Alain Viltard

Evidence of two regimes of easterly waves over West Africa and the tropical Atlantic

Synoptic-scale easterly waves at 700 hPa have been studied over West Africa and the tropical Atlantic for the summers of 1979–1995. Spectral analyses (Fast Fourier and Wavelet Transforms) of 700 hPa meridional wind component along 17.5°N enable to point out two band-periods, between 3 and 5 days and between 6 and 9 days. An example of each wave is shown. Composite analysis confirms the evidence of two such regimes of easterly waves during summer over West Africa and the tropical Atlantic. The 6–9-day wave regime differ from the 3–5-day wave regime by larger anticyclonic cells originated from the Libyan and the Azores areas, and by more northern tracks.

Comparison of the African and the 6-9 day wave-like disturbance patterns over West-Africa and the tropical Atlantic during summer 1985

SummaryUsing the ECMWF and NMC analyses, this study documents the composite structures of the African and of the 6–9 day waves. In spite of the fact that the two types of disturbances develop over almost the same area, i.e. Central and West Africa and the tropical Atlantic, during the same season, i.e. summer, in spite of the fact that they have almost the same East-West velocity, i.e. 7–8 degree longitude per day, the structures of the two waves are very different.At 12.5°N, the African wave has an amplitude maximum in the meridional wind component whilst the zonal wind component is almost unperturbed. On the contrary, in the 6–9 day wave, at 12.5°N and also at 12.5°S, the zonal wind component has an amplitude maximum whilst the meridional wind component is very small and there is an amplitude maximum for the meridional wind component at the equator and 20°N.

A Synoptic-Scale Wave of 6–9-Day Period in the Atlantic Tropical Troposphere during Summer 1981

Abstract Using the ECMWF analyses, this study documents a tropospheric synoptic-scale wave in the tropical Atlantic during summer 1981. It has a 6–9-day period and a westward velocity of about 8.5° longitude per day. The wave structure has features similar to a mixed Rossby-gravity wave. It is suggested that it may be a normal mode of the earth atmosphere.

Characteristics of North African 6–9 Day Waves during Summer 1981

Abstract This study applies compositing methods to ECMWF model analyses during July and August 1981 to determine the structure of synoptic scale waves with periods of 6–9 days. Over northern Africa, these ware intermittent in time, are most regular in period and amplitude west of the Greenwich meridian in a narrow longitude band, though there is a suggestion of their existence between 15°E and 0°. The average wavelength is 5000 km and the westward velocity is 6°–7° longitude day−1. The horizontal wind pattern in midtroposphere (700 and 500 hPa) consists of two vortices of opposite signs on either side of 12.5°N, with maximum perturbation of the zonal wind component at this latitude, and maximum perturbation of the meridional wind component at 5°–7.5°N and at 17.5°–20°N. Temperature is modulated by the passage of the waves throughout the troposphere. This structure is very different from that of the 3‐4 day African wave.

Statistical Analysis of Wind Velocity in an Easterly Wave over West Africa

Abstract Results of a statistical analysis of wind data collected in West Africa during the summer of 1974 (GATE) are presented. A comparison of the wind meridional components in Bamako and Dakar shows that the easterly wave velocities at 700 mb during the period 30 August–20 September 1974 are approximately equal to those at 200 mb and that the waves are in phase opposition at these two levels. It is suggested that these wave features are favorable to the existence of cloud system formations which were observed in a greater number during GATEs third active phase than during the two previous phases.

Systematic Errors of the ECMWF Operational Model over Tropical Africa

Abstract Five-day forecasts using the high-resolution T213L31 Operational Global Spectral Model of the European Centre for Medium-Range Weather Forecasts and associated analyses for summer 1995 are compared in this study to assess the model drift. It is shown here that there is extensive model warming of the lower troposphere especially over the Sahel (+7.5°C) and cooling of the middle troposphere in the tropical Southern Hemisphere. The surface heat wave episode diagnosed by the Climate Prediction Center in summer 1995 was severely overestimated (11°C). A likely cause of this major error is the inaccuracy of convection and boundary layer parameterizations in the case of extreme events. It is found that the model overestimates upper-level subtropical westerlies (+3 m s−1), low-level monsoon winds, and the Hadley circulation. The African easterly jet is weaker in the model than observed probably because initial easterly zonal wind errors evolve rapidly leading to underestimation of the jet at medium range....

Rainfall and the 6–9 day wave-like disturbance in West-Africa during summer 1989

SummaryUsing ECMWF analyses and daily rain amounts of 569 stations in Western Africa for summer 1989, the study documents the composite structure of the 6–9 day oscillation and its influence on rain. Rain is modulated by vorticity as displayed in the wave composite. There are rainfall maxima coincident with cyclonic vorticity and rainfall minima coincident with anticyclonic vorticity at the 700hPa level, at 17.5°N and 7.5°N.

Daily Precipitation Forecasting in Dakar Using the NCEP–NCAR Reanalyses

Abstract In order to predict the daily rain amount at Dakar at 1–5-day lead times, 65 thermodynamical and dynamical indices are computed at each grid point for the area 15°S–30°N, 30°W–30°E. The data used are NCEP–NCAR reanalyses and daily rainfall obtained by averaging over 21 rain gauges near Dakar, for 23 Augusts (1968–90). At each lead time and each grid point, a Pearson product–moment correlation coefficient r is computed between each index and the rainfall over 17 Augusts (1968–84). Predictive regression equations are developed including the 65 indices taken at the grid points where their r value is at a maximum. The prediction skill is tested over six Augusts (1985–90). The variance (R2) explained is 42% for the 1-day lead time, it decreases slowly up to the 4-day lead time (35%), and it is 28% at the 5-day lead time. The skill is better than when climatological data are used to predict rain amount. Among the predictors that appear most frequently in the predicting equations are lifting condensatio...

Influence of the 6-9 day wave disturbances on temperature, vorticity and cloud cover over the tropical Atlantic during summer 1985

SummaryUsing ECMWF and NCEP/NCAR analysis and reanalysis data sets, 6–9 day wave-like oscillations have been described in the African and tropical Atlantic troposphere during the summers of 1981 and 1985. In spite of several kinematic analogies, their structure was quite different from the African waves. In this paper we study the connection between the 6–9 day oscillations and the cloud cover with help of the ISCCP-Cl data set. The largest cloud cover oscillation (±30%) occurs in the 800–680 hPa layer, a maximum (minimum) is connected with a cyclonic (anticyclonic) circulation. The influence of the 6–9 day oscillation on horizontal water vapor flux and temperature is also displayed.

A fast moving easterly wave of the west Africa troposphere

SummaryThis paper documents a tropospheric synoptic-scale wavelike disturbance over Northern Africa and the tropical Atlantic during summer, in the 3.–5.day band period, different from the African wave. It has a velocity of 10–12 degree longitude per day instead of 6–8, a slighter shorter period, i.e. 3.5 instead of 4.5 days. It is best seen between 5°–7° N whilst the African wave has its largest amplitude about 12° N. The data used are NCEP/NCAR reanalyses and radiosonde data.