Characteristics of Tropical Convection over the Ocean near Kwajalein

Jasmine Cetrone Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Robert A. Houze Jr. Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

Radar observations have been analyzed to determine characteristics of convection over the oceanic region around Kwajalein in the tropical western Pacific. Generally, the echo areas, heights, and durations exhibited lognormal distributions. Heights were greater under conditions of higher midtropospheric humidity and correlated with echo area, with a wide spread of values. Mergers and splits often truncated echo lifetimes.

The most surprising result was the distribution of orientation angles of echo lines, which statistically verify the shear-parallel and shear-normal modes of convective line organization seen elsewhere over tropical oceans. The two modes typically coexisted within the area of radar coverage, indicating a potential difficulty in predicting line orientation in terms of large-scale variables.

The largest members of the echo population were mesoscale convective systems (MCSs), which had large stratiform components. They occurred in environments of increased humidity and higher midlevel buoyancy associated with westward-propagating synoptic-scale disturbances. MCSs moved with the easterlies in low to midlevels and exhibited the jump updraft, overturning updraft, and subsiding midlevel inflow characteristic of tropical oceanic MCSs.

The Kwajalein Experiment (KWAJEX) radar echo population resembles that of the eastern tropical Atlantic in terms of the shapes of the distributions of area, height, and lifetime, the prevalence of echo mergers and splits, and the tendency to form shear-parallel and shear-normal lines. These characteristics appear to be endemic to oceanic convective populations. Large-scale conditions appear to modulate these basic population characteristics rather than qualitatively alter them.

Corresponding author address: Jasmine Cetrone, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: jasmine@atmos.washington.edu

Abstract

Radar observations have been analyzed to determine characteristics of convection over the oceanic region around Kwajalein in the tropical western Pacific. Generally, the echo areas, heights, and durations exhibited lognormal distributions. Heights were greater under conditions of higher midtropospheric humidity and correlated with echo area, with a wide spread of values. Mergers and splits often truncated echo lifetimes.

The most surprising result was the distribution of orientation angles of echo lines, which statistically verify the shear-parallel and shear-normal modes of convective line organization seen elsewhere over tropical oceans. The two modes typically coexisted within the area of radar coverage, indicating a potential difficulty in predicting line orientation in terms of large-scale variables.

The largest members of the echo population were mesoscale convective systems (MCSs), which had large stratiform components. They occurred in environments of increased humidity and higher midlevel buoyancy associated with westward-propagating synoptic-scale disturbances. MCSs moved with the easterlies in low to midlevels and exhibited the jump updraft, overturning updraft, and subsiding midlevel inflow characteristic of tropical oceanic MCSs.

The Kwajalein Experiment (KWAJEX) radar echo population resembles that of the eastern tropical Atlantic in terms of the shapes of the distributions of area, height, and lifetime, the prevalence of echo mergers and splits, and the tendency to form shear-parallel and shear-normal lines. These characteristics appear to be endemic to oceanic convective populations. Large-scale conditions appear to modulate these basic population characteristics rather than qualitatively alter them.

Corresponding author address: Jasmine Cetrone, Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: jasmine@atmos.washington.edu

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