A Simple Model of the Life Cycle of Mesoscale Convective Systems Cloud Shield in the Tropics

International audienceMesoscale convective systems (MCSs) are important to the water and energy budget of the tropical climate and are essential ingredients of the tropical circulation. MCSs are readily observed in satellite infrared geostationary imagery as cloud clusters that evolve in time from small structures to well-organized large patches of cloud shield before dissipating. The MCS cloud shield is the result of a large ensemble of mesoscale dynamical, thermodynamical, and microphysical processes. This study shows that a simple parametric model can summarize the time evolution of the morphological characteristics of the cloud shield during the life cycle of the MCS. It consists of a growth-decay linear model of the cloud shield and is based on three parameters: the time of maximum extent, the maximum extent, and the duration of the MCS. It is shown that the time of maximum is frequently close to the middle of the life cycle and that the correlation between maximum extent and duration is strong all over the tropics. This suggests that 1 degree of freedom is left to summarize the life cycle of the MCS cloud shield. Such a model fits the observed MCS equally well, independent of the duration, size, location, and propagation characteristics, and its relevance is assessed for a large number of MCSs over three boreal summer periods over the whole tropical belt. The scaling of this simple model exhibits weak (strong) regional variability for the short-(long-) lived systems indicative of the primary importance of the internal dynamics of the systems to the large-scale environment for MCS sustainability

Moisture environment of a mesoscale cloud system: a case study from the 2006 AMMA campaign

International audienceWater vapor of the free troposphere is one of the dominant greenhouse gases in the atmosphere and is part of a strong feedback on climate change. One of the main processes that govern the distribution of water vapor in the tropical belt is deep convection that redistributes the moisture in the upper levels. The processes at play are nevertheless not well understood. In particular the relative role of the physics (convective detrainment) versus the microphysics (evaporation of cloud condensate) in building the moisture distribution in the free troposphere of the ITCZ deserves inquiry. Two channels onboard the Meteosat Second Generation (MSG) satellites were designed to observed the water vapor of two overlapping layers of the free troposphere: in the upper levels (~500-200 hPa) with channel 5 (6.2µm) in the mid-levels (~700-300 hPa) with channel 6 (7.3µm). Under clear sky conditions or in the case of low-level cloudiness, the measured brightness temperatures are interpreted in terms of vertically integrated humidities "Upper Tropospheric Humidity" and "Free Tropospheric Humidity" (channels 5 and 6 resp.). MSG observations also provide documentation on the cloudiness through a cloud classification (upper, mid and low level clouds) developed by the meteorological center of Lannion and a mesoscale convective systems (MCS) tracking algorithm developed at LMD. These data have been retrieved during the Special Observing Period of AMMA at the MSG nominal resolution (3km at nadir, 15min), thus allowing to study the links between UTH/FTH and the cloudiness at the scale of the convective systems. The analyses are performed over the Niamey area (Niger), during the active phase of the 2006 monsoon (July 24th - 30th), and reveal a strong link between the variations of humidity (both UTH and FTH) and the frequency of high clouds, the maximum being reached for a lag of 6 to 7h, thus suggesting the role of detrainment from those clouds in the humidification of the free troposphere. Other analyses concerning a specific convective system observed near Djougou (Benin) on July 28th and the distribution of humidity in its close environment will be shown

Moisture environment of a mesoscale cloud system: a case study from the 2006 AMMA campaign

International audienceWater vapor of the free troposphere is one of the dominant greenhouse gases in the atmosphere and is part of a strong feedback on climate change. One of the main processes that govern the distribution of water vapor in the tropical belt is deep convection that redistributes the moisture in the upper levels. The processes at play are nevertheless not well understood. In particular the relative role of the physics (convective detrainment) versus the microphysics (evaporation of cloud condensate) in building the moisture distribution in the free troposphere of the ITCZ deserves inquiry. Two channels onboard the Meteosat Second Generation (MSG) satellites were designed to observed the water vapor of two overlapping layers of the free troposphere: in the upper levels (~500-200 hPa) with channel 5 (6.2µm) in the mid-levels (~700-300 hPa) with channel 6 (7.3µm). Under clear sky conditions or in the case of low-level cloudiness, the measured brightness temperatures are interpreted in terms of vertically integrated humidities "Upper Tropospheric Humidity" and "Free Tropospheric Humidity" (channels 5 and 6 resp.). MSG observations also provide documentation on the cloudiness through a cloud classification (upper, mid and low level clouds) developed by the meteorological center of Lannion and a mesoscale convective systems (MCS) tracking algorithm developed at LMD. These data have been retrieved during the Special Observing Period of AMMA at the MSG nominal resolution (3km at nadir, 15min), thus allowing to study the links between UTH/FTH and the cloudiness at the scale of the convective systems. The analyses are performed over the Niamey area (Niger), during the active phase of the 2006 monsoon (July 24th - 30th), and reveal a strong link between the variations of humidity (both UTH and FTH) and the frequency of high clouds, the maximum being reached for a lag of 6 to 7h, thus suggesting the role of detrainment from those clouds in the humidification of the free troposphere. Other analyses concerning a specific convective system observed near Djougou (Benin) on July 28th and the distribution of humidity in its close environment will be shown

Moisture environment of a mesoscale cloud system: a case study from the 2006 AMMA campaign

International audienceWater vapor of the free troposphere is one of the dominant greenhouse gases in the atmosphere and is part of a strong feedback on climate change. One of the main processes that govern the distribution of water vapor in the tropical belt is deep convection that redistributes the moisture in the upper levels. The processes at play are nevertheless not well understood. In particular the relative role of the physics (convective detrainment) versus the microphysics (evaporation of cloud condensate) in building the moisture distribution in the free troposphere of the ITCZ deserves inquiry. Two channels onboard the Meteosat Second Generation (MSG) satellites were designed to observed the water vapor of two overlapping layers of the free troposphere: in the upper levels (~500-200 hPa) with channel 5 (6.2µm) in the mid-levels (~700-300 hPa) with channel 6 (7.3µm). Under clear sky conditions or in the case of low-level cloudiness, the measured brightness temperatures are interpreted in terms of vertically integrated humidities "Upper Tropospheric Humidity" and "Free Tropospheric Humidity" (channels 5 and 6 resp.). MSG observations also provide documentation on the cloudiness through a cloud classification (upper, mid and low level clouds) developed by the meteorological center of Lannion and a mesoscale convective systems (MCS) tracking algorithm developed at LMD. These data have been retrieved during the Special Observing Period of AMMA at the MSG nominal resolution (3km at nadir, 15min), thus allowing to study the links between UTH/FTH and the cloudiness at the scale of the convective systems. The analyses are performed over the Niamey area (Niger), during the active phase of the 2006 monsoon (July 24th - 30th), and reveal a strong link between the variations of humidity (both UTH and FTH) and the frequency of high clouds, the maximum being reached for a lag of 6 to 7h, thus suggesting the role of detrainment from those clouds in the humidification of the free troposphere. Other analyses concerning a specific convective system observed near Djougou (Benin) on July 28th and the distribution of humidity in its close environment will be shown

Moisture environment of a mesoscale cloud system: a case study from the 2006 AMMA campaign

International audienceWater vapor of the free troposphere is one of the dominant greenhouse gases in the atmosphere and is part of a strong feedback on climate change. One of the main processes that govern the distribution of water vapor in the tropical belt is deep convection that redistributes the moisture in the upper levels. The processes at play are nevertheless not well understood. In particular the relative role of the physics (convective detrainment) versus the microphysics (evaporation of cloud condensate) in building the moisture distribution in the free troposphere of the ITCZ deserves inquiry. Two channels onboard the Meteosat Second Generation (MSG) satellites were designed to observed the water vapor of two overlapping layers of the free troposphere: in the upper levels (~500-200 hPa) with channel 5 (6.2µm) in the mid-levels (~700-300 hPa) with channel 6 (7.3µm). Under clear sky conditions or in the case of low-level cloudiness, the measured brightness temperatures are interpreted in terms of vertically integrated humidities "Upper Tropospheric Humidity" and "Free Tropospheric Humidity" (channels 5 and 6 resp.). MSG observations also provide documentation on the cloudiness through a cloud classification (upper, mid and low level clouds) developed by the meteorological center of Lannion and a mesoscale convective systems (MCS) tracking algorithm developed at LMD. These data have been retrieved during the Special Observing Period of AMMA at the MSG nominal resolution (3km at nadir, 15min), thus allowing to study the links between UTH/FTH and the cloudiness at the scale of the convective systems. The analyses are performed over the Niamey area (Niger), during the active phase of the 2006 monsoon (July 24th - 30th), and reveal a strong link between the variations of humidity (both UTH and FTH) and the frequency of high clouds, the maximum being reached for a lag of 6 to 7h, thus suggesting the role of detrainment from those clouds in the humidification of the free troposphere. Other analyses concerning a specific convective system observed near Djougou (Benin) on July 28th and the distribution of humidity in its close environment will be shown

Robust observational quantification of the contribution of mesoscale convective systems to rainfall in the tropics

International audienceSatellite 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. The study focuses on two main mesoscale characteristics of the systems: duration and propagation. Overall, the systems lasting more than 12h are shown to account for around 75% of the tropical rainfall. 60% of the rainfall is due to systems traveling more than 250km, 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, our analysis provides a simple metric for evaluating high resolution numerical simulations of the tropical water budget. Furthermore, our results are shown to be robust to the selection of the satellite rainfall products

Summertime Climatology of Mesoscale Convective Systems over West Africa from 24-years of METEOSAT observations

International audienceThe West African monsoon hydrological cycle and heat budget strongly depends on the Mesoscale Convective Systems (MCS). The analysis of the morphology of these tropical convective systems has received much attention in the past decades mainly thanks to the advent of geostationary infrared data. A number of definitions have been proposed yielding to a complex corpus of knowledge. In this context, a unique climatology of Mesoscale Convective Systems has been computed from the Meteosat IR observations, over tropical Africa, during the summer monsoon from 1983 to 2006 based on a simple definition of the MCS. Using a brightness temperature threshold at 233°K, cold cloud clusters are detected every 30 minutes. Then, an overlap technique is used on these segmented images to determine the life cycles of all the clusters. This tracking algorithm computes morphological parameters of the cloud clusters like duration, velocity, size of the cold cloud shield, local time of genesis, distribution of brightness temperature, cumulated area … A simple classification of the clusters is then developed based on the duration of the systems and their propagation speed. Four classes of Convective Systems are formed using threshold selected on a physical basis of 9 hour and 10 m/s. The climatological features of these 4 classes of systems are shown. In order to investigate the relationship between the convective systems variability and the seasonal rainfall, MCS distributions are then related to the rainfall thanks to the use of the Global Precipitation Climatology Project estimates. The analysis reveals that a linear combination of the occurrence of each class of systems can explain a significant part of the rainfall interannual variability over most of West Africa

Summertime Climatology of Mesoscale Convective Systems over West Africa from 24-years of METEOSAT observations

International audienceThe West African monsoon hydrological cycle and heat budget strongly depends on the Mesoscale Convective Systems (MCS). The analysis of the morphology of these tropical convective systems has received much attention in the past decades mainly thanks to the advent of geostationary infrared data. A number of definitions have been proposed yielding to a complex corpus of knowledge. In this context, a unique climatology of Mesoscale Convective Systems has been computed from the Meteosat IR observations, over tropical Africa, during the summer monsoon from 1983 to 2006 based on a simple definition of the MCS. Using a brightness temperature threshold at 233°K, cold cloud clusters are detected every 30 minutes. Then, an overlap technique is used on these segmented images to determine the life cycles of all the clusters. This tracking algorithm computes morphological parameters of the cloud clusters like duration, velocity, size of the cold cloud shield, local time of genesis, distribution of brightness temperature, cumulated area … A simple classification of the clusters is then developed based on the duration of the systems and their propagation speed. Four classes of Convective Systems are formed using threshold selected on a physical basis of 9 hour and 10 m/s. The climatological features of these 4 classes of systems are shown. In order to investigate the relationship between the convective systems variability and the seasonal rainfall, MCS distributions are then related to the rainfall thanks to the use of the Global Precipitation Climatology Project estimates. The analysis reveals that a linear combination of the occurrence of each class of systems can explain a significant part of the rainfall interannual variability over most of West Africa

Summertime Climatology of Mesoscale Convective Systems over West Africa from 24-years of METEOSAT observations

International audienceThe West African monsoon hydrological cycle and heat budget strongly depends on the Mesoscale Convective Systems (MCS). The analysis of the morphology of these tropical convective systems has received much attention in the past decades mainly thanks to the advent of geostationary infrared data. A number of definitions have been proposed yielding to a complex corpus of knowledge. In this context, a unique climatology of Mesoscale Convective Systems has been computed from the Meteosat IR observations, over tropical Africa, during the summer monsoon from 1983 to 2006 based on a simple definition of the MCS. Using a brightness temperature threshold at 233°K, cold cloud clusters are detected every 30 minutes. Then, an overlap technique is used on these segmented images to determine the life cycles of all the clusters. This tracking algorithm computes morphological parameters of the cloud clusters like duration, velocity, size of the cold cloud shield, local time of genesis, distribution of brightness temperature, cumulated area … A simple classification of the clusters is then developed based on the duration of the systems and their propagation speed. Four classes of Convective Systems are formed using threshold selected on a physical basis of 9 hour and 10 m/s. The climatological features of these 4 classes of systems are shown. In order to investigate the relationship between the convective systems variability and the seasonal rainfall, MCS distributions are then related to the rainfall thanks to the use of the Global Precipitation Climatology Project estimates. The analysis reveals that a linear combination of the occurrence of each class of systems can explain a significant part of the rainfall interannual variability over most of West Africa