scholarly journals Radar Observations of Convective System Variability in Relationship to African Easterly Waves during the 2006 AMMA Special Observing Period

2009 ◽  
Vol 137 (12) ◽  
pp. 4136-4150 ◽  
Author(s):  
Rosana Nieto Ferreira ◽  
Thomas Rickenbach ◽  
Nick Guy ◽  
Earle Williams
Keyword(s):  
Rainfall Variability ◽  
Convective Available Potential Energy ◽  
Squall Line ◽  
Convective System ◽  
African Easterly Waves ◽  
The West ◽  
African Monsoon ◽  
Easterly Waves ◽  
Stratiform Rain ◽  
African Easterly Jet

Abstract A radar-based analysis of the structure, motion, and rainfall variability of westward-propagating squall-line mesoscale convective systems (SLMCSs) in Niamey, Niger, during the African Monsoon Multidisciplinary Activities (AMMA) 2006 special observing period is combined with an analysis of 700-mb (hPa) winds and relative vorticity to study the relationship between SLMCSs and African easterly waves (AEWs). Radar results show that SLMCSs were the most important rainmakers in Niamey and accounted for about 90% of the rainfall despite being present less than 17% of the time. Analysis of the 700-mb synoptic-scale flow revealed that during the 2006 West African monsoon season the African easterly jet vacillated between about 10° and 15°N on time scales of 1–2 weeks. AEWs followed the jet as it vacillated north and south, thereby producing two preferred paths for AEWs propagating past Niamey’s longitude, a northern track along 8°–16°N and a southern track along 2°–6°N. It was found that Niamey SLMCSs occurred westward of the trough of AEWs propagating along either track. The properties of SLMCSs must then be placed in the context of their location relative to these two AEW tracks, rather than in the trough and ridge pattern of a single AEW track. Radar analysis further indicated that although the total amounts of rainfall produced by SLMCSs occurring in both African easterly jet latitude regimes were similar, significant structural differences occurred between the two groups of systems. SLMCSs that formed to the west of AEW troughs propagating along the northern track had a significantly larger mean stratiform rain fraction in an environment of lower convective available potential energy when compared with the SLMCSs that occurred to the west of the troughs of AEWs in the southern track. The authors conclude that AEWs that propagated farther north provided a more favorable environment for stratiform rain production in Niamey SLMCSs than those AEWs located farther south. These results may be helpful to studies of the two-way interaction between AEWs and convection in West Africa.

African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon

2006 ◽  
Vol 27 (2-3) ◽  
pp. 319-332 ◽  
Author(s):  
Christophe Lavaysse ◽  
Arona Diedhiou ◽  
Henri Laurent ◽  
Thierry Lebel
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Convective Activity ◽  
African Easterly Waves ◽  
The West ◽  
African Monsoon ◽  
Easterly Waves

scholarly journals Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic

2017 ◽  
Vol 145 (2) ◽  
pp. 599-616 ◽  
Author(s):  
Abdou L. Dieng ◽  
Saidou M. Sall ◽  
Laurence Eymard ◽  
Marion Leduc-Leballeur ◽  
Alban Lazar
Keyword(s):  
West African ◽  
Dynamical Structure ◽  
African Easterly Waves ◽  
The South ◽  
The West ◽  
Easterly Waves ◽  
West African Coast ◽  
African Coast ◽  
Tropical Depressions ◽  
The Relationship

In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems). By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.

scholarly journals The Influence of African Easterly Waves on Convection over Tropical Africa and the East Atlantic

2015 ◽  
Vol 144 (1) ◽  
pp. 171-192 ◽  
Author(s):  
Matthew A. Janiga ◽  
Chris D. Thorncroft
Keyword(s):  
Convective System ◽  
Western Coast ◽  
African Easterly Waves ◽  
Shallow Convection ◽  
East Atlantic ◽  
Tropical Africa ◽  
Easterly Waves ◽  
Convective Systems ◽  
Conditional Instability ◽  
Rain Rates

Abstract Using data from the Tropical Rainfall Measuring Mission (TRMM), the modulation of convection by African easterly waves (AEWs) is investigated over regions of the east Atlantic and tropical Africa. To explain the modulation of convection, the large-scale environment (lift, moisture, conditional instability, and shear) is also examined as a function of AEW phase in each region. Over semiarid portions of tropical Africa, unconditional rain rates are greatest in the northerly phase of AEWs due to the strong adiabatic forcing for ascent. Along the Guinea Coast, the western coast of Africa, and over the east Atlantic—where forcing for ascent is weaker—rainfall is shifted toward the trough where the air is moist. Significant contrasts in the characteristics of convection as a function of AEW phase—comparable in magnitude to regional contrasts—are also observed. In all regions, large and high echo-top convective systems are more sensitive to AEW phase than small and low echo-top systems. In semiarid regions, deep convection and large high echo-top convective systems account for a large fraction of the rainfall in the ridge and northerlies. Stratiform and small low echo-top convective systems dominate in the trough and southerlies. Convective system height and conditional rain rates increase with conditional instability and system sizes may increase with shear. Over the east Atlantic, stratiform fractions and convective system sizes and echo-top heights are greatest in the trough while the ridge is dominated by shallow convection. This is primarily related to the presence of moist air in the trough and dry air in the ridge.

scholarly journals A Characterization of African Easterly Waves on 2.5–6-Day and 6–9-Day Time Scales

2013 ◽  
Vol 26 (18) ◽  
pp. 6750-6774 ◽  
Author(s):  
Man-Li C. Wu ◽  
Oreste Reale ◽  
Siegfried D. Schubert
Keyword(s):  
Time Scales ◽  
Ensemble Empirical Mode Decomposition ◽  
African Easterly Waves ◽  
Pressure System ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
Easterly Wave ◽  
Mode Decomposition ◽  
The Tropics

Abstract This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6–9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation.

scholarly journals The Impact of Low-Level Moisture Errors on Model Forecasts of an MCS Observed during PECAN

2017 ◽  
Vol 145 (9) ◽  
pp. 3599-3624 ◽  
Author(s):  
John M. Peters ◽  
Erik R. Nielsen ◽  
Matthew D. Parker ◽  
Stacey M. Hitchcock ◽  
Russ S. Schumacher
Keyword(s):  
Convective Available Potential Energy ◽  
Mesoscale Convective System ◽  
Squall Line ◽  
Convective System ◽  
Low Level ◽  
Eastern Flank ◽  
Western Flank ◽  
Mesoscale Convective ◽  
Outflow Boundary ◽  
The Impact

This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved southeastward along this OFB as a squall line. The western flank of the MCS remained quasi stationary approximately 100 km north of the system’s OFB and produced localized flooding. A total of 16 radiosondes were launched near the MCS’s eastern flank and 4 were launched near the MCS’s western flank. Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 h prior to the arrival of the squall line. In contrast, the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to their underrepresentation of moisture. Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed. MCS displacement error was strongly correlated with models’ underrepresentation of low-level moisture and their associated overrepresentation of the vertical distance between a parcel’s initial height and its level of free convection ([Formula: see text], which is correlated with CIN). The overpredicted [Formula: see text] in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso- α-scale lift before these parcels initiated convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.

Variability and Evolution of African Easterly Wave Structures and Their Relationship with Tropical Cyclogenesis over the Eastern Atlantic

2015 ◽  
Vol 143 (12) ◽  
pp. 4975-4995 ◽  
Author(s):  
Alan Brammer ◽  
Chris D. Thorncroft
Keyword(s):  
Coastal Region ◽  
Lower Troposphere ◽  
Absolute Number ◽  
West African ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
The West ◽  
Easterly Waves ◽  
West African Coast ◽  
African Coast

Abstract African easterly waves (AEWs) are objectively tracked between West Africa and the tropical Atlantic based on the CFSRv2 data for 1979 to 2012. The characteristics of the troughs of the AEWs at the West African coast are explored and related to whether they favor tropical cyclogenesis over the eastern Atlantic. A logistic regression model was used to determine the optimum combination of predictors that relate AEW characteristics to tropical cyclogenesis. The most skillful model for genesis over the eastern Atlantic consisted of four variables of the AEWs dynamics over the coastal region and the absolute number of days from the peak in the AEW season. Using this diagnostic an equal number of favorable developing and nondeveloping waves were compared through a composite difference analysis. Favorable developing waves had significantly higher moisture content in the lower troposphere to the northwest of the trough as they exited the West African coast compared to favorable nondeveloping waves. Trajectory analysis for all the waves revealed that as the AEWs transition over the West African coast the troughs are typically open to the environment ahead and to the northwest of the trough. For developing waves this means that moist air is ingested into the lower levels of the system, while for nondeveloping waves dry air is ingested. At this point in the AEW life cycle this difference may be fundamental in determining whether a favorable wave can develop or not.

scholarly journals A Characterization of Cold Pools in the West African Sahel

2016 ◽  
Vol 144 (5) ◽  
pp. 1923-1934 ◽  
Author(s):  
M. Provod ◽  
J. H. Marsham ◽  
D. J. Parker ◽  
C. E. Birch
Keyword(s):  
Potential Temperature ◽  
West African ◽  
Energy Deficit ◽  
Squall Line ◽  
Cold Pool ◽  
The West ◽  
Early Season ◽  
African Monsoon ◽  
Cold Pools

Cold pools are integral components of squall-line mesoscale convective systems and the West African monsoon, but are poorly represented in operational global models. Observations of 38 cold pools made at Niamey, Niger, during the 2006 African Monsoon Multidisciplinary Analysis (AMMA) campaign (1 June–30 September 2006), are used to generate a seasonal characterization of cold pool properties by quantifying related changes in surface meteorological variables. Cold pools were associated with temperature decreases of 2°–14°C, pressure increases of 0–8 hPa, and wind gusts of 3–22 m s−1. Comparison with published values of similar variables from the U.S. Great Plains showed comparable differences. The leading part of most cold pools had decreased water vapor mixing ratios compared to the environment, with moister air, likely related to precipitation, approximately 30 min behind the gust front. A novel diagnostic used to quantify how consistent observed cold pool temperatures are with saturated or unsaturated descent from midlevels [fractional evaporational energy deficit (FEED)] shows that early season cold pools are consistent with less saturated descents. Early season cold pools were relatively colder, windier, and wetter, consistent with drier midlevels, although this was only statistically significant for the change in moisture. Late season cold pools tended to decrease equivalent potential temperature from the pre–cold pool value, whereas earlier in the season changes were smaller, with more increases. The role of cold pools may therefore change through the season, with early season cold pools more able to feed subsequent convection.

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

2011 ◽  
Vol 24 (20) ◽  
pp. 5378-5396 ◽  
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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