Analysis of the in situ and MODIS albedo variability at multiple timescales in the Sahel

Abstract

[1] The variability of the Sahelian albedo is investigated through the combined analysis of 5 years of in situ radiation data from the African Monsoon Multidisciplinary Analysis northernmost sites and remotely sensed albedo from 7 years of Moderate Resolution Imaging Spectroradiometer data. Both data sets are found to be in good agreement in terms of correlation and bias. The drivers of albedo variability are identified by means of in situ measurements of biological and physical properties of the land surface collected over a network of 29 long-term survey sites. Short-term variability is dominated by changes in the spectral composition of incident radiation, which reflects aerosol optical depth and integrated water content, and changes in soil moisture, which have a short-lived effect (1 d). Bush fires cause a marked decrease of albedo of the order of 10 d, whereas a dry season storm event is suspected to have increased albedo through litter and soil surface abrasion. Seasonal plant growth causes the largest changes in rainy season albedo, and displays a large interannual variability: Because of the 2004 drought, albedo increases steadily from late 2003 to early 2005 at latitude 15°N. Grazing pressure is found to impact albedo mostly in the dry season. Dry season albedo is controlled by the amount of litter and standing dead phytomass hiding the bright soils. Thus rainfall anomalies have a direct effect on albedo through plant growth but also a lagged effect caused by above normal amounts of dry phytomass that can persist until the arrival of the next monsoon. EOF analysis and Hovmüller diagrams show these effects to be present on a large scale.

Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling

“…This value is consistent with recent post-fire albedo found in Africa by Govaerts et al (2002). Also, the ten-day removal period is corroborated by the results presented in the previously mentioned work by Samain et al (2008).…”

“…In addition to altering the vegetation, soil reflectance was lowered based on the same amount of burned area fraction and survival rate to simulate the effect of ash and charcoal depositions associated with wildfires (Govaerts et al 2002;Samain et al 2008). Ground darkening was maintained for a certain period, after which the soil albedo was returned to its normal (unburned) values to mimic the action of char removal by wind and precipitation.…”

“…Through a combined analysis of 5-year in situ radiation data and remotely sensed albedo, Samain et al (2008) showed that wildfires caused a marked decreased of surface albedo of about 10 days in the Sahel. They determined that the reduction of ground reflectance is mainly due to accumulation of charred material that is quickly dispersed by wind.…”

“…The impact of burned areas on the surface albedo has been investigated by several observational, remote-sensing and modeling studies (e.g., Govaerts et al 2002;Jin and Roy 2005;Myhre et al 2005;Samain et al 2008;Lyons et al 2008;Bowman et al 2009). Most of these studies have indicated that wildfires and resulting burned areas tend to decrease surface albedo at short time scales due to the deposition and ash and charcoal (Govaerts et al 2002;Myhre et al 2005).…”

The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes

“…If the albedo of the soil surface is significantly different from above-canopy albedo, for example, any changes in vegetation cover that may expose the soil can be important (Berbet & Costa, 2003). Rainfall events can also reduce the magnitude of above-canopy albedo by as much as 50% of its dry-peak, and are more persistent if the downpour occurs in the morning than in the afternoon, because the soil surface has time to drain and dry during the night (Samain et al, 2008). Surface albedo is also controlled by the intrinsic structural and optical properties of vegetated canopies, e.g.…”