Formation of African Easterly Waves and Mesoscale Convective Systems over Eastern Africa and its Implication to Tropical Cyclogenesis over Eastern Atlantic Ocean

2009 ◽  
Author(s):  
Yuh-Lang Lin ◽  
Hakeem M. Oluseyi
Keyword(s):  
Atlantic Ocean ◽  
Mesoscale Convective Systems ◽  
Eastern Africa ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Convective Systems ◽  
Eastern Atlantic Ocean ◽  
Mesoscale Convective

Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

2021 ◽  
Author(s):  
Kelly M. Núñez Ocasio ◽  
Alan Brammer ◽  
Jenni L. Evans ◽  
George S. Young ◽  
Zachary L. Moon
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
Eastern Africa ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Vertically Aligned ◽  
Somali Jet

AbstractEastern Africa is a common region of African easterly wave (AEW) onset and AEW early-life. How the large-scale environment over east Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: an anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, deeper wave pouch, and arguably more AEW and Mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are more likely to be southern-track AEWs than non-developing AEWs.

scholarly journals Observations of Coastally Transitioning West African Mesoscale Convective Systems during NAMMA

2012 ◽  
Vol 2012 ◽  
pp. 1-25
Author(s):  
Bradley W. Klotz ◽  
Paul Kucera
Keyword(s):  
West African ◽  
Radar Reflectivity ◽  
Mesoscale Convective Systems ◽  
Environmental Stability ◽  
Tropical Cyclogenesis ◽  
Easterly Waves ◽  
Convective Systems ◽  
Maximum Reflectivity ◽  
Multidisciplinary Analysis ◽  
Mesoscale Convective

Observations from the NASA 10 cm polarimetric Doppler weather radar (NPOL) were used to examine structure, development, and oceanic transition of West African Mesoscale Convective Systems (MCSs) during the NASA African Monsoon Multidisciplinary Analysis (NAMMA) to determine possible indicators leading to downstream tropical cyclogenesis. Characteristics examined from the NPOL data include echo-top heights, maximum radar reflectivity, height of maximum radar reflectivity, and convective and stratiform coverage areas. Atmospheric radiosondes launched during NAMMA were used to investigate environmental stability characteristics that the MCSs encountered while over land and ocean, respectively. Strengths of African Easterly Waves (AEWs) were examined along with the MCSs in order to improve the analysis of MCS characteristics. Mean structural and environmental characteristics were calculated for systems that produced TCs and for those that did not in order to determine differences between the two types. Echo-top heights were similar between the two types, but maximum reflectivity and height and coverage of intense convection (>50 dBZ) are all larger than for the TC producing cases. Striking differences in environmental conditions related to future TC formation include stronger African Easterly Jet, increased moisture especially at middle and upper levels, and increased stability as the MCSs coastally transition.

scholarly journals Precipitation and Mesoscale Convective Systems: Explicit versus Parameterized Convection over Northern Africa

2018 ◽  
Vol 146 (3) ◽  
pp. 797-812 ◽  
Author(s):  
Irene Reinares Martínez ◽  
Jean-Pierre Chaboureau
Keyword(s):  
Central Africa ◽  
Northern Africa ◽  
Mesoscale Convective Systems ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Convective Systems ◽  
Convective Clouds ◽  
Infrared Signature ◽  
Mesoscale Convective ◽  
Excess Moisture

Abstract Precipitating systems are analyzed during a dust event from 9 to 14 June 2006 over northern Africa. A common analysis is applied to satellite observations and two Meso-NH simulations: one convection permitting (grid spacing x = 2.5 km) and the other with parameterized convection (x = 20 km). The precipitating systems are identified as cloud objects and classified as deep convective clouds (DCCs) or other clouds according to their infrared signature. Large DCCs [hereafter named mesoscale convective systems (MCSs)] are tracked, characterized in terms of precipitation and thermodynamic profiles, and analyzed in southern West Africa (SWA), central Africa, and Ethiopia. Precipitation is mostly observed along 0°–15°N, with 71% of the total precipitation produced by all DCCs and 55% by long-lived MCSs. It shows a marked diurnal cycle with a peak in the evening, mainly due to long-lived MCSs, which are characterized by an increase in size, zonal speed, and duration from east to west, with the largest, fastest, and longest-lived ones found over SWA. This is due to an enhanced African easterly jet (AEJ) and monsoon flow leading to stronger shear and greater conditional instability. The simulation with parameterized convection fails to distribute precipitation correctly. The convection-permitting simulation captures most of the observed precipitation features, but lacks the increase in organization of the long-lived MCSs over SWA. Excess moisture in a too zonal AEJ flow suggests that the long-lived MCSs in SWA are poorly located with respect to African easterly waves. The convection-permitting model improves the representation of precipitation but without fully resolving the long-lived MCSs.

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.

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 Tropical Atlantic Hurricanes, Easterly Waves, and West African Mesoscale Convective Systems

2010 ◽  
Vol 2010 ◽  
pp. 1-13 ◽  
Author(s):  
Yves K. Kouadio ◽  
Luiz A. T. Machado ◽  
Jacques Servain
Keyword(s):  
West Africa ◽  
West African ◽  
Mesoscale Convective Systems ◽  
Tropical Atlantic ◽  
Atmospheric Conditions ◽  
Easterly Waves ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Sst Anomaly ◽  
Atlantic Hurricanes

The relationship between tropical Atlantic hurricanes (Hs), atmospheric easterly waves (AEWs), and West African mesoscale convective systems (MCSs) is investigated. It points out atmospheric conditions over West Africa before hurricane formation. The analysis was performed for two periods, June–November in 2004 and 2005, during which 12 hurricanes (seven in 2004, five in 2005) were selected. Using the AEW signature in the 700 hPa vorticity, a backward trajectory was performed to the African coast, starting from the date and position of each hurricane, when and where it was catalogued as a tropical depression. At this step, using the Meteosat-7 satellite dataset, we selected all the MCSs around this time and region, and tracked them from their initiation until their dissipation. This procedure allowed us to relate each of the selected Hs with AEWs and a succession of MCSs that occurred a few times over West Africa before initiation of the hurricane. Finally, a dipole in sea surface temperature (SST) was observed with a positive SST anomaly within the region of H generation and a negative SST anomaly within the Gulf of Guinea. This SST anomaly dipole could contribute to enhance the continental convergence associated with the monsoon that impacts on the West African MCSs formation.

scholarly journals African Easterly Waves and Cyclonic Activity over the Eastern Atlantic: Composite and Case Studies

2011 ◽  
Vol 2011 ◽  
pp. 1-14 ◽  
Author(s):  
Moctar Camara ◽  
Arona Diedhiou ◽  
Amadou Gaye
Keyword(s):  
Atlantic Ocean ◽  
Deep Convection ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Eastern Atlantic Ocean ◽  
West African Coast ◽  
Thermodynamic Conditions ◽  
Unstable Environment ◽  
Rawinsonde Data

This study aims to understand the main differences over the African continent and the Eastern Atlantic Ocean between African Easterly Waves (AEWs) associated with Atlantic cyclones (developing AEWs) and non-developing AEWs. A statistical study showed that most of the named cyclones generated near the West African coast have a long lifecycle and all are associated with intense AEWs. Using NCEP/NCAR reanalyses, a composite study of the characteristics of developing AEWs is carried out and compared to those of non-developing AEWs. Developing AEWs exhibit the greatest baroclinic and barotropic conversions which are known to be the main processes involved in AEWs growth suggesting that these AEWs are stronger than the non-developing ones. Moreover, the developing AEWs are characterized by the existence of a relatively more unstable environment over West Africa and the Atlantic Ocean. A case study using rawinsonde data showed that the developing AEW is associated with dynamic and thermodynamic conditions conducive for deep convection and subsequent cyclogenesis compared to the non-developing AEW case.

scholarly journals In Situ Initiation of East Pacific Easterly Waves in a Regional Model

2017 ◽  
Vol 74 (2) ◽  
pp. 333-351 ◽  
Author(s):  
Adam V. Rydbeck ◽  
Eric D. Maloney ◽  
Ghassan J. Alaka
Keyword(s):  
Wrf Model ◽  
Warm Pool ◽  
Early Morning ◽  
Mesoscale Convective Systems ◽  
Weather Research And Forecasting ◽  
Easterly Waves ◽  
Convective Systems ◽  
East Pacific ◽  
Mesoscale Convective

Abstract The in situ generation of easterly waves (EWs) in the east Pacific (EPAC) is investigated using the Weather Research and Forecasting (WRF) Model. The sensitivity of the model to the suppression of EW forcing by locally generated convective disturbances is examined. Specifically, local forcing of EWs is removed by reducing the terrain height in portions of Central and South America to suppress robust sources of diurnal convective variability, most notably in the Panama Bight. High terrain contributes to the initiation of mesoscale convective systems in the early morning that propagate westward into the EPAC warm pool. When such mesoscale convective systems are suppressed in the model, EW variance is significantly reduced. This result suggests that EPAC EWs can be generated locally in association with higher-frequency convective disturbances, and these disturbances are determined to be an important source of EPAC EW variability. However, EPAC EW variability is not completely eliminated in such sensitivity experiments, indicating the importance for other sources of EW forcing, namely, EWs propagating into the EPAC from West Africa. Examination of the EW vorticity budget in the model suggests that nascent waves are zonally elongated and amplified by horizontal advection and vertical stretching of vorticity. Changes in the mean state between the control run and simulation with reduced terrain height also complicate interpretation of the results.

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