Andrew J. Dowdy

Tropical Cyclone Climatology of the South Pacific Ocean and Its Relationship to El Nino-Southern Oscillation

AbstractClimatological features of tropical cyclones in the South Pacific Ocean have been analyzed based on a new archive for the Southern Hemisphere. A vortex tracking and statistics package is used to examine features such as climatological maps of system intensity and the change in intensity with time, average tropical cyclone system movement, and system density. An examination is presented of the spatial variability of these features, as well as changes in relation to phase changes of the El Nino–Southern Oscillation phenomenon. A critical line is defined in this study based on maps of cyclone intensity to describe the statistical geographic boundary for cyclone intensification. During El Nino events, the critical line shifts equatorward, while during La Nina events the critical line is generally displaced poleward. Regional variability in tropical cyclone activity associated with El Nino–Southern Oscillation phases is examined in relation to the variability of large-scale atmospheric or oceanic varia...

Changes in the Risk of Extratropical Cyclones in Eastern Australia

AbstractThe east coast of Australia is a region of the world where a particular type of extratropical cyclone, known locally as an east coast low, frequently occurs with severe consequences such as extreme rainfall, winds, and waves. The likelihood of formation of these storms is examined using an upper-tropospheric diagnostic applied to three reanalyses and three global climate models (GCMs). Strong similarities exist among the results derived from the individual reanalyses in terms of their seasonal variability (e.g., winter maxima and summer minima) and interannual variability. Results from reanalyses indicate that the threshold value used in the diagnostic method is dependent on the spatial resolution. Results obtained when applying the diagnostic to two of the three GCMs are similar to expectations given their spatial resolutions, and produce seasonal cycles similar to those from the reanalyses. Applying the methodology to simulations from these two GCMs for both current and future climate in respons...

Atmospheric and Fuel Moisture Characteristics Associated with Lightning-Attributed Fires

AbstractA systematic examination is presented of the relationship between lightning occurrence and fires attributed to lightning ignitions. Lightning occurrence data are matched to a database of fires attributed to lightning ignition over southeastern Australia and are compared with atmospheric and fuel characteristics at the time of the lightning occurrence. Factors influencing the chance of fire per lightning stroke are examined, including the influence of fuel moisture and weather parameters, as well as seasonal and diurnal variations. The fuel moisture parameters of the Canadian Fire Weather Index System are found to be useful in indicating whether a fire will occur, given the occurrence of lightning. The occurrence of “dry lightning” (i.e., lightning that occurs without significant rainfall) is found to have a large influence on the chance of fire per lightning stroke. Through comparison of the results presented here with the results of studies from other parts of the world, a considerable degree of ...

An analysis of tropical cyclone occurrence in the Southern Hemisphere derived from a new satellite-era data set

Satellite remote sensing is vital to accurately estimate tropical cyclone (TC) parameters such as position and intensity. A TC archive for the Southern Hemisphere (SH) has been prepared for the ‘satellite era’, and it now consists of best track data, covering name (and/or unique identification number), position and intensity (in terms of central pressure). Based on these best track data, TC climatologies for the South Indian Ocean (SIO) and South Pacific Ocean (SPO) are investigated. Variability in TC activity is examined depending on different phases of the El Niño-Southern Oscillation (ENSO). The outcomes demonstrate prospects for improving the regional seasonal prediction of TC activity. The growth in the availability of satellite data is found to have a significant influence on the quantity and quality of available TC data, particularly in relation to the intensity of TCs. To provide a means of accessing detailed information and data on historical TCs in the SH, a specialized website for disseminating results and data was developed using the OpenLayers platform. This allows dynamic map navigation of detailed information for user-selected regions.

Estimating trends and seasonality in Australian monthly lightning flash counts

We present the results of a statistical analysis of lightning characteristics in mainland Australia for the period from approximately 1988 to 2012, based on monthly lightning flash count (LFC) series obtained from a network of 19 Comite Internationale des Grands Reseaux Electriques, 500 Hz peak transmission filter circuit sensors. The temporal structures of the series are examined in terms of detecting and characterizing seasonal cycles, long-term trends, and changes in seasonality over time. A generalized additive modeling approach is used to ensure that the estimated structures are determined by the data, rather than by the constraints of any assumed mathematical form for the trends and seasonal cycle. Results indicate strong seasonality at all sites, the presence of long-term trends at 16 sites, and interactions between trend and seasonality (corresponding to changes in seasonality over time) at 13 sites. The most systematic change corresponds to a progressive deepening of the seasonal cycle (i.e., an ongoing decline in winter lightning flash counts) and is most noticeable across southern Australia (south of 30°S). These results are consistent with previous analyses that have detected decreasing atmospheric instability during the austral winter since the mid-1970s. This is associated with increasing mean sea level pressure and declining rainfall.

Diagnosing indicators of large-scale forcing of east-coast cyclogenesis

Extra-tropical cyclones that develop near the east coast of Australia often have severe consequences such as flash flooding and damaging winds and seas, as well as beneficial consequences such as being responsible for heavy rainfall events that contribute significantly to total rainfall and runoff. There is subjective evidence that the development of most major events, commonly known as East Coast Lows, is associated with the movement of a high amplitude upper-tropospheric trough system over eastern Australia. This paper examines a number of upper-tropospheric diagnostic quantities that might provide a basis for preparing a climatology of the large-scale drivers of east-coast cyclogenesis. A preliminary climatology of these diagnostic quantities, based on ECMWF interim reanalyses, is compared with a database of observed East Coast Low events. The potential application of these diagnostics to global climate model simulations of past and future climates is also discussed.

Energetics and Dynamics of Subtropical Australian East Coast Cyclones: Two Contrasting Cases

AbstractThe subtropical east coast region of Australia is characterized by the frequent occurrence of low pressure systems, known as east coast lows (ECLs). The more intense ECLs can cause severe d...

Climatological Variability of Fire Weather in Australia

AbstractLong-term variations in fire weather conditions are examined throughout Australia from gridded daily data from 1950 to 2016. The McArthur forest fire danger index is used to represent fire weather conditions throughout this 67-yr period, calculated on the basis of a gridded analysis of observations over this time period. This is a complementary approach to previous studies (e.g., those based primarily on model output, reanalysis, or individual station locations), providing a spatially continuous and long-term observations-based dataset to expand on previous research and produce climatological guidance information for planning agencies. Long-term changes in fire weather conditions are apparent in many regions. In particular, there is a clear trend toward more dangerous conditions during spring and summer in southern Australia, including increased frequency and magnitude of extremes, as well as indicating an earlier start to the fire season. Changes in fire weather conditions are attributable at lea...

Tropical cyclone track direction climatology and its intraseasonal variability in the Australian region

Aspects of tropical cyclone (TC) activity, such as the influence of the El Nino-Southern Oscillation and the variability of TC genesis location, have been examined in numerous previous studies. However, relatively few studies have examined aspects such as the influence of intraseasonal variability on TC track direction. Here we focus on a number of knowledge gaps relating to observed TC track directionality and intraseasonal variability in the Australian region. Climatological examinations are presented for TC track directional variability throughout the Australian region. In contrast to previous studies that have focused on the mean direction of TC movement in this region, TC tracks are examined here based on the full spectrum of track directions for a given location or region. Variability in initial TC track directions is investigated, including an examination of the influence of the Madden Julian oscillation (MJO). It is demonstrated that there is a considerable degree of seasonal and intraseasonal variation in TC motion in this region. These variations result from variations in genesis location throughout the TC season, as well as zonal wind anomalies associated with the influence of the MJO on the steering flow winds. Anomalous westerly steering flow occurs during MJO phases 4-5 and anomalous easterly flow during phases 8-1 in the western basin, with resulting changes in the proportion of TC tracks in each direction during these phases of the MJO. The results presented here are intended to provide improved seasonal TC activity guidance and enhanced resilience to TC impacts.

Exploratory analysis of lightning-ignited wildfires in the Warren Region, Western Australia

An exploratory analysis of lightning-ignited wildfire data for the Warren Region of Western Australia was carried out for the period from April 1976 to December 2016. Temporal patterns in the series were examined in terms of characterizing the seasonal cycle, and detecting long-term trends and changes in seasonality over time. A generalized additive modelling approach was used to ensure that temporal features were determined by the data rather than a priori assumed mathematical forms (e.g. linear or low-order polynomial functions). The spatial organization of the data was evaluated using concepts from the theory of stochastic point processes. Results indicate a strong seasonality in the monthly lightning ignition series, the presence of a long-term trend and an interaction between trend and seasonality. There is also strong evidence of spatial variation in the number of ignitions per unit area in terms of location and distance from nearest ignition. Within the Warren Region, observation platforms for fire detection and reporting protocols have remained stable over the period of record, and changes in land use are unlikely to have altered the pattern of lightning ignition. Thus, the above results might reflect an interplay between: landscape attributes (e.g. vegetation classes, elevation, slope, aspect); changes in rainfall and fuel moisture; changes in fuel management practices; and, perhaps, an increase in the frequency of dry thunderstorms and fire weather conditions.