Par l'intermediaire de leurs interactions multiples avec le rayonnement, les nuages ont un ef... more Par l'intermediaire de leurs interactions multiples avec le rayonnement, les nuages ont un effet preponderant sur la prevision du temps a long terme. Toutefois, dans les modeles climatiques, les nuages sont encore mal simules. L'evolution actuelle de la modelisation qui tend vers une representation plus realiste de leur contenu en eau liquide pose le probleme de sa repartition a l'interieur de la maille des modeles. A partir de donnees satellitaires (METEOSAT B2 (30 km de resolution) et LANDSAT-5 TM (30 m de resolution)), nous avons quantifie les influences respectives de la repartition horizontale de l'eau liquide et de la forme des nuages sur la determination du bilan radiatif du systeme terre-atmosphere. Lorsque la forme des nuages est negligee, leur impact sur le bilan radiatif est, en general, surestime dans le domaine solaire et sous-estime dans le domaine tellurique. En assimilant, de plus, les nuages a une couche plane parallele horizontalement homogene a l'echelle de la maille (tels qu'ils sont introduits dans les modeles climatiques) leur impact sur les bilans radiatifs solaire et tellurique est systematiquement surestime. Les deux effets induisent des biais consequents dans la determination du bilan radiatif solaire ; dans l'infrarouge thermique ils peuvent eventuellement se compenser. Dans le cas des nuages hauts (Cirrus), la principale difficulte pour le calcul du transfert radiatif vient moins de la forme des nuages que de la meconnaissance de leurs proprietes optiques et microphysiques. Dans la derniere partie de ce travail, nous avons cherche a developper une methode de determination de la taille des cristaux de glace a partir de donnees satellaires (AVHRR ; 1 km de resolution spatiale). Bien qu'insuffisante pour determiner leur forme et/ou leur taille, cette methode permet de fixer des limites a leur variabilite.
Bulletin of the American Meteorological Society, 2019
In the past two decades, more than 150 jet engine power-loss and damage events have been attribut... more In the past two decades, more than 150 jet engine power-loss and damage events have been attributed to a phenomenon known as ice crystal icing (ICI). Ingestion of large numbers of ice particles into the engine core are thought to be responsible for these events, which typically occur at high altitudes near large convective systems in tropical air masses. In recent years, scientists, engineers, aviation regulators, and airlines from around the world have collaborated to better understand the relevant meteorological processes associated with ICI events, solve critical engineering problems, develop new certification standards, and devise mitigation strategies for the aviation industry. One area of research is the development of nowcasting techniques based on available remote sensing technology and numerical weather prediction (NWP) models to identify areas of high ice water content (IWC) and enable the provision of alerts to the aviation industry. Multiple techniques have been develope...
. Through their multiple interactions with radiation, clouds have an important impact on the clim... more . Through their multiple interactions with radiation, clouds have an important impact on the climate. Nonetheless, the simulation of clouds in climate models is still coarse. The present evolution of modeling tends to a more realistic representation of the liquid water content; thus the problem of its subgrid scale distribution is crucial. For a convective cloud field observed during ICE 89, Landsat TM data (resolution: 30m) have been analyzed in order to quantify the respective influences of both the horizontal distribution of liquid water content and cloud shape on the Earth radiation budget. The cloud field was found to be rather well-represented by a stochastic distribution of hemi-ellipsoidal clouds whose horizontal aspect ratio is close to 2 and whose vertical aspect ratio decreases as the cloud cell area increases. For that particular cloud field, neglecting the influence of the cloud shape leads to an over-estimate of the outgoing longwave flux; in the shortwave, it leads to an over-estimate of the reflected flux for high solar elevations but strongly depends on cloud cell orientations for low elevations. On the other hand, neglecting the influence of cloud size distribution leads to systematic over-estimate of their impact on the shortwave radiation whereas the effect is close to zero in the thermal range. The overall effect of the heterogeneities is estimated to be of the order of 10 W m-2 for the conditions of that Landsat picture (solar zenith angle 65°, cloud cover 70%); it might reach 40 W m-2 for an overhead sun and overcast cloud conditions.
<p>Radiative Budget, essential to the monitoring of climate change, can be ... more <p>Radiative Budget, essential to the monitoring of climate change, can be investigated with ERB-dedicated instruments like the Clouds and the Earth Radiant Energy System (CERES) instrument (Wielicki, 1996). On the other side, non-dedicated instruments, such as POLDER-3/PARASOL measuring narrowband radiances, can also be used advantageously to obtain shortwave albedos and fluxes (Buriez et al, 2007; Viollier et al, 2002).</p><p>We present here a comparison between the shortwave fluxes and albedos derived from POLDER-3 and those derived from CERES flying aboard Aqua, chosen as a reference.</p><p>Monthly means of shortwave fluxes computed from the measurements of the two instruments are first set side by side. They show a good agreement in the all-sky case. However, after December 2009, the values from POLDER-3 display a slight drift which coincides with the lowering of the orbit of the PARASOL satellite and the modification of its overpass time in comparison to the other satellites of the A-Train mission. In clear sky situations, greater differences between POLDER and CERES shortwave fluxes are observed, especially over land regions, and the drift increases faster after 2009.</p><p>A second comparison is presented, between instantaneous albedos. For the period of coincident observations between POLDER-3 and CERES/Aqua, there is a good correlation between both products. This correlation deteriorates when the comparison is extended after 2009, as the values given by POLDER-3 increase. This result is expected, as the albedo is a function of the Solar Zenith Angle.</p><p>The slope of the increase of instantaneous albedo values is higher than for the diurnally extrapolated, monthly averaged shortwave fluxes. This tends to show that the POLDER algorithm leading to the monthly means of diurnal shortwave albedos moderates the increase of instantaneous shortwave albedo values but it doesn’t completely compensate for the effects of the drift of the instrument.</p><p> </p>
Par l'intermediaire de leurs interactions multiples avec le rayonnement, les nuages ont un ef... more Par l'intermediaire de leurs interactions multiples avec le rayonnement, les nuages ont un effet preponderant sur la prevision du temps a long terme. Toutefois, dans les modeles climatiques, les nuages sont encore mal simules. L'evolution actuelle de la modelisation qui tend vers une representation plus realiste de leur contenu en eau liquide pose le probleme de sa repartition a l'interieur de la maille des modeles. A partir de donnees satellitaires (METEOSAT B2 (30 km de resolution) et LANDSAT-5 TM (30 m de resolution)), nous avons quantifie les influences respectives de la repartition horizontale de l'eau liquide et de la forme des nuages sur la determination du bilan radiatif du systeme terre-atmosphere. Lorsque la forme des nuages est negligee, leur impact sur le bilan radiatif est, en general, surestime dans le domaine solaire et sous-estime dans le domaine tellurique. En assimilant, de plus, les nuages a une couche plane parallele horizontalement homogene a l'echelle de la maille (tels qu'ils sont introduits dans les modeles climatiques) leur impact sur les bilans radiatifs solaire et tellurique est systematiquement surestime. Les deux effets induisent des biais consequents dans la determination du bilan radiatif solaire ; dans l'infrarouge thermique ils peuvent eventuellement se compenser. Dans le cas des nuages hauts (Cirrus), la principale difficulte pour le calcul du transfert radiatif vient moins de la forme des nuages que de la meconnaissance de leurs proprietes optiques et microphysiques. Dans la derniere partie de ce travail, nous avons cherche a developper une methode de determination de la taille des cristaux de glace a partir de donnees satellaires (AVHRR ; 1 km de resolution spatiale). Bien qu'insuffisante pour determiner leur forme et/ou leur taille, cette methode permet de fixer des limites a leur variabilite.
Bulletin of the American Meteorological Society, 2019
In the past two decades, more than 150 jet engine power-loss and damage events have been attribut... more In the past two decades, more than 150 jet engine power-loss and damage events have been attributed to a phenomenon known as ice crystal icing (ICI). Ingestion of large numbers of ice particles into the engine core are thought to be responsible for these events, which typically occur at high altitudes near large convective systems in tropical air masses. In recent years, scientists, engineers, aviation regulators, and airlines from around the world have collaborated to better understand the relevant meteorological processes associated with ICI events, solve critical engineering problems, develop new certification standards, and devise mitigation strategies for the aviation industry. One area of research is the development of nowcasting techniques based on available remote sensing technology and numerical weather prediction (NWP) models to identify areas of high ice water content (IWC) and enable the provision of alerts to the aviation industry. Multiple techniques have been develope...
. Through their multiple interactions with radiation, clouds have an important impact on the clim... more . Through their multiple interactions with radiation, clouds have an important impact on the climate. Nonetheless, the simulation of clouds in climate models is still coarse. The present evolution of modeling tends to a more realistic representation of the liquid water content; thus the problem of its subgrid scale distribution is crucial. For a convective cloud field observed during ICE 89, Landsat TM data (resolution: 30m) have been analyzed in order to quantify the respective influences of both the horizontal distribution of liquid water content and cloud shape on the Earth radiation budget. The cloud field was found to be rather well-represented by a stochastic distribution of hemi-ellipsoidal clouds whose horizontal aspect ratio is close to 2 and whose vertical aspect ratio decreases as the cloud cell area increases. For that particular cloud field, neglecting the influence of the cloud shape leads to an over-estimate of the outgoing longwave flux; in the shortwave, it leads to an over-estimate of the reflected flux for high solar elevations but strongly depends on cloud cell orientations for low elevations. On the other hand, neglecting the influence of cloud size distribution leads to systematic over-estimate of their impact on the shortwave radiation whereas the effect is close to zero in the thermal range. The overall effect of the heterogeneities is estimated to be of the order of 10 W m-2 for the conditions of that Landsat picture (solar zenith angle 65°, cloud cover 70%); it might reach 40 W m-2 for an overhead sun and overcast cloud conditions.
<p>Radiative Budget, essential to the monitoring of climate change, can be ... more <p>Radiative Budget, essential to the monitoring of climate change, can be investigated with ERB-dedicated instruments like the Clouds and the Earth Radiant Energy System (CERES) instrument (Wielicki, 1996). On the other side, non-dedicated instruments, such as POLDER-3/PARASOL measuring narrowband radiances, can also be used advantageously to obtain shortwave albedos and fluxes (Buriez et al, 2007; Viollier et al, 2002).</p><p>We present here a comparison between the shortwave fluxes and albedos derived from POLDER-3 and those derived from CERES flying aboard Aqua, chosen as a reference.</p><p>Monthly means of shortwave fluxes computed from the measurements of the two instruments are first set side by side. They show a good agreement in the all-sky case. However, after December 2009, the values from POLDER-3 display a slight drift which coincides with the lowering of the orbit of the PARASOL satellite and the modification of its overpass time in comparison to the other satellites of the A-Train mission. In clear sky situations, greater differences between POLDER and CERES shortwave fluxes are observed, especially over land regions, and the drift increases faster after 2009.</p><p>A second comparison is presented, between instantaneous albedos. For the period of coincident observations between POLDER-3 and CERES/Aqua, there is a good correlation between both products. This correlation deteriorates when the comparison is extended after 2009, as the values given by POLDER-3 increase. This result is expected, as the albedo is a function of the Solar Zenith Angle.</p><p>The slope of the increase of instantaneous albedo values is higher than for the diurnally extrapolated, monthly averaged shortwave fluxes. This tends to show that the POLDER algorithm leading to the monthly means of diurnal shortwave albedos moderates the increase of instantaneous shortwave albedo values but it doesn’t completely compensate for the effects of the drift of the instrument.</p><p> </p>
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