Thesis of Simonne Guilbert
Soutenance de thèse
Amphithéâtre Pierre Glorieux
Comparison between POLDER / PARASOL and CERES / AQUA shortwave fluxes: improvement of POLDER / PARASOL shortwave fluxes
Abstract :
In the context of climate change, it is essential to estimate precisely and be able to monitor over time the energy balance of the Earth at the top of the atmosphere and at the surface.
In terms of measurement, obtaining a correct estimate of the radiative balance requires a precise determination of the shortwave (solar) and longwave (infrared) radiative fluxes. The objective of this thesis is to assess the solar radiative fluxes obtained from the French radiometer POLDER on board the PARASOL microsatellite supported by CNES. A first part of the thesis presents a comparison between the operational products computed from POLDER observations with the reference fluxes obtained through the broadband radiometers CERES on the NASA space platforms Aqua and Terra.
The comparisons are made over two periods: first a period with coincident measurements (2005-2009), then a second period which corresponds to the drift of the PARASOL satellite (2010-2013). We show that this drift had an impact on the observations, with strong repercussions on the calculated fluxes. Over the period of coincidence of the measurements, POLDER fluxes are very close to the fluxes from CERES for two of the products studied (CERES SSF1deg and CERES SYN1deg) with relative differences under 2% until December 2009. After 2010, the relative difference increases with the drift. A land/ocean compensation effect is also revealed.
The results obtained through these comparisons led us to study in detail the component of the algorithm used to obtain the monthly means of POLDER shortwave fluxes. This part of the algorithm is the diurnal extrapolation, used to estimate a value of albedo at all hours of the day from a single observation using models that are scene-dependent. The models used for the operational products were built using four months of POLDER-1 observations (1996-1997). We decided to take advantage of the data obtained throughout the entire PARASOL mission to improve these models. The shortwave fluxes obtained with the new models show less dependence on the drift over oceans but a drift is still present over lands. These results led to several propositions that could improve POLDER's shortwave fluxes, mainly by increasing the number of POLDER models.
This work, based on measurements from POLDER-3, which was stopped in December 2013, but whose data is available, will be largely reusable for the future multispectral, multi-angular and polarized radiometer 3MI, developed by ESA and EUMETSAT and which will fly onboard the EPS-SG mission supported by EUMETSAT from 2024, for approximately 20 years. Keywords : Radiative fluxes,Shortwave fluxes,Radiation Budget,POLDER,CERES
In terms of measurement, obtaining a correct estimate of the radiative balance requires a precise determination of the shortwave (solar) and longwave (infrared) radiative fluxes. The objective of this thesis is to assess the solar radiative fluxes obtained from the French radiometer POLDER on board the PARASOL microsatellite supported by CNES. A first part of the thesis presents a comparison between the operational products computed from POLDER observations with the reference fluxes obtained through the broadband radiometers CERES on the NASA space platforms Aqua and Terra.
The comparisons are made over two periods: first a period with coincident measurements (2005-2009), then a second period which corresponds to the drift of the PARASOL satellite (2010-2013). We show that this drift had an impact on the observations, with strong repercussions on the calculated fluxes. Over the period of coincidence of the measurements, POLDER fluxes are very close to the fluxes from CERES for two of the products studied (CERES SSF1deg and CERES SYN1deg) with relative differences under 2% until December 2009. After 2010, the relative difference increases with the drift. A land/ocean compensation effect is also revealed.
The results obtained through these comparisons led us to study in detail the component of the algorithm used to obtain the monthly means of POLDER shortwave fluxes. This part of the algorithm is the diurnal extrapolation, used to estimate a value of albedo at all hours of the day from a single observation using models that are scene-dependent. The models used for the operational products were built using four months of POLDER-1 observations (1996-1997). We decided to take advantage of the data obtained throughout the entire PARASOL mission to improve these models. The shortwave fluxes obtained with the new models show less dependence on the drift over oceans but a drift is still present over lands. These results led to several propositions that could improve POLDER's shortwave fluxes, mainly by increasing the number of POLDER models.
This work, based on measurements from POLDER-3, which was stopped in December 2013, but whose data is available, will be largely reusable for the future multispectral, multi-angular and polarized radiometer 3MI, developed by ESA and EUMETSAT and which will fly onboard the EPS-SG mission supported by EUMETSAT from 2024, for approximately 20 years. Keywords : Radiative fluxes,Shortwave fluxes,Radiation Budget,POLDER,CERES
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