The vitality and germination of seeds

Duvel, J. W. T.

doi:10.5962/bhl.title.35433

Cited by 13 publications

(11 citation statements)

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“…Convection (OLR minima) is located ahead of the trough and has a its maximum on 22 August. From the description of these interactions between the disturbances in the wind eld, the AEJ and the convection, it can be concluded that the present case-study corresponds to a 3-5-day easterly wave (Reed et al1977;Chen and Ogura 1982;Duvel 1990).…”

Section: Synoptic Scalementioning

confidence: 99%

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Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

Redelsperger

Diongue

Diedhiou

et al. 2002

Quart J Royal Meteoro Soc

114

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SUMMARYA reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX-SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case-study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity eld is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity eld features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aṏ r mountains propagating south-westward at 15 m s ¡1 . The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium-Range Weather Forecasts re-analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX-SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented.

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Section: Synoptic Scalementioning

confidence: 99%

“…This and many other studies have shed light on the structure and origin of the AEW as well as on its modulation of convection (e.g. Chen and Ogura 1982;Duvel 1990). However, the dependence of the AEW structures on various environmental parameters (e.g.…”

Section: Introductionmentioning

confidence: 99%

Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

Redelsperger

Diongue

Diedhiou

et al. 2002

Quart J Royal Meteoro Soc

114

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show abstract

“…Rather than use sparse rain-gauge data, many researchers have based their studies of Sahelian precipitation systems on TIR satellite imagery (e.g. Desbois et al 1988;Duvel 1989Duvel ,1990Laing and Fritsch 1993;Rowell and Milford 1993). The geostationary Meteosat satellite, centred on the equator at the Greenwich Meridian, has proved particularly useful in this respect as it provides half-hourly TIR images (10.5-12.5 µm).…”

Section: Observing Land-atmosphere Coupling Using Satellite Data (A) mentioning

confidence: 99%

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

Taylor

Parker

Lloyd

et al. 2005

Q. J. R. Meteorol. Soc.

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SUMMARYCloud-free brightness temperatures from satellite are used to interpret changes in the state of the land surface energy balance from day to day across the tropical semi-arid region of North Africa. The method provides an assessment of the spatial and temporal patterns in the surface characteristics. The variability in brightness temperature is closely linked to precipitation, as inferred from available rain-gauge data and satellite cold-cloud imagery. Temperatures drop sharply after rainfall, and increase gradually in subsequent days consistent with surface drying. The analysis shows that surface temperatures (and therefore fluxes) are well-organized at the synoptic scale, and can be linked to the westward propagation of mesoscale convective systems and African easterly waves (AEWs).The modulation of rainfall and cloud cover by weather systems is a key element in producing variability in surface fluxes of heat and moisture. To examine the relationships between surface variability and the atmosphere, a composite of anomalously warm (and dry) surface conditions with spatial coherence over a given longitude band is constructed from satellite data covering a single wet season. Operational analyses are used to examine atmospheric anomalies associated with these composited 'hotspots'. Surface moisture variations on the scale of several hundred km and larger can generate anomalous heat-lows during the day and, according to the operational analyses, generate cyclonic vorticity overnight. These vortices appear to be responsible for the observed modulation of cold cloud in the vicinity of the hotspot, and may influence the characteristics of AEWs in the Northern Sahel. Furthermore, theoretical ideas suggest that surface hotspots are likely to trigger a westward-propagating Rossbywave response; this process is demonstrated here in a simple numerical model.

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“…There is known to be a strong diurnal cycle of deep convection over the land (e.g. Houze and Betts 1981;Duvel 1989) which adds a mesoscale diurnal forcing to the WAM. Recent studies (Diongue et al 2002;Redelsperger et al 2002) have highlighted the significance of scale interactions between such systems and their synoptic environment.…”

Section: Introductionmentioning

confidence: 99%

The diurnal cycle of the West African monsoon circulation

Parker

Burton

Diongue-Niang

et al. 2005

Quart J Royal Meteoro Soc

275

307

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SUMMARYUsing numerical model analyses, it is shown that there is a coherent diurnal cycle of the West African monsoon winds. As has been observed in previous studies of arid and semi-arid areas, the winds are at their weakest in the afternoon when the convective boundary layer (CBL) is deep, and intensify overnight when the boundary-layer turbulence is much weaker. This diurnal cycle is maximized in the northern part of the monsoon layer, where the meridional pressure gradient and the diurnal cycle of the CBL are both strong.The diurnal cycle can also be resolved in surface and upper-air data, which show how the nocturnal meridional circulation acts to stratify the lower part of the monsoon layer. In contrast, mixing in the daytime CBL acts to maintain the baroclinicity, as has been observed in laboratory flows. This pattern has implications for the efficiency of the monsoon circulation in the continental water budget, as well as in mixing of trace gases and aerosols between the surface layer and the free troposphere. Vertical mixing occurs by day, while meridional advection, with isentropic upgliding and downgliding, is most efficient at night.Finally, high-resolution observations from the JET2000 experiment are used to show that there is mesoscale structure in the diurnally varying monsoon circulation. In the nocturnal flows, local circulations have been observed and appear to represent a response to recent deep convective events. In contrast, the daytime CBL properties at these scales have been shown in a previous study to map closely onto patterns of soil moisture, with horizontal advection playing a weaker role.

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The vitality and germination of seeds

Cited by 13 publications

References 0 publications

Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

Observations of synoptic-scale land surface variability and its coupling with the atmosphere

The diurnal cycle of the West African monsoon circulation

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