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T. D. Keenan

Abstract

A general method of objective tropical cyclone guidance is proposed in which the forecast variables are categories of tropical cyclone motion. Using this approach, information on more than one scenario is presented, a potentially important diagnostic consideration in uncertain forecast situations, e.g., when conflicts exist between aids or when a major track change is possible. As a first step to this type of forecasting, motion categories were defined using tercile ranges of zonal and meridional Australian region storm motion. Discriminant functions developed using both past track and current synoptic data then provided probability forecasts of each category. Forecasts for each zonal and meridional category were obtained 12, 24, 36 and 48 h in advance. With this simple approach, the accuracy of zonal (meridional) classification on the dependent data ranged from 76 (69)% at 12 h to 59 (57)% at 48 h. A no-skill category assignment would yield a figure of 33%. Classification accuracy was generally best in the below- and above-average groups where extreme storm motion and the largest forecast errors occurred.

Position forecasts were also derived for each category of motion using multiple linear regression equations. Thus, in addition to the probability information aimed at a general description of the storm's behavior, position forecasts specifically developed for each category of motion were also available. Objective use of these position forecasts resulted in lower errors than obtained with the best aid available to tropical cyclone forecasters in the Australian region, especially for situations in which some big forecast errors occurred, i.e., large movement cases.

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T. D. Keenan and L. R. Brody

Abstract

Time-longitude representations of Japanese Geostationary Meteorology Satellite (GMS) infrared imagery indicate the existence of major synoptic-scale banding of convection within the Australian summer monsoon. This bands can be interpreted as active and break phases of major convective activity within the monsoon. This study relates the occurrence of convection and its organization into synoptic-scale bands to observable flow features.

GMS Digital Equivalent Blackbody Temperature (TBB) data and wind fields from the Australian Numerical Meteorology Research Centre (ANMRC) tropical analysis scheme for the 1978/79 and 1983/84 seasons were composited relative to wind field surges and bands of enhanced and suppressed convective activity. Some low-level wind surges in the South China Sea produced a modulation in the convective activity within the preexisting bands but did not seem to be associated with their formation. Surges in the Southern Hemisphere trade-wind easterlies and the southerly jet off the west coast of Australia were not associated with any major change in convective activity. The organization of the convection into synoptic-scale bands was associated with the Southern Hemisphere 200 mb flow. Areas of enhanced convective banding were cast of upper-level tropospheric troughs. The troughs and associated subtropical jet streaks had amplified from the south, interacting and enhancing the monsoonal convection. West of the trough, in the region of subsiding air, the convection was suppressed. Independent studies taken from the 1984/85 season showed that this type of interaction was discernable for individual cases.

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T. D. Keenan and J. I. Templeton

Abstract

Radiosonde soundings obtained within 180 km of tropical cyclones in the Australian region are compared to similar sets of hurricane and typhoon data. The mean storm profiles in each region reflect the background structure of each storm basin. Comparison of the storm groups as deviations from the background profiles, reveals a very similar structure, although some unresolved differences are found in the upper troposphere. The mean tropical cyclone sounding shows boundary layer characteristics consistent with the continental location of most Australian radiosonde stations. A mean tropical cyclone profile derived from island data only, shows more boundary layer moisture and greater potential instability than the mean sounding obtained from all the Australian data.

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Peter T. May and Thomas D. Keenan

Abstract

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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T. D. Keenan and R. E. Carbone

Abstract

Warm season cold cloud-top climatology in the Austral–Indonesian region is examined for evidence of propagating modes of precipitation that originate from elevated heat sources and the diurnal heating cycle. Using satellite-inferred cloudiness from the period 1996–2001 as a proxy for rainfall, this coherent regeneration process and subsequent event propagation is found to consistently occur from the midlatitudes (30°–40°S) to the tropics (10°–20°S) in the Austral region.

Given favorable environmental shear at midlatitudes, long-lived eastward-propagating events are observed to occur regularly with a span and duration typically larger than observed by Carbone et al. The genesis of these events, while intermittent, is directly related to elevated heat sources and the diurnal cycle, similar to the United States. However, given the relatively flat terrain of Australia, an elevated heat source is often insufficient, thus increasing the relative influence of transient synoptic forcing.

In the tropics, the thermal forcing associated with elevated terrain found over the islands of the Maritime Continent and the land–sea interface is increasingly dominant on daily basis. While eastward- and westward-propagating events are found in the more varied environment of the monsoon regime, evidence for meridionally propagating modes is also found. In this manner, complex interactions occur that modify the location and timing of clouds that develop over neighboring oceanic and continental locations. The impact of convection initially linked to the New Guinea highlands and subsequently impacting the Java Sea region is particularly evident affecting the observed diurnal cycle.

The subtropics show characteristics intermediate between the above extremes. With the seasonal cycle, the spring environment favors eastward-propagating events but in summer there is an increasing frequency of diurnally forced quasi-stationary development over elevated terrain enhanced by favorable synoptic conditions. Overall the subtropical summer events have a shorter duration and span than their spring counterparts. The increased environmental steering winds and shear in spring are thought to be the primary reason.

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Peter T. May, J. D. Kepert, and T. D. Keenan

Abstract

Tropical Cyclone Ingrid had a distinctly asymmetric reflectivity structure with an offshore maximum as it passed parallel to and over an extended coastline near a polarimetric weather radar located near Darwin, northern Australia. For the first time in a tropical cyclone, polarimetric weather radar microphysical analyses are used to identify extensive graupel and rain–hail mixtures in the eyewall. The overall microphysical structure was similar to that seen in some other asymmetric storms that have been sampled by research aircraft. Both environmental shear and the land–sea interface contributed significantly to the asymmetry, but their relative contributions were not determined. The storm also underwent very rapid changes in tangential wind speed as it moved over a narrow region of open ocean between a peninsula and the Tiwi Islands. The time scale for changes of 10 m s−1 was of the order of 1 h. There were also two distinct types of rainbands observed—large-scale principal bands with embedded deep convection and small-scale bands located within 50 km of the eyewall with shallow convective cells.

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R. J. Potts, T. D. Keenan, and P. T. May

Abstract

The characteristics of radar echoes for 12 thunderstorm days in the vicinity of Sydney, Australia, in the summer of 1995/96 have been examined using an objective methodology for storm identification and tracking. The spatial distribution of identified storms shows a maximum in frequency and intensity along the east side of the mountains that lie inland from the coast. Characteristics such as storm volume, area, and height are shown to have a lognormal frequency distribution. Reflectivity also has a skewed frequency distribution with a prevalence of lower reflectivity storms. Both the maximum reflectivity and storm height are shown to be correlated with the logarithm of storm volume. Although small storms predominate, the bulk of precipitation flux comes from the relatively few large-scale storms. It is also shown that storms generally move or propagate in a direction slightly to the left of the mass-weighted mean wind for the surface-to-300-hPa layer at a speed slightly less than the mean speed. Furthermore the deviation of the storm to the left of the mean layer wind increases and the standard deviation decreases as the storm size increases.

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N. A. Shaw, I. A. Bourne, and T. D. Keenan

Abstract

Acoustic radar records are reproduced to show the effects which may result from varying system parameters such as transmitted power, pulse length, pulse repetition frequency and recording techniques. Improved results can often be achieved by such variation but examples of records which are open to misinterpretation are also given.

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P. T. May, T. D. Keenan, D. S. Zrnić, L. D. Carey, and S. A. Rutledge

Abstract

A 5-cm wavelength (C band) polarimetric radar was deployed during the MCTEX (Maritime Continent Thunderstorm Experiment) field program. This paper investigates the use of the C-band data for quantitative rainfall measurements with particular emphasis on specific differential phase (K DP) and traditional reflectivity-based rain-rate estimates in moderate to high rain rates (10–200 mm h−1). Large values of backscatter differential phase shift are occasionally seen in these data, thus resonance scattering effects are important. A consensus algorithm for K DP estimation in these cases is described. The rain-rate estimates are compared with the data from a d-scale rain gauge network. The K DP estimates are shown to produce the highest quality data, although variations in drop size distribution characteristics have a significant effect on the rain estimates. When corrections are applied for beam blockage and attenuation, good agreement can also be obtained with Z–R-based estimates. The attenuation corrections were made using a polarimetric variable, total differential phase, which provides an estimate of the total water content along the path. The polarimetric estimates of total accumulation also show excellent agreement.

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T. D. Keenan, L. D. Carey, D. S. Zrnić, and P. T. May

Abstract

The sensitivity of polarimetric variables at a 5-cm wavelength to raindrop size and axial ratio is examined using T-matrix modeling of the scattering process for gamma raindrop size distributions fitted to tropical rainfall collected at Darwin, Australia. These simulations demonstrate that, while specific differential phase (K DP)–based estimates of rainfall, attenuation (A H), and differential attenuation are less affected by drop size distribution (DSD) variations, large drop occurrence can have significant impacts. Attenuation is sensitive to the occurrence of large drops, which can produce anomalously high values associated with resonance effect scattering. The polarimetric variables are sensitive to the relation between the equivolume diameter and axial ratio. Variations in the assumed form of the raindrop axial ratio can result in significant biases in rainfall and attenuation. Combined rainfall estimators, which include differential reflectivity (Z DR), such as R(K DP, Z DR) and R(A H, Z DR) are more robust to both DSD and raindrop axial ratio variations. The results also demonstrate that polarimetric techniques employed to classify the phase of hydrometeors are sensitive to the assumed raindrop axial ratio.

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