precipitation (TS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), bow echoes (BE), and embedded lines (EL). Nonlinear and linear systems composed 44.7% and 55.3% of total MCSs, respectively, but there was no primary linear mode. All MCS morphologies attained their peak occurrence in July, except BE systems, which peaked in June. On average, TS and PS modes had relatively longer lifespans than did other modes. Significant differences in MCS-produced severe weather existed between dry and moist environments. High winds and hail events were mainly observed in dry environments, and in contrast, short-term intense precipitation occurred more frequently in moist environments. BE systems generated the most severe weather on average, while most TS systems were attendant with short-term intense precipitation and high winds. EL and PS systems were most frequently associated with extreme short-time intense precipitation (≥50 mm h−1) as these systems preferentially developed in moist environments. BE systems generally occurred under strong low-level shear and intermediately moist conditions. LS systems were observed in weak low-level shear, whereas EL systems often developed in relatively stable conditions and weak low- to middle-level shear. The largest instability was present in the environment for NS systems. The environmental parameters for TS systems featured the largest differences between the dry and moist cases.

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