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Abstract:
There are several sources of forecast uncertainty when modelling fine scale weather phenomena such as supercell thunderstorms. These sources include the uncertainty associated with the initial and boundary conditions, and uncertainty arising from model deficiencies. Model deficiencies can be due, for example, to the various assumptions and parameterisations used in the model physics. Model physics play an important role in determining resulting forecasts and highlight the need for model diversity in short-range ensemble forecasting systems. The approach in this work is to assess the variability in single model forecasts of the April 1999 supercell (for example, the variability in hail size, updraft/downdraft strength and temperature drop at the surface), by investigating the sensitivity to parameterisations within the ice physics package used in the non-hydrostatic version of The University of New South Wales numerical weather prediction model (HIRES), which was developed by Professor Lance Leslie (Centre for Environmental Modelling and Prediction, UNSW). For supercell thunderstorms, the ice microphysics is extremely important in representing the various phases of water vapour that may lead to the development of large hail. With the full HIRES NWP system and using the real-time operational data, the devastating supercell hailstorm of 14 April 1999 in Sydney is examined. Results will be shown that indicate sensitivity to track and hail size.
About the Speaker:
Milton is a meteorologist with the Bureau of Meteorology in Sydney where he has many years of research and operational experience. Milton is also affiliated with CEMAP in the School of Mathematics (UNSW) where he has been engaged in joint research with Professor Lance Leslie (Director of CEMAP) and other CEMAP personnel for over 10 years. More recently, in 2002, Milton spent several months at the USA National Severe Storms Laboratory engaged in research on aspects of short range ensemble forecasting of severe storms.
About the Speaker:
Jimmy Deguara has been a weather enthusiast for many years and has chased storms since 1993 with Michael Bath and then David Croan more recently. Chuck Doswell's visit at his first AMOS meeting in 1989 and his talk on Chasing on the High Plains certainly urged him to storm chase.
The highlight of the storm chasing calendar occurred in 2001 when David Croan and Jimmy Deguara intercepted 4 tornadoes in one day including a ½ mile wide monster violent tornado. Consequently, the US Tornado Alley was added to the annual storm chasing calendar.
With regards to hailstorms, 2003 was another good year in Australia with 3 giant hail events forecasted and intercepted. In total, Jimmy has experienced 6 hailstorms spawning giant hail. Jimmy is committed to maintaining his website with Michael Bath that aims to educate the public in severe weather awareness and safety.
Abstract:
Oceanic planetary (Rossby) waves play a significant role in ocean dynamics. They maintain and influence the strong western boundary currents, are the main oceanic response to changes in atmospheric forcing and are an indicator of the length of time that anomalous conditions persist. There is evidence that these waves influence the ocean biosphere and climate. However, due to their small sea surface signature (<0.1 m) and slow propagation speeds (<0.1 m/s), detection of these waves was nearly impossible prior to the advent of satellite altimetry. With more than a decade of altimeter data from the TOPEX/POSEIDON satellite and prior missions, it is now possible to examine planetary waves at a basin wide or global scale with centimetre accuracy from sea surface height anomalies. Analysis of altimeter data has shown significant inconsistencies between planetary wave theory and observations and are a valuable resource in the south Pacific ocean where in situ observations are often scarcer than for most other oceans. Of particular interest is whether the unique bathymetry of the south Pacific basin interacts with the planetary waves.
Altimeter data from the Topex/Poseidon and ERS missions are examined to investigate planetary wave variability in the South Pacific basin using spectral techniques. Analysis of the planetary wave signal suggests that there is interaction with bathymetric features throughout the entire South Pacific Ocean. Variability in the energy of the westward propagating signal suggests topographic steering. Interaction with ridges influences the speed and meridional extent of the variability.
About the speaker:
Angela Maharaj is a PhD student at Macquarie University with the Department of Physical Geography and is supervised by Dr. Neil Holbrook, Macquarie University and Professor Richard Coleman, University of Tasmania. She received a Bachelor of Science (Honours) from Macquarie University in 1998 and worked for the Department of Physical Geography as a Scientific Officer in 1999 before accepting an Australian Postgraduate Award and enrolling in the Doctor of Philosophy degree. She is engaged in collaborative research with Dr. Paolo Cipollini at Southampton Oceanography Centre, UK. The work being presented here is part of her thesis research.
Abstract:
The climate changes for many reasons and determining the causes of regional climate change can be extremely difficult. A sudden reduction in rainfall occurred in the southwest of Western Australia in the mid-20th Century. This reduced inflows to the Perth water supply by about 120 GL (42%) and led to an acceleration of projects to develop new water sources at a cost of about US$300 million. The reduction in rainfall was coincident with warmer temperatures. A major analysis of these changes, facilitated by the Western Australian government indicated that the changes in temperature were likely caused by the enhanced greenhouse effect and the changes in rainfall were likely caused by a large scale reorganization of the atmospheric circulation.
We explore an alternative hypothesis, that large scale land cover change explains the observed changes in rainfall and temperature. We use three high resolution mesoscale model configurations forced at the boundaries to simulate (for each model) five July climates for each of natural and current land cover. We find that land cover change explains up to 50% of the observed warming. Following land cover change we also find, in every simulation, a reduction in rainfall over southwest Western Australia, and an increase in rainfall inland, that matches the observations well. We show that the reduced surface roughness following land cover change largely explains the simulated changes in rainfall by increasing moisture divergence over southwest Western Australia and increasing moisture convergence inland. Increased horizontal wind magnitudes and suppressed vertical velocities over southwest Western Australia reduces the likelihood of precipitation. Inland, moisture convergence and increased vertical velocities lead to an increase in rainfall.
Our results highlight the dangers of attributing regional climate change to any single cause and emphasizes the need to approach understanding of changes in the atmosphere from an Earth System Science perspective.