Nicolas Viltard

Rain Retrieval from TMI Brightness Temperature Measurements Using a TRMM PR–Based Database

This study focuses on improving the retrieval of rain from measured microwave brightness temperatures and the capability of the retrieved field to represent the mesoscale structure of a small intense hurricane. For this study, a database is constructed from collocated Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the TRMM Microwave Imager (TMI) data resulting in about 50 000 brightness temperature vectors associated with their corresponding rain-rate profiles. The database is then divided in two: a retrieval database of about 35 000 rain profiles and a test database of about 25 000 rain profiles. Although in principle this approach is used to build a database over both land and ocean, the results presented here are only given for ocean surfaces, for which the conditions for the retrieval are optimal. An algorithm is built using the retrieval database. This algorithm is then used on the test database, and results show that the error can be constrained to reasonable levels for most of the observed rain ranges. The relative error is nonetheless sensitive to the rain rate, with maximum errors at the low and high ends of the rain intensities (60% and 30%, respectively) and a minimum error between 1 and 7 mm h 1 . The retrieval method is optimized to exhibit a low total bias for climatological purposes and thus shows a high standard deviation on point-to-point comparisons. The algorithm is applied to the case of Hurricane Bret (1999). The retrieved rain field is analyzed in terms of structure and intensity and is then compared with the TRMM PR original rain field. The results show that the mesoscale structures are indeed well reproduced even if the retrieved rain misses the highest peaks of precipitation. Nevertheless, the mesoscale asymmetries are well reproduced and the maximum rain is found in the correct quadrant. Once again, the total bias is low, which allows for future calculation of the heat sources/sinks associated with precipitation production and evaporation.

The Megha-Tropiques mission: a review after three years in orbit

The Megha-Tropiques mission is operating a suite of payloads dedicated to the documentation of the water and energy cycles in the intertropical region in a low inclination orbit. The satellite was launched in October, 2011 and we here review the scientific activity after the first three years of the mission. The microwave sounder (SAPHIR) and the broad band radiometer (SCARAB) are functioning nominally and exhibit instrumental performances well within the original specifications. The microwave imager, MADRAS, stopped acquisition of scientific data on January 26th, 2013 due to a mechanical failure. During its 16 months of operation, this radiometer experienced electrical issues making its usage difficult and delayed its validation. A suite of geophysical products has been retrieved from the Megha-Tropiques payloads, ranging from TOA radiative flux to water vapor profiles and instantaneous rain rates. Some of these geophysical products have been merged with geostationary data to provide, for instance, daily accumulation of rainfall all over the intertropical region. These products compare favorably with references from ground based or space-borne observation systems. The contribution of the mission unique orbit to its scientific objectives is investigated. Preliminary studies indicate a positive impact on both, humidity Numerical Weather Prediction forecasts thanks to the assimilation of SAPHIR Level 1 data, and on the rainfall estimation derived from the Global Precipitation Mission constellation. After a long commissioning phase, most of the data and the geophysical products suite are validated and readily available for further scientific investigation by the international community.

The Megha-Tropiques mission

The Megha-Tropiques satellite is devoted to the study of the atmospheric water cycle in the tropics and its relation to the radiative budget. It is aiming to study both the energy and water budget of the intertropical band and the life cycle of the convective complexes in the Tropics. The orbit of the satellite allows it to sample several times per day the zone from 23°N to 23°S, where most of the precipitation of the planet and large energy exchanges occur. The three instruments of the mission are a microwave imager, a microwave water vapor sounder and a radiative budget instrument. The launch of this mission by an Indian Rocket is foreseen in 2006-2007. It will hopefully coincide with the time frame of the Global Precipitation Mission, allowing to improve its tropical coverage.

Robust Observational Quantification of the Contribution of Mesoscale Convective Systems to Rainfall in the Tropics

AbstractSatellite estimation of precipitation and satellite-derived statistics of mesoscale convective systems (MCS) are analyzed conjunctively to quantify the contribution of the various types of MCS to the water budget of the tropics. This study focuses on two main mesoscale characteristics of the systems: duration and propagation. Overall, the systems lasting more than 12 h are shown to account for around 75% of the tropical rainfall, and 60% of the rainfall is due to systems traveling more than 250 km, a typical GCM grid. A number of regional features are also revealed by factoring in the convective systems’ morphological parameters in the water budget computation. These findings support the challenging effort to account for such mesoscale features when considering the theory on the future evolution of the water budget as well as the physical parameterizations of climate models. Finally, this analysis provides a simple metric for evaluating high-resolution numerical simulations of the tropical water b...

RONSARD Radar: Implementation of Dual Polarization on a C-Band Doppler Weather Radar

The French C-band meteorological Doppler radar Recherche sur les Orages et Nuages par un Systeme Associe de Radars Doppler (RONSARD) was recently equipped with dual polarization. This modification required, on the one hand, an additional receiver and, on the other hand, a new design for the antenna geometry in order to decrease strongly the sidelobe level. This new radar configuration allows us to choose between two complementary modes: (1) the previous single-polarization mode, still preserved with fast Fourier transform calculations of the first three momentums of the Doppler spectrum, i.e., horizontal (H) reflectivity, radial wind velocity, and wind velocity variance calculated both in precipitation areas and clear air areas (depending on the gate), and (2) the dual-polarization mode with pulse pair processing of H and vertical reflectivities and velocities, differential phase shift, and coherence coefficient. Moreover, both modes work over contiguous gates along each direction, allowing fine radial range resolution. Thanks to the flexibility between these two modes, the RONSARD radar becomes a new major facility aimed at studying the dynamics-microphysics interactions within precipitation and their environment.

Evolution of the Distribution of Upper-Tropospheric Humidity over the Indian Ocean: Connection with Large-Scale Advection and Local Cloudiness

AbstractThe spatial and temporal distribution of upper-tropospheric humidity (UTH) observed by the Sounder for Atmospheric Profiling of Humidity in the Intertropics by Radiometry (SAPHIR)/Megha-Tropiques radiometer is analyzed over two subregions of the Indian Ocean during October–December over 2011–14. The properties of the distribution of UTH were studied with regard to the phase of the Madden–Julian oscillation (active or suppressed) and large-scale advection versus local production of moisture. To address these topics, first, a Lagrangian back-trajectory transport model was used to assess the role of the large-scale transport of air masses in the intraseasonal variability of UTH. Second, the temporal evolution of the distribution of UTH is analyzed using the computation of the higher moments of its probability distribution function (PDF) defined for each time step over the domain. The results highlight significant differences in the PDF of UTH depending on the phase of the MJO. The modeled trajectorie...

3D wind field retrieval from spaceborne Doppler radar

Numerous space missions carrying a radar are presently envisioned, particularly to study tropical rain systems. Among those missions, BOITATA is a joint effort between Brazil (INPE/AEB) and France (CNES). The goal is to embark a Doppler radar with scanning possibilities onboard a low-orbiting satellite. This instrument should be implemented in addition to a Passive Microwave Radiometer (PMR) between 19 and 183 GHz, an improved ScaraB-like broadband radiometer, a mm/submm PMR and a lightning detection instrument. This package would be meant to document the feedback of the ice microphysics on the rain systems life cycle and on their heat and radiative budgets. Since the microphysics and the water and energy budgets are strongly driven by the dynamics, the addition of a Doppler radar with scanning possibilities could provide precious information (3D wind and rain fields). It would allow us to build a large statistics of such critical information over the entire tropics and for all the stages of development of the convection. This information could be used to better understand the tropical convection and to improve convection parameterization relevant for cloud and climate models and associated applications such as now-casting and risk prevention. The present work focuses on the feasibility to retrieve 3D winds in precipitating areas from such a radar. A simulator of some parts of the spaceborne radar is developed to estimate the precision on the retrieved wind field depending on the scanning strategies and instrumental parameters and to determine the best sampling parameters.

A spaceborne Doppler radar for 3D winds mapping inside clouds

This paper presents the design of two spaceborne Doppler radar for 3D winds mapping inside cloud at 94 GHz: first one corresponds to radar with high quality of the retrieved 3D components of the wind field without scanning capability (fixed antenna with two reflectors); second one corresponds to scanning Doppler radar with swath width capability thanks to conical scanning antenna (rotation of all the instrument).

Estimated precipitation and latent heating profiles from combined Tropical Rainfall Measuring Mission TMI/PR/VIRS observations

The November 27, 1997 launch of the Tropical Rainfall Measuring Mission (TRMM) observatory provides the first opportunity to perform combined passive microwave/radar/infrared remote sensing of precipitation from satellite. A method for retrieving vertical precipitation/latent heating profiles using combined measurements from the TRMM Microwave Imager (TMI), Precipitation Radar (PR), and Visible and Infrared Scanner (VIRS) is presented. The TMI is a nine-channel passive microwave radiometer with dual polarization channels at 10.65, 19.35, 37, and 85.5 GHz and a vertical-polarization channel at 21.3 GHz. The PR is a 13.8 GHz single-parameter radar. The VIRS radiometer operates at visible/infrared wavelengths of 0.63, 1.61, 3.75, 10.8, and 12 microns. The TMI channels are to a large extent affected by the vertically-integrated cloud and precipitation water contents within clouds, while the PR senses the backscatter of radiation from precipitation within 0.25 km thick contiguous cloud layers. The VIRS, although not sensitive to precipitation directly, yields high-resolution measurements of cloud top temperature and overall cloud geometry. The combined TMI/PR/VIRS observations provide complementary information regarding the cloud morphology and spatial distribution of liquid/ice phase precipitation in tropical weather systems. From this information, the vertical and horizontal distributions of latent heating may be inferred.