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Thomas D. Keenan
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Thomas D. Keenan and Michael Fiorino

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The Fleet Numerical Oceanography Center (FNOC) Numerical Variational Analysis (NVA) has been used to develop statistical–synoptic tropical cyclone forecast schemes for the Southern Hemisphere. Results, from a limited sample, showed that the best objective techniques were at least as good as the official Joint Typhoon Warning Center (JTWC) forecasts.

Development and testing of the aids also indicated that approaches used in the Northern Hemisphere did not necessarily lead to better forecasts. Geographically orientated, rather than track-orientated coordinate systems, were found to have lower errors, especially with operational data. Stratification of the datasets by geographical location did not result in any significant improvements in the forecasting accuracy. Similarly, use of prediction equations based on storm position relative to the subtropical ridge did not result in the improvement found for the Northern Hemisphere.

A comparison of techniques using height versus wind field data for the synoptic forcing component of the equations showed lowest errors with the mass field predictors. However, the differences were not statistically significant and this finding may have been a result of data quality rather than the field type.

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Greg J. Holland and Thomas D. Keenan

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Thomas D. Keenan and Steven A. Rutledge

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Observations undertaken on 12 January 1990 at Darwin, Australia (12°S, 130°E), are used to document the structure of a monsoonal rainband in a low-CAPE, low-shear tropical environment. Dual-Doppler radar analyses are employed to investigate the structure and kinematics of the convective and stratiform regions. A system with the characteristics of a relatively short-lived squall line in which warm rain processes play a significant role in the production of precipitation is evident. Planetary boundary layer cold-pool production is important in the organization and motion of the system. A trailing stratiform region is evident with a mean updraft-downdraft circulation, but is composed of in situ decaying convective cells. A storm-relative mesoscale cyclonic circulation is also observed within the stratiform cloud. This vortex was maintained by thermodynamically induced midlevel convergence, convectively generated storm-scale circulations, and their interaction with the background monsoon now.

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Peter T. May and Thomas D. Keenan

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Polarimetric radar data have been used to produce microphysical classifications. This kind of analysis is run in a real-time mode from several research radars, including the C-band polarimetric (C-Pol) radar in Darwin, Australia. However, these classifications have had very little systematic evaluation with independent data. Using surface data is often difficult because of sampling issues, particularly for hail. The approach taken here is to use a combination of 50- and 920-MHz wind profiler estimates of rain and hail to provide validation data for the radar pixels over the profiler. The profilers also observe signals associated with lightning, and some comparisons are made between lightning occurrence and the radar measurements of graupel. The retrievals of hail–rain mixtures are remarkably robust; there are some issues regarding other microphysical classes, however, including difficulties in detecting melting snow layers in stratiform rain. These difficulties are largely due to the resampling of the radar volume data onto a grid and to poor separation of the snow classes.

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Greg J. Holland, Thomas D. Keenan, and Geoff D. Crane

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Observations are presented of a phenomenal upper tropospheric mesoscale temperature perturbation arising from the interaction between dissipating Tropical Cyclone Kerry (1979) and a midlatitude trough in the westerlies, which we describe as a black hole from its appearance on satellite imagery. We propose that this perturbation arose from dynamically forced subsidence along a confluence between the environments flow and outflow from a major convective complex. The frequency of occurrence and subsequent adjustment of the atmosphere is described and discussed.

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Steven A. Rutledge, Earle R. Williams, and Thomas D. Keenan

DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carried out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the “maritime continent,” experiences both monsoon and “break” period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potentional energy) and total flash rate is presented and discussed to explain this observation.

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Peter T. May, A. R. Jameson, Thomas D. Keenan, and Paul E. Johnston

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This paper describes the results of an experiment that combines the data from a 5-cm-wavelength polarimetric radar and multiple-frequency wind profilers to examine the polarimetric signatures associated with the microphysical structure of several relatively shallow thunderstorms and also to examine quantitative rainfall measurements made with the polarimetric radar. These shallow storms produce considerable amounts of centimeter-sized hail. The presence and size of this hail are deduced from the wind profiler data. The melting hail particles produce a distinctive polarimetric signature with large values of differential reflectivity Z DR and suppressed values of the correlation coefficient between the signals at horizontal and vertical polarization. Comparisons between the mass-weighted mean drop diameter and differential reflectivity have been performed and show reasonable agreement with theoretical expectations, although the observed Z DR are somewhat smaller than expected. This may be associated with the theoretical assumption of the Pruppacher–Beard oblateness relationship even though there is evidence to suggest that real raindrops may be less oblate on average in convective rain. Quantitative polarimetric rainfall estimators have been compared with rainfall rates derived from the profiler drop size distribution retrievals and show reasonably good agreement when reflectivity values are matched.

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Peter T. May, A. R. Jameson, Thomas D. Keenan, Paul E. Johnston, and Chris Lucas

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An experiment combining wind profiler and polarimetric radar analyses of intense, but shallow, tropical thunderstorms has been performed. These storms are important as they are very common along many tropical coasts and islands and are sometimes the precursors to large intense multicellular storms such as occur over the Tiwi Islands north of Darwin, Australia. All the storms sampled had a similar structure, with intense updrafts on the periphery of the cells producing significant-sized hail and downdrafts in the storm center. The hail concentrations are relatively small, but have a large effect on the radar reflectivity and polarimetric measurands because of the size (10–20 mm). It is this hail melting that produces characteristic Z DR columns in the polarimetric radar data.

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Richard H. Johnson, Steven L. Aves, Paul E. Ciesielski, and Thomas D. Keenan

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The organizational modes of convection over the northern South China Sea (SCS) during the onset of the summer monsoon are documented using radar and sounding data from the May–June 1998 South China Sea Monsoon Experiment (SCSMEX). The onset occurred in mid-May with a rapid increase in deep convection over a 10-day period, accompanied by a major shift in the circulation over the east Asian region.

Analysis of Bureau of Meteorology Research Centre (BMRC) radar data from Dongsha Island reveals a wide range of organizational modes of convection over the northern SCS. Proximity sounding data indicate that lower- and middle-level vertical wind shears exerted a dominant control over the orientation of convective lines within mesoscale convective systems in this region, as has been found in the Australian monsoon region and the equatorial western Pacific. The results are consistent with the conceptual model of LeMone et al. based on the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), except two new organizational modes have been identified: shear-parallel bands for strong low-level shear and weak midlevel shear when there is weak instability and the air is dry aloft, and shear-parallel bands for strong shears in both layers when the shear vectors are in the same direction. Midlatitude influences, namely, the passage of troughs over southern China, likely contributed to these two additional modes.

The stratiform rain fraction from the convective systems during the monsoon onset period was relatively small (26%) compared to the estimated average of about 40% for the entire Tropics. This small fraction is attributed to the weak instability during the onset period and relatively dry air in the upper troposphere.

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