Seasonal characterization on the diurnal cycle of convection frequency over Southeastern South America under different low-jet conditions

PAOLA SALIO; MATILDE NICOLINI

Foz do Iguaçu, Brazil

Conferencia; 8th Inter. Conference on Southern Hemisphere Meteorology and Oceanography; 2006

American Meteorological Society

Seasonal characterization on the diurnal cycle of convection frequency over Southeastern South America under different low-jet conditions. Paola Salio and Matilde Nicolini Centro de Investigaciones del Mar y la Atmósfera. CONICET/UBA. Departamento de Ciencias de la Atmósfera y los Océanos. Universidad de Buenos Aires. Buenos Aires. Argentina Abstract The objective of the present work is to characterize the diurnal cycle of the convection frequency and explore possible changes during the year. Given previous evidences of the existence of a close relationship between convection and low-level jet conditions a distinction is made between different synoptic situations. The diurnal cycle of convection frequency during all seasons in the Southeast of South America (SESA) is studied using a three year sample of satellite data (2000-2003) with high horizontal and temporal resolution. IR-brightness temperature is used to calculate the relative convection frequency. The geographical distribution of convection frequency represented by an IR brightness temperature below -55 ºC (218 ºK) and -38°C (235°K) is obtained with half an hour resolution during all seasons. Also, the convection frequency is calculated over different synoptic conditions characterized by the evidence or not of a low-level jet (LLJ) over SESA region. Three synoptic situations are categorized: South American Low-Level Jet events (SALLJ), LLJ Argentina events (LLJA) and cases without the evidence of LLJ over SESA (No LLJ). These situations are classified using operational analyses (GDAS - Global Data Assimilation System) with high spatial and 6-hours temporal resolution. A criteria similar to Bonner (1968) is used to identify SALLJ and LLJA events. The convection located south of 23°S shows a strong relationship to the SALLJ events and displays an evidently nocturnal character during spring and fall; while in summer a different behavior is observed since two phases of maximum convection are present: one nocturnal and another in the afternoon. This behavior shows a strong relationship between the time of the maximum in the low level jet and convection during the night whereas in the evening during summer the convection is influenced by the radiative heating. The nocturnal maximum over eastern Argentina and the daytime maximum over the slopes of the Andes is coherent with the organized convection trajectories toward the east – northeast found in previous studies. During winter, the presence of temperature lower than -55°C is unusual, except during SALLJ events when this convection frequency becomes significant with values higher than 10%. Convection frequency for LLJA and NOLLJ events during the warm season and fall displays frequencies below 2% on the whole day, emphasizing the low probability of convection formation during these events in subtropical areas. On other side, in tropical areas a strong activation of convection in cases LLJA is evident. Corresponding First author address: Paola Salio: Centro de Investigaciones del Mar y la Atmósfera. Ciudad Universitaria, Pabellón II, 2do piso. (1428) Buenos Aires, Argentina. e-mail: salio@cima.fcen.uba.ar