• Aceituno, P., 1988: On the functioning of the Southern Oscillation in the South American sector. Part I: Surface climate. Mon. Wea. Rev., 116 , 505524.

    • Search Google Scholar
    • Export Citation
  • Arakawa, A., , and V. R. Lamb, 1977: Computational design of the basic dynamical processes of the UCLA general circulation model. Methods Comput. Phys., 17 , 173265.

    • Search Google Scholar
    • Export Citation
  • Baumhefner, D. P., , and D. J. Perkey, 1982: Evaluation of lateral boundary errors in a limited-domain model. Tellus, 34 , 409428.

  • Berbery, E. H., , and E. A. Collini, 2000: Springtime precipitation and water vapor flux over southeastern South America. Mon. Wea. Rev., 128 , 13281346.

    • Search Google Scholar
    • Export Citation
  • Betts, A. K., , and M. T. Miller, 1986: A new convective adjustment scheme. Part II: Single column tests GATE wave, BOMEX, and Arctic air-mass data. Quart. J. Roy. Meteor. Soc., 112 , 693703.

    • Search Google Scholar
    • Export Citation
  • Black, T. L., 1994: The new NMC mesoscale Eta Model: Description and forecast examples. Wea. Forecasting, 9 , 265277.

  • Blackadar, A. K., 1957: Boundary layer wind maxima and their significance for the growth of nocturnal inversions. Bull. Amer. Meteor. Soc., 38 , 283290.

    • Search Google Scholar
    • Export Citation
  • Bleeker, W., , and M. J. Andre, 1951: On diurnal variation of precipitation, particularly over central U.S.A., and its relation to large-scale orographic circulation systems. Quart. J. Roy. Meteor. Soc., 77 , 260271.

    • Search Google Scholar
    • Export Citation
  • Bonner, W. D., 1968: Climatology of low-level jet. Mon. Wea. Rev., 96 , 833850.

  • Bonner, W. D., , and J. Paegle, 1970: Diurnal variation of boundary layer winds over the south-central United States in summer. Mon. Wea. Rev., 98 , 735744.

    • Search Google Scholar
    • Export Citation
  • Chen, F., , K. Z. Janjic, , and K. Mitchel, 1997: Impact of the atmospheric surface-layer parameterizations in the new land-surface scheme of the NCEP mesoscale Eta model. Bound.-Layer Meteor., 85 , 391421.

    • Search Google Scholar
    • Export Citation
  • Douglas, M. W., , M. Nicolini, , and C. Saulo, 1999: The low-level jet at Santa Cruz, Bolivia during January–March 1998: Pilot balloon observation and model comparisons. Preprints, 10th Symp. on Global Change, Dallas, TX, Amer. Meteor. Soc., 223–226.

    • Search Google Scholar
    • Export Citation
  • Fels, S. B., , and M. D. Schwartzkopf, 1975: The simplified exchange approximation: A new method for radiative transfer calculations. J. Atmos. Sci., 32 , 14751488.

    • Search Google Scholar
    • Export Citation
  • Garreaud, R. D., 2000: Cold air incursions over subtropical South America: Mean structure and dynamics. Mon. Wea. Rev., 128 , 25442559.

    • Search Google Scholar
    • Export Citation
  • Garreaud, R. D., , and J. M. Wallace, 1997: The diurnal march of convective cloudiness over the Americas. Mon. Wea. Rev., 125 , 31573171.

    • Search Google Scholar
    • Export Citation
  • Garreaud, R. D., , and J. M. Wallace, 1998: Summertime incursions of midlatitude air into subtropical and tropical South America. Mon. Wea. Rev., 126 , 27132733.

    • Search Google Scholar
    • Export Citation
  • Gerrity, J. P., , T. L. Black, , and R. E. Treadon, 1994: The numerical solution of the Mellor–Yamada level 2.5 turbulent kinetic energy equation in the Eta Model. Mon. Wea. Rev., 122 , 16401646.

    • Search Google Scholar
    • Export Citation
  • Giorgi, F., 1990: Simulation of regional climate using a limited area model nested in a general circulation model. J. Climate, 3 , 941963.

    • Search Google Scholar
    • Export Citation
  • Giorgi, F., , and G. T. Bates, 1989: The climatological skill of a regional model over complex terrain. Mon. Wea. Rev., 117 , 23252347.

  • Grimm, A. M., , S. E. T. Ferraz, , and J. Gomes, 1998: Precipitation anomalies in Southern Brazil associated with El Niño and La Niña events. J. Climate, 11 , 28632880.

    • Search Google Scholar
    • Export Citation
  • Grimm, A. M., , V. R. Barros, , and M. E. Doyle, 2000: Climate variability in southern South America associated with El Niño and La Niña events. J. Climate, 13 , 3557.

    • Search Google Scholar
    • Export Citation
  • Hastenrath, S., 1978: On modes of tropical circulations and climate anomalies. J. Atmos. Sci., 35 , 22222231.

  • Holton, J. R., 1967: The diurnal boundary layer wind oscillation above the sloping terrain. Tellus, 19 , 199205.

  • Horel, J. D., , and A. G. Cornejo-Garrido, 1986: Convection along the coast of northern Peru during 1983: Special and temporal variation of clouds and rainfall. Mon. Wea. Rev., 114 , 20912105.

    • Search Google Scholar
    • Export Citation
  • Janjic, Z. I., 1990: The step-mountain coordinate: Physical package. Mon. Wea. Rev., 118 , 14291443.

  • Janjic, Z. I., 1994: The step-mountain coordinate model: Further developments of the convection, viscous sublayer, and turbulence closure schemes. Mon. Wea. Rev., 122 , 927945.

    • Search Google Scholar
    • Export Citation
  • Ji, Y., , and A. D. Vernekar, 1997: Simulation of the Asian summer monsoons 1987 and 1988 with a regional model nested in a global CGM. J. Climate, 10 , 19651979.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors. 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 , 437471.

  • Kirtman, B. P., , J. Shukla, , B. Huang, , Z. Zhu, , and E. K. Schneider, 1997: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 125 , 789808.

    • Search Google Scholar
    • Export Citation
  • Kousky, V. E., 1980: Diurnal rainfall variation in northeast Brazil. Mon. Wea. Rev., 108 , 488498.

  • Kousky, V. E., , and M. T. Kayano, 1994: Principal modes of outgoing longwave radiation and 250-mb circulation for the South American sector. J. Climate, 7 , 11311143.

    • Search Google Scholar
    • Export Citation
  • Lacis, A. A., , and J. E. Hansen, 1974: A parameterization of absorption of solar radiation in the earth's atmosphere. J. Atmos. Sci., 31 , 118133.

    • Search Google Scholar
    • Export Citation
  • Liebmann, B., , J. D. Glick, , J. A. Marengo, , V. E. Kousky, , H. C. Wainer, , and O. Massambani, 1998: A comparison of rainfall, outgoing longwave radiation, and divergence over the Amazon basin. J. Climate, 11 , 28982909.

    • Search Google Scholar
    • Export Citation
  • Liebmann, B., , J. L. Kiladis, , J. A. Marengo, , T. Ambrizzi, , and J. D. Glick, 1999: Submonthly convective variability over South America and South Atlantic convergence zone. J. Climate, 12 , 18771890.

    • Search Google Scholar
    • Export Citation
  • Lobocki, L., 1993: A procedure for derivation of surface layer bulk relationships from simplified second-order closer models. J. Appl. Meteor., 32 , 126138.

    • Search Google Scholar
    • Export Citation
  • Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61 , 13741387.

  • Marengo, J. A., 1992: Interannual variability of surface climate in the Amazon basin. Int. J. Climatol., 12 , 853863.

  • Marengo, J. A., , and S. Hastenrath, 1993: Case studies of extreme climatic events in the Amazon basin. J. Climate, 6 , 617627.

  • Marengo, J. A., , B. Liebmann, , I. Wainer, , and V. Kousky, 1999: On the characteristics of the season in Amazonia. Preprints, 10th Symp. on Global Change, Dallas, TX, Amer. Meteor. Soc., 219–222.

    • Search Google Scholar
    • Export Citation
  • Mellor, G. L., , and T. Yamada, 1974: A hierarchy of turbulence closure models for boundary layers. J. Atmos. Sci., 31 , 17911806.

  • Mesinger, F., 1984: A blocking technique for representation of mountains in atmospheric models. Riv. Meteor. Aeronaut., 44 , 195202.

  • Mesinger, F., , Z. L. Janjic, , S. Nickovic, , D. Gavrilov, , and D. Deaven, 1988: The step-mountain coordinate: Model description and performance for cases of Alpine Lee cyclogenesis and for a case of an Appalachian redevelopment. Mon. Wea. Rev., 116 , 14931518.

    • Search Google Scholar
    • Export Citation
  • Misra, V., , P. A. Dirmeyer, , B. P. Kirtman, , H-M. H. Juang, , and M. Kanamitsu, 2000: Regional simulation of interannual variability over South America. COLA Tech. Rep. 85, 42 pp.

    • Search Google Scholar
    • Export Citation
  • Nogues-Paegle, J., , and K-C. Mo, 1997: Alternating wet and dry conditions over South America during summer. Mon. Wea. Rev., 125 , 279291.

    • Search Google Scholar
    • Export Citation
  • Poveda, G., , and O. J. Mesa, 2000: On the existence of Lloro (the rainiest locality on Earth): Enhanced ocean–land–atmosphere interaction by a low level jet. Geophys. Res. Lett., 27 , 16751678.

    • Search Google Scholar
    • Export Citation
  • Reynolds, R. W., , and T. M. Smith, 1994: Improved global sea surface temperature anomalies using optimum interpolation. J. Climate, 7 , 929948.

    • Search Google Scholar
    • Export Citation
  • Rogers, E., , T. L. Black, , D. G. Deaven, , and G. J. DiMego, 1996: Changes to the operational early Eta analysis/forecast system at the National Centers for Environmental Prediction. Wea. Forecasting, 11 , 391413.

    • Search Google Scholar
    • Export Citation
  • Ropelewski, C. F., , and M. S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Niño/Southern Oscillation. Mon. Wea. Rev., 115 , 16061626.

    • Search Google Scholar
    • Export Citation
  • Ropelewski, C. F., , and M. S. Halpert, 1989: Precipitation pattern associated with the high index phase of Southern Oscillation. J. Climate, 2 , 268284.

    • Search Google Scholar
    • Export Citation
  • Saulo, A. C., , M. Nicolini, , and S. C. Chou, 2000: Model characterization of the South American low-level flow during 1997–1998 spring–summer season. Climate Dyn., 16 , 867881.

    • Search Google Scholar
    • Export Citation
  • Seluchi, M. E., , and J. Marengo, 2000: Tropical–midlatitude exchange of air masses during summer and winter in South America: Climatic aspects and examples of intense events. Int. J. Climatol., 20 , 11671190.

    • Search Google Scholar
    • Export Citation
  • Stensrud, D. J., 1996: Importance of low-level jets to climate: A review. J. Climate, 9 , 16981711.

  • Uvo, C. B., , C. A. Repelli, , S. E. Zebiak, , and Y. Kushnir, 1998: The relationship between tropical Pacific and Atlantic SST and northeast Brazil monthly precipitation. J. Climate, 11 , 551562.

    • Search Google Scholar
    • Export Citation
  • Velasco, I., , and J. M. Fritsch, 1987: Mesoscale convective complexes in the Americas. J. Geophys. Res., 92 , 95919613.

  • Vera, C. S., , and P. K. Vigliarolo, 2000: A diagnostic study of cold air outbreaks over South America. Mon. Wea. Rev., 128 , 324.

  • Vernekar, A. D., , and Y. Ji, 1999: Simulation of onset and intraseasonal variability of two contrasting summer monsoons. J. Climate, 12 , 17071725.

    • Search Google Scholar
    • Export Citation
  • Warner, T. T., , R. A. Peterson, , and R. E. Treadon, 1997: A tutorial on lateral boundary conditions as a basic and potentially serious limitation to regional numerical weather prediction. Bull. Amer. Meteor. Soc., 78 , 25992618.

    • Search Google Scholar
    • Export Citation
  • Willmott, C. J., cited 2001: South American climate data. [Available online at http://climate.geog.udel.edu/∼climate.].

  • Xie, P., , and P. A. Arkin, 1997: A 17-year monthly analysis based on gauge observations, satellite systems, and numerical model outputs. Bull. Amer. Meteor. Soc., 78 , 25392558.

    • Search Google Scholar
    • Export Citation
  • Zhao, Q., , T. L. Black, , and M. E. Baldwin, 1997: Implementation of the cloud prediction scheme in the Eta Model at NCEP. Wea. Forecasting, 12 , 697712.

    • Search Google Scholar
    • Export Citation
  • Zhou, J., , and K-M. Lau, 2001: Interannual and decadal variability of principal modes of summer rainfall over South America. Int. J. Climatol., 21 , 16231644.

    • Search Google Scholar
    • Export Citation
  • Zolman, J. L., , E. J. Zipser, , and K. I. Mohr, 2000: A comparison of tropical mesoscale convective systems in El Niño and La Niña. J. Climate, 13 , 33143326.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 16 16 4
PDF Downloads 15 15 4

Low-Level Jets and Their Effects on the South American Summer Climate as Simulated by the NCEP Eta Model

View More View Less
  • 1 Department of Meteorology, University of Maryland at College Park, College Park, Maryland
  • 2 Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland
© Get Permissions
Restricted access

Abstract

The National Centers for Environmental Prediction (NCEP) Eta Model (80 km, 38L) is used to simulate the tropical South American summer (January–March) climate for 1983, 1985, 1987, 1989, and 1991 using lateral boundary conditions from the NCEP–National Center for Atmospheric Research (NCAR) reanalysis. Simulations of the lower tropospheric circulation and precipitation are analyzed to study the variability on diurnal, intraseasonal, and interannual timescales. The results are compared with observations and previous studies.

The Eta Model produces better regional circulation details, such as low-level jets (LLJs), than does the reanalysis because of its higher resolution, more realistic topography and coastal geometry, and because of its ability to realistically simulate the effects of mesoscale circulation on the time-mean flow. The model detects not only the LLJ east of the Andes Mountains and the LLJ west of northern Cordillera Occidental, which have been reported in previous studies, but it also detects three distinct LLJs just north of the equator embedded in the strong northeasterly trade winds over Colombia, Venezuela, and Guiana. All the LLJs show strong diurnal variability with a nocturnal maximum. The LLJ east of the Andes Mountains brings warm moist air from the Amazon basin to the Gran Chaco region where the jet exits. The moisture convergence in the jet exit region creates favorable conditions for precipitation. Hence, the precipitation over the region also shows strong diurnal variability with a nocturnal maximum. The LLJs just north of the equator bring moisture from the tropical Atlantic Ocean, the western Caribbean Sea, and the Gulf of Panama to their exit regions over the northern Amazon basin and west coasts of Colombia and Ecuador. The precipitation over these regions also has diurnal variability with a nocturnal maximum. The diurnal variability of precipitation over most of the Tropics has an afternoon rainfall maximum except for regions influenced by LLJs, which have a nocturnal rainfall maximum. The intraseasonal variability of the LLJs is episodic with an approximate period of 20 days. The interannual variability of the LLJs is dominated by the ENSO cycle. The LLJ east of the Andes Mountains is stronger in the warm phase of ENSO than in the cold phase. However, the model has some difficulty simulating the observed relationship between the strength of LLJ and precipitation, but the model succeeds in the case of LLJs just north of the equator. For example, these LLJs are weaker in the warm phase of ENSO than in the cold phase. Hence, during the warm (cold) phase of ENSO, dry (wet) conditions normally occur over the northern part of the Amazon basin, which is the exit region of these LLJs.

Corresponding author address: Dr. Anandu D. Vernekar, Dept. of Meteorology, University of Maryland, College Park, MD 20742. Email: adv@atmos.umd.edu

Abstract

The National Centers for Environmental Prediction (NCEP) Eta Model (80 km, 38L) is used to simulate the tropical South American summer (January–March) climate for 1983, 1985, 1987, 1989, and 1991 using lateral boundary conditions from the NCEP–National Center for Atmospheric Research (NCAR) reanalysis. Simulations of the lower tropospheric circulation and precipitation are analyzed to study the variability on diurnal, intraseasonal, and interannual timescales. The results are compared with observations and previous studies.

The Eta Model produces better regional circulation details, such as low-level jets (LLJs), than does the reanalysis because of its higher resolution, more realistic topography and coastal geometry, and because of its ability to realistically simulate the effects of mesoscale circulation on the time-mean flow. The model detects not only the LLJ east of the Andes Mountains and the LLJ west of northern Cordillera Occidental, which have been reported in previous studies, but it also detects three distinct LLJs just north of the equator embedded in the strong northeasterly trade winds over Colombia, Venezuela, and Guiana. All the LLJs show strong diurnal variability with a nocturnal maximum. The LLJ east of the Andes Mountains brings warm moist air from the Amazon basin to the Gran Chaco region where the jet exits. The moisture convergence in the jet exit region creates favorable conditions for precipitation. Hence, the precipitation over the region also shows strong diurnal variability with a nocturnal maximum. The LLJs just north of the equator bring moisture from the tropical Atlantic Ocean, the western Caribbean Sea, and the Gulf of Panama to their exit regions over the northern Amazon basin and west coasts of Colombia and Ecuador. The precipitation over these regions also has diurnal variability with a nocturnal maximum. The diurnal variability of precipitation over most of the Tropics has an afternoon rainfall maximum except for regions influenced by LLJs, which have a nocturnal rainfall maximum. The intraseasonal variability of the LLJs is episodic with an approximate period of 20 days. The interannual variability of the LLJs is dominated by the ENSO cycle. The LLJ east of the Andes Mountains is stronger in the warm phase of ENSO than in the cold phase. However, the model has some difficulty simulating the observed relationship between the strength of LLJ and precipitation, but the model succeeds in the case of LLJs just north of the equator. For example, these LLJs are weaker in the warm phase of ENSO than in the cold phase. Hence, during the warm (cold) phase of ENSO, dry (wet) conditions normally occur over the northern part of the Amazon basin, which is the exit region of these LLJs.

Corresponding author address: Dr. Anandu D. Vernekar, Dept. of Meteorology, University of Maryland, College Park, MD 20742. Email: adv@atmos.umd.edu

Save