Probabilistic discrimination between large-scale environments of intensifying and decaying African Easterly Waves

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.

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Investigating the Factors That Contribute to African Easterly Wave Intensity Forecast Uncertainty in the ECMWF Ensemble Prediction System

Monthly Weather Review ◽

10.1175/mwr-d-18-0071.1 ◽

2019 ◽

Vol 147 (5) ◽

pp. 1679-1698

Author(s):

Travis J. Elless ◽

Ryan D. Torn

Keyword(s):

Large Scale ◽

Ensemble Prediction ◽

Prediction System ◽

African Easterly Waves ◽

Ensemble Prediction System ◽

Easterly Waves ◽

Convectively Coupled Equatorial Waves ◽

Easterly Wave ◽

Equatorial Wave ◽

Diabatic Processes

Abstract Although there have been numerous studies documenting the processes/environments that lead to the intensification of African easterly waves (AEWs), only a few of these studies investigated the effect of those processes or the environment on the predictability of AEWs. Here, the large-scale modulation of AEW intensity predictability is evaluated using the 51-member ECMWF ensemble prediction system (EPS) during an active AEW period (July–September 2011–13). Forecasts are stratified based on the 72-h AEW intensity standard deviation (SD) to evaluate hypotheses for how different processes contribute to large forecast SD. While large and small SD forecasts are associated with similar baroclinic and barotropic energy conversions, forecasts with large SD are characterized by higher relative humidity values downstream of the AEW trough. These areas of higher humidity are also associated with higher precipitation and precipitation SD, suggesting that uncertainty associated with diabatic processes could be linked with large AEW intensity SD. Although water vapor is a strong function of longitude and phase of convectively coupled equatorial waves, the cases with large and small SD are characterized by similar longitude and wave phase, suggesting that AEWs occurring in certain locations or convectively coupled equatorial wave phases are not more or less predictable.

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Observations of Seven African Easterly Waves in the East Atlantic during 2006

Journal of the Atmospheric Sciences ◽

10.1175/2009jas3118.1 ◽

2010 ◽

Vol 67 (1) ◽

pp. 26-43 ◽

Cited By ~ 26

Author(s):

Jonathan Zawislak ◽

Edward J. Zipser

Keyword(s):

Large Scale ◽

High Amplitude ◽

African Easterly Waves ◽

Easterly Waves ◽

Wave Trough ◽

Wind Structure ◽

Easterly Wave ◽

Global Data Assimilation System ◽

Low Amplitude ◽

The Relationship

Abstract The African Monsoon Multidisciplinary Analyses (AMMA) experiment and its downstream NASA extension, NAMMA, provide an unprecedented detailed look at the vertical structure of consecutive African easterly waves. During August and September 2006, seven easterly waves passed through the NAMMA domain: two waves developed into Tropical Cyclones Debby and Helene, two waves did not develop, and three waves were questionable in their role in the development of Ernesto, Florence, and Gordon. NCEP Global Data Assimilation System (GDAS) analyses are used to describe the track of both the vorticity maxima and midlevel wave trough associated with each of the seven easterly waves. Dropsonde data from NAMMA research flights are used to describe the observed wind structure and as a tool to evaluate the accuracy of the GDAS to resolve the structure of the wave. Finally, satellite data are used to identify the relationship between convection and the organization of the wind structure. Results support a necessary distinction between the large-scale easterly wave trough and smaller-scale vorticity centers within the wave. An important wave-to-wave variability is observed: for NAMMA waves, those waves that have a characteristically high-amplitude wave trough and well-defined low-level circulations (well organized) may contain less rainfall, do not necessarily develop, and are well resolved in the analysis, whereas low-amplitude (weakly organized) NAMMA waves may have stronger vorticity centers and large persistent raining areas and may be more likely to develop, but are not well resolved in the analysis.

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Effects of Saharan Dust on African Easterly Waves: The Impact of Aerosol-Affected Satellite Radiances on Data Assimilation

10.5194/acp-2021-129 ◽

2021 ◽

Author(s):

Dustin Francis Phillip Grogan ◽

Cheng-Hsuan Lu ◽

Shih-Wei Wei ◽

Sheng-Po Chen

Keyword(s):

Data Assimilation ◽

Large Scale ◽

West African Monsoon ◽

Forecast Model ◽

Saharan Dust ◽

African Easterly Waves ◽

Radiative Effects ◽

Easterly Waves ◽

Brightness Temperatures ◽

The Impact

Abstract. This study incorporates time-varying aerosols into satellite radiance calculations within the Global Data Assimilation System (GDAS) to investigate its impact on African easterly waves (AEWs) and their environment. Comparison of analysis fields from the aerosol-aware experiment and an aerosol-blind control during August 2017 showed that the aerosol-affected radiances accelerated the African easterly jet and West African monsoon flow; warmed the Saharan boundary layer; and modified the AEW vorticity structure, with increases in the northern circulation and decreases in the southern circulation. Analysis fields from each experiment were used in the Global Forecast System (GFS) to examine differences in forecasting two AEW cases that developed hurricanes over the Atlantic, but were structurally different over North Africa. The aerosol-aware experiment reduced errors in forecasting the AEW case whose northern circulation interacted with a large-scale Saharan dust plume; neutral improvement was found for the other AEW, which did not contain a northern circulation nor interacted with a dust plume. The changes to the analysis fields by the aerosol-aware assimilation are reminiscent of dust radiative effects that operate on AEWs and their environment. That is, the aerosol-affected radiances produce corrections to the brightness temperatures that modify the analysis fields like dust aerosols that are radiatively coupled to the atmospheric variables in the forecast model. We show qualitatively that dust radiative effects are captured by the aerosol-affected radiances for the AEW case that interacted with a dust plume, which served to improve forecasts of the wave downstream.

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The Madden–Julian Oscillation’s Influence on African Easterly Waves and Downstream Tropical Cyclogenesis

Monthly Weather Review ◽

10.1175/mwr-d-10-05028.1 ◽

2011 ◽

Vol 139 (9) ◽

pp. 2704-2722 ◽

Cited By ~ 56

Author(s):

Michael J. Ventrice ◽

Chris D. Thorncroft ◽

Paul E. Roundy

Keyword(s):

Large Scale ◽

Tropical Cyclogenesis ◽

Tropical Atlantic ◽

Madden Julian Oscillation ◽

African Easterly Waves ◽

Tropical Africa ◽

Easterly Waves ◽

African Easterly Wave ◽

Easterly Wave ◽

Equatorial Rossby Waves

The influence of the Madden–Julian oscillation (MJO) over tropical Africa and Atlantic is explored during the Northern Hemisphere summer months. The MJO is assessed by using real-time multivariate MJO (RMM) indices. These indices divide the active convective signal of the MJO into 8 phases. Convection associated with the MJO is enhanced over tropical Africa during RMM phases 8, 1, and 2. Convection becomes suppressed over tropical Africa during the subsequent RMM phases (phases 3–7). African convective signals are associated with westward-propagating equatorial Rossby waves. The MJO modulates African easterly wave (AEW) activity. AEW activity is locally enhanced during RMM phases 1–3 and suppressed during RMM phases 6–8. Enhanced AEW activity occurs during periods of enhanced convection over tropical Africa, consistent with stronger or more frequent triggering of AEWs as well as more growth associated with latent heat release. Enhanced AEW activity occurs during the low-level westerly wind phase of the MJO, which increases the cyclonic shear on the equatorward side of the AEJ, increasing its instability. Atlantic tropical cyclogenesis frequency varies coherently with the MJO. RMM phases 1–3 show the greatest frequency of tropical cyclogenesis events whereas phases 7 and 8 show the least. RMM phase 2 is also the most likely phase to be associated with a train of three or more tropical cyclones over the tropical Atlantic. This observed evolution of tropical cyclogenesis frequency varies coherently with variations in AEW activity and the large-scale environment.

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Land–Atmosphere Coupling Sensitivity to GCMs Resolution: A Multimodel Assessment of Local and Remote Processes in the Sahel Hot Spot

Journal of Climate ◽

10.1175/jcli-d-20-0303.1 ◽

2021 ◽

Vol 34 (3) ◽

pp. 967-985

Author(s):

Omar V. Müller ◽

Pier Luigi Vidale ◽

Benoît Vannière ◽

Reinhard Schiemann ◽

Retish Senan ◽

...

Keyword(s):

Soil Moisture ◽

High Resolution ◽

Large Scale ◽

Hot Spot ◽

Moisture Flux ◽

Horizontal Wind ◽

African Easterly Waves ◽

Atmospheric Conditions ◽

Easterly Waves ◽

Duration Comparison

AbstractLand–atmosphere interactions are often interpreted as local effects, whereby the soil state drives local atmospheric conditions and feedbacks originate. However, nonlocal mechanisms can significantly modulate land–atmosphere exchanges and coupling. We make use of GCMs at different resolutions (low ~1° and high ~0.25°) to separate the two contributions to coupling: better represented local processes versus the influence of improved large-scale circulation. We use a two-legged metric, complemented by a process-based assessment of four CMIP6 GCMs. Our results show that weakening, strengthening, and relocation of coupling hot spots occur at high resolution globally. The northward expansion of the Sahel hot spot, driven by nonlocal mechanisms, is the most notable change. The African easterly jet’s horizontal wind shear is enhanced in JJA due to better resolved orography at high resolution. This effect, combined with enhanced easterly moisture flux, favors the development of African easterly waves over the Sahel. More precipitation and soil moisture recharge produce strengthening of the coupling, where evapotranspiration remains controlled by soil moisture, and weakening where evapotranspiration depends on atmospheric demand. In SON, the atmospheric influence is weaker, but soil memory helps to maintain the coupling between soil moisture and evapotranspiration and the relocation of the hot spot at high resolution. The multimodel agreement provides robust evidence that atmospheric dynamics determines the onset of land–atmosphere interactions, while the soil state modulates their duration. Comparison of precipitation, soil moisture, and evapotranspiration against satellite data reveals that the enhanced moistening at high resolution significantly reduces model biases, supporting the realism of the hot-spot relocation.

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A Recurrence Analysis of Multiple African Easterly Waves during Summer 2006

Current Topics in Tropical Cyclone Research ◽

10.5772/intechopen.86859 ◽

2020 ◽

Author(s):

Tiffany Reyes ◽

Bo-Wen Shen

Keyword(s):

Large Scale ◽

Mesoscale Model ◽

Recurrence Plot ◽

Temporal Variations ◽

Local Analysis ◽

Lead Times ◽

African Easterly Waves ◽

Easterly Waves ◽

Weather Forecasts

Accurate detection of large-scale atmospheric tropical waves, such as African easterly waves (AEWs), may help extend lead times for predicting tropical cyclone (TC) genesis. Since observed AEWs have comparable but slightly different periods showing spatial and temporal variations, local analysis of frequencies and amplitudes of AEWs is crucial for revealing the role of AEWs in the modulation of TC genesis. To achieve this goal, we investigate the recurrence plot (RP) method. A recurrence is defined when the trajectory of a state returns to the neighborhood of a previously visited state. To verify implementation of the RP method in Python and its capability for revealing a transition between different types of solutions, we apply the RP to analyze several idealized solutions, including periodic, quasiperiodic, chaotic and limit cycle solutions, and various types of solutions within the three- and five-dimensional Lorenz models. We then extend the RP analysis to two datasets from the European Centre for Medium-Range Weather Forecasts global reanalysis and global mesoscale model data in order to reveal the recurrence of multiple AEWs during summer 2006. Our results indicate that the RP analysis effectively displays the major features of time-varying oscillations and the growing or decaying amplitudes of multiple AEWs.

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The Interaction between African Easterly Waves and Different Types of Deep Convection and Its Influence on Atlantic Tropical Cyclones

Atmosphere ◽

10.3390/atmos13010005 ◽

2021 ◽

Vol 13 (1) ◽

pp. 5

Author(s):

Bantwale D. Enyew ◽

Ademe Mekonnen

Keyword(s):

Large Scale ◽

Deep Convection ◽

African Easterly Waves ◽

Large Area ◽

Easterly Waves ◽

Past Work ◽

Development Region ◽

Different Types ◽

Tightly Coupled ◽

Cumulus Congestus

This study revisited the association of African easterly waves (AEWs) to Atlantic tropical cyclone (TC) development using weather states (WSs) from the International Satellite Cloud Climatology Project, National Hurricane Center best track hurricane data (HURDAT2), and reanalysis products. The WS data are used as a proxy for two different types of deep convection. This study covers July–October 1984–2009. Statistical analysis based on HURDAT2 and objectively tracked AEWs has shown that a small fraction (~20%) of the AEWs that propagate from Africa serve as TC precursors. About 80% of the AEWs from the continent were non-developing. As in the past work, our study showed an important difference between developing and non-developing AEWs. Composites based on developing AEWs revealed well-organized large scale deep convection (one type, composed of mesoscale systems and thick anvil clouds) is tightly coupled to the AEW trough, while scattered, less well-organized deep convection (second type, isolated cumulonimbus and cumulus congestus clouds) dominated a large area downstream of the developing AEW trough. Developing AEWs propagate westwards while strengthening. In contrast, non-developing AEWs showed that the peak well-organized deep convection is located either behind (to the east of) or far ahead (to the west) of the AEW trough (peaks values are not in close proximity). Moreover, well-organized deep convections associated with non-developing AEWs were weaker than those associated with developing AEWs. The results indicated that convective activity ahead of the non-developing AEWs is weak. Positive relative humidity (RH) anomalies dominate the area around AEWs and downstream over the main TC development region. In contrast, negative RH dominated the main TC development region ahead of non-developing AEWs, suggesting an unfavorable environment downstream of the AEWs. The results also showed that developing AEWs maintained stronger features in the lower and middle troposphere, while non-developing AEWs exhibited weaker structures, in agreement with past work. (Supplemental information related to this paper is available at the journal’s website of this edition).

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