Contribution of cold fronts to seasonal rainfall in simulations over the southern La Plata Basin

Abstract

This work investigates how regional climate models REMO and RegCM4 simulate the frequency of cold fronts (CFs) over southern Brazil and the contribution of these systems to the seasonal precipitation over the southern part of La Plata Basin (SLPB) in South America. Simulations were driven by the ERA-Interim reanalysis and compared with local observations (from Rio Grande station; RG), ERA-Interim, and rainfall analysis from the Climate Prediction Center (CPC). CF identification was carried out objectively by considering (1) the turning of the meridional wind component from north to south and (2) a decrease in air temperature between 1 d before and 1 d after the CF passage. For the period 1991−2008, a mean of 55.3, 53.1 and 51.8 CFs yr −1 were identified, respectively, for RG (observed), REMO and RegCM4. These values show a small bias in the simulated annual frequency of CFs (around −5%), but the underestimation reaches −17% during summer in the RegCM4. In the simulations, it is possible to find some association between seasonal bias of rainfall and the bias of rainfall during CF periods (defined as extending from 2 d before to 2 d after the CF passage) over SLPB. Except in summer, RegCM4 presents stronger biases than REMO for rainfall for both CF periods and overall seasonal climatology. For the winter season, the CF composites indicate that deeper low pressure and larger availability of moisture at low levels (from 2 d before to the day of the CF) contribute to the small underestimation of rainfall in REMO (−5%) compared with RegCM4 (−25%).

Sensitivity of simulated South America climate to the land surface schemes in RegCM4

“…2, the underestimation of the precipitation over La Plata Basin (southern Brazil, Uruguay and northeastern Argentina) by RegCLM and RegBATS is a common error in many RCMs Menendez et al 2010;Reboita et al 2014;Llopart et al 2014) and it is attributed to a set of factors. Some authors have associated this precipitation deficit with the surface specifications in the model , the convective parameterization scheme , a weaker-than-observed low level jet east of the Andes Mountains (Fernandez et al 2006a), differences in the frequency and intensity of cyclones (Reboita et al 2010) and cold fronts (Jesus et al 2016).…”

Climatology of surface baroclinic zones in the coast of Brazil

“…In order to estimate the percentage of precipitation associated with each kind of system in the region with a well-defined dry and wet period, São Paulo was the city chosen for this analysis. For this reason, the same methodology by Silva et al (2014), de Jesus et al (2016) and Andrade (2017) was applied, i.e., the precipitation from a classical cold front is one that occurs in the period between a day before (pre-frontal rain) and a day after (post-frontal rain) the passage of the cold front. The same method was applied for the precipitation associated with subtropical fronts and baroclinic troughs.…”

Assessing the Moisture Transports Associated With Nocturnal Low-Level Jets in Continental South America

“…These smaller correlations may be associated with the moisture sink anomalies occupying larger areas over the ocean than in the continent (Figures 9I-L), while the opposite occurs for precipitation anomalies (Figures 11I-L). A greater increase in precipitation over southern Brazil in the austral spring (Figure 11L) during the NLLJs in Brazil-SE may be associated with a higher occurrence of cold fronts crossing the south/southeast of Brazil, reinforcing the formation of clouds and precipitation over the region (Rodrigues et al, 2004;De Jesus et al, 2016).…”

“…Most of the studied SALLJs, identified at some points in eastern of Bolivia and Paraguay, is the NCJE, due to its role in transporting moisture from the tropical to subtropical South America (Marengo et al, 2004;Vera et al, 2006;Algarra et al, 2019;Montini et al, 2019). The moisture transport in SALLJ organizes deep convective systems in the form of a mesoscale convective complex and also contributes to the extreme precipitation events associated with cyclones over the La Plata basin (Velasco and Fritsch, 1987;Nicolini and Saulo, 2006;Salio et al, 2007;Cavalcanti et al, 2009;Reboita et al, 2010;Reboita et al, 2015;Carril et al, 2016;De Jesus et al, 2016), giving this region the highest occurrence of thunderstorms in the continent (Morales Rodriguez, 2019). In South America, moisture transport has been studied by using the Eulerian approach, i.e., the vertically integrated moisture flux (Arraut et al, 2012;Satyamurty et al, 2013;Zemp et al, 2014;Montini et al, 2019) or the Lagrangian one that computes the change of moisture following the air particles (Dirmeyer et al, 2009;Durán-Quesada et al, 2010;Van der Ent et al, 2010;Gimeno et al, 2012;Martinez and Dominguez, 2014;Zemp et al, 2014;Algarra et al, 2019).…”