Dynamic Downscaling of the North American Monsoon with the NCEP–Scripps Regional Spectral Model from the NCEP CFS Global Model

Abstract

Abstract The June–September (JJAS) 2000–07 NCEP coupled Climate Forecasting System (CFS) global hindcasts are downscaled over the North and South American continents with the NCEP–Scripps Regional Spectral Model (RSM) with anomaly nesting (AN) and without bias correction (control). A diagnosis of the North American monsoon (NAM) in CFS and RSM hindcasts is presented here. RSM reduces errors caused by coarse resolution but is unable to address larger-scale CFS errors even with bias correction. CFS has relatively weak Great Plains and Gulf of California low-level jets. Low-level jets are strengthened from downscaling, especially after AN bias correction. The RSM NAM hydroclimate shares similar flaws with CFS, with problematic diurnal and seasonal variability. Flaws in both diurnal and monthly variability are forced by erroneous convection-forced divergence outside the monsoon core region in eastern and southern Mexico. NCEP reanalysis shows significant seasonal variability errors, and AN shows little improvement in regional-scale flow errors. The results suggest that extreme caution must be taken when the correction is applied relative to reanalyses. Analysis also shows that North American Regional Reanalysis (NARR) NAM seasonal variability has benefited from precipitation data assimilation, but many questions remain concerning NARR’s representation of NAM.