Estimation of surface albedo increase during the eighties Sahel drought from Meteosat observations
Introduction
The Sahel is a narrow band in West Africa, between the Sahara desert at 18°N and Savannah (grass and open forest) and equatorial forest at about 15°N. The devastating drought in that region during the 70s and the 80s is among the most undisputed and largest recent climate event recognized by the research community. This dramatic climate event has generated numerous sensitivity analyses on land-atmosphere feedback mechanisms with contradicting conclusions on surface albedo response to precipitation changes. Although the potential of space-based observations to derive surface albedo maps has long been recognized, it is routinely retrieved only since 2001 from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and the Multiangle Imaging SpectroRadiometer (MISR) onboard the Terra platform (Jin et al., 2003, Martonchik et al., 1998). Consequently, long-term inter-annual surface albedo variability is still poorly quantified. This situation results in part from the scarceness of space instruments dedicated to land surface observations before the late 1990s, when systematic space-borne observations of the land surface were essentially limited to data acquired by geostationary meteorological satellites and a few polar platforms.
Pinty et al. (2000b) recently demonstrated the potential of geostationary satellites to derive reliable surface albedo maps. The high temporal sampling of geostationary satellites allows to account for both the atmospheric scattering effects and the anisotropy of the surface reflectance when data are accumulated during the course of the day (Pinty et al., 2000a). This novel approach opens new avenues in the exploitation of geostationary satellite observations for climate studies since their corresponding archives often cover two decades or more.
Additionally, dedicated efforts have been undertaken to increase the reliability of the calibration of the archived Meteosat data, the European operational geostationary satellites, which observe the African continent every half an hour continuously since 1983 (Govaerts et al., 2004). These improvements have permitted the generation of reliable and temporally consistent long-time series of climate data record of surface albedo.
This study takes thus advantage of the generation of this climate data record to quantify the impact of Sahel drought on surface albedo changes. In particular, it has been possible to compare surface albedo seasonal variations in 1984, the driest year of the 80s, with current values, i.e., year 2003, corresponding to normal precipitation conditions. The objective of this paper is thus to quantify surface albedo changes resulting from precipitation deficit in the Sahel region, comparing surface albedo derived from these two specific years. Section (2) of this paper illustrates the impact of precipitation seasonal cycle over the Sahel region on vegetation growth and senescence and how surface albedo is affected by these changes. Processes that might affect these seasonal cycles on an inter-annual basis are discussed in Section (3). Global tropical ocean circulation and temperature seem to play an important role in determining the importance of the Sahel precipitation during the rainy season. Past efforts to quantify the magnitude of albedo response to precipitation changes are reviewed in Section (4) and reveal a large range of different values. Section (5) describes the surface albedo data set that has been generated to quantify these changes. Results, displayed in Section (6), show detailed information on the geographical extension and magnitude of the surface albedo increase resulting from the 80s drought.
Observed surface albedo changes exhibit geographical variations that might explain the wide range of different values found in the literature.
Section snippets
Relationships between precipitation, vegetation and surface albedo
Climate and atmospheric dynamic in the Sahel region are determined by the west-African monsoon resulting from the alternation of the southwesterly and northeasterly winds, which corresponds to a North-South migration of the Intertropical Convergence Zone (ITCZ). During the northward shift of the ITCZ, rainfalls occur over Sahel during a short but intense wet season, ranging from late June to mid September, which induces fast vegetation growth. The onset of the dry season that follows the rainy
The '70s and '80s Sahel drought mechanism
Sahel has been affected by dramatic rainfall deficit from the late 1960s to the late 1980s. Fig. (2) shows the June–October precipitation relative difference between 1984 and 2003 derived from GPCP data set. Charney et al. (1977) proposed an explanation for these precipitation shortages. According to these authors, precipitation deficit results from a vegetation removal by overgrazing, which, in turn, leads to an increase in surface albedo and a decrease in precipitation as the surface becomes
Magnitude of albedo response to precipitation changes
The magnitude of the albedo response to precipitation changes remains largely unknown. Attempts to analyse these responses can be categorized into three types of approaches. The first one, essentially based on Charney's hypothesis, consists in sensitivity analysis with prescribed surface albedo change. In their famous paper, Charney et al. (1977) used a drastic albedo change of about 0.2, i.e., from 0.14 to 0.35. With a very simple general circulation model, these authors demonstrated that such
Deriving surface albedo from Meteosat observations
Operational meteorological satellites are playing an increasing role to monitor key climate variables thanks to the duration of past and future missions. This has been recognised by the Global Climate Observing System (GCOS) who established a list of Essential Climate Variables (ECV) that characterize the state of the global climate system and its evolution resulting from natural and anthropogenic forcing (GCOS, 2003, GCOS, 2006). These ECVs, covering atmospheric, oceanic and terrestrial
Results
In order to assess surface albedo response to precipitation change in the Sahel, albedos corresponding to a dry (1984) and a wet year (2003) have been compared. In Section (5), we saw that the surface albedo derived from different spacecraft might be subject to uncertainty resulting from radiometric limitation of the spacecraft radiometer designed more than 30 years ago. Hence, to remove any possible remaining biases between the data set derived from Meteosat-2 and -7 observations, the mean
Conclusion
This study took advantages of recent algorithmic improvements and dedicated calibration efforts which have allowed a better quantitatively analyses of the archived Meteosat data. It represents a comprehensive attempt to quantify surface albedo changes in the Sahel region resulting from the 80s drought. It has been shown that the deficit in JJASO precipitation that occurred between 12°N and 18°N translates into an increase in broadband surface albedo of about 0.06 with a mean zonal difference
Acknowledgment
The authors would like to thank Dr. A. Giannini for fruitful comments on this study.
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