The Center for Australian Weather and Climate Research

Melbourne, Australia

The Center for Australian Weather and Climate Research

Melbourne, Australia
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Ying K.,Beijing Normal University | Ying K.,CAS Institute of Atmospheric Physics | Zhao T.,CAS Institute of Atmospheric Physics | Quan X.-W.,University of Colorado at Boulder | And 2 more authors.
Climate Dynamics | Year: 2015

The interannual variability of seasonal precipitation in eastern China from fall to following spring is examined for the period of 1951–2004 based on observations at 106 stations. The temporal variability of seasonal mean values is decomposed into intraseasonal (fast) and slow (potentially predictable) components. EOF analysis is then applied to both the fast and predictable components. We find that (1) the most predictable signal migrates in a north–south direction along with the annual cycle of the monsoon in east China, while spatial patterns of the leading fast modes does not change much; (2) the predictable signal of precipitation in eastern China is associated with anomalous atmospheric circulation patterns having more zonally symmetric structures while the fast time-varying precipitation components are accompanied by wavy anomalous atmospheric circulation patterns; (3) the most predictable signal has an apparent 1-season lagged correlation with the interannual variation of sea surface temperature associated with El Niño/Southern Oscillation; (4) The fast rainfall component is largely attributed to the intraseasonal variabilities of the Siberian High over the Eurasian continent and the subtropical high associated with the Western-Pacific-Oscillation-like variabilities over the North Pacific; and (5) The ENSO signal in the fall seasonal precipitation persisted throughout the entire 54-year period while the signal in winter intensified significantly after the mid-1970s. This is attributed to the weaker/stronger intensification of ENSO anomalies in the tropical Pacific during the fall/winter. © 2014, Springer-Verlag Berlin Heidelberg.

Shahrban M.,Monash University | Walker J.P.,Monash University | Wang Q.J.,CSIRO | Seed A.,The Center for Australian Weather and Climate Research | Steinle P.,The Center for Australian Weather and Climate Research
Hydrological Sciences Journal | Year: 2016

Assessment of forecast precipitation is required before it can be used as input to hydrological models. Using radar observations in southeastern Australia, forecast rainfall from the Australian Community Climate Earth-System Simulator (ACCESS) was evaluated for 2010 and 2011. Radar rain intensities were first calibrated to gauge rainfall data from four research rainfall stations at hourly time steps. It is shown that the Australian ACCESS model (ACCESS-A) overestimated rainfall in low precipitation areas and underestimated elevated accumulations in high rainfall areas. The forecast errors were found to be dependent on the rainfall magnitude. Since the cumulative rainfall observations varied across the area and through the year, the relative error (RE) in the forecasts varied considerably with space and time, such that there was no consistent bias across the study area. Moreover, further analysis indicated that both location and magnitude errors were the main sources of forecast uncertainties on hourly accumulations, while magnitude was the dominant error on the daily time scale. Consequently, the precipitation output from ACCESS-A may not be useful for direct application in hydrological modelling, and pre-processing approaches such as bias correction or exceedance probability correction will likely be necessary for application of the numerical weather prediction (NWP) outputs. © 2016, Taylor & Francis. All rights reserved.

Asseng S.,University of Florida | McIntosh P.C.,CSIRO | Wang G.,The Center for Australian Weather and Climate Research | Khimashia N.,CSIRO
European Journal of Agronomy | Year: 2012

Achievable grain yields can vary widely between seasons in rain-fed agriculture. Adjusting N fertiliser inputs according to achievable grain yields could reduce over-fertilisation in low-yielding seasons and allow increasing gross margins in potential high-yielding seasons. Seasonal rainfall forecasts from the coupled ocean-atmosphere global circulation model POAMA were skill tested and employed for N fertiliser decision making in the Western Australian wheat-belt. The POAMA seasonal rainfall forecast showed significant skill in forecasting rainfall season types in southern regions of the Western Australian wheat-belt. This skill resulted in about A$50ha -1 of additional benefits when used in N management decisions in wheat cropping. However, such a forecast should not be used without considering other systems knowledge available to farmers. Combining a forecast with systems information such as initial soil water conditions can be crucial in obtaining value from a forecast. Another important factor to consider is the risk behaviour of farmers, where the gross margin from additional fertiliser is expected to exceed the cost by a factor of two or more. Finally, variations in fertiliser cost and wheat prices are critical in determining the benefits from using a forecast system for management decisions in agriculture. Using a forecast for only the wet season-type can further increase a forecast value because the additional gains in wet seasons are often higher than the savings from reduced fertiliser in dry seasons. It is expected that skilful seasonal forecasting systems will become increasingly valuable in regions where rainfall is decreasing because they help to capture benefits in the declining number of potentially high-yielding seasons and minimise the losses in the increasing number of low-yielding seasons. © 2012.

Sandery P.A.,The Center for Australian Weather and Climate Research | Sakov P.,The Center for Australian Weather and Climate Research | Majewski L.,The Center for Australian Weather and Climate Research
Ocean Modelling | Year: 2014

We investigate the performance of an eddy resolving regional ocean forecasting system of the East Australian Current (EAC) for both ensemble optimal interpolation (EnOI) and ensemble Kalman filter (EnKF) with a focus on open boundary model nesting solutions. The performance of nesting into a global re-analysis; nesting into the system's own analysis; and nesting into a free model is quantified in terms of forecast innovation error. Nesting in the global reanalysis is found to yield the best results. This is closely followed by the system that nests inside its own analysis, which seems to represent a viable practical option in the absence of a suitable analysis to nest within. Nesting into a global reanalysis without data assimilation and nesting into an unconstrained model were both found to be unable to constrain the mesoscale circulation at all times. We also find that for a specific interior area of the domain where the EAC separation takes place, there is a mixture of results for all the systems investigated here and that, whilst the application of EnKF generates the best results overall, there are still times when not even this method is able to constrain the circulation in this region with the available observations. © 2014 Elsevier Ltd.

Uslu B.,National Oceanic and Atmospheric Administration | Uslu B.,University of Washington | Power W.,Institute of Geological & Nuclear Sciences | Greenslade D.,The Center for Australian Weather and Climate Research | And 2 more authors.
Pure and Applied Geophysics | Year: 2011

On 15 July 2009, a Mw 7. 8 earthquake occurred off the New Zealand coast, which by serendipitous coincidence occurred while the International Tsunami Symposium was in session in Novosibirsk, Russia. The earthquake generated a tsunami that propagated across the Tasman Sea and was detected in New Zealand, Australia and as far away as the US West coast. Small boats close to the epicenter were placed in jeopardy, but no significant damage was observed despite a measured run-up height of 2. 3 m in one of the Sounds in close proximity to the source (Wilson in GNS Science Report 46:62 2009). Peak-to-trough tsunami heights of 55 cm were measured at Southport, Tasmania and a height of 1 m was measured in Jackson Bay, New Zealand. The International Tsunami Symposium provided an ideal venue for illustration of the value of immediate real-time assessment and provided an opportunity to further validate the real time forecasting capabilities with the scientific community in attendance. A number of agencies with responsibility for tsunami forecast and/or warning, such as the NOAA Center for Tsunami Research, the Pacific Tsunami Warning Center, GNS Science in New Zealand, the Australian Bureau of Meteorology and the European Commission Joint Research Centre were all represented at the meeting and were able to demonstrate the use of state of the art numerical models to assess the tsunami potential and provide warning as appropriate. © 2011 Springer Basel AG.

Rotstayn L.D.,CSIRO | Rotstayn L.D.,The Center for Australian Weather and Climate Research | Collier M.A.,CSIRO | Collier M.A.,The Center for Australian Weather and Climate Research | And 9 more authors.
International Journal of Climatology | Year: 2010

We assess the simulation of Australian mean climate and rainfall variability in a new version of the CSIRO coupled ocean-atmosphere global climate model (GCM). The new version, called Mark 3.6 (Mk3.6), differs from its recent predecessors (Mk3.0 and Mk3.5) by inclusion of an interactive aerosol scheme, which treats sulfate, dust, sea salt and carbonaceous aerosol. Other changes include an updated radiation scheme and a modified boundary-layer treatment. Comparison of the mean summer and winter climate simulations in Mk3.6 with those in Mk3.0 and Mk3.5 shows several improvements in the new version, especially regarding winter rainfall and sea-level pressure. The improved simulation of Australian mean seasonal climate is confirmed by calculation of a non-dimensional skill score (the 'M-statistic'), using data from all four seasons. However, the most dramatic improvement occurs in the model's simulation of the leading modes of annual rainfall variability, which we assess using empirical orthogonal teleconnections (EOTs). Compared to its predecessors and several international GCMs, Mk3.6 is best able to capture the spatial pattern of the leading rainfall mode, which represents variability due to the El Nino Southern Oscillation (ENSO). Mk3.6 is also best able to capture the spatial pattern of the second rainfall mode, which corresponds to increased rainfall in the northwest, and decreased rainfall over eastern Australia. We propose a possible mechanism for the improved simulation of rainfall variability in terms of the role of interactive dust in Mk3.6. By further suppressing convection over eastern Australia during El Nino events, dust feedbacks may enhance rainfall variability there, in tune with the model's ENSO cycle. This suggests that an interactive aerosol treatment may be important in a GCM used for the study of Australian climate change and variability. Mechanistic sensitivity studies are needed to further evaluate this hypothesis. © 2009 Royal Meteorological Society.

Whan K.,Australian National University | Whan K.,University of New South Wales | Timbal B.,The Center for Australian Weather and Climate Research | Lindesay J.,Australian National University
International Journal of Climatology | Year: 2014

The intensity and position of the sub-tropical ridge (STR) have strong relationships with rainfall variability in southern Australia. The combined effect of intensity and position in March-April-May (MAM) and June-July-August (JJA) is the focus of this research. Linear statistics were used first: area-averaged and Australia-wide spatial correlations of STR intensity and position with precipitation in south-west eastern Australia reveal that STR intensity has a much stronger and more widespread relationship with precipitation in both seasons. Over time, these relationships vary in magnitude and spatial extent with the sign of the correlation changing between two 50-year epochs. These nonlinearities were investigated further using classification trees. Area-averaged precipitation data (terciles) for south-west eastern Australia was classified on the basis of STR intensity and position. In both seasons the classification trees identify STR intensity as the primary partition defining the dry group, supporting the linear analysis. In the transition season of MAM, the time of year when the mean position of the STR is more southerly, STR position is important in distinguishing between a 'winter-like' and a 'summer-like' wet groups, providing STR intensity is low. Vector wind analyses were computed to explain the composite seasonal precipitation anomaly results in terms of different circulation patterns associated with these two wet groups. The frequency of wet and dry cases in each group was examined with changes evident over the recent years. The research confirms that STR intensity is more important than STR position in explaining inter-annual rainfall variability across southern Australia but also demonstrates the additional role of STR position in MAM. These results explain the low correlation between rainfall and STR position and why this relationship has evolved during the 20th century as the mean location of the STR has shifted south in MAM. © 2013 Royal Meteorological Society.

Dowdy A.J.,The Center for Australian Weather and Climate Research
Atmospheric Science Letters | Year: 2014

Tropical cyclone (TC) observations are used to examine changes in the TC climatology of the Australian region. The ability to investigate long-term changes in TC numbers improves when the El Niño-Southern Oscillation (ENSO) is considered. Removing variability in TC numbers associated with ENSO shows a significant decreasing trend in TC numbers at the 93-98% confidence level. Additionally, there is some indication of a temporal change in the relationship between ENSO and TC numbers, with ENSO accounting for about 35-50% of the variance in TC numbers during the first half of the study period, but only 10% during the second half. © 2014 Royal Meteorological Society.

Sandery P.A.,The Center for Australian Weather and Climate Research | O'Kane T.J.,The Center for Australian Weather and Climate Research | O'Kane T.J.,CSIRO
Quarterly Journal of the Royal Meteorological Society | Year: 2014

A coupled ocean-atmosphere dynamical ensemble prediction system is used to study coupled initialization and bred cyclic modes in the case of Tropical Cyclone (TC) Yasi. Ocean initial perturbations are constructed to identify the fastest-growing nonlinear modes in the ocean response to the TC. The ensemble provides a characterization of how initial and evolving dynamical ocean perturbations influence the coupled system through surface fluxes under extreme conditions. Results show how sea-surface temperature perturbations project into atmospheric perturbations of pressure and moisture content within the storm environment. By calculating the local bred vector dimension for ocean-surface velocity, we show that a low-dimensional subspace forms along the track of TC Yasi. The iterative approach to coupled initialization used in this study generates cyclic modes that are embedded on to the dynamics of regions critical to the coupled ocean-atmosphere TC dynamics. The ensemble mean forecasted sea-surface temperature and sea-surface height associated with the ocean response is in better agreement with observations, despite the biases the coupled model inherits from its component models. Both model and observations reveal a twin cold core structure in the ocean wake of TC Yasi. © 2013 Royal Meteorological Society.

Sakov P.,The Center for Australian Weather and Climate Research | Sandery P.A.,The Center for Australian Weather and Climate Research
Ocean Modelling | Year: 2015

This study compares two regional eddy resolving ocean reanalysis systems, based on the ensemble Kalman filter (EnKF) and ensemble optimal interpolation (EnOI), focusing on data assimilation aspects. Both systems are configured for the Tasman Sea using the same ocean model with 0.1° resolution and commonly available observations of satellite altimetry, sea surface temperature and subsurface temperature and salinity. The primary goals are to quantify the difference in performance of the EnKF and EnOI and investigate how important this difference might be from an oceanographic perspective. We find that both systems generally constrain mesoscale circulation in the region, with some exceptions for the East Australian Current separation region, the most energetic and chaotic part of the domain. Overall, the EnKF is found to consistently outperform the EnOI, producing on average 9-21% smaller innovations. The EnKF also has better forecast skill relative to the persisted analysis than the EnOI. For SST the EnKF forecast outperforms persisted analysis by about 17%, which indicates that the surface circulation is mainly constrained. The EnKF and EnOI are shown to produce qualitatively different increments of unobserved or sparsely observed variables; however, we find only moderate improvements of the EnKF over EnOI in subsurface temperature fields when compared against withheld XBT observations. We attribute this lack of a major improvement in subsurface reconstruction to the inability of the EnKF to linearly constrain the system due to initialisation shock, model error caused by open boundaries, and possibly insufficient observations. © 2015 Elsevier Ltd.

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