The contributions of fronts, lows and thunderstorms to southern Australian rainfall

This is the final version. Available from the publisher via the DOI in this record.The correction to this article is available in ORE at http://hdl.handle.net/10871/126081A systematic analysis of the main weather types influencing southern Australian rainfall is presented for the period 1979–2015. This incorporates two multi-method datasets of cold fronts and low pressure systems, which indicate the more robust fronts and lows as distinguished from the weaker and less impactful events that are often indicated only by a single method. The front and low pressure system datasets are then combined with a dataset of environmental conditions associated with thunderstorms, as well as datasets of warm fronts and high pressure systems. The results demonstrate that these weather types collectively account for about 86% of days and more than 98% of rainfall in Australia south of 25° S. We also show how the key rain-bearing weather systems vary throughout the year and for different regions, with the co-occurrence of simultaneous lows, fronts and thunderstorm conditions particularly important during the spring and summer months in southeast Australia

Are greenhouse gas signals of Northern Hemisphere winter extra-tropical cyclone activity dependent on the identification and tracking algorithm?

For Northern Hemisphere extra-tropical cyclone activity, the dependency of a potential anthropogenic climate change signal on the identification method applied is analysed. This study investigates the impact of the used algorithm on the changing signal, not the robustness of the climate change signal itself. Using one single transient AOGCM simulation as standard input for eleven state-of-the-art identification methods, the patterns of model simulated present day climatologies are found to be close to those computed from re-analysis, independent of the method applied. Although differences in the total number of cyclones identified exist, the climate change signals (IPCC SRES A1B) in the model run considered are largely similar between methods for all cyclones. Taking into account all tracks, decreasing numbers are found in the Mediterranean, the Arctic in the Barents and Greenland Seas, the mid-latitude Pacific and North America. Changing patterns are even more similar, if only the most severe systems are considered: the methods reveal a coherent statistically significant increase in frequency over the eastern North Atlantic and North Pacific. We found that the differences between the methods considered are largely due to the different role of weaker systems in the specific methods

IMILAST: a community effort to intercompare extratropical cyclone detection and tracking algorithms

The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of weak cyclones, and distribution in some densely populated regions. Consistency between methods is better for strong cyclones than for shallow ones. Two case studies of relatively large, intense cyclones reveal that the identification of the most intense part of the life cycle of these events is robust between methods, but considerable differences exist during the development and the dissolution phases

Variability and Trends of Global Atmospheric Frontal Activity and Links with Large-Scale Modes of Variability

Abstract Presented here is a global analysis of frontal activity variability derived from ERA-Interim data over the 34-yr period of January 1979–March 2013 using a state-of-the-art frontal tracking scheme. In December–February over that epoch, there is a northward shift of frontal activity in the Pacific in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the largest trends are identified in the austral summer and are manifested by a southward shift of frontal activity over the Southern Ocean. Variability of frontal behavior is found to be closely related to the main modes of atmospheric circulation, such as the North Atlantic Oscillation (NAO) for the Atlantic–European sector in the NH and the southern annular mode (SAM) in the middle and high latitudes of the SH. A signal associated with El Niño and hence emanating from the tropics is also apparent in the behavior of frontal systems over the Pacific by a reduction in the number of fronts in the middle South Pacific and intensification of frontal activity in high and low latitudes throughout the year. It is shown in general that the associations of the large-scale modes with frontal variability are much stronger than with cyclones. This indicates that the quantification of the behavior of fronts is an important component of understanding the climate system. At the very high latitudes, it is also shown here that, in the recent years of rapid sea ice reduction in the Arctic, there have been fewer summer fronts observed over the Canadian Arctic

Sub-synoptic-scale features associated with extreme surface gusts during the South Australia storm of September 2016 – part I: characteristics of the event

Winds are one of the major meteorological contributors to deaths, damage and insured losses in Australia. A ‘freak storm’ hit the state of South Australia on 28 September 2016, causing state‐wide blackouts and leaving 1.7 million people without power. In the first part of this two‐part study, we analyse this event and find that it was indeed extreme, deepening more explosively than all but two Adelaide‐affecting extratropical cyclones over the past 37 years and exhibiting the lowest central pressure. This generated hurricane force winds, with the central South Australia site of Neptune Island recording a gust of over 120kmh−1. We show that this storm potentially contained a sting jet. Such jets are well known as a cause of major damage across Europe, and this is the first study which investigates whether a sting jet can be produced over Australia. The main deepening of the system occurred over the Great Australian Bight, so if a sting jet did form and make it to the surface, it was not the cause of the state‐wide damage. However, the cyclone did contain numerous extreme gust‐producing mesoscale features, as explored in part II of this paper (Earl and Simmons, 2018)

Antarctic Peninsula warm winters influenced by Tasman Sea temperatures

The Antarctic Peninsula of West Antarctica was one of the most rapidly warming regions on the Earth during the second half of the 20th century. Changes in the atmospheric circulation associated with remote tropical climate variabilities have been considered as leading drivers of the change in surface conditions in the region. However, the impacts of climate variabilities over the mid-latitudes of the Southern Hemisphere on this Antarctic warming have yet to be quantified. Here, through observation analysis and model experiments, we reveal that increases in winter sea surface temperature (SST) in the Tasman Sea modify Southern Ocean storm tracks. This, in turn, induces warming over the Antarctic Peninsula via planetary waves triggered in the Tasman Sea. We show that atmospheric response to SST warming over the Tasman Sea, even in the absence of anomalous tropical SST forcing, deepens the Amundsen Sea Low, leading to warm advection over the Antarctic Peninsula

The sensitivity of characteristics of cyclone activity to identification procedures in tracking algorithms

The IMILAST project (‘Intercomparison of Mid-Latitude Storm Diagnostics’) was set up to compare low-level cyclone climatologies derived from a number of objective identification algorithms. This paper is a contribution to that effort where we determine the sensitivity of three key aspects of Northern Hemisphere cyclone behaviour [namely the number of cyclones, their intensity (defined here in terms of the central pressure) and their deepening rates] to specific features in the automatic cyclone identification. The sensitivity is assessed with respect to three such features which may be thought to influence the ultimate climatology produced (namely performance in areas of complicated orography, time of the detection of a cyclone, and the representation of rapidly propagating cyclones). We make use of 13 tracking methods in this analysis. We find that the filtering of cyclones in regions where the topography exceeds 1500 m can significantly change the total number of cyclones detected by a scheme, but has little impact on the cyclone intensity distribution. More dramatically, late identification of cyclones (simulated by the truncation of the first 12 hours of cyclone life cycle) leads to a large reduction in cyclone numbers over the both continents and oceans (up to 80 and 40%, respectively). Finally, the potential splitting of the trajectories at times of the fastest propagation has a negligible climatological effect on geographical distribution of cyclone numbers. Overall, it has been found that the averaged deepening rates and averaged cyclone central pressure are rather insensitive to the specifics of the tracking procedure, being more sensitive to the data set used (as shown in previous studies) and the geographical location of a cyclone

Seasonal aspects of an objective climatology of anticyclones affecting the mediterranean

An objective climatology of anticyclones over the greater Mediterranean region is presented based on the Interim ECMWF Re-Analysis (ERA-Interim) for a 34-yr period (1979-2012) and the Melbourne University automatic identification and tracking algorithm. The scheme's robustness and reliability for the transient extratropical propagation of anticyclones, with the appropriate choices of parameter settings, has been established and the results obtained here present new research perspectives on anticyclonic activity affecting the Mediterranean. Properties of Mediterranean anticyclones, such as frequency, generation and dissipation, movement, scale, and depth are investigated. The highest frequency of anticyclones is found over continental areas, while the highest maritime frequency occurs over closed basins exhibiting also maxima of anticyclogenesis. There is a significant seasonality in system density and anticyclogenesis maxima, this being associated with the seasonal variations of the larger-scale atmospheric circulation that affect the greater Mediterranean region. © 2014 American Meteorological Society