Relationship between easterly waves and mesoscale convective systems over the Sahel

Mathon, Vincent; Diedhiou, Arona; Laurent, H.

doi:10.1029/2001gl014371

Cited by 27 publications

(31 citation statements)

References 13 publications

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“…Earlier studies by Burpee (1974), Duvel (1990), Mathon et al (2002b) and Fink and Reiner (2003) observed enhanced convective precipitation ahead of the AEW ridge for systems north of 12.5°N. Machado et al (1993) found that, over the ITCZ, the wave trough phase promotes the formation of large convective clusters more than it favours the initiation of convection.…”

Section: Phase Speed Relationships Of Convective Cloud Streaks and Eamentioning

confidence: 99%

“…Machado et al (1993) found that, over the ITCZ, the wave trough phase promotes the formation of large convective clusters more than it favours the initiation of convection. The concentration of convection and precipitation at or behind the trough, observed mainly in the northern Sahel, is primarily influenced by moisture availability -the northward transport of moisture from the ocean and equatorial forests (Burpee, 1974;Duvel, 1990;Diedhou et al, 2001;Mathon et al, 2002b). Diedhou et al (2001) found that, for waves along reference latitude 12.5°N, positive rainfall anomalies are highest in the trough where convection is associated with maximum lowlevel convergence and cyclonic vorticity, while near 5°N rainfall anomalies are maximized ahead of the trough.…”

Section: Phase Speed Relationships Of Convective Cloud Streaks and Eamentioning

confidence: 99%

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The propagation and diurnal cycles of deep convection in northern tropical Africa

Laing

Carbone

Levizzani

et al. 2008

Quart J Royal Meteoro Soc

150

132

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ABSTRACT:The propagation and diurnal cycle of organized convection in northern tropical Africa are examined using five years (1999)(2000)(2001)(2002)(2003) of digital infrared imagery for May-August. Reduced-dimension techniques are used to document the properties of cold clouds -proxies for deep convection and precipitation. Large-scale environments are diagnosed from global analyses.Organized convection in Africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h. A substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration. Episodes with 36 h duration and 1472 km span recur at a one-per-day interval. Most episodes have phase speed of 10-20 m s −1 , which is faster than most African easterly waves. Convective episodes tend to initiate in the lee of high terrain, consistent with thermal forcing from elevated heat sources. Average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed-phase arrival of systems propagating from the east. Propagation occurs with moderate low-to mid-tropospheric shear, which varies with the African easterly jet migration and West African monsoon phases. Frequent deep convection occurs with local shear maxima near high terrain. For the peak monsoon period and for 10°W-10°E, where easterly waves and convective systems are frequent, 35% of cold cloud episodes occur east of the wave trough compared with about 24% to the west. Based on the coherent behaviour of organized, propagating convection, inferences may be made regarding the prediction of precipitation beyond one or two days.

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Section: Phase Speed Relationships Of Convective Cloud Streaks and Eamentioning

confidence: 99%

Section: Phase Speed Relationships Of Convective Cloud Streaks and Eamentioning

confidence: 99%

The propagation and diurnal cycles of deep convection in northern tropical Africa

Laing

Carbone

Levizzani

et al. 2008

Quart J Royal Meteoro Soc

150

132

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“…The evolution of these synoptic patterns is important for several reasons: the AEWs modulate the muchneeded convective rainfall (e.g. Mathon et al, 2002), they influence the uptake and transport of mineral dust from the Sahara (e.g. Karyampudi and Carlson, 1988;Karyampudi et al, 1999) and they propagate westwards over the tropical Atlantic where they act as precursors for more than 80% of intense hurricanes (Landsea, 1993).…”

Section: Introductionmentioning

confidence: 99%

A simple model of coupled synoptic waves in the land surface and atmosphere of the northern Sahel

Parker

2008

Quart J Royal Meteoro Soc

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A simple dynamic model is developed to describe the observed interactions between the atmosphere and the soil moisture patterns of the northern Sahel. In the model, the atmosphere follows quasi-geostrophic dynamics, while landatmosphere coupling is described by simple linear relationships. Dry surfaces heat the atmospheric boundary layer, while wet surfaces cool the boundary layer, relative to the equilibrium state of the atmosphere and land surface. In turn, cloud processes, which are assumed to maximise in the cool, humid phase of an atmospheric disturbance, cool the land surface through wetting (rainfall) and reduction of the incoming solar flux. These assumptions lead to a linear system which can be solved numerically to obtain modal solutions, and the adjoints (optimal excitation) of these. Moist convective influences on the atmospheric state are not explored in detail. The coupling with the land surface leads to the existence of unstable modes, which do not exist in the atmosphere-only part of the system. Solutions can be easterly or westerly propagating, according to wave number, with the longer waves tending to be easterly. Propagation relies on a favourable configuration between the atmospheric and soil moisture anomalies: easterly propagation requires the surface temperature pattern to be shifted to the east of the atmospheric temperature pattern. In contrast, optimal excitation of the fastest-growing mode occurs when the atmospheric pattern has a thermal anomaly lying to the east of a strong surface temperature (and moisture) anomaly. These results have value for weather prediction, and indicate the usefulness of soil moisture data for forecasters.

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“…Their contribution to the total rainfall varies significantly with respect to latitude, accounting for as much as 80%-90% of the annual rainfall in the Sahelian climate zone (12°-18°N) (Dhonneur 1981;Mathon et al 2002a). In the wetter climate zones to the south, squall lines account for about half of the rainfall in the Sudanian climate zone (9°-12°N) (Eldridge 1957;Omotosho 1985) and as little as 16%-32% along the Guinea coast (Acheampong 1982;Omotosho 1985).…”

Section: Introductionmentioning

confidence: 99%

“…The sys- tems were "advective" in the sense that they formed outside of the UOV domain and propagated into the UOV not less than 2 h after forming. The term "organized convective system" was defined by Mathon et al (2002b) and uses infrared satellite-derived statistics to formally specify the size, minimum lifetime, and propagation rate of the cloud cluster. Fink et al (2006) assumed for their analysis that most of the 52 type Ia systems they tracked over central and western North Africa in the rainy season of 2002 were actually classical West African squall lines at some stage of their lifetime.…”

mentioning

confidence: 99%

Use of a Rain Gauge Network to Infer the Influence of Environmental Factors on the Propagation of Quasi-Linear Convective Systems in West Africa

Schrage

Fink

2007

Weather and Forecasting

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The West African squall line is a key quasi-linear storm system that brings much of the precipitation observed in the data-poor Sudanian climate zone. Squall lines propagate at a wide range of speeds and headings, but the lack of operational radar stations in the region makes quantifying the propagation of the squall lines difficult. A new method of estimating the propagation rate and heading for squall lines is proposed. Based on measurements of the time of onset of precipitation (OOP) at a network of rain gauge stations, an estimate of the propagation characteristics of the squall line can be inferred. By combining estimates of propagation rate with upper-air observations gathered at a nearby radiosonde station, the impact of various environmental factors on the propagation characteristics of West African squall lines is inferred. Results suggest that the propagation speed for West African squall lines is related to the conditions at midtropospheric levels, where dry air and an enhanced easterly flow favor faster propagation. Northerly anomalies at these levels are also associated with faster propagation. When applied to West African squall lines, the correlations between these environmental factors and the speed of propagation are significantly higher than those of methods developed for mesoscale convective systems in other parts of the world.

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Relationship between easterly waves and mesoscale convective systems over the Sahel

Cited by 27 publications

References 13 publications

The propagation and diurnal cycles of deep convection in northern tropical Africa

The propagation and diurnal cycles of deep convection in northern tropical Africa

A simple model of coupled synoptic waves in the land surface and atmosphere of the northern Sahel

Use of a Rain Gauge Network to Infer the Influence of Environmental Factors on the Propagation of Quasi-Linear Convective Systems in West Africa

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