Albedo response to a change in the precipitation regime in the Sahel region

By Alessio Lattanzio

The Sahel region is a transition belt between the Sahara desert in the north and the tropical savannas to the south, spanning from the Atlantic Ocean to the Red Sea. The population living in this region suffered from heavy droughts in the 1970s and the 1980s.

It is known that low rainfall and human activities, such as overgrazing and deforestation, and fires cause depletion of vegetation, which leads to a brighter surface (higher surface albedo) observable from space. As early as 1977, Charney et al. suggested a mechanism relating the increase in surface albedo to a decrease in precipitation.

This rather local effect is coupled with a complex large scale feedback mechanism (Figure 1), triggered by variations at interhemispheric scale of sea surface temperature that regulate the strength of the African Monsoon, which is essential for Sahel rainfall (Giannini, 2003; Zeng, 2003).

The baseline dataset for this study is the Meteosat Surface Albedo (MSA) Climate Data Record (CDR) (EUMETSAT, 2009), that describes the evolution of the surface albedo with a temporal sampling of about 10 days. These data, together with rainfall data, from the Global Precipitation Climatology Project (GPCP), were used to assess the impact of changes in rainfall on land surface albedo, as described by Govaerts and Lattanzio, 2008.

In addition, proxy data for vegetation growth were determined from the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), derived from Wide Field-of-view Sensor (SeaWiFS) observations.

Figure 2 shows the existence of a seasonal relationship between the three quantities, in particular, there is a one-month lag between rainfall and vegetation growth. The surface albedo follows the changes in surface vegetation coverage.

To demonstrate the magnitude of albedo response to large positive or negative anomalies in rainfall, two contrasting years were analysed. Time-series of the rainfall index during the period June to October over the Sahel region (Janowiak, 1988), as shown in Figure 3, reveal two years, 1984 and 2003, since the start of satellite observations, that exhibit a large difference in precipitation amounts — with 1984 being a very dry year and 2003 being a rather wet year.

The Meteosat derived land surface albedo during the period August, September, October of the dry year (1984) and the wet year (2003) were compared. Figure 4 shows the difference map of mean surface albedo. Differences due to the retrieval uncertainty have been removed in order to clearly highlight the real albedo ASO change between 1984 and 2003 (Govaerts and Lattanzio, 2007).

The map in Figure 4 highlights a generally lower surface albedo in 2003 compared to 1984. The absolute magnitude of the difference is not geographically uniform and varies over the Sahel belt. Govaerts and Lattanzio, 2008 concluded that regions particularly affected by the 1980s drought are essentially located into a narrow band of about 2° latitude along 16°N running from 18°W up to 20°E.

Within this geographical area, the mean broadband surface albedo decreases from 0.35±0.04 in 1984 to 0.27±0.03 in 2003, which corresponds to an absolute difference of 0.08±0.05, because of the lack of rainfall of 200 mm/year over the season in 1984.

Recently, EUMETSAT published a new Geostationary Surface Albedo (GSA) climate record (EUMETSAT, 2020a, 2020b, 2020c), which further improved the quality of the surface albedo retrievals, as compared to the first climate record (EUMETSAT, 2009) used in this study.

The major improvement originates from the usage of a cloud mask significantly reducing errors due to undetected clouds in the first version of the climate record. In addition, the new data record spans a much longer period that enables further studies of the African climate.

Benefits

This study shows the key role that observations and retrieval of geophysical parameters from geostationary satellites might have in climate studies. In particular in regions such as the Sahel, and in general in a belt of 30 degrees north and south of the equator, where the number of measurements from polar satellite instruments is often too low to address variations of parameters like the land surface albedo.

Data used

The table below shows the data used for this study:

References

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