Abstract
A prequel study showed that dynamic downscaling using a regional climate model (RCM) over Africa improved the Goddard Institute for Space Studies Atmosphere-Ocean Global Climate Model (GISS AOGCM: ModelE) simulation of June–September rainfall patterns over Africa. The current study applies bias corrections to the lateral and lower boundary data from the AOGCM driving the RCM, based on the comparison of a 30-year simulation to the actual climate. The analysis examines the horizontal pattern of June–September total accumulated precipitation, the time versus latitude evolution of zonal mean West Africa (WA) precipitation (showing monsoon onset timing), and the latitude versus altitude cross-section of zonal winds over WA (showing the African Easterly Jet and the Tropical Easterly Jet). The study shows that correcting for excessively warm AOGCM Atlantic sea-surface temperatures (SSTs) improves the simulation of key features, whereas applying 30-year mean bias corrections to atmospheric variables driving the RCM at the lateral boundaries does not improve the RCM simulations. We suggest that AOGCM climate projections for Africa should benefit from downscaling by nesting an RCM that has demonstrated skill in simulating African climate, driven with bias-corrected SST.
Highlights
IntroductionForecasts Interim reanalysis (ERA-I) data, 1989–2009: (1) An “oceanic” phase from November to mid-April, featuring a broad rain band with peak values just north of the Equator; (2) a “coastal” phase from mid-April to the end of June, featuring a rainfall peak in the coastal region around 4◦ N (over the ocean); (3) a transitional “break” phase during early July, when West African (WA) rainfall is at a minimum; and (4) a Sahelian phase between mid-July and September, when a more intense rainfall peak is established in the Sahel region around 10◦ N
We suggest that AOGCM climate projections for Africa should benefit from downscaling by nesting an regional climate model (RCM) that has demonstrated skill in simulating African climate, driven with bias-corrected sea-surface temperatures (SSTs)
(Tropical Rainfall Measuring Mission) (Huffman et al 2007) [38], and Figure 1b shows the corresponding distribution of RM3nc precipitation forced by uncorrected ModelE LBCs and SST, which was previously shown to be a big improvement over the ModelE results
Summary
Forecasts Interim reanalysis (ERA-I) data, 1989–2009: (1) An “oceanic” phase from November to mid-April, featuring a broad rain band with peak values just north of the Equator; (2) a “coastal” phase from mid-April to the end of June, featuring a rainfall peak in the coastal region around 4◦ N (over the ocean); (3) a transitional “break” phase during early July, when WA rainfall is at a minimum; and (4) a Sahelian phase between mid-July and September, when a more intense rainfall peak is established in the Sahel region around 10◦ N This organization is consistent with Gu and Adler (2004) [2], who described the break in West African rainfall during phase 3. Sahel precipitation is statistically linked to (1) the long-term warming trend of global SSTs (global warming is GW), which is influenced by natural and anthropogenic forcing;
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