School of Earth Sciences - Research Publications
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ItemCoral-reef-derived dimethyl sulfide and the climatic impact of the loss of coral reefs(COPERNICUS GESELLSCHAFT MBH, 2021-04-20)Abstract. Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a comparable amount of DMS, which is unaccounted for in models. It has further been hypothesised that the coral reef source of DMS may modulate regional climate. This hypothesis presents a particular concern given the current threat to coral reefs under anthropogenic climate change. In this paper, a global climate model with online chemistry and aerosol is used to explore the influence of coral-reef-derived DMS on atmospheric composition and climate. A simple representation of coral-reef-derived DMS is developed and added to a common DMS surface water climatology, resulting in an additional flux of 0.3 Tg yr−1 S, or 1.7 % of the global sulfur flux from DMS. By comparing the differences between both nudged and free-running ensemble simulations with and without coral-reef-derived DMS, the influence of coral-reef-derived DMS on regional climate is quantified. In the Maritime Continent–Australian region, where the highest density of coral reefs exists, a small decrease in nucleation- and Aitken-mode aerosol number concentration and mass is found when coral reef DMS emissions are removed from the system. However, these small responses are found to have no robust effect on regional climate via direct and indirect aerosol effects. This work emphasises the complexities of the aerosol–climate system, and the limitations of current modelling capabilities are highlighted, in particular surrounding convective responses to changes in aerosol. In conclusion, we find no robust evidence that coral-reef-derived DMS influences global and regional climate.
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ItemSub-seasonal to seasonal prediction of rainfall extremes in Australia(WILEY, 2020-04-13)
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ItemExtreme rainfall in New Zealand and its association with Atmospheric Rivers(Institute of Physics (IoP), 2021-04-01)Atmospheric rivers (ARs) are narrow and elongated regions of enhanced horizontal water vapour transport. Considerable research on understanding Northern Hemisphere ARs and their relationship with extreme precipitation has shown that ARs have a strong association with heavy rainfall and flooding. While there has been very little work on ARs in the Southern Hemisphere, global climatologies suggest that ARs are equally as common in both hemispheres. New Zealand in particular is located in a region of high AR frequency. This study aims to test the hypothesis that ARs play a significant role in heavy precipitation and flooding events in New Zealand. We used a recently developed AR identification method and daily station data across New Zealand to test for the concurrence of ARs and extreme rainfall. We found that, at each of the eleven stations analysed, at least seven to all ten of the top ten heaviest precipitation days between 1980 and 2018 were associated with AR conditions. Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR events. These results have important implications for understanding extreme rainfall in New Zealand, and ultimately for predicting some of the most hazardous events in the region. This work also highlights that more research on ARs in New Zealand is needed.
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ItemDoes Lower‐Stratospheric Shear Influence the Mesoscale Organization of Convection?(American Geophysical Union (AGU), 2021-02-16)Organized mesoscale convection is important for many atmospheric phenomena and hazards, however the understanding of its governing mechanisms is incomplete. Theories explaining mesoscale organization rely on the interaction between convection outflows and lower‐tropospheric wind shear. Here a new mechanism is presented, where lower‐stratospheric wind shear is shown to influence mesoscale organization. The mechanism is linked to coupling between convection and gravity waves, with the stratosphere playing a role in shaping the tropospheric wave spectrum. The key result is that lower‐stratospheric shear creates a preference for organized systems propagating in the same direction as the shear vector by weakening the systems propagating in the opposite direction to the shear. This result has important implications for stratosphere‐troposphere interactions, numerical modeling, and understanding of convective organization in general.
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ItemOn the Robustness of Annual Daily Precipitation Maxima Estimates Over Monsoon Asia(Frontiers Media SA, 2020)Understanding precipitation extremes over Monsoon Asia is vital for water resource management and hazard mitigation, but there are many gaps and uncertainties in observations in this region. To better understand observational uncertainties, this study uses a high-resolution validation dataset to assess the consistency of the representation of annual daily precipitation maxima (Rx1day) over land in 13 observational datasets from the Frequent Rainfall Observations on Grids (FROGS) database. The FROGS datasets are grouped into three categories: in situ-based and satellite-based with and without corrections to rain gauges. We also look at three sub-regions: Japan, India, and the Maritime Continent based on their different station density, orography, and coastal complexity. We find broad similarities in spatial and temporal distributions among in situ-based products over Monsoon Asia. Satellite products with correction to rain gauges show better general agreement and less inter-product spread than their uncorrected counterparts. However, this comparison also reveals strong sub-regional differences that can be explained by the quantity and quality of rain gauges. High consistency in spatial and temporal patterns are observed over Japan, which has a dense station network, while large inter-product spread is found over the Maritime Continent and India, which have sparser station density. We also highlight that while corrected satellite products show improvement compared to uncorrected products in regions of high station density (e.g., Japan) they have mixed success over other regions (e.g., India and the Maritime Continent). In addition, the length of record available at each station can also affect the satellite correction over these poorly sampled regions. Results of the additional comparison between all considered datasets and the sub-regional high resolution dataset remain the same, indicating that the overall quality of the station network has implications for the reliability of the in situ-based products derived and also the satellite products that use a correction to in situ data. Given these uncertainties in observations, there is no single best dataset for assessment of Rx1day in Monsoon Asia. In all cases we recommend users understand how each dataset is produced in order to select the most appropriate product to estimate precipitation extremes to fit their purpose.
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ItemEnvironment and Mechanisms of Severe Turbulence in a Midlatitude Cyclone(American Meteorological Society, 2020-11-01)A large midlatitude cyclone occurred over the central United States from 0000 to 1800 UTC 30 April 2017. During this period, there were more than 1100 reports of moderate-or-greater turbulence at commercial aviation cruising altitudes east of the Rocky Mountains. Much of this turbulence was located above or, otherwise, outside the synoptic-scale cloud shield of the cyclone, thus complicating its avoidance. In this study we use two-way nesting in a numerical model with finest horizontal spacing of 370 m to investigate possible mechanisms producing turbulence in two distinct regions of the cyclone. In both regions, model-parameterized turbulence kinetic energy compares well to observed turbulence reports. Despite being outside of hazardous large radar reflectivity locations in deep convection, both regions experienced strong modification of the turbulence environment as a result of upper-tropospheric/lower-stratospheric (UTLS) convective outflow. For one region, where turbulence was isolated and short lived, simulations revealed breaking of ~100-km horizontal-wavelength lower-stratospheric gravity waves in the exit region of a UTLS jet streak as the most likely mechanism for the observed turbulence. Although similar waves occurred in a simulation without convection, the altitude at which wave breaking occurred in the control simulation was strongly affected by UTLS outflow from distant deep convection. In the other analyzed region, turbulence was more persistent and widespread. There, overturning waves of much shorter 5–10-km horizontal wavelengths occurred within layers of gradient Richardson number < 0.25, which promoted Kelvin–Helmholtz instability associated with strong vertical shear in different horizontal locations both above and beneath the convectively enhanced UTLS jet.
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ItemThe Sensitivity of Atmospheric River Identification to Integrated Water Vapor Transport Threshold, Resolution, and Regridding Method(AMER GEOPHYSICAL UNION, 2020-10-27)Atmospheric rivers (ARs) are elongated narrow bands of enhanced water vapor that can cause intense rainfall and flooding. ARs only appeared in the literature the last 30 years, and there has been much debate about how to define ARs and how to identify them. As a result, a wide range of AR identification algorithms have been produced with variations in the conditions required for an object to be classified as an AR and differences in the input data. One of the key conditions in most AR identification algorithms is a minimum threshold of water vapor flux, along with geometric criteria. The aim of this study is to explore uncertainties in global AR identification based on a single integrated water vapor transport (IVT)-based identification method. We conduct a sensitivity analysis under one algorithmic framework to explore the effects of different IVT thresholds, input data resolutions, and regridding methods during the Years of Tropical Convection operational analysis (May 2008 to April 2010). We found that the resolution and regridding method affects the number of ARs identified but the seasonal cycle is maintained. AR identification is highly sensitive to the choice of IVT threshold; importantly, the commonly used 250 kg m−1 s−1 IVT threshold is not appropriate for global studies with detection methods that also include a restrictive geometric condition as this combination can lead to the strongest systems failing to be identified. The uncertainties within a single AR detection method and input data parameters may be as large as uncertainties across AR detection methodologies.
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ItemGlobal and regional impacts differ between transient and equilibrium warmer worlds(NATURE PUBLISHING GROUP, 2020-01-01)under exclusive licence to Springer Nature Limited. There has recently been interest in understanding the differences between specific levels of global warming, especially the Paris Agreement limits of 1.5 °C and 2 °C above pre-industrial levels. However, different model experiments1–3 have been used in these analyses under varying rates of increase in global-average temperature. Here, we use climate model simulations to show that, for a given global temperature, most land is significantly warmer in a rapidly warming (transient) case than in a quasi-equilibrium climate. This results in more than 90% of the world’s population experiencing a warmer local climate under transient global warming than equilibrium global warming. Relative to differences between the 1.5 °C and 2 °C global warming limits, the differences between transient and quasi-equilibrium states are substantial. For many land regions, the probability of very warm seasons is at least two times greater in a transient climate than in a quasi-equilibrium equivalent. In developing regions, there are sizable differences between transient and quasi-equilibrium climates that underline the importance of explicitly framing projections. Our study highlights the need to better understand differences between future climates under rapid warming and quasi-equilibrium conditions for the development of climate change adaptation policies. Yet, current multi-model experiments1,4 are not designed for this purpose.
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ItemLocally forced convection in sub‐kilometre scale simulations with the Unified Model and WRF(Wiley, 2020)This study evaluates the performance and benefits of kilometre and sub‐kilometre scale convection permitting simulations over tropical Australia. Focusing on an extended Monsoon break period we can directly compare Unified Model (UM) and Weather Research and Forecasting model (WRF) simulations to CPOL radar observations and soundings. We show that the two models have different behaviour, and both are different to observations. Whereas WRF produces daily squall lines whether or not they occurred in observations, the UM primarily generates small but intense storms. The UM and WRF produce qualitatively different surface density currents at different times in the diurnal cycle. Once the density currents are present, the models also show different behaviour in relation to convective initiation. While higher resolution helps in the distribution of total precipitation over the domain, most characteristics do not change with higher resolutions, and model difference are always larger than resolution differences. While CAPE/CIN does not seem to be important to explain model differences, our findings point to the evolution of density currents in the boundary layer as most important source of model errors and differences.
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ItemShear-Parallel Tropical Convective Systems: Importance of Cold Pools and Wind Shear(American Geophysical Union, 2020-06-28)The impact of cold pools on line-orientated convective systems is assessed using idealized simulations of tropical oceanic convection under weak, moderate, and strong wind shear regimes. Cold pools are weakened by suppressing evaporation in the shallow subcloud layer. Analysis of objectively identified convective systems reveals that the convection with weaker cold pools is more often oriented parallel, rather than perpendicular, to the wind shear. The cold pool-induced orientation changes are most pronounced in the strong shear environment. Interactions between convective orientation and the tropical atmosphere are assessed. Simulations with shear-parallel convection demonstrate more top-of-atmosphere upwelling longwave radiation and less reflected shortwave radiation due to changes in convective anvils, faster-propagating larger-scale gravity waves, narrower cross-shear moisture distributions, and differences in convective momentum fluxes. The results highlight critical interactions across convective scales, mesoscales, and climate scales, as well as avenues for parameterizing structural modes of mesoscale-organized convection in global models.