This paper presents the use of trace-based performance visualization of a large scale atmospheric model, the Ocean-Land-Atmosphere Model (OLAM). The trace was obtained with the libRastro library, and the visualization was done with Paj´e. The use of visualization aimed to analyze OLAM's performance and to identify its bottlenecks. Especially, we are interested in the model's I/O operations, since it was proved to be the main issue for the model's performance. We show that most of the time spent in the output routine is spent in the close operation. With this information, we delayed this operation until the next output phase, obtaining improved I/O performance.