Exchanges - Scientific Contributions - Exchanges - Scientific Contributions - * Contribution to Exchanges No. 29, March 2004 Modeling Studies Related to SALLJEX * Jan Paegle 1 , Tercio Ambrizzi 2 , Ernesto H. Berbery 3 , C. Campetella, René Garreaud 4 , Dirceu 5 Herdies, Jose Marengo 5 , Claudio Menendez 6 , Matilde Nicolini 6 , R. Porfirio da Rocha, J. Ruiz, Celeste S. Saulo 6 , Marcelo Seluchi 7 , and Pedro L. Silva Dias 2 1 U. Utah, Salt Lake City, USA 2 U. São Paulo, São Paulo, Brazil 3 U. Maryland, College Park, USA 4 U. Santiago de Chile, Santiago de Chile, Chile 5 CPTEC, San Jose dos Campos, Brazil 6 U. Buenos Aires, Buenos Aires, Argentina 7 INPE, Cachoeira Paulista, São Paulo, Brazil corresponding e-mail: jpaegle@met.utah.edu Introduction Prediction of weather and short-range climate evolution over South America is characterized by different obser- vation problems than those found in North America. The SALLJEX project focused upon special problems posed by observation gaps in the Southern Hemisphere. In addition to observation issues, the steep Andes orography and the low latitude of most of the continent provide new concerns relative to forecast problems for North America. Remote sensing is particularly impor- tant in the Southern Hemisphere, where the forecast skill now approaches levels found in the Northern Hemi- sphere. This has been accomplished through better and more efficient use of vast amounts of satellite data, pri- marily over the oceans. The impact of an enhanced ob- serving network in the SALLJEX area is a major chal- lenge that may lead to improvement of models, data assimilation procedures, and better use of remote sens- ing products over continents. There remains a question whether data-deficient regions of the oceans or of the continents most strongly limit the southern observing system. This is being addressed within CPTEC, INPE, and the University of Sao Paulo in Brazil, in the University of Buenos Aires, CIMA, and the Servicio Meteorologico in Argentina, at the Univer- sity of Chile, and also by investigators in North America using SALLJEX observations to study the value of re- gional surface-based observations. The field experiment was supported by regional, real-time model simulations provided by researchers within these organizations. Real-time comparison between model results and ob- servations was routinely made at the operations center of SALLJEX in Santa Cruz de la Sierra in Bolivia. Model forecasts were used for guidance in planning special ob- serving and intensive observing periods, and were at times found to be quite accurate for planning purposes, but were less adequate at other times, and the experi- ment has led to systematic intercomparisons of models designed to prioritize observational and modeling en- hancements required for improved numerical guidance. Dynamical perspectives Past research by Silva Dias and collaborators has shown that basic features of low-level summer circulations in tropical regions are often well-described by linear mod- els that include the effect of observed heat sources. These and other studies also point out significant discrepan- cies over South America that cannot be resolved in mod- els lacking the topographic effect. A number of studies confirm that the lower tropospheric flow is most affected by the steep mountains, while the upper level flow is barely influenced. While most past studies have emphasized topographic modifications of Amazon Basin wind systems and the effect of heating east of the Andes, some investigators emphasize correlation of the East Andes LLJ with El Nino events of the Pacific Ocean, and with the strength of the upper westerlies. Cyclonic circulation surrounds the Andes in the lower-troposphere in both summer and winter, and this supports a poleward east Andes LLJ in Fig. 1: Mesoscale Convective System as observed over the SALLJEX area on January 18, 2003 at 13 UTC by the GOES- 8? satellite.