School of Earth Sciences - Research Publications
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ItemSynoptic controls upon δ18O in southern Tasmanian precipitation(American Geophysical Union, 2008)An event-based record of 18 O in precipitation at Margate in Tasmania, Australia, was analysed using 3d Lagrangian trajectories and composites of ERA40 850hPa geopotential height for the years 1994 - 2002. Trajectory analysis found that moisture entrainment occurs during the 48 hour period prior to arrival for all precipitating air masses at Margate. The rate of entrainment was greatest for events of high rainfall/high depletion, contributing up to 19% of vapour in the air mass in summer and up to 47% in winter. The majority of air masses were found to be advected from the Southern Ocean, however, for high rainfall/high depletion events during summer 30% of air masses were found to approach from the Tasman Sea (east of 155E longitude). Mean isotope ratios were less depleted by 1.4‰ for these events indicating mixing with less depleted air. High rainfall/high depletion events were also associated with negative geopotential anomalies east of Tasmania.
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ItemObservation and modeling of stable water isotopes as diagnostics of rainfall dynamics over southeastern Australia(AMER GEOPHYSICAL UNION, 2009-12-09)A unified approach of observation and modeling was applied to the investigation of three circulation types that typically bring rain to southeastern Australia. Observations from the Melbourne University Network of Isotopes in Precipitation of high‐resolution variations in the ratios of 18O and 2H were collected for (1) mixed frontal, (2) convective, and (3) stratiform precipitation events. Isotopic content of precipitation varied over both high and low frequencies because of influences from local variations in rain intensity and rainout by large‐scale precipitation. Deuterium excess showed a weak relationship with rainfall amount on intraevent time scales but was stronger under convective rainfall conditions. As a supplement to the observations, a version of the National Center for Atmospheric Research Community Atmosphere Model running an isotope hydrology scheme simulated the mixed frontal and stratiform events by nudging with reanalyses. The simulations represented well the evolution of vapor profiles of 18O and deuterium excess. Trajectories for the mixed frontal case illustrated the structure of the vapor profiles, revealing a convergence of air masses from different source regions. Deuterium excess in precipitation was represented less accurately by the model, indicating a possible shortcoming in the parameterization of postcondensation processes in the general circulation model. By combining observations and modeling in this way, detail of the structure and history of the events was provided that would be unavailable from the sampling of precipitation alone.
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ItemBiases in the calculation of Southern Hemisphere mean baroclinic eddy growth rate(American Geophysical Union, 2009)The maximum Eady growth rate measure of baroclinic instability is very commonly used in the literature. Its average is usually calculated directly from the time-mean flow. It is suggested here that this approach is not entirely suitable, but rather one should obtain the Eady growth rates at all relevant synoptic times and average these. It is found at the 850 hPa level in the Southern Hemisphere that the time-mean of the instantaneous rates exceed those calculated from the time-mean field over much of the mid and high latitudes, and the difference is even more marked at 500 hPa. At both levels the axes of the maxima Eady growth rates are displaced to the south. Some implications are discussed, including the need for caution when diagnosing changes in cyclone properties from changes in Eady growth rate calculated directly from the time-mean flow in climate change model simulations.
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ItemNew perspectives on the synoptic development of the severe October 1992 Nome storm(AMER GEOPHYSICAL UNION, 2009-07-14)Understanding the characteristics of storms that impact the Alaska region is of importance to emergency planning. The 5–7 October 1992 storm was a severe event which cost Nome, a town in Alaska, $6 million dollars. We will explore its characteristics with the aid of two established cyclone tracking schemes: the NOAA CPC current operational algorithm and the University of Melbourne algorithm. Manual tracking was performed as a control. The essential features are captured by both algorithms, but they differ in the genesis and lysis location. The NOAA algorithm broke the storm into two separate events. Synoptic development of the storm was influenced by a blocking high that affected how the tracking algorithms handled the event. A synoptic re‐examination of this storm is presented in terms of the depth, Laplacian and radius of the system. These new results present a fresh perspective on the intensity and longevity of this dramatic storm.
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ItemExtraordinary September Arctic sea ice reductions and their relationships with storm behavior over 1979-2008(American Geophysical Union, 2009)Dramatic changes have been observed in Arctic sea ice, cyclone behavior and atmospheric circulation in recent decades. Decreases in September ice extent have been remarkable over the last 30 years, and particularly so in very recent times. The analysis reveals that the trends and variability in September ice coverage and mean cyclone characteristics are related, and that the strength (rather than the number) of cyclones in the Arctic basin is playing a central role in the changes observed in that region, especially in the last few years. The findings reinforce suggestions that the decline in the extent and thickness of Arctic ice has started to render it particularly vulnerable to future anomalous cyclonic activity and atmospheric forcing.