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Jean Philippe Duvel

Abstract

Some aspects of the interannual and the diurnal variations of the convection over the tropical Africa and the Atlantic Ocean are derived using Meteosat data. The study is based on four summer months (June, July, August and September) of three years from 1983 to 1985, for regions of 2.5°×2.5° extending from 5°S to 25°N and from 50°E to 50°W. Using ECMWF (European Centre for Medium Range Weather Forecasts) analyses, mean cloud fields and interannual changes are interpreted in terms of dynamical forcing and feedback. Anomalies in the thermal wind at 700 mb between 1985 (relatively wet year) and the two other years are consistent with previous results based on more contrasted wet and dry years.

Defining the high cloudiness by a threshold in the infrared signal, the amplitude of the diurnal variation is maximum over land with larger values over regions of large mean fractional cloudiness corresponding generally to regions of highlands. The diurnal cycle of high clouds is generally not sinusoidal and the period of development is shorter than the period of dissipation. Over land the maximum high cloud coverage occurs between 1800 LST and midnight and the minimum between 0900 LST and noon. Over oceanic coastal region the maximum high cloudiness is maximum around local noon. The phase, however, becomes more noisy far from land areas.

The diurnal composite of the infrared histogram of selected regions gives additional information. A striking result is the general presence of a large concentration of midlevel cloud with tops typically near 500 mb. These clouds have a maximum development near sunrise and a minimum in the afternoon. This particular diurnal phase modifies their radiative forcing, giving an enhancement of their greenhouse effect at the expense of their albedo effect. Another striking result is the existence of a strong coherent diurnal cycle of the cloudiness over all oceanic convergence areas. This diurnal behavior of the cloudiness is basically the same over all ocean regions studied and is compatible with results obtained for regions of the tropical Pacific.

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Jean Philippe Duvel

Abstract

Using Meteosat data and European Centre for Medium-range Weather Forecast (ECMWF) analyses, we examine easterly waves and their relation with the cloudiness over West Africa and the tropical Atlantic Ocean for three summers (June, July, August and September 1983-85). Spectral analysis of the low-level meridional wind in the 2.8–5.1 day band reveals maximum wave amplitude near the West African coast at 20°N. During August and September the wave amplitude is larger than during June and July and a secondary maximum appears around 7.5°N.

Composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ECMWF analyses. The phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions. This phase varies however grossly in four large land and ocean regions, centered at 7.5° and 17.5°N, respectively, for which we derive vertical cross sections of the wave modulation of the atmospheric state and of the vertical distribution of clouds for the summer of 1985.

For land and oceanic regions around 7.5°N, the larger deep convective activity at and ahead of the wave trough is well related to the maximum low-level convergence and high-level (200 mb) divergence. At latitudes near 17.5°N over the Saharo-Sahelian region, the deep convection has a primary maximum ¼ wavelength east of the trough, and a secondary maximum cast of the ridge. At and ahead of the trough axis there is highly suppressed cloud condition over Saharo-Sahelian regions consistent with a strong shallow dry convection described by ECMWF analyses. For oceanic trade regions near 17.5°N, the cloudiness is maximum during the phase of maximum southerly wind. This study also shows that the wave modulation of analyzed temperature and moisture profiles is in reasonable agreement with previous results and the observed cloud modulation.

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Jean-Philippe Duvel

Abstract

Using 38 years of the ERA-Interim dataset, an objective tracking approach is used to analyze the origin, characteristics, and cyclogenesis efficiency (CE) of synoptic-scale vortices initiated over West Africa and the Atlantic Ocean. Vortices initiated over the ocean at a given pressure level often result from a vertical expansion of a “primary” vortex track initiated earlier over West Africa. Low-level (850 hPa) primary vortices are initiated mainly in July near the Hoggar Mountains (24°N, 5°E), while midlevel (700 hPa) primary vortices are initiated mainly in August–September near the Guinea Highlands (10°N, 10°W). The CE of all these vortices is about 10% in July and 30% in August. The average CE is, however, smaller for low-level “Hoggar” vortices because they peak in July when the cyclogenesis potential index of the Atlantic Ocean is weak. Seasonal and interannual modulations of the cyclogenesis is related more to this index than to the number of vortices crossing the West African coast. Cyclogenesis is nearly equally distributed between the coast and 60°W, but the part of the cyclogenesis due to vortices initiated over West Africa decreases from 80% near the coast to about 30% at 60°W. The most probable delay between the vortex vertical expansion and cyclogenesis is 2 days, but it can be up to 10 days. This analysis also confirms previous results, such as the larger CE for vortices extending at low levels over the continent at 10°N, or the delayed and therefore west-shifted cyclogenesis of low-level “Hoggar” vortices.

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Jean-Philippe Duvel

Abstract

Using ERA-Interim global atmospheric reanalysis, an original tracking approach is developed to follow tropical low pressure systems from the early tropical depression (TD) stage up to possible intensification into developed tropical cyclones (TCs). The different TC stages are identified using the IBTrACS dataset. This approach detects many more TD initiations compared to IBTrACS alone and thus gives a more comprehensive dataset to study the cyclogenesis by considering separately TD initiations and the probability of intensification.

In the south Indian Ocean (SIO), the MJO modulation of the number of TCs is primarily due to the modulation of the number of TD initiations and secondarily to the probability of their intensification. The TD initiations are more probable at 55°, 75°, and 95°E and can be primarily attributed to the development of an unstable cyclonic meridional shear of the zonal wind at low levels. The reinforcement of this shear results from (i) a heat low, related to a precipitation anomaly, which triggers westerly winds equatorward of the initiation region and (ii) an easterly wind strengthening south of the initiation regions due either to a reinforcement of the subtropical high (for western and central SIO) or to a large-scale depression over the western Maritime Continent (for eastern SIO). Over the western and central SIO, the concomitance of precipitation and subtropical high anomalies at the origin of the shear reinforcement could be partly stochastic, giving a weaker relation with MJO and ENSO. Over the eastern SIO, the large-scale MJO (and ENSO) perturbation pattern alone can reinforce the shear, giving a larger modulation of the number of TD initiations.

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