Yvon Lemaître

A New Analysis for the Retrieval of Three-Dimensional Mesoscale Wind Fields from Multiple Doppler Radar

Abstract The present study is devoted to a new analysis of the Doppler information from multiple Doppler radar scannings aimed at studying the three-dimensional wind field at mesoscale. This analysis referred to as MANDOP (for Multiple ANalytical DOPpler) is mathematically described. An application of the method to simulated and to real data is also presented in the paper, including a comparison with VAD and DVAD analyses and with observations from radiosoundings and sodar. The method consists in expanding the three wind components in terms of orthonormal functions. Physical constraints, such as the boundary condition at ground level and the continuity equation, are included in the analysis as variational constraints in order to improve the vertical component retrieval. Solving the resulting linear system provides the coefficients for the analytical expansion of the wind. Thus, the wind, as well as its derivatives and associated physical parameters (e.g., vorticity, pressure, and temperature perturbations...

A Radar Simulator for High-Resolution Nonhydrostatic Models

A full radar simulator for high-resolution (1–5 km) nonhydrostatic models has been developed within the research nonhydrostatic mesoscale atmospheric (Meso-NH) model. This simulator is made up of building blocks, each of which describes a particular physical process (scattering, beam bending, etc.). For each of these blocks, several formulations have been implemented. For instance, the radar simulator offers the possibility to choose among Rayleigh, Rayleigh–Gans, Mie, or T-matrix scattering methods, and beam bending can be derived from an effective earth radius or can depend on the vertical gradient of the refractive index of air. Moreover, the radar simulator is fully consistent with the microphysical parameterizations used by the atmospheric numerical model. Sensitivity experiments were carried out using different configurations for the simulator. They permitted the specification of an observation operator for assimilation of radar reflectivities by high-resolution nonhydrostatic numerical weather prediction systems, as well as for their validation. A study of the flash flood of 8–9 September 2002 in southeastern France, which was well documented with volumetric data from an S-band radar, serves to illustrate the capabilities of the radar simulator as a validation tool for a mesoscale model.

Three-Dimensional Variational Data Analysis to Retrieve Thermodynamical and Dynamical Fields from Various Nested Wind Measurements

Abstract The present study is devoted to a new analysis of wind measurements from dropsonding and/or radiosonding of Doppler information from multiple Doppler radar scanning and of other wind measurements (sodar, dynamical sensors on board aircraft, and instruments at ground) aimed at retrieving three-dimensional thermodynamical and dynamical fields both in clear air and in precipitating areas of mesoscale phenomena. This analysis, well suited to assimilate data from differing platforms specified at differing spatial/temporal resolutions, is based on the analytical and variational concept of the Multiple Analytical Doppler (MANDOP) analysis and thus is an extension of it. This new analysis presents many advantages, including the same as MANDOP and others well adapted for the verification or the initialization of a mesoscale cloud model. An application to simulated and to real data extracted from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment database is presented in the ...

Retrieval of Kinematic Fields Using a Single-Beam Airborne Doppler Radar Performing Circular Trajectories

Abstract The present study is devoted to new analyses of single-beam airborne Doppler radar data referred to as SAVAD (single-beam airborne velocity–azimuth display) and double SAVAD. These techniques permit processing of Doppler radial velocities from circular trajectories performed by the aircraft. As in the VAD and double VAD analyses for ground-based radars, these SAVAD and double SAVAD analyses permit diagnosis of crucial mesoscale kinematic and thermodynamic parameters such as the horizontal wind, its horizontal divergence, its stretching and shearing deformations, the vertical air motion, the mean fall velocity of the hydrometeors, the mesoscale vertical vorticity, and horizontal gradients of pressure and temperature perturbations. These analyses are mathematically described. An application of the method to simulated data and to a real dataset extracted from the TOGA COARE database is also presented.

Scale Interactions Involved in the Initiation, Structure, and Evolution of the 15 December 1992 MCS Observed during TOGA COARE. Part I: Synoptic-Scale Processes

Abstract This paper, the first of a series, examines the synoptic-scale mechanisms involved in the initiation, structure, and evolution of a mesoscale convective system observed during TOGA COARE. This study relies upon the use of the Japanese Geosynchronous Meteorological Satellite-4 imagery and ECMWF model outputs, from which diagnostic parameters are derived and interpreted. This mesoscale convective system consists initially of two groups of convective entities that progressively move toward each other and merge. It is shown that the synoptic-scale flow creates a favorable environment for the formation of this large convective system through the production of convective available potential energy (CAPE) by horizontal advection and the enhancement of low-level convergence in the region where the convective system formed. Moreover, the general evolution of the system is found to be governed by the synoptic-scale circulation and, more precisely, by the temporal evolution of CAPE and low-level convergence...

Mesoscale Convective Systems in Relation to African and Tropical Easterly Jets

AbstractThis paper documents the interaction processes between mesoscale convective systems (MCS), the tropical easterly jet (TEJ), and the African easterly jet (AEJ) over West Africa during the monsoon peak of 2006 observed during the African Monsoon Multidisciplinary Analyses (AMMA) project. The results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle. A systematical study reveals that intense and long-lived MCSs correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of TEJ and in exit regions of AEJ. A case study on a particularly well-documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence, favoring the persistence of MCSs.

Retrieval of kinematic fields from dual-beam airborne radar data gathered in circular trajectories during the FASTEX Experiment

Abstract The present paper describes the vertical structure of the wind field obtained by analysis of “purls,” that is, circular trajectories regularly performed by airborne dual-beam Doppler radars within the FASTEX frontal cyclones. Kinematic information on these systems are obtained using a new analysis scheme named DAVAD (Dual-Beam Antenna Velocity Azimuth Display). Using this scheme, it is possible to obtain the mesoscale wind field and its first-order derivatives, that is, the horizontal divergence (thus the vertical velocity), the stretching and shearing deformations, and the vertical component of vorticity. A unique advantage of this analysis is that it also provides a direct estimate of the terminal fall velocity of the hydrometeors. All these parameters are crucial for validation and initialization of mesoscale and large-scale models. The capabilities of this method and the best conditions for its application are assessed through simulations. Finally, an example of application of the scheme on t...