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    Sub-synoptic scale features associated with extreme surface gusts during the South Australia Storm of September 2016-Part I: characteristics of the event
    Earl, N ; Simmonds, I ; Rudeva, I (WILEY, 2019-08)
    Abstract 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).
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    Sub synoptic scale features of the South Australia Storm of September 2016-Part II: analysis of mechanisms driving the gusts
    Earl, N ; Simmonds, I (WILEY, 2019-09)
    Abstract An extreme extratropical cyclone (ETC) struck South Australia on 28 September 2016, causing state‐wide blackouts and damage. In the second part of this two‐part study, we examine the extreme surface wind producing mechanisms within the ETC. ETCs have been extensively studied in the Northern Hemisphere (particularly in western Europe), highlighting the gust‐producing mesoscale features within. Before now, no Southern Hemisphere ETC has been examined in this way. There were a number of extreme gust‐producing features within the ETC, comparable to those observed in storms over western Europe. One such feature was a convective line, which caused many of the most extreme gusts and knocked out the state power grid. However, dry slot convection also contributed to the extremes, and this feature rarely causes extreme gusts in ETCs over the UK. Thus, further analysis is warranted to examine whether this is a common extreme‐gust‐producing ETC feature over Southern Australia. The strongest winds recorded throughout the event occurred on 29 September, and these were associated with the cold conveyor belt which spiralled around the low‐pressure centre.
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    Spatial and Temporal Variability and Trends in 2001-2016 Global Fire Activity
    Earl, N ; Simmonds, I (AMER GEOPHYSICAL UNION, 2018-03-16)
    Abstract Fire regimes across the globe have great spatial and temporal variability, and these are influence by many factors including anthropogenic management, climate, and vegetation types. Here we utilize the satellite‐based “active fire” product, from Moderate Resolution Imaging Spectroradiometer (MODIS) sensors, to statistically analyze variability and trends in fire activity from the global to regional scales. We split up the regions by economic development, region/geographical land use, clusters of fire‐abundant areas, or by religious/cultural influence. Weekly cycle tests are conducted to highlight and quantify part of the anthropogenic influence on fire regime across the world. We find that there is a strong statistically significant decline in 2001–2016 active fires globally linked to an increase in net primary productivity observed in northern Africa, along with global agricultural expansion and intensification, which generally reduces fire activity. There are high levels of variability, however. The large‐scale regions exhibit either little change or decreasing in fire activity except for strong increasing trends in India and China, where rapid population increase is occurring, leading to agricultural intensification and increased crop residue burning. Variability in Canada has been linked to a warming global climate leading to a longer growing season and higher fuel loads. Areas with a strong weekly cycle give a good indication of where fire management is being applied most extensively, for example, the United States, where few areas retain a natural fire regime.
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    A high-resolution climatological study on the comparison between surface explosive and ordinary cyclones in the Mediterranean
    Kouroutzoglou, J ; Flocas, HA ; Hatzaki, M ; Keay, K ; Simmonds, I (SPRINGER HEIDELBERG, 2014-10)
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    Large scale features and energetics of the hybrid subtropical low 'Duck' over the Tasman Sea
    Pezza, AB ; Garde, LA ; Paixao Veiga, JA ; Simmonds, I (SPRINGER, 2014-01)
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    Sensitivity of the distribution of thunderstorms to sea surface temperatures in four Australian east coast lows
    Chambers, CRS ; Brassington, GB ; Walsh, K ; Simmonds, I (SPRINGER WIEN, 2015-10)
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    Vertical characteristics of cyclonic tracks over the eastern Mediterranean during the cold period of the year
    Flocas, HA ; Kountouris, P ; Kouroutzoglou, J ; Hatzaki, M ; Keay, K ; Simmonds, I (SPRINGER WIEN, 2013-05)
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    Polar Climate Change as Manifest in Atmospheric Circulation
    Screen, JA ; Bracegirdle, TJ ; Simmonds, I (SPRINGER HEIDELBERG, 2018-12)
    PURPOSE OF REVIEW: Dynamic manifestations of climate change, i.e. those related to circulation, are less well understood than are thermodynamic, or temperature-related aspects. However, this knowledge gap is narrowing. We review recent progress in understanding the causes of observed changes in polar tropospheric and stratospheric circulation, and in interpreting climate model projections of their future changes. RECENT FINDINGS: Trends in the annular modes reflect the influences of multiple drivers. In the Northern Hemisphere, there appears to be a "tug-of-war" between the opposing effects of Arctic near-surface warming and tropical upper tropospheric warming, two predominant features of the atmospheric response to increasing greenhouse gases. Future trends in the Southern Hemisphere largely depend on the competing effects of stratospheric ozone recovery and increasing greenhouse gases. SUMMARY: Human influence on the Antarctic circulation is detectable in the strengthening of the stratospheric polar vortex and the poleward shift of the tropospheric westerly winds. Observed Arctic circulation changes cannot be confidently separated from internal atmospheric variability.
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    The Australian Northwest Cloudband: Climatology, Mechanisms, and Association with Precipitation
    Reid, KJ ; Simmonds, I ; Vincent, CL ; King, AD (AMER METEOROLOGICAL SOC, 2019-10-01)
    Australian northwest cloudbands (NWCBs) are continental-scale bands of continuous cloud that stretch from northwest to southeast Australia. In earlier studies, where the characteristics of NWCBs and their relationship with precipitation were identified from satellite imagery, there was considerable uncertainty in the results due to limited quality and availability of data. The present study identifies NWCBs from 31 years of satellite data using a pattern-matching algorithm. This new climatology is the longest record based entirely on observations. Our findings include a strong annual cycle in NWCB frequency, with a summer maximum and winter minimum, and a statistically significant increase in annual NWCB days over the period 1984–2014. Physical mechanisms responsible for NWCB occurrences are explored to determine whether there is a fundamental difference between summer and winter NWCBs as hypothesized in earlier studies. Composite analyses are used to reveal that a key difference between these is their genesis mechanisms. Whereas summer NWCBs are triggered by cyclonic disturbances, winter NWCBs tend to form when meridional sea surface temperature gradients trigger baroclinic instability. It was also found that while precipitation is enhanced over parts of Australia during a cloudband day, it is reduced in other regions. During a cloudband day, precipitation extremes are more likely over northwest, central, and southeast Australia, while the probability of extreme precipitation decreases in northeast and southwest Australia. Additionally, cold fronts and NWCBs can interact, leading to enhanced rainfall over Australia.