Rémy Roca

Evidence for control of Atlantic subtropical humidity by large scale advection

The interplay between large scale dynamics and tropospheric moisture is investigated. A simple conceptual model of the sources and sinks of humidity is used to reconstruct, using a backward Lagrangian trajectory technique, the water vapor distribution in the tropical and subtropical free troposphere. Satellite data in the water vapor channel from both Meteosat-3 and Meteosat-4 satellites are then used to validate the model following a model-to-satellite approach over the whole Atlantic ocean. There is excellent agreement between simulations and observations in the drier regions, but the simulated brightness temperature exhibits a warm bias within and near moist, convective regions. This bias is most probably due to the neglect of cloud effects in reconstructing the simulated brightness temperature, rather than to a dry bias in the simulation. A second advective simulation, performed with monthly mean rather than full transient winds, led to a substantially drier subtropics. This calculation demonstrates the importance of synoptic scale transient eddies in determining the humidity of the subtropical dry zones. It is speculated on this basis that discontinuous changes in synoptic eddy activity could provide a mechanism for rapid global climate changes.

An intercomparison of 10-day satellite precipitation products during West African monsoon

In the frame of the African Monsoon Multidisciplinary Analyses (AMMA) programme, a specific rainfall algorithm (EPSAT-SG; Estimation of Precipitation by SATellites – Second Generation) was developed for the requirements of the scientific community and an intercomparison exercise was undertaken to assess the performance of various rainfall analyses to help users of satellite precipitation estimates to take into consideration the limitations of these products. The intercomparison exercise presented in this article includes three regional precipitation products as well as seven operational global products that are publicly available and easily accessible on websites. This study has been performed using validation data from rain gauge observations analyses on the Sahelian region provided by the AGRHYMET centre, for three rainy seasons from 2004 to 2006. The 10 different satellite-based precipitation products are verified against the same reference ground-based dataset of 10-day rainfall accumulations at the 0.5° × 0.5° latitude–longitude resolution. The performance of the different precipitation algorithms is assessed according to various indicators such as the behaviour of the precipitation distributions, several statistical parameters and spatial distribution of the errors. All the statistical results indicate that the three ‘near-real-time’ products (3B42-RT, CPC MORPHing technique (CMORPH) and Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN)) have a poorer performance than the other products considered for intercomparison. In fact these algorithms cannot make use of useful inputs such as rain gauge observations that are not available at near real time. It is noted that the simple basic Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI) product performs better, with a higher skill score index. The three products Global Precipitation Climatology Project (GPCP)-1dd, Global Satellite Mapping of Precipitation (GSMaP)-MVK and Tropical Rainfall Measuring Mission (TRMM)-3B42 obtain better statistical results but the best results are obtained by the precipitation products created specifically for this African region. The EPSAT-SG product has the best performance according to several statistical criteria including skill score, coefficient of determination and root mean square (RMS) error whereas the Rain Fall Estimation (RFE)-2.0 estimates offer the best match with validation estimates in term of distribution and bias. The Tropical Applications of Meteorology using SATellite and other data (TAMSAT) estimates have also similar statistically good results.

An Algorithm for the Detection and Tracking of Tropical Mesoscale Convective Systems Using Infrared Images From Geostationary Satellite

This paper focuses on the tracking of mesoscale convective systems (MCS) from geostationary satellite infrared data in the tropical regions. In the past, several automatic tracking algorithms have been elaborated to tackle this problem. However, these techniques suffer from limitations in describing convection at the “true” scale and in depicting coherent MCS life cycles (split and merge artifacts). To overcome these issues, a new algorithm called Tracking Of Organized Convection Algorithm through a 3-D segmentatioN has been developed and is presented in this paper. This method operates in a time sequence of infrared images to identify and track MCS and is based on an iterative process of 3-D segmentation of the volume of infrared images. The objective of the new tracking algorithm is to associate the convective core of an MCS to its anvil cloud in the spatiotemporal domain. The technique is applied on various case studies over West Africa, Bay of Bengal, and South America. The efficiency of the new algorithm is established from an analysis of the case studies and via a statistical analysis showing that the cold cloud shield defined by a 235-K threshold in the spatiotemporal domain is decomposed into realistic MCSs. In comparison with an overlap-based tracking algorithm, the analysis reveals that MCSs are detected earlier in life cycle and later in their dissipation stages. Moreover, MCSs identified are not anymore affected by split and merge events along their life cycles, allowing a better characterization of their morphological parameters along their life cycles.

Direct comparison of meteosat water vapor channel data and general circulation model results

Following a model to satellite approach, this study points out the ability of the general circulation model (GCM) of the Laboratoire de Meteorologie Dynamique to reproduce the observed relationship between tropical convection and subtropical moisture in the upper troposphere. Those parameters are characterized from Meteosat water vapor equivalent brightness temperatures (WVEBT) over a monthly scale. The simulated WVEBT field closely resembles to the observed distribution. The pure water vapor features and the convective areas are well located and their seasonal variations are captured by the model. A dry (moist) bias is found over convective (subsiding) areas, whereas the model globally best acts over Atlantic ocean than over Africa. The observed and simulated seasonal variations show that an extension of the ITCZ is correlated to a moistening of the upper troposphere in subtropical areas. Those results imply a positive large scale relationship between convective and subsiding areas in both observation and simulation, and suggest the relevance of our approach for further climatic studies.

A study of the free tropospheric humidity interannual variability using meteosat data and an advection-condensation transport model.

Water vapor in the midtroposphere is an important element for the earth radiation budget. Despite its importance, the relative humidity in the free troposphere is not very well documented, mainly because of the difficulties associated with its measurements. A new long-term archive of free tropospheric humidity (FTH) derived from the water vapor channel of the Meteosat satellite from 1983 to 2005 is introduced. Special attention is dedicated to the long-term homogeneity and the definition of the retrieval layer. It is shown to complement the existing databases and is used to establish the climatology of FTH. Interannual variability is then evaluated for each season by using a normalized interannual standard deviation. This normalization approach reveals the importance of the relative variability of the dry areas to the moist regions. In consequence, emphasis is on the driest area of the region. Focusing on composites of the moist and dry seasons of the time series, the authors demonstrate that the 500-hPa relative humidity field, reconstructed using an idealized Lagrangian model, is a good proxy for the FTH variability there. The analysis of the origin of the air mass, using the back trajectory model, points out that lateral mixing between the deep tropics and extratropical latitudes takes place over this area, as advocated in previous theoretical studies. Systematic estimation of this large-scale mixing shows that, indeed, a significant part of the interannual variability of the free tropospheric humidity in this subtropical region stems from the amount of mixing of air originating from the deep tropics versus extratropical latitudes. The importance of this mechanism in the general understanding of the FTH distribution and variability is then discussed.

Evaluation of the distribution of subtropical free tropospheric humidity in AMIP‐2 simulations using METEOSAT water vapor channel data

In the framework of the Atmospheric Model Intercomparison Project (AMIP) phase 2, we have established a diagnostic of the free tropospheric humidity (FTH) distribution using METEOSAT data over the 1984– 1995 period for 14 climate models. The methodology of evaluation follows a two step ‘‘model-to-satellite’’ approach. First the raw METEOSAT ‘‘Water Vapor’’ radiances are simulated from the model profiles of temperature and humidity using the RTTOV-7 radiative transfer model. Second, the radiances are converted into FTH using the same coefficients as in the satellite product offering a direct comparison. The analysis is focused on the dry subtropical areas observed by METEOSAT: the Eastern Mediterranean and the tropical South Atlantic Ocean. Most of the models reproduce the observed seasonal cycle both in terms of phasing and magnitude, despite an overall moist bias. A few models are in close agreement with the satellite data. The magnitude of the satellite estimated inter-annual variability is also generally captured by models. Again, a small subset of models shows close agreement with the observations. This comparison suggests general improvements of the models with respect to the AMIP-1 simulations.

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.

A clear‐sky radiance archive from Meteosat “water vapor” observations

A long-term archive of clear-sky Meteosat “water vapor” observations, covering the July 1983 to February 1997 period with a 3 hourly time step and a spatial resolution of 0.625°, is presented. Cloud clearing is performed using a scene selection procedure based on the International Satellite Cloud Climatology Project DX product. In this procedure low cloud scenes are kept because of the negligible contribution of the low atmospheric layer in this spectral band. Cloud contamination is shown to have little influence on the clear-sky radiance (CSR) field and is mainly confined to the continental Intertropical Convergence Zone with values less than 0.5 K. This scene selection yields to a significantly enhanced sampling with respect to pure clear-sky in the subtropical high regions. Homogenization of the 14 year database is performed in accordance with existing technique. A comparison to the operational radiosondes archive indicates a small bias of 0.3 K that is stable throughout the period. A first analysis of the CSR variability reveals that the intraseasonal variance over the subtropical dry regions has a strong seasonal cycle in the Northern Hemisphere that is not observed in the Southern Hemisphere. Such a data set completes the ones currently available to document the water vapor variability of the troposphere from climatic down to regional and daily scales.

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...