Kathleen L. McInnes

Objectively Determined Resolution-Dependent Threshold Criteria for the Detection of Tropical Cyclones in Climate Models and Reanalyses

Objectively derived resolution-dependent criteria are defined for the detection of tropical cyclones in model simulations and observationally based analyses. These criteria are derived from the wind profiles of observed tropical cyclones, averaged at various resolutions. Both an analytical wind profile model and two-dimensional observed wind analyses are used. The results show that the threshold wind speed of an observed tropical cyclone varies roughly linearly with resolution. The criteria derived here are compared to the numerous different criteria previously employed in climate model simulations. The resulting method provides a simple means of comparing climate model simulations and reanalyses.

Southwest Western Australian winter rainfall and its association with Indian Ocean climate variability.

Southwest Western Australia (SWWA) has experienced a significant decrease in winter rainfall since the late 1960s. This decrease is unexplained and the resultant problem of reduced water storage has been compounded by the lack of any useful predictive skill at the seasonal time scale. This study uses recent gridded, historical data and simple linear correlation in order to evaluate the importance of links between rainfall and both mean sea level pressure (MSLP) and sea-surface temperature (SST) patterns over the Indian Ocean. The decrease in rainfall is linked to decreases in the density of low-pressure systems in the region and to increases in both MSLP and SST over the southern Indian Ocean. Warmer SSTs and increases in MSLP are associated with the observed long-term changes, but changes in these variables do not explain a great deal of the observed interannual variability. Greenhouse-induced climate change is not regarded as a likely explanation for the observed decrease, however, the existence of links with both MSLP and SSTs suggests the existence of coupled air–sea interactions over the southern Indian Ocean which may be relevant at decadal or multi-decadal timescales. A major difficulty with defining any such processes is the relative sparseness of data at high latitudes in the Southern Hemisphere. This should be partly alleviated as more recent high quality data becomes available over time. Copyright

Impact of Sea-level Rise and Storm Surges on a Coastal Community

A technique to evaluate the risk of storm tides (the combination of a storm surge and tide) under present and enhanced greenhouse conditions has been applied to Cairns on the north-eastern Australian coast. The technique combines a statistical model for cyclone occurrence with a state-of-the-art storm surge inundation model and involves the random generation of a large number of storm tide simulations. The set of simulations constitutes a synthetic record of extreme sea-level events that can be analysed to produce storm tide return periods. The use of a dynamic storm surge model with overland flooding capability means that the spatial extent of flooding is also implicitly modelled. The technique has the advantage that it can readily be modified to include projected changes to cyclone behaviour due to the enhanced greenhouse effect. Sea-level heights in the current climate for return periods of 50, 100, 500 and 1000 years have been determined to be 2.0 m, 2.3 m, 3.0 m and 3.4 m respectively. In an enhanced greenhouse climate (around 2050), projected increases in cyclone intensity and mean sea-level see these heights increase to 2.4 m, 2.8 m, 3.8 m and 4.2 m respectively. The average area inundated by events with a return period greater than 100 years is found to more than double under enhanced greenhouse conditions.

Using Sea Level Rise Projections for Urban Planning in Australia

Abstract This study deals with incorporating predictions of sea level rise into practical municipal planning. Predictions of global mean sea level rise can be made with more confidence than many other aspects of climate change science. The world has warmed in the past century, and as a result global mean sea level has risen and is expected to continue to rise. Even so, there are significant uncertainties regarding predictions of sea level. These arise from two main sources: the future amount of greenhouse gases in the atmosphere, and the ability of models to predict the impact of increasing concentrations of greenhouse gases. Current knowledge regarding the effect of global warming on sea level rise is reviewed. Global mean sea level is expected to rise by 3–30 cm by 2040, and 9–88 cm by 2100. An important remaining uncertainty is the future contribution of surface water storage (for example, lakes and reservoirs) to changes in sea level. In addition, there are also significant local sea level effects that need to be taken account in many regions of the globe, including isostatic and tectonic effects. The thermal expansion component of sea level rise is also likely to vary regionally, due to regional differences in the rate of downward mixing of heat and to changes in ocean currents. The current state of planning for sea level rise in Australia is reviewed. While not all coastal municipalities include sea level rise in their planning schemes, the recent adoption in a number of States of new planning schemes with statutory authority creates a changed planning environment for local government. Coastal urban planning needs to take sea level rise into account because its effects will be apparent during the typical replacement time of urban infrastructure such as buildings (before about 70 years). For local planning, ideally a risk assessment methodology may be employed to estimate the risk caused by sea level rise. In many locations, planning thresholds would also have to be considered in the light of possible changes in storm surge climatology due to changes in storm frequency and intensity, and (in some locations) changes to return periods of riverine flooding. In the medium term (decades), urban beaches will need beach re-nourishment and associated holding structures such as sea walls. Changes in storm and wave climatology are crucial factors for determining future coastal erosion.

Use of Representative Climate Futures in impact and adaptation assessment

A key challenge for climate projection science is to serve the rapidly growing needs of impact and adaptation assessments (hereafter risk assessments) in an environment where there are substantial differences in the regional projections of climate models, an expanding number of potentially relevant climate model results, and a desire amongst many users to limit the number of future climate scenarios in their assessments. While it may be attractive to select a small number of climate models based on their ability to replicate current climate, there is no robust method for doing this. We outline and illustrate a method that addresses this challenge in a different way. The range of plausible future climates simulated by climate models is classified into a small set of Representative Climate Futures (RCFs) and the relative likelihood of these estimated. For each region, the RCFs are then used as a framework in which to classify more detailed information, such as available climate model and downscaled data sets. Researchers wishing to apply the RCFs in risk assessments can then choose to use a subset of RCFs, such as the “most likely”, “high risk” and “least change” cases for their impact system. Preparation and analysis of future climate data sets can therefore be confined to those models whose simulations best represent the selected RCFs. This significantly reduces the number of models involved, and potentially the effort required to undertake the risk assessment. Consistently applied within a region, RCFs, rather than individual climate models, can become the boundary objects which anchor discussion between the climate science and risk assessment communities, simplifying communication. Since the RCF descriptions need not change as new climate model results emerge, they can also provide a stable framework for assimilating risk assessments undertaken at different times with different sets of climate models. Systematic application of this approach requires various challenges to be addressed, such as robustly classifying future regional climates into a small set and estimating likelihoods.

Past and Future Changes in Extreme Sea Levels and Waves

(1) Proudman Oceanographic Laboratory, Liverpool, UK (plw@pol.ac.uk) (2) The Hadley Centre, Met Office, UK (jason.lowe@metoffice.gov.uk) (3) Geophysical Fluid Dynamics Laboratory, Princeton, USA (tom.knutson@noaa.gov) (4) The Hadley Centre, Met Office, UK (ruth.mcdonald@metoffice.gov.uk) (5) CSIRO, Aspendale, Australia (kathleen.mcinnes@csiro.au) (6) GKSS, Geesthacht, Germany (woth@gkss.de) (7) GKSS, Geesthacht, Germany (storch@gkss.de) (8) Proudman Oceanographic Laboratory, Liverpool, UK (jaw@pol.ac.uk) (9) Environment Canada, Downsview, Canada (val.swail@ec.gc.ca) (10) Dalhousie University, Halifax, Canada (natacha.bernier@phys.ocean.dal.ca) (11) P.P. Shirshov Institute of Oceanology, Moscow, Russia (gul@sail.msk.ru) (12) Proudman Oceanographic Laboratory, Liverpool, UK (kevinh@pol.ac.uk) (13) National Institute of Oceanography, Goa, India (unni@darya.nio.org) (14) University of Hobart, Tasmania, Australia (john.hunter@utas.edu.au)

Atmospheric convective plumes emanating from leads: 1. Thermodynamic structure

A one-dimensional model with detailed radiative transfer, a second-order closure scheme for turbulence, and bulk cloud microphysics has been employed to simulate the convective atmosphere above a wide open lead. The development of a thermal internal boundary layer (TIBL) associated with the atmospheric convection above a wide open lead is modeled for several different temperature profiles characteristic of the Arctic winter. Large differences in the predicted top of the TIBL are observed between two simple diagnostic models and the time-dependent model introduced here. The modeled thermodynamic processes (i.e., radiative transfer and condensation/evaporation) occurring within the TIBL above an open lead are shown to account for as much as 20% of the predicted TIBL top height. Radiative heating dominates the heat budget in the lowest model level while sensible and latent heating occur throughout the TIBL. The production of cloud water affects the modeled TIBL top height via enhanced radiative cooling near the top of the TIBL and by latent heat released during condensation and freezing. Under stable conditions the TIBL top height above an open lead increases with time as a decaying exponential. The initial temperature of a surface-based isothermal layer can account for up to 50% of the variations in TIBL top height. The depth of the surface-based inversion layer and the stability of the atmosphere above the inversion are both important controlling factors in TIBL development while the temperature difference across the inversion layer is slightly less important.

The impact of eastern Australian cut‐off lows on coastal sea levels

Cut-off lows that develop off the east coast of Australia are a major cause of elevated coastal sea levels in this region. Their duration often exceeds a day and the combination of elevated sea levels with the high rainfall that commonly accompanies these events means that coastal flooding can be a major hazard. The processes contributing to higher-than-normal sea levels are a combination of storm surge and breaking wave setup. Three events are modelled using an atmospheric model, storm surge model and wave setup model. The storm surges resulting from the depressions are found to explain between one-third and one-half of the measured sea-level residuals at the coast and are shown to develop in a regime of coast-parallel winds that produce onshore Ekman transport. The timing of the storm surges are well captured in two of the events in which the atmospheric depressions are of broad spatial scale and well simulated by the atmospheric model. However, a poor simulation by the atmospheric model of the location of the third low, which is of smaller spatial scale, results in an inaccurate timing of the onset of the surge. The wave setup model produces sea levels at the coast that are around 11% of the incident rms wave heights. The total modelled sea-level residual, obtained by adding the modelled wave setup to the storm surge, is qualitatively similar to the measured sea-level residual. However, the modelled values are higher, especially at Sydney where the tide gauge is situated in a sheltered harbour location. In such sheltered locations, it is expected that some attenuation of the wave setup would occur and suggests that sea levels on the open coast could be considerably higher during such events.

On regional dynamical downscaling for the assessment and projection of temperature and precipitation extremes across Tasmania, Australia

The ability of an ensemble of six GCMs, downscaled to a 0.1° lat/lon grid using the Conformal Cubic Atmospheric Model over Tasmania, Australia, to simulate observed extreme temperature and precipitation climatologies and statewide trends is assessed for 1961–2009 using a suite of extreme indices. The downscaled simulations have high skill in reproducing extreme temperatures, with the majority of models reproducing the statewide averaged sign and magnitude of recent observed trends of increasing warm days and warm nights and decreasing frost days. The warm spell duration index is however underestimated, while variance is generally overrepresented in the extreme temperature range across most regions. The simulations show a lower level of skill in modelling the amplitude of the extreme precipitation indices such as very wet days, but simulate the observed spatial patterns and variability. In general, simulations of dry extreme precipitation indices are underestimated in dryer areas and wet extremes indices are underestimated in wetter areas. Using two SRES emissions scenarios, the simulations indicate a significant increase in warm nights compared to a slightly more moderate increase in warm days, and an increase in maximum 1- and 5-day precipitation intensities interspersed with longer consecutive dry spells across Tasmania during the twenty-first century.

GCM Simulations of Eastern Australian Cutoff Lows

Abstract The ability of the CSIRO-9 General Circulation Model (GCM) to capture surface cutoff lows over eastern Australia is investigated by comparing composites of ten GCM cases with ten observed lows. The lows are also studied individually to compare their development and movement, as well as synoptic features, which may have been smoothed out in the compositing process. Finally, the incidence of all such lows in the 1 × CO2 and 2 × CO2 simulations are examined to determine the possible effects a doubling of CO2 will have on their occurrence. The GCM surface lows were found to develop from an upper-level cutoff low in a manner similar to the observed lows. In both sets, this development took place between the upper-level subtropical and polar jets in all seasons except summer, where only one jet was evident. Latent heat release appeared to play an important role in the intensification of the surface lows. The main difference between the two sets of cutoff lows was that the GCM surface lows tended to dev...