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Jasmine Cetrone and Robert A. Houze Jr.

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.

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Jasmine Cetrone and Robert A. Houze Jr.

Abstract

The anvil clouds of tropical squall-line systems over West Africa have been examined using cloud radar data and divided into those that appear ahead of the leading convective line and those on the trailing side of the system. The leading anvils are generally higher in altitude than the trailing anvil, likely because the hydrometeors in the leading anvil are directly connected to the convective updraft, while the trailing anvil generally extends out of the lower-topped stratiform precipitation region. When the anvils are subdivided into thick, medium, and thin portions, the thick leading anvil is seen to have systematically higher reflectivity than the thick trailing anvil, suggesting that the leading anvil contains numerous larger ice particles owing to its direct connection to the convective region. As the leading anvil ages and thins, it retains its top. The leading anvil appears to add hydrometeors at the highest altitudes, while the trailing anvil is able to moisten a deep layer of the atmosphere.

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Peter N. Blossey, Christopher S. Bretherton, Jasmine Cetrone, and Marat Kharoutdinov

Abstract

Three-dimensional cloud-resolving model simulations of a mesoscale region around Kwajalein Island during the Kwajalein Experiment (KWAJEX) are performed. Using observed winds along with surface and large-scale thermodynamic forcings, the model tracks the observed mean thermodynamic soundings without thermodynamic nudging during 52-day simulations spanning the whole experiment time period, 24 July–14 September 1999. Detailed comparisons of the results with cloud and precipitation observations, including radar reflectivities from the Kwajalein ground validation radar and International Satellite Cloud Climatology Project (ISCCP) cloud amounts and radiative fluxes, reveal the biases and sensitivities of the model’s simulated clouds. The amount and optical depth of high cloud are underpredicted by the model during less rainy periods, leading to excessive outgoing longwave radiation (OLR) and insufficient albedo. The simulated radar reflectivities tend to be excessive, especially in the upper troposphere, suggesting that simulated high clouds are precipitating large hydrometeors too efficiently. Occasionally, large-scale advective forcing errors also seem to contribute to upper-level cloud and relative humidity biases. An extensive suite of sensitivity studies to different microphysical and radiative parameterizations is performed, with surprisingly little impact on the results in most cases.

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Robert A. Houze Jr., Shuyi S. Chen, Wen-Chau Lee, Robert F. Rogers, James A. Moore, Gregory J. Stossmeister, Michael M. Bell, Jasmine Cetrone, Wei Zhao, and S. Rita Brodzik

The Hurricane Rainband and Intensity Change Experiment (RAINEX) used three P3 aircraft aided by high-resolution numerical modeling and satellite communications to investigate the 2005 Hurricanes Katrina, Ophelia, and Rita. The aim was to increase the understanding of tropical cyclone intensity change by interactions between a tropical cyclone's inner core and rainbands. All three aircraft had dual-Doppler radars, with the Electra Doppler Radar (ELDORA) on board the Naval Research Laboratory's P3 aircraft, providing particularly detailed Doppler radar data. Numerical model forecasts helped plan the aircraft missions, and innovative communications and data transfer in real time allowed the flights to be coordinated from a ground-based operations center. The P3 aircraft released approximately 600 dropsondes in locations targeted for optimal coordination with the Doppler radar data, as guided by the operations center. The storms were observed in all stages of development, from tropical depression to category 5 hurricane. The data from RAINEX are readily available through an online Field Catalog and RAINEX Data Archive. The RAINEX dataset is illustrated in this article by a preliminary analysis of Hurricane Rita, which was documented by multiaircraft flights on five days 1) while a tropical storm, 2) while rapidly intensifying to a category 5 hurricane, 3) during an eye-wall replacement, 4) when the hurricane became asymmetric upon encountering environmental shear, and 5) just prior to landfall.

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