Fabrice Chane-Ming

Vertical short-scale structures in the upper tropospheric and lower stratospheric temperature and ozone at la Réunion Island (20.8°S 55.3°E)

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.

On Developing a Tropical Cyclone Archive and Climatology for the South Indian and South Pacific Oceans

Tropical cyclones (TCs) are the most dangerous and damaging weather phenomena to regularly affect countries in the South Indian (SIO) and the South Pacific (SPO) Oceans. The year-to-year impact varies, and historical records demonstrate significant interannual variability in TC frequency and spatial distribution of TC tracks. Additionally, the climate is changing on a global scale (IPCC 2007) and it is important to understand how a warmer climate may affect TC activity. Numerous studies on TC activity in various regions of the northern and southern hemispheres have been completed with the aim of developing TC climatologies and establishing driving forces behind TC temporal and spatial variability.

Remote Sensing for Improved Forecast of Typhoons

This chapter presents the advantages of remote sensing in various aspects of tropical cycles or typhoons for the purpose of improved forecasts. A variety of variables influencing the typhoons become our concerns and sequentially investigated. First of all, to effectively predict the rainfall associated with a landfalling typhoon, the ground-based Global Positioning System (GPS) zenith total delay is combined with Doppler radar data through data assimilation algorithms. Subsequently, discussions on a natural phenomenon of interactions among two typhoons with and without tropical depressions (TDs) are elaborated. Remote sensing imagery and image processing techniques are applied to analyze relevant interactions and physical responses, including TDs’ appearance, development, interaction and how they merge. Then, the application of remote sensing observational data in numerical modeling for the study of atmospheric gravity waves, especially during the occurrence of asymmetric tropical cyclones is presented. Finally, a brief introduction is given to the oceanic surface wind measurement from different satellites with already demonstrated or potential impacts on typhoon simulations and predictions. Note that atmospheric and oceanic parameters derived from observations of Global Navigation Satellite System (GNSS) receivers onboard low Earth orbit (LEO) satellites, i.e., FORMOSAT-3/COSMIC and to-be-launched FORMOSAT-7/COSMIC-2, are also discussed within the selected topics. The importance of implementing remote sensing technology in the investigation and forecast of typhoons is the conclusion.

Raman water-vapor lidar implemented on an existing lidar system in the southern tropics

A Raman lidar dedicated to night-time tropospheric water-vapor high-resolution measurements is currently being developed at Réunion 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. The choice of a visible laser excitation wavelength was initially a constraint, in view of the weakness of the Raman scattering process that is the basis of the development of this instrument, but many arguments also plead for such a choice. After describing the water-vapor measurement method of this lidar, which is straightforward in principle, we stress on the main delicate underlying issues related to this method. A precise description of the optical parts of the lidar system is then given that emphasizes the importance of the rejection of the elastically backscattered signals in the Raman channels. Finally, we list the most important future works concerning the validation and calibration stages of this instrument that is intended to become an atmospheric surveillance instrument on a medium term.

Wavelet techniques applied to lidar temperature profiles in the middle atmosphere to study gravity waves

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.