Graham A. Mills

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 ...

Dynamics and Prediction of a Mesoscale Extreme Rain Event in the Baiu Front over Kyushu, Japan

Abstract A high-resolution version of the limited-area primitive equations model of the Bureau of Meteorology Research Centre is used to investigate the dynamics and prediction of the mesoscale, extreme rain event of 1 August 1993 over Kyushu. The model is multiply nested, uses high-order numerics, includes sophisticated parameterizations of physical processes, and has a capability to use cloud imagery from the Japanese Geostationary Meteorological Satellite (GMS) to initialize the vertical motion field. The short-term forecasts of the timing and distribution of rainfall are very encouraging. However, they are sensitive to many components of the forecast system, particularly the diabatic initialization and boundary layer parameterization. Large-scale flow changes during the event were 1) the maintenance of the subtropical ridge to the south of Japan and the development of a trough over Korea, which combined to maintain a warm and moist southwesterly flow over Kyushu, 2) the movement of a cold low from wes...

A High-Resolution Prediction Study of Two Typhoons at Landfall

Abstract The landfall on the China coast of two typhoons has been studied using both observational diagnostics and a 30-km, 19-level prediction system. The typhoons vary in size and in the amount of rainfall they produced after landfall. Use is made of a synthetic typhoon insertion scheme and the relatively dense observational network around China to provide objective analyses. The authors illustrate that, in the forecast model, as the typhoon’s outer circulation impinges on the China coast, an outer cloud band develops offshore, parallel to the coast, extending cyclonically north and east. During this time, the inner circulation weakens. As the outer cloud band strengthens, the center of the circulation approaches it from the east and there is a short-term intensification of both the inner region vertical motion and vorticity. Following landfall rapid dissipation of the core occurs. Using detailed diagnostics from the model integration, the changing thermal structure of the forecast vortex is illustrated...

The Development of the Australian Air Quality Forecasting System: Current Status

The AAQFS is routinely providing highspatial resolution air quality forecasts for guidance for the EPAs in Melbourne and Sydney. Case studies of photochemical smog events in Melbourne and Sydney have given encouraging agreement with observations. Meteorologically the two airsheds present different challenges: in Melbourne it is important to predict the onset and strength of the Port Phillip Bay breeze and the Bass Strait sea breeze; in Sydney is it important to predict the onset and strength of the Tasman sea breeze, the pollution plume trajectory for flow over complex terrain and the effects of local photochemical smog production and inter-regional transport. In the case studied for Sydney there was an additional complication of a synoptic-scale wind surge called the Southerly Buster. For both airsheds, the interaction between synoptic-scale forcing and mesoscale circulations can strongly influence the characteristics of an air pollution event and thus the meteorological model must be able to accurately simulate these interactions. In general, the LCC photochemical mechanism gave better predictions of the 1-hour ozone peak than the GRS mechanism. Improvements to the GRS mechanism and emissions inventory and online modelling of emissions and photochemistry are being developed and implemented. Work on the meteorological model to improve surface winds, soil moisture analysis and boundary-layer height also continues. We have yet to establish the limits of predictability of the system.

Changes in the Risk of Cool-Season Tornadoes over Southern Australia due to Model Projections of Anthropogenic Warming

Abstract Anthropogenic climate change is likely to be felt most acutely through changes in the frequency of extreme meteorological events. However, quantifying the impact of climate change on these events is a challenge because the core of the climate change science relies on general circulation models to detail future climate projections, and many of these extreme events occur on small scales that are not resolved by climate models. This note describes an attempt to infer the impact of climate change on one particular type of extreme meteorological event—the cool-season tornadoes of southern Australia. The Australian Bureau of Meteorology predicts threat areas for cool-season tornadoes using fine-resolution numerical weather prediction model output to define areas where the buoyancy of a near-surface air parcel and the vertical wind shear each exceed specified thresholds. The diagnostic has been successfully adapted to coarser-resolution climate models and applied to simulations of the current climate, a...

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.