Assessing the dynamic-downscaling ability over South America using the intensity-scale verification technique

The National Centers for Environmental Prediction (NCEP) ETA regional circulation model (RCM) was one-way nested in the T62 NCEP general circulation model for a series of 3-month simulations of the austral summer and winter over South America (SA). The intensity-scale verification technique (ISVT), based on the scale decomposition of precipitation skill score and energy relative difference, was used to quantitatively assess the dynamic-downscaling ability of seasonal precipitation and inter-annual precipitation difference. The ISVT showed that the RCM was able to add value to summer and winter rainfall forecasts over southern South America. Largest improvements were associated to precipitation events at spatial scales of about 400-800 km and rainfall rates above 4 mm day(-1). In general, downscaling failed to yield better results over northern South America. In terms of inter-annual precipitation difference, the RCM produced better results over southern South America, by simulating the increase in intense small-scale events in the wet years. Analysis of meridional moisture flux associated with the South American low-level jet (SALLJ) suggested that the Andean topography plays an important role in the RCM's rainfall simulations over the La Plata basin. A sensitivity test with lowered Andean topography heights produced weaker moisture advection by the SALLJ, and lower rainfall totals over that basin for both seasons. During summer, results showed that the reduced rainfall was associated with deteriorated simulations of midto large-scale precipitation events, whereas during winter it was associated with deteriorated simulations of smaller-scale events. Copyright. (C) 2010 Royal Meteorological Society