Gaëlle Clain

Analysis of the origin of the distribution of CO in the subtropical southern Indian Ocean in 2007

We show carbon monoxide (CO) distributions at different vertical levels over the subtropical southern Indian Ocean, analyzing an observation campaign using Fourier transform infrared (FTIR) solar absorption spectrometry performed in 2007 at Reunion Island (21°S, 55°E). The CO pollution levels detected by the FTIR measurements during the campaign show a doubling of the CO total columns during the Southern Hemisphere biomass burning season. Using correlative data from the Measurement of Pollution in the Troposphere instrument and back trajectories analyses, we show that the potential primary sources for CO throughout the troposphere in 2007 are southern Africa (June-August) and South America (September-October). A secondary potential contribution from Southeast Asia and Indonesia-Malaysia was identified in the upper troposphere, especially in July and September. We examine the relation between the Asian monsoon anticyclone seasonal cycle and this result. We also investigate the relative contribution of different areas across the globe to the CO concentration in the subtropical southern Indian Ocean in 2007 using backward simulations combining the Lagrangian model FLEXPART 6.2, the Global Fire Emissions Database (GFEDv2.1) and the Emission Database for Global Atmospheric Research (EDGARv3.2-FT2000). We confirm the predominance of the African and South American contributions in the CO concentration in the southern subtropical Indian Ocean below 11 km. We show that CO transported from Australia makes only a small contribution to the total CO concentration observed over Reunion Island, and that the long-range transport of CO coming from Southeast Asia and Indonesia-Malaysia is important, especially from June until September in the upper troposphere.

Occurrence of monsoon depressions in the Southwest Indian Ocean: Synoptic descriptions and stratosphere to troposphere exchange investigations

This study focuses on Monsoon Depressions (MD) in the Southwest Indian Ocean (SWIO) basin. A MD is a subcategory of tropical depressions. Some aspects of MD dynamical characteristics differ from those of the classical tropical depressions. A MD displays a broader horizontal extension (>1000 km), and its structure is marked by deep convection clusters that are generally poorly organized. These convective clusters often develop as a wide convection belt in the eastern semicircle of the system. The MDnucleus is characterized by an area of weak winds about 200 km wide and by a hazy central cloud system, surrounded by a larger belt of strong winds. In the SWIO basin, between 2000 and 2008, 5 systems, out of the 100 tropical cyclones that were followed by RSMC La Reunion, comply with these criteria and then have been identified as MD. Using potential vorticity from European Centre for Medium‐Range Weather Forecasts analyses as stratospheric tracer, stratospheric signatures have been identified in the troposphere for the five events, involving different dynamical mechanisms: tropopause fold between an anticyclone and MD, tropopause fold between MD and a jet front system, and cut‐off low. The dynamical structure of MD, especially the external belt of strong winds, seems to favor tropopause disturbances and leads to stratosphere-troposphere exchange.

An Assessment of SAPHIR Calibration Using Quality Tropical Soundings

AbstractThe Sondeur Atmospherique du Profil d’Humidite Intertropicale par Radiometrie (SAPHIR) instrument on board the Megha-Tropiques (MT) platform is a cross-track, multichannel microwave humidity sounder with six channels near the 183.31-GHz water vapor absorption line, a maximum scan angle of 42.96° (resulting in a maximum incidence angle of 50.7°), a 1700-km-wide swath, and a footprint resolution of 10 km at nadir. SAPHIR L1A2 brightness temperature (BT) observations have been compared to BTs simulated by the radiative transfer model (RTM) Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV-10), using in situ measurements from radiosondes as input. Selected radiosonde humidity observations from the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year (CINDY)–Dynamics of the Madden–Julian Oscillation (DYNAMO) campaign (September 2011–March 2012) were spatiotemporally collocated with MT overpasses. Although several...

Validation of Upper-Tropospheric Humidity from SAPHIR on board Megha-Tropiques Using Tropical Soundings

This paper describes the Upper Tropospheric Humidity (UTH) product derived from brightness temperature (BT) measurements of the SAPHIR radiometer onboard the Megha-Tropiques satellite. Under non-scattering conditions, the observations from three channels of SAPHIR, located at ±0.2, ±1.1 and ±2.8GHz respectively around the 183.31GHz strong water vapor absorption band, are interpreted into three different UTHs following a well-established method, thus describing the humidity content of the upper to mid troposphere. The evaluation of the UTHs is performed using reference UTHs defined from relative humidity profiles from radiosoundings of two field campaigns: the CINDY/DYNAMO/AMIE field experiment and a Megha-Tropiques dedicated campaign in Ouagadougou (Burkina Faso) during the summer 2012. A budget of the various uncertainties associated to each component of the methodology of evaluation (such as the radiometric sensitivity and the radiative transfer computations) was performed to achieve a more robust comparison between the two UTH estimates. The comparison between the reference UTHs and the SAPHIR UTHs reveals small global biases lower than 2%RH on average, with correlation coefficients between 0.86 and 0.89. Taking into account the individual uncertainties gives root-mean-square errors of regressions ranging between 0.92% and 4.71%. These three UTHs provide a vertical distribution of the relative humidity, which is suitable to study various temporal and spatial scales of the tropical variability. The signature of a mesoscale convective system on its environment is briefly presented to illustrate the capability of this new dataset.