Recent numerical studies in stratospheric dynamics and its variability as well as climate, have highlighted the need of more observational analyses to improve simulation of the West African monsoon (WAM). In this paper, activity and spectral characteristics of short-scale vertical waves (wavelengths <4 km) are analysed in equatorial coastal and tropical lower stratosphere during the WAM. A first detailed description of such waves over West Africa is derived from high-resolution vertical profiles of temperature and horizontal wind obtained during Intensive Observation Period of the African Monsoon Multidisciplinary Analyses (AMMA) Campaign 2006. Monthly variation of wave energy density is revealed to trace the progression of the inter-tropical convergence zone (ITCZ) over West Africa. Mesoscale inertia gravity-waves structures with vertical and horizontal wavelengths of 1.5–2.5 and 400–1100 km respectively and intrinsic frequencies of 1.1–2.2 f or periods <2 days are observed in the tropical LS with intense activity during July and August when the WAM is installed over the tropical West Africa. Over equatorial region, gravity waves with intrinsic frequencies of 1.4–4 f or periods <5.2 days, vertical wavelength of 2.1 km and long horizontal wavelengths of 1300 km are intense during the WAM coastal phase. From July to October, gravity waves with intrinsic frequencies of 1.2–3.8 f or periods <6 days, vertical wavelength of 2.1 km and horizontal wavelengths of 1650 km are less intense during the WAM Sahelian phase of the WAM, March–June. Unlike potential energy density, kinetic energy density is observed to be a good proxy for the activity of short-scale vertical waves during the WAM because quasi-inertial waves are dominant. Long-term wave activity variation from January 2001 to December 2009, highlights strong year-to-year variation superimposed on convective activity and quasi-biennial oscillation-like variations especially above tropical stations.

Gravity waves (GWs) with horizontal wavelengths of 32-2000 km are investigated during tropical cyclone (TC) Ivan (2008) in the southwest Indian Ocean in the upper troposphere (UT) and the lower stratosphere (LS) using observational data sets, radiosonde and GPS radio occultation data, ECMWF analyses and simulations of the French numerical model Meso-NH with vertical resolution < 150 m near the surface and 500 m in the UT/LS. Observations reveal dominant low-frequency GWs with short vertical wavelengths of 0.7-3 km, horizontal wavelengths of 80-400 km and periods of 4.6-13 h in the UT/LS. Continuous wavelet transform and image-processing tools highlight a wide spectrum of GWs with horizontal wavelengths of 40-1800 km, short vertical wavelengths of 0.6-3.3 km and periods of 20 min-2 days from modelling analyses. Both ECMWF and Meso-NH analyses are consistent with radiosonde and GPS radio occultation data, showing evidence of a dominant TC-related quasi-inertia GW propagating eastward east of TC Ivan with horizontal and vertical wavelengths of 400-800 km and 2-3 km respectively in the LS, more intense during TC intensification. In addition, the Meso-NH model produces a realistic, detailed description of TC dynamics, some high-frequency GWs near the TC eye, variability of the tropospheric and stratospheric background wind and TC rainband characteristics at different stages of TC Ivan. A wave number 1 vortex Rossby wave is suggested as a source of dominant inertia GW with horizontal wavelengths of 400-800 km, while shorter scale modes (100-200 km) located at northeast and southeast of the TC could be attributed to strong localized convection in spiral bands resulting from wave number 2 vortex Rossby waves. Meso-NH simulations also reveal GW-related clouds east of TC Ivan.

Using the French non-hydrostatic mesoscale numerical model Méso-NH, intense tropical cyclone (TC) Dina (2002) is simulated to investigate the forcing caused by the steep orography of Réunion island (20.8° S, 55.5° E) in the southwest Indian Ocean. The model initialised by a bogus vortex derived from Doppler radar observations reproduces quite well the dynamical characteristics of TC Dina approaching the island and provides some clues on the orographic influence on the structure and the evolution of the TC. The presence of the island is observed to stabilise the cyclonic circulation by damping the natural elliptical eyewall rotation and forcing the flow circulation. Initially, the cyclonic flow is blocked upwind of the orography which induces a convergence associated with upward vertical velocities, intense precipitation and maximum horizontal winds along the upwind slopes of the island. A mountain wave, generated over the highest terrains, is associated with downward motions on the lee side. When the strongest winds reach the island, the flow changes its behaviour from passing around to over the island. Non-dimensional flow parameters in agreement with recent theories are calculated to explain TC track.

Conventional and wavelet methods are combined to characterize gravity-waves (GWs) produced by two intense tropical cyclones (TCs) in the upper troposphere and lower stratosphere (UT/LS) from GPS winsonde data. Analyses reveal large contribution of GWs induced by TCs to wave energy densities in the UT/LS. An increase in total energy density of about 30% of the climatological energy density in austral summer was estimated in the LS above Tromelin during TC Dina. Four distinct periods in GW activity in relation with TC Faxai stages is observed in the UT. Globally, GWs have periods of 6 h–2.5 days, vertical wavelenghts of 1–3 km and horizontal wavelengths <1000 km in the UT during the evolution of TCs. Horizontal wavelengths are longer in the LS and about 2200 km during TCs. Convective activity over the basin and GW energy density were modulated by mixed equatorial waves of 3–4 days, 6–8 days and 10–13 days confirmed by Hövmöller diagram, Fourier and wavelet analyses of OLR data. Moreover, location of GW sources is below the tropopause height when TCs are intense otherwise varies at lower tropospheric heights depending on the strength of convection. Finally, the maximum surface wind speeds of TCs Dina and Faxai can be linearly estimated with total energy densities.

Tropical cyclone (TC) activity is diagnosed through convective gravity waves (GWs) observed in the upper troposphere (UT)/lower stratosphere (LS) above Tromelin island (15.53°S, 54.31°E) in the tropical southwest Indian Ocean. Monthly and weekly GW total energy densities derived from daily GPS windsonde data are compared with Outgoing Longwave Radiation (OLR) and TC hours in the vicinity of Tromelin. A relationship between GW energy density and TC activity is observed in the LS, for the TC season 2001/2002. Moreover TCs (local convection) produce GWs with total energy density mostly higher (lower) than 12 J kg−1. A 10‐season climatology (1997/1998–2006/2007) confirms that large values of GW total energy density in the LS are associated with weak values of OLR during the TC passage. Monthly total, kinetic and potential GW energy densities within 2000 km radius of Tromelin can be estimated using linear relationships with TC hours for a threshold of above 6 TC days per month. A linear relationship also exists between weekly GW total energy density in the LS and the activity of intense TCs above a threshold of 2 TC days per week within 1000 km radius of Tromelin. GW energy density in the LS could be used as a possible index to investigate TC activity in the UT/LS.

Gravity wave activity is studied with six years radio sounding data from some of the African meteorological stations located in the 18° N and 05° S latitude domains. This study focuses on gravity wave parameters (e.g., total energy, kinetic and potential energies, fraction of upward propagating energy) above Bamako, Mali (12°53’ N, 07°95’ W). Climatology and seasonal variation are analyzed in the lower stratosphere and upper troposphere. Sources and variability are also examined in relation to convection indices and monsoons.

Gravity wave activity is analysed in the lower stratosphere using 6 year radiosonde data (2001–2006) above two meteorological stations in the West African tropical region such as Niamey (13.47° N; 2.16° E) and Ouagadougou (12.35° N; 1.51° W). Monthly total energy density of gravity waves is computed with temperature and horizontal wind perturbations to highlight the West African Monsoon period from June to September. Comparison with monthly total energy density calculated with temperature only supports that observed small-scale temperature and wind perturbations are mostly associated with gravity waves in the lower stratosphere especially for large values during the wet season. Above the two sites, monthly evolution of gravity wave total energy density reveals a maximum intensity of gravity wave activity in July during the West African Monsoon period. Indicators of convective activity such as mean Outgoing Longwave Radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) rain rates reveal to be adequate monsoon proxies to be compared to gravity wave energy intensity during the West African Monsoon.

Vertical temperature profiles obtained by radiosonde and Raman lidar measurements are used to investigate a climatology of total energy density of gravity waves (GW) in the Upper Troposphere (UT) and the Lower Stratosphere (LS) from 1992 to 2004 above Mahé (4° S, 55° E), Tromelin (15° S, 54° E) and La Réunion (21° S, 55° E) located in the tropical South-West Indian Ocean. The commonly used spectral index value (p≈5/3) of the intrinsic frequency spectrum is used for calculating estimated total energy density in the UT and LS. Estimated total energy density provides good estimation of total energy density in the LS but underestimates total energy density by one half in the UT above Mahé and Tromelin probably due to the activity of near-inertial frequency waves. Estimated total energy density reveals a strong seasonal variability as a function of latitude and convection as an evident active source of GW activity in the LS in austral summer. Above La Réunion, a semi-annual GW activity is observed in the LS with the signature of the subtropical barrier in the UT. Moreover, radiosondes and Raman lidar provide consistent GW surveys in the UT/LS at heights<23 km above La Réunion.

A Raman lidar dedicated to night-time tropospheric water-vapor high-resolution measurements has been developed at Reunion island in the south-western Indian Ocean. To our knowledge, it is the first permanent instrument of its kind in this tropical region. The geophysical and instrumental interests and issues on the radiative, dynamical and chemical plans for such a measurement, specially in the tropics, are obvious. This summary aims to give a brief technical description of the lidar system and to emphasize the importance of the rejection of the elastically backscattered signals in the Raman channels of this instrument that is intended to become an atmospheric surveillance system on a medium term.

Characteristics of inertia-gravity waves are analysed in high-resolution vertical profiles of temperature, winds and ozone collected at Macquarie Island (54degreesS, 159degreesE) during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) observation programme in 1994. Two particular techniques are outlined to identify gravity-wave modes. The first is based on the continuous wavelet transform and seeks altitudes where the atmospheric fluctuations satisfy the gravity-wave polarisation relations in the temperature and wind soundings. The second analyses the phase and amplitude relationship of small-scale wavelike signatures seen in temperature and ozone profiles. The wavelet method identified gravity-wave modes with vertical wavelengths of 1-10 km, horizontal wavelengths of 50-1000 km and intrinsic frequencies of 1-2 f. Both methods reveal the presence of dominant modes with vertical wavelength <4 km in the upper troposphere and lower stratosphere over Macquarie Island. The energy activity of observed modes agrees well with the seasonal cycle of the upper level jet at 10 km height. These techniques together with classical spectral methods are applied to the case study of 25 October 1994 for which three quasi-monochromatic coherent modes with upwardly propagating wave energy are detected in the lower stratosphere.

The time and spatial evolution of gravity‐wave characteristics are analysed using wavelets in vertical profiles of temperature and winds at Tromelin Island (15.53°S, 54.31°E) during the passage of the intense tropical cyclone Hudah in the Southern Ocean Indian Basin in 2000. Inertia‐gravity waves were observed in the upper troposphere and the lower stratosphere with dominant vertical wavelengths of 1.5–3 km, horizontal wavelengths <2000 km and periods of 0.6–1.6 days. Large amounts of gravity‐wave energy were detected during landfalls of the tropical cyclone. The distribution of total energy indicates that mesoscale convective structures such as tropical cyclones are important gravity‐wave sources in the upper troposphere.

The capabilities of the continuous wavelet transform (CWT) and the multiresolution analysis (MRA) are presented in this work to measure vertical gravity wave characteristics. Wave properties are extracted from the first data set of Rayleigh lidar obtained between heights of 30 km and 60 km over La Reunion Island (21°S, 55°E) during the Austral winter in 1994 under subtropical conditions. The altitude-wavelength representations deduced from these methods provide information on the time and spatial evolution of the wave parameters of the observed dominant modes in vertical profiles such as the vertical wavelengths, the vertical phase speeds, the amplitudes of temperature perturbations and the distribution of wave energy. The spectra derived from measurements show the presence of localized quasi-monochromatic structures with vertical wavelengths <10 km. Three methods based on the wavelet techniques show evidence of a downward phase progression. A first climatology of the dominant modes observed during the Austral winter period reveals a dominant night activity of 2 or 3 quasi-monochromatic structures with vertical wavelengths between 1/2 km from the stratopause, 3/4 km and 6/10 km observed between heights of 30 km and 60 km. In addition, it reveals a dominant activity of modes with a vertical phase speed of −0.3 m/s and observed periods peaking at 3/4 h and 9 h. The characteristics of averaged vertical wavelengths appear to be similar to those observed during winter in the southern equatorial region and in the Northern Hemisphere at mid-latitudes.

The distribution and the nature of vertical short‐scale structures observed in ozone and temperature are investigated in the upper troposphere and the lower stratosphere at La Reunion Island located in the vicinity of the southern subtropical barrier by using wavelet‐based methods. A climatology of dominant wavelike patterns with short vertical wavelengths reveals the presence of localized structures on both the ozone and the temperature perturbations, extracted from ozonesonde and temperature data, up to the middle stratosphere. Some case studies are presented to identify the nature of short‐scale structures with 1‐ to 5‐km vertical wavelengths in the troposphere and the stratosphere. A climatology of short‐scale structures induced by gravity waves and the horizontal advection shows that short‐scale structures are mainly detected in the middle and upper troposphere and in the lower stratosphere. The weak value of the coefficient R(z) that links the ozone and temperature perturbations induced by gravity waves is a major limit to detecting such short‐scale structures above 21‐km altitude. Some structures with vertical wavelengths ranging from 1 to 5 km are attributed to gravity waves produced by convection in summer and the subtropical jet in winter, or quasi‐horizontal large‐scale motions from both sides of the subtropical barrier.

This paper puts forward wavelet based tools such as continuous wavelet transform, multiresolution and wavelet packet methods to study gravity wave motions and extract vertical characteristics (wavelength, phase speed) versus time and height in the middle atmosphere (30-60 km height) from vertical temperature perturbation profiles provided by a Rayleigh lidar. Different dominant quasi-monochromatic structures with downward phase progression inferior to 1 m/s are identified and extracted to be studied separately. Wave dissipation is observed in the stratopause with production of short waves less than 2 km at the boundary of the lower mesosphere. Longer waves have smaller phase speed than shorter waves.