Mesoscale Convective Systems over Southeastern South America and Their Relationship with the South American Low-Level Jet

Monthly Weather Review - Tập 135 Số 4 - Trang 1290-1309 - 2007
Paola Salio1, Matilde Nicolini1, Edward J. Zipser2
1Centro de Investigaciones del Mar y la Atmósfera, CONICET/UBA, and Departamento de Ciencias de la Atmósfera y los Océanos, Universidad de Buenos Aires, Buenos Aires, Argentina
2Department of Meteorology, University of Utah. Salt Lake City, Utah

Tóm tắt

Abstract Prior studies have shown that the low-level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems (MCSs) over the Great Plains of the United States. The South American low-level jet (SALLJ) over southeastern South America (SESA) has an analogous role, advecting heat and moisture from the Amazon basin southward into the central plains of southeastern South America, generating ideal environmental conditions for convection initiation and growth into MCSs. This research has two purposes. One is to describe the characteristics of a 3-yr MCS sample in South America, south of the equator, and its related geographical distribution of convection frequency. The other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large MCSs, specifically those that mature under SALLJ conditions. High horizontal and temporal resolution satellite images are used to detect MCSs in the area for the period 1 September 2000–31 May 2003. Operational 1° horizontal resolution fields from NCEP are used to examine the environment associated with the systems and for the same period. Differences between tropical and subtropical MCSs in terms of size, diurnal cycle, and duration are found. Tropical MCSs are smaller, shorter in duration, and are characterized by a diurnal cycle mainly controlled by diurnal radiative heating. Subtropical MCSs show a preference for a nocturnal phase at maturity over Argentina, which contrasts with a tendency for a daytime peak over Uruguay and southern Brazil. In all seasons, at least one subtropical MCS developed in 41% of the SALLJ days, whereas in the days with no SALLJ conditions this percentage dropped to 12%. This result shows the importance of the synoptic conditions provided by the SALLJ for the development of MCSs and motivates the study of the atmospheric large-scale structure that evolves in close coexistence between SALLJ and subtropical organized convection at the mature stage. The large-scale environment associated with large long-lived MCSs during SALLJ events over SESA evolves under thermodynamic and dynamic forcings that are well captured by the compositing analysis. Essential features are low-level convergence generated by an anomalous all-day-long strong low-level jet prior to the development of the system, overlapped by high-level divergence associated with the anticyclonic flank of the entrance of an upper-level jet streak. This provides the dynamical forcing for convection initiation in an increasingly convectively unstable atmosphere driven by an intense and persistent horizontal advection of heat and moisture at low levels. These processes act during at least one diurnal cycle, enabling gradual building of optimal conditions for the formation of the largest organized convection in the subtropical area. The frequency of convection culminates in a geographically concentrated nocturnal maximum over northeast Argentina on the following day (MCS–SALLJ day). The northeastward displacement and later dissipation of subtropical convection are affected by a northward advance of a baroclinic zone, which is related to horizontal cold advection and divergence of moisture flux at low levels, both contributing to the stabilization of the atmosphere.

Từ khóa


Tài liệu tham khảo

Augustine, 1988, Mesoscale convective complexes over the United States during 1985., Mon. Wea. Rev., 116, 685, 10.1175/1520-0493(1988)116<0685:MCCOTU>2.0.CO;2

Berbery, 2000, Springtime precipitation and water vapor flux over southeastern South America., Mon. Wea. Rev., 128, 1328, 10.1175/1520-0493(2000)128<1328:SPAWVF>2.0.CO;2

Berbery, 2002, The hydrologic cycle of the La Plata Basin in South America., J. Hydrometeor., 3, 630, 10.1175/1525-7541(2002)003<0630:THCOTL>2.0.CO;2

Campetella, 2002, The influence of the Andes Mountains on the South American low level flow., Geophys. Res. Lett., 29, 10.1029/2002GL015451

Cohen, 1995, Environmental conditions associated with Amazonian squall lines: A case study., Mon. Wea. Rev., 123, 3163, 10.1175/1520-0493(1995)123<3163:ECAWAS>2.0.CO;2

Cotton, 1989, A composite model of mesoscale convective complexes., Mon. Wea. Rev., 117, 765, 10.1175/1520-0493(1989)117<0765:ACMOMC>2.0.CO;2

Douglas, 1998, Observational evidences of a low level jet east of the Andes during January–March 1998., Meteorologica, 23, 63

Fritsch, J. M., and G. S.Forbes, 2001: Mesoscale convective systems. Severe Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc., 323–356.

Janowiak, 2001, A real-time global half hourly pixel resolution infrared dataset and its applications., Bull. Amer. Meteor. Soc., 82, 205, 10.1175/1520-0477(2001)082<0205:ARTGHH>2.3.CO;2

Johnson, R., and B. E.Mapes, 2001: Mesoscale processes and severe convective weather. Severe Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc., 71–122.

Liebmann, 2004, Subseasonal variations of rainfall in South America in the vicinity of the low-level jet east of the Andes and comparison to those in the South Atlantic convergence zone., J. Climate, 17, 3829, 10.1175/1520-0442(2004)017<3829:SVORIS>2.0.CO;2

Machado, 2004, The convective system area expansion over Amazonia and its relationships with convective system life duration and high-level wind divergence., Mon. Wea. Rev., 132, 714, 10.1175/1520-0493(2004)132<0714:TCSAEO>2.0.CO;2

Machado, 1998, Life cycle variations of mesoscale convective systems over the Americas., Mon. Wea. Rev., 126, 1630, 10.1175/1520-0493(1998)126<1630:LCVOMC>2.0.CO;2

Maddox, 1983, Large-scale meteorological conditions associated with midlatitude, mesoscale convective complexes., Mon. Wea. Rev., 111, 1475, 10.1175/1520-0493(1983)111<1475:LSMCAW>2.0.CO;2

Marengo, 2002, The South American low level jet east of the Andes during the 1999 LBATRMM and LBA-WET AMC campaign., J. Geophys. Res., 107, 10.1029/2001JD001188

Marengo, 2004, Climatology of the low level jet east of the Andes as derived from the NCEP–NCAR reanalyses: Characteristics and temporal variability., J. Climate, 17, 2261, 10.1175/1520-0442(2004)017<2261:COTLJE>2.0.CO;2

Mota, G. V. , 2003: Characteristics of rainfall and precipitation features defined by the Tropical Rainfall Measuring Mission over South America. Ph.D. dissertation, University of Utah, 215 pp.

Nesbitt, 2000, A census of precipitation features in the Tropics using TRMM: Radar, ice scattering, and lightning observations., J. Climate, 13, 4087, 10.1175/1520-0442(2000)013<4087:ACOPFI>2.0.CO;2

Nicolini, M., and A. C.Saulo, 2000: ETA characterization of the 1997–98 warm season Chaco jet cases. Preprints, Sixth Int. Conf. on Southern Hemisphere Meteorology and Oceanography, Santiago, Chile, Amer. Meteor. Soc., 330–331.

Nicolini, 2006, Modeled Chaco low-level jets and related precipitation patterns during the 1997–1998 warm season., Meteor. Atmos. Phys., 94, 129, 10.1007/s00703-006-0186-7

Nicolini, 1993, Diurnal oscillations of low-level jets, vertical motion, and precipitation: A model case study., Mon. Wea. Rev., 121, 2588, 10.1175/1520-0493(1993)121<2588:DOOLLJ>2.0.CO;2

Nicolini, 2002, Strong South American low level jet events characterization during warm season and implications for enhanced precipitation., Meteorologica, 27, 59

Nicolini, M., P.Salio, G.Ulke, J.Marengo, M.Douglas, J.Paegle, and E.Zipser, 2004: South American low level jet diurnal cycle and three dimensional structure. CLIVAR Exchanges, No. 9, International CLIVAR Project Office, Southampton, United Kingdom, 6–8.

Nieto Ferreira, 2003, Variability of South American convective cloud systems and tropospheric circulation during January–March 1998 and 1999., Mon. Wea. Rev., 131, 961, 10.1175/1520-0493(2003)131<0961:VOSACC>2.0.CO;2

Nogués-Paegle, 1997, Alternating wet and dry conditions over South America during summer., Mon. Wea. Rev., 125, 279, 10.1175/1520-0493(1997)125<0279:AWADCO>2.0.CO;2

Paegle, J. , 2000: American low level jets in observation and theory: The ALLS project. Preprints, Sixth Int. Conf. on Southern Hemisphere Meteorology and Oceanography, Santiago, Chile, Amer. Meteor. Soc., 161–162.

Salio, P. , 2002: Characterization of extreme low level jet events east of the Andes using reanalysis (in Spanish). Ph.D. thesis, University of Buenos Aires, 231 pp.

Salio, 2002, Chaco low level jet events characterization during the austral summer season by ERA reanalysis., J. Geophys. Res., 107, 10.1029/2001JD001315

Salio, P., E. J.Zipser, M.Nicolini, and C.Liuct, 2004: Diurnal cycle of mesoscale convective systems over Southeastern South America. Preprints, 14th Int. Precipitation and Cloud Conf., Bologna, Italy, International Association of Meteorology and Atmospheric Science, 1844–1847.

Saulo, 2004, A case study of a Chaco low-level jet event., Mon. Wea. Rev., 132, 2669, 10.1175/MWR2815.1

Silva Dias, 1999, Storms in Brazil.

Siqueira, 2004, Influence of the frontal systems on the day-to-day convection variability over South America., J. Climate, 17, 1754, 10.1175/1520-0442(2004)017<1754:IOTFSO>2.0.CO;2

Velasco, 1987, Mesoscale convective complexes in the Americas., J. Geophys. Res., 92, 9591, 10.1029/JD092iD08p09591

Vera, 2002, Cold season synoptic-scale waves over subtropical South America., Mon. Wea. Rev., 130, 684, 10.1175/1520-0493(2002)130<0684:CSSSWO>2.0.CO;2

Vera, 2006, The South American Low-Level Jet Experiment., Bull. Amer. Meteor. Soc., 87, 63, 10.1175/BAMS-87-1-63

Thông tin
Thông tin xuất bản

Monthly Weather Review

Tập 135 Số 4

1290-1309

Thông tin tác giả