Influence of the Andes over a cut-off low's life cycle

ALEJANDRO ANIBAL GODOY; NORMA E. POSSIA; CLAUDIA M. CAMPETELLA; MARCELO E. SELUCHI

Melbourne - Australia

Conferencia; 9th International Conference on Southern Hemisphere Meteorology and Oceanography; 2009

American Meteorological Society

Upper level cut-off lows (COLs) are synoptic patterns affecting southern South America with a frequency close to 17 cases per year. COLs reach maximum intensity more frequently in a region between 30º-45ºS and 68º-80ºW. Between 25 March and 2 April 2007  a COL developed over this region maintained stationary off the central coast of Chile for 96 hours corresponding to the segregation and mature stages. The main purpose of the present work is to investigate a possible influence of the Andes Cordillera over the life cycle of this particular COL event. The Eta/CPTEC model was used in order to simulate the whole life cycle. Two 8 days runs were performed with a horizontal resolution of 40 km  and 38 vertical levels, both initialized on 25 March: with topography (CTRL) and without topography (NT). Results of the CTRL run showed that the model reproduced adequately the life cycle and the three-dimensional COL structure, although it underestimates its depth. The NT run showed the major differences with CTRL start off in the segregation stage, showing a development according a classic mid-latitude baroclinc wave. The difference between NT and CTRL runs can summarize as follow: i) at lower levels, the absence of the Andes weaken the northerly flow with low level jet characteristics on the lee of the Andes and therefore weakening the warm advection over central Argentina, ii) at 300 hPa, the COL in the NT run shows an eastward movement during its segregation phase, while the CTRL run resembles the stationary behavior of the observed event, iii) the whole life cycle of the COL in the NT run is 3 days shorter than the CTRL run Upper level cut-off lows (COLs) are synoptic patterns affecting southern South America with a frequency close to 17 cases per year. COLs reach maximum intensity more frequently in a region between 30º-45ºS and 68º-80ºW. Between 25 March and 2 April 2007  a COL developed over this region maintained stationary off the central coast of Chile for 96 hours corresponding to the segregation and mature stages. The main purpose of the present work is to investigate a possible influence of the Andes Cordillera over the life cycle of this particular COL event. The Eta/CPTEC model was used in order to simulate the whole life cycle. Two 8 days runs were performed with a horizontal resolution of 40 km  and 38 vertical levels, both initialized on 25 March: with topography (CTRL) and without topography (NT). Results of the CTRL run showed that the model reproduced adequately the life cycle and the three-dimensional COL structure, although it underestimates its depth. The NT run showed the major differences with CTRL start off in the segregation stage, showing a development according a classic mid-latitude baroclinc wave. The difference between NT and CTRL runs can summarize as follow: i) at lower levels, the absence of the Andes weaken the northerly flow with low level jet characteristics on the lee of the Andes and therefore weakening the warm advection over central Argentina, ii) at 300 hPa, the COL in the NT run shows an eastward movement during its segregation phase, while the CTRL run resembles the stationary behavior of the observed event, iii) the whole life cycle of the COL in the NT run is 3 days shorter than the CTRL run