Cavados em médios níveis no hemisfério sul: climatologia, energética e ciclogênese em superfície / Midtropospheric trough in southern hemisphere: climatology, energetic and surface cyclogenesis

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Southern Hemisphere and its relationship with the Southern America surface cyclogenesis. An objective method of trough identification and tracking was developed, which was applied for 24 years of NCEP/NCAR reanalysis 500 hPa geopotencial height data. We found, in mean, 868 troughs per year, with positive trend. The winter season (summer) presented greater (smaller) number. During the summer, the troughs were more active in 60°S-40°S band, while in the winter this band extended from 30°S to Antarctic continent. The trough genesis presented 3 maxima of occurrence: i) over the region of the Drake Passage and South Atlantic Ocean, ii) over the Indian Ocean along 50°S, and iii) over the Southwestern Pacific Ocean between 150°E and 150°W. The trough termination was more dispersed than the genesis, but presenting also 3 maxima: i) over the west of Andes, ii) to the south of the African continent, and iii) to the south of Australia. The troughs have presented the mean life of 4,3 days, increasing in the subtropical (high) latitudes. The genesis and termination of troughs occurred preferentially in southwestern and northwestern flow, respectively, however in environments of similar baroclinicity. For the 1999-2003 winters, all situations have been analyzed where one 500 hPa troughs, traveling over the South America, generated surface cyclogenesis over Uruguay or neighborhood. Two composites were created, named B composite (with 25 situations) and C composite (with 13 situations). The results showed that before (during) the surface cyclogenesis the midtropospheric troughs had less (more) intensity in the composite B than C; the same occurred for the kinetic energy. Of all variables evaluated in the low, middle and upper troposphere, southward heat flux in low levels only presented differences between the two composites, being more intense in composition C than B. The volume-averaged of the conversion terms showed that the baroclinic conversion was dominant in two composites while the term of downstream development was unfavorable for the cyclogenesis in composite C and relatively more important for the initial phase in the composite B cyclogenesis. Two trough cases that crossed the South America but did not generate surface cyclogenesis were compared with these two composites. The case 1 with synoptic characteristics of the composite B and case 2 with of the composite C. In case 1, the low, middle and upper troposphere conditions were less favorable for cyclogenesis than in the composite B. The kinetic energy in the case 1 was less than that observed in the composite B. The term associated with the downstream development, although was counterbalanced for the barotropic decay, justifying the small energy growth. In case 2, the horizontal temperature gradient, the Eady growth rate and the vertical motion were favorable to cyclogenesis, while the heat flux in low levels, and the divergence and the negative vorticity advection in upper levels were less favorable. The kinetic energy as well as all the terms of energy conversion were greater in case 2 than in the composite C. The term associated with the downstream development was important in the beginning of the period. The forecast for 5 days of the General Atmospheric Circulation Model of the CPTEC/COLA, for trough cases that developed on February 2004, was evaluated through the kinetic energy budget. The results showed that the baroclinic conversion and the convergence of ageostrophic energy flux were greater in the forecasting than in the NCEP/NCAR reanalysis. The residue was elevated in the beginning of the forecasting and rapidly reduced in first 12 h.

ASSUNTO(S)

downstream development meteorologia middle level troughs ciclogênese em superfície baroclinic instability southern hemisphere cavados em médios níveis energética surface cyclogenesis hemisfério sul desenvolvimento corrente abaixo instabilidade baroclínica meteorology energetic

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