Projected changes in African easterly wave intensity and track in response to greenhouse forcing

The first episodes of floods caused by heavy rainfall during the major rainy season in 2018 occurred in Accra (5.6°N and 0.17°W), a coastal town, and Kumasi (6.72°N and 1.6°W) in the forest region on the 18th and 28th of June, respectively. We applied the Weather Research and Forecasting (WRF) model to investigate and examine the meteorological dynamics, which resulted in the extreme rainfall and floods that caused 14 deaths, 34076 people being displaced with damaged properties, and economic loss estimated at $168,289 for the two cities according to the National Disaster Management Organization (NADMO). The slow-moving thunderstorms lasted for about 8 hours due to the weak African Easterly Wave (AEW) and Tropical Easterly Jet (TEJ). Results from the analysis showed that surface pressures were low with significant amount of moisture influx aiding the thunderstorms intensification, which produced 90.1 mm and 114.6 mm of rainfall over Accra and Kumasi, respectively. We compared the rainfall amount from this event to the historical rainfall data to investigate possible changes in rainfall intensities over time. A time series of annual daily maximum rainfall (ADMR) showed an increasing trend with a slope of 0.45 over Accra and a decreasing trend and a slope of –0.07 over Kumasi. The 95th percentile frequencies of extreme rainfall with thresholds of 45.10 mm and 42.16 mm were analyzed for Accra and Kumasi, respectively, based on the normal distribution of rainfall. Accra showed fewer days with more heavy rainfall, while Kumasi showed more days with less heavy rainfalls.

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Convection in an African Easterly Wave over West Africa and the eastern Atlantic: A model case study of Helene (2006)

Quarterly Journal of the Royal Meteorological Society ◽

10.1002/qj.566 ◽

2010 ◽

Vol 136 (S1) ◽

pp. 364-396 ◽

Cited By ~ 25

Author(s):

Juliane Schwendike ◽

Sarah C. Jones

Keyword(s):

West Africa ◽

Model Case ◽

African Easterly Wave ◽

Easterly Wave

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Genesis of Hurricane Julia (2010) within an African Easterly Wave: Developing and Nondeveloping Members from WRF–LETKF Ensemble Forecasts

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-13-0187.1 ◽

2014 ◽

Vol 71 (7) ◽

pp. 2763-2781 ◽

Cited By ~ 8

Author(s):

Stefan F. Cecelski ◽

Da-Lin Zhang ◽

Takemasa Miyoshi

Keyword(s):

Surface Pressure ◽

Mesoscale Convective System ◽

Convective System ◽

Ensemble Forecasts ◽

Upper Troposphere ◽

African Easterly Wave ◽

Easterly Wave ◽

Mesoscale Convective ◽

Rossby Radius Of Deformation ◽

Faster Development

Abstract In this study, the predictability of and parametric differences in the genesis of Hurricane Julia (2010) are investigated using 20 mesoscale ensemble forecasts with the finest resolution of 1 km. Results show that the genesis of Julia is highly predictable, with all but two members undergoing genesis. Despite the high predictability, substantial parametric differences exist between the stronger and weaker members. Notably, the strongest-developing member exhibits large upper-tropospheric warming within a storm-scale outflow during genesis. In contrast, the nondeveloping member has weak and more localized warming due to inhibited convective development and a lack of a storm-scale outflow. A reduction in the Rossby radius of deformation in the strongest member aids in the accumulation of the warmth, while little contraction takes place in the nondeveloping member. The warming in the upper troposphere is responsible for the development of meso-α-scale surface pressure falls and a meso-β surface low in the strongest-developing member. Such features fail to form in the nondeveloping member as weak upper-tropospheric warming is unable to induce meaningful surface pressure falls. Cloud ice content is nearly doubled in the strongest member as compared to its nondeveloping counterparts, suggesting the importance of depositional heating of the upper troposphere. It is found that the stronger member undergoes genesis faster due to the lack of convective inhibition near the African easterly wave (AEW) pouch center prior to genesis. This allows for the faster development of a mesoscale convective system and storm-scale outflow, given the already favorable larger-scale conditions.

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Internal processes within the African Easterly Wave system

Quarterly Journal of the Royal Meteorological Society ◽

10.1002/qj.2420 ◽

2014 ◽

Vol 141 (689) ◽

pp. 1121-1136 ◽

Cited By ~ 14

Author(s):

D. Emmanuel Poan ◽

Jean-Philippe Lafore ◽

Romain Roehrig ◽

Fleur Couvreux

Keyword(s):

Wave System ◽

African Easterly Wave ◽

Easterly Wave

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Intermittent African Easterly Wave Activity in a Dry Atmospheric Model: Influence of the Extratropics

Journal of Climate ◽

10.1175/jcli-d-11-00049.1 ◽

2011 ◽

Vol 24 (20) ◽

pp. 5378-5396 ◽

Cited By ~ 19

Author(s):

Stephanie Leroux ◽

Nicholas M. J. Hall ◽

George N. Kiladis

Keyword(s):

Storm Track ◽

Atmospheric Model ◽

Convective Heating ◽

African Easterly Waves ◽

Easterly Waves ◽

Global Circulation ◽

African Easterly Jet ◽

African Easterly Wave ◽

The North ◽

Easterly Wave

Abstract A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (Thorncroft et al.; Leroux and Hall). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.

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Case Study of a Developing African Easterly Wave during NAMMA: An Energetic Point of View

Journal of the Atmospheric Sciences ◽

10.1175/2009jas3009.1 ◽

2009 ◽

Vol 66 (10) ◽

pp. 2991-3020 ◽

Cited By ~ 14

Author(s):

Joël Arnault ◽

Frank Roux

Keyword(s):

West Africa ◽

Point Of View ◽

Finite Domain ◽

African Easterly Wave ◽

Easterly Wave ◽

Multidisciplinary Analysis ◽

Energy Cycle ◽

Convective Scale ◽

Energetic Point ◽

Energy Equations

Abstract The West African perturbation that subsequently evolved into Hurricane Helene (2006) during NASA’s African Monsoon Multidisciplinary Analysis (NAMMA), 15 August–14 September 2006, and AMMA’s third special observing period (SOP-3), 15–29 September 2006, has been simulated with the nonhydrostatic Méso-NH model using parameterized convection. The simulated disturbance evolved over West Africa and the adjacent eastern tropical Atlantic through interactions between different processes at the convective scale, mesoscale, and synoptic scale. The aim of this paper is to quantify the energetics of the simulated disturbance. A set of energy equations is first developed in the hydrostatic case to solve the limitations of Lorenz’s analysis when applied to a finite domain. It is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the Méso-NH results. Application to the simulated pre-Helene disturbance allows one to determine the most important terms in these equations. These simplifications are taken into account to derive an energy cycle including barotropic and baroclinic conversions of eddy kinetic energy. The development of the simulated system was found to result from barotropic–baroclinic growth over West Africa and barotropic growth over the tropical eastern Atlantic. It is suggested that most of these energy conversions were the result of an adjustment of the wind field in response to the pressure decrease, presumably caused by convective activity.

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Genesis of Pre–Hurricane Felix (2007). Part I: The Role of the Easterly Wave Critical Layer

Journal of the Atmospheric Sciences ◽

10.1175/2009jas3420.1 ◽

2010 ◽

Vol 67 (6) ◽

pp. 1711-1729 ◽

Cited By ~ 70

Author(s):

Zhuo Wang ◽

M. T. Montgomery ◽

T. J. Dunkerton

Keyword(s):

Large Scale ◽

Focal Point ◽

Model Simulation ◽

Deep Convection ◽

Wrf Model ◽

Lower Troposphere ◽

Critical Layer ◽

African Easterly Wave ◽

Easterly Wave ◽

Vorticity Balance

Abstract The formation of pre–Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in Part II of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities. In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the “wave pouch” from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors’ earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.

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