Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses |
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Authors: | Soline Bielli Remy Roca |
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Institution: | 1. Canadian Network for Regional Climate Modelling and Diagnostics, Université du Québec à Montréal @OURANOS, 550 Rue Sherbrooke Ouest, 19e, Montreal, QC, H3A-1B9, Canada 2. Laboratoire de Météorologie Dynamique, Tour 45-55, 3eme étage, Case Postale 99, 4 place Jussieu, 75252, Paris Cedex 05, France
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Abstract: | NCEP/GFS analysis is used to investigate the scale dependence and the interplay between the terms of the atmospheric water
budget over West Africa using a dedicated decomposition methodology. The focus is on a 2-month period within the active monsoon
period of 2006. Results show that the dominant scales of seasonal mean precipitation and moisture flux divergence over West
Africa during the monsoon period are large scales (greater than 1,400 km) except over topography, where mean values of small
scales (smaller than 900 km) are strong. Correlations between moisture flux divergences in monsoon and African Easterly Jet
layers and precipitation indicate that precipitation is strongly correlated to moisture flux divergence via both large-scale
and small-scale processes, but the correlation signal is quite different depending on the region and vertical layer considered.
The analysis of the scales associated with the rainfall and the local evaporation over 3 different regions shows that positive
correlation exists over the ocean between precipitation and evaporation especially at large scale. Over the continent south
of the Sahel, the correlation is negative and driven by large scale. Over the northern part of Sahel, positive correlation
is found, only at small scales during the active monsoon period. Lag correlation reveals that the maximum evaporation over
the Sahel occurs 1–3 days after the maximum precipitation with maximum contribution from small-scale processes during the
first day. This study shows that NCEP/GFS reproduces well the known atmospheric water budget features. It also reveals a new
scale dependence of the relative role of each term of the atmospheric water budget. This indicates that such scale decomposition
approach is helpful to clarify the functioning of the water cycle embedded in the monsoon system. |
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