Tropical South America–Atlantic Sector Convective Margins and Their Relationship to Low-Level Inflow

Benjamin R. Lintner Department of Atmospheric and Oceanic Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California

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J. David Neelin Department of Atmospheric and Oceanic Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California

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Abstract

The edges or margins of tropical convective zones are hypothesized to be sensitive to low-level inflow conditions. The present study evaluates where and to what extent convective margin variability is sensitive to low-level inflow variability using observed precipitation and reanalysis wind and total precipitable water data over the tropical South America–Atlantic sector in austral summer. Composite analysis based on an inflow measure defined by projecting low-level monthly-mean atmospheric boundary layer (ABL) or lower free troposphere (LFT) winds onto either mean horizontal precipitation or precipitable water gradients shows widespread contraction of the edges of convection zones in the direction of stronger convection for anomalously strong low-level inflow; such behavior is consistent with enhanced import of relatively dry air along the edges of convection zones. However, the distinction between ABL and LFT winds may be significant regionally, for example, along the Atlantic ITCZ’s northern margin. Back trajectory analysis is employed to estimate source regions of low-level air masses arriving at margin points over time scales (2–4 days) during which low-level air masses are expected to retain some memory of initial moisture conditions while also undergoing diabatic modification. Probability distribution functions of mean precipitation values encountered along trajectories facilitate objective quantification of the frequency with which trajectories approach the margin from drier areas outside the convection zone. While margin points in the ABL are strongly dominated by inflow (i.e., trajectories originating outside of the convection zone), points in the LFT may show inflow, outflow, or mixed inflow–outflow conditions. LFT locations dominated by inflow trajectories generally correspond to regions with composites exhibiting the clearest signatures of LFT wind variability on precipitation.

* Current affiliation: Department of Environmental Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey

Corresponding author address: Dr. Benjamin R. Lintner, Department of Environmental Sciences, Rutgers, The State University of New Jersey, 14 College Farm Road, New Brunswick, NJ 08901-8551. Email: lintner@envsci.rutgers.edu

Abstract

The edges or margins of tropical convective zones are hypothesized to be sensitive to low-level inflow conditions. The present study evaluates where and to what extent convective margin variability is sensitive to low-level inflow variability using observed precipitation and reanalysis wind and total precipitable water data over the tropical South America–Atlantic sector in austral summer. Composite analysis based on an inflow measure defined by projecting low-level monthly-mean atmospheric boundary layer (ABL) or lower free troposphere (LFT) winds onto either mean horizontal precipitation or precipitable water gradients shows widespread contraction of the edges of convection zones in the direction of stronger convection for anomalously strong low-level inflow; such behavior is consistent with enhanced import of relatively dry air along the edges of convection zones. However, the distinction between ABL and LFT winds may be significant regionally, for example, along the Atlantic ITCZ’s northern margin. Back trajectory analysis is employed to estimate source regions of low-level air masses arriving at margin points over time scales (2–4 days) during which low-level air masses are expected to retain some memory of initial moisture conditions while also undergoing diabatic modification. Probability distribution functions of mean precipitation values encountered along trajectories facilitate objective quantification of the frequency with which trajectories approach the margin from drier areas outside the convection zone. While margin points in the ABL are strongly dominated by inflow (i.e., trajectories originating outside of the convection zone), points in the LFT may show inflow, outflow, or mixed inflow–outflow conditions. LFT locations dominated by inflow trajectories generally correspond to regions with composites exhibiting the clearest signatures of LFT wind variability on precipitation.

* Current affiliation: Department of Environmental Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey

Corresponding author address: Dr. Benjamin R. Lintner, Department of Environmental Sciences, Rutgers, The State University of New Jersey, 14 College Farm Road, New Brunswick, NJ 08901-8551. Email: lintner@envsci.rutgers.edu

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