Precipitation diurnal cycle and summer climatology assessment over South America: An evaluation of Regional Climate Model version 3 simulations

Abstract

Regional Climate Model version 3 (RegCM3) simulations of 17 summers (1988–2004) over part of South America south of 5°S were evaluated to identify model systematic errors. Model results were compared to different rainfall data sets (Climate Research Unit (CRU), Climate Prediction Center (CPC), Global Precipitation Climatology Project (GPCP), and National Centers for Environmental Prediction (NCEP) reanalysis), including the five summers mean (1998–2002) precipitation diurnal cycle observed by the Tropical Rainfall Measuring Mission (TRMM)‐Precipitation Radar (PR). In spite of regional differences, the RegCM3 simulates the main observed aspects of summer climatology associated with the precipitation (northwest‐southeast band of South Atlantic Convergence Zone (SACZ)) and air temperature (warmer air in the central part of the continent and colder in eastern Brazil and the Andes Mountains). At a regional scale, the main RegCM3 failures are the underestimation of the precipitation in the northern branch of the SACZ and some unrealistic intense precipitation around the Andes Mountains. However, the RegCM3 seasonal precipitation is closer to the fine‐scale analyses (CPC, CRU, and TRMM‐PR) than is the NCEP reanalysis, which presents an incorrect north‐south orientation of SACZ and an overestimation of its intensity. The precipitation diurnal cycle observed by TRMM‐PR shows pronounced contrasts between Tropics and Extratropics and land and ocean, where most of these features are simulated by RegCM3. The major similarities between the simulation and observation, especially the diurnal cycle phase, are found over the continental tropical and subtropical SACZ regions, which present afternoon maximum (1500–1800 UTC) and morning minimum (0900–1200 UTC). More specifically, over the core of SACZ, the phase and amplitude of the simulated precipitation diurnal cycle are very close to the TRMM‐PR observations. Although there are amplitude differences, the RegCM3 simulates the observed nighttime rainfall in the eastern Andes Mountains, over the Atlantic Ocean, and also over northern Argentina. The main simulation deficiencies are found in the Atlantic Ocean and near the Andes Mountains. Over the Atlantic Ocean the convective scheme is not triggered; thus the rainfall arises from the grid‐scale scheme and therefore differs from the TRMM‐PR. Near the Andes, intense (nighttime and daytime) simulated precipitation could be a response of an incorrect circulation and topographic uplift. Finally, it is important to note that unlike most reported bias of global models, RegCM3 does not trigger the moist convection just after sunrise over the southern part of the Amazon.

Climate change in Central America and Mexico: regional climate model validation and climate change projections

“…As a result, the dry season precipitation bias is reduced in the RCM in these regions (regions 5 and 6; see Table 3). Underestimation of precipitation along the leeward slopes (region 5) and overestimation along the windward slopes (region 6) is also detected in a PRECIS study over South America (Urrutia and Vuille 2009;Buytaert et al 2010) and other RCM studies as well (Caldwell et al 2009;da Rocha et al 2009). …”

Uncertainties in climate change projections and regional downscaling in the tropical Andes: implications for water resources management

“…The overestimation of precipitation along the eastern Andean slope is a problem common to all RCMs and related to excessive orographic uplift due to strong easterly winds (e.g., da Rocha et al, 2009;Insel et al, 2010;Urrutia and Vuille, 2009). It should be kept in mind, however, that the gridded observational precipitation products such as the climatology data have significant uncertainties arising from space-time gaps in the observational records, instrumentation and calibration problems, and sampling inadequacies.…”

Severe Convection Features in the Amazon Basin: A TRMM-Based 15-Year Evaluation

“…The WAS subregion continues to experience these nocturnal occurrences throughout the rainy season. The physical mechanisms for nocturnal systems in both regions may be related to the local mountain valley circulation in a similar setting as the one observed in the more subtropical latitudes associated with MCS (e.g., Velasco and Fritsch, 1987) and seen in simulations such as the of DaRocha et al (2009). The South American Low Level Jet-SALLJ (Vera et al, 2006) flows from the north and supplies the MCS that develop further south with heat and moisture.…”

“…In OND, when the greatest amount of PSC occurs, the winds are from the north. In this situation Figure 2 shows PSC forming close to the Andes, possibly due to the confluence of the large scale flow with local circulations due to the Andes slopes topography, as suggested by DaRocha et al (2009) using a regional model simulation. The same mechanism may be affecting the WAS subregion.…”