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Frederiksen C.S.,Collaboration for Australian Weather and Climate Research | Zheng X.,Beijing Normal University | Zheng X.,CAS Institute of Atmospheric Physics | Grainger S.,Collaboration for Australian Weather and Climate Research
Climate Dynamics | Year: 2015

A methodology is proposed that allows an estimate of the contribution of decadal noise to the inter-decadal variability of climate variables. By removing this decadal noise, or unpredictable decadal variability, from the total inter-decadal variability, the residual variability is more likely to be associated with potentially predictable slow processes. This residual variability may be considered to be potentially predictable. We apply the method to sea surface temperature from a 1000 year (850–1850 AD) experiment with the CCSM4 model forced by solar variations, volcanic aerosols, orbital forcings, land use and greenhouse gas concentrations. Our analysis shows large potential SST inter-decadal predictability in the extra-tropical regions in both hemispheres, as well as in the subtropical/tropical Indian Ocean, the western Pacific and the Atlantic. In the tropical eastern Pacific, inter-decadal variability is dominated by unpredictable decadal noise. The two leading unpredictable modes of inter-decadal variability have features related to the two leading ENSO modes, and the PDO. The four leading potentially predictable modes of inter-decadal variability are shown to be related to the external forcing, the IPO, inter-hemispheric SST fluctuations and the AMO, and decadal/multi-decadal AO/NAO variability. © 2015 Springer-Verlag Berlin Heidelberg

Gregory P.A.,Collaboration for Australian Weather and Climate Research | Rikus L.J.,Collaboration for Australian Weather and Climate Research
Journal of Applied Meteorology and Climatology | Year: 2016

Forecast solar exposure fields produced by the Australian Bureau of Meteorology's updated numerical weather prediction systems were validated against multiple sites for the 2012 calendar year. The updated systems are denoted as the Australian Community Climate and Earth-System Simulator (ACCESS) model and became operational in August 2010. The systems are based on the Met Office's Unified Model and feature improved assimilation methods and radiation parameterizations that were expected to greatly improve forecasts of solar exposure several days in advance. In this study forecasts of global, direct, and diffuse exposure from the mesoscale model ACCESS-A were validated. Statistics were generated for all-sky and clear-sky conditions. Additionally, evaluation of the model's forecast exposure through single-layer low clouds was conducted. Results show an improvement in global forecasts relative to the older operational model; however, forecasts of diffuse and direct exposure still suffer from large biases. These can be attributed to the choices of the asymmetry factor used in the two-stream approximation for incoming radiation, which determines scattering of the direct beam through clouds. © 2016 American Meteorological Society.

Grainger S.,Collaboration for Australian Weather and Climate Research | Frederiksen C.S.,Collaboration for Australian Weather and Climate Research | Zheng X.,CAS Institute of Atmospheric Physics
Climate Dynamics | Year: 2016

An analysis is made of the coherent patterns, or modes, of interannual variability of Southern Hemisphere 500 hPa geopotential height field under current and projected climate change scenarios. Using three separate multi-model ensembles (MMEs) of coupled model intercomparison project phase 5 (CMIP5) models, the interannual variability of the seasonal mean is separated into components related to (1) intraseasonal processes; (2) slowly-varying internal dynamics; and (3) the slowly-varying response to external changes in radiative forcing. In the CMIP5 RCP8.5 and RCP4.5 experiments, there is very little change in the twenty-first century in the intraseasonal component modes, related to the Southern annular mode (SAM) and mid-latitude wave processes. The leading three slowly-varying internal component modes are related to SAM, the El Niño–Southern oscillation (ENSO), and the South Pacific wave (SPW). Structural changes in the slow-internal SAM and ENSO modes do not exceed a qualitative estimate of the spatial sampling error, but there is a consistent increase in the ENSO-related variance. Changes in the SPW mode exceed the sampling error threshold, but cannot be further attributed. Changes in the dominant slowly-varying external mode are related to projected changes in radiative forcing. They reflect thermal expansion of the tropical troposphere and associated changes in the Hadley Cell circulation. Changes in the externally-forced associated variance in the RCP8.5 experiment are an order of magnitude greater than for the internal components, indicating that the SH seasonal mean circulation will be even more dominated by a SAM-like annular structure. Across the three MMEs, there is convergence in the projected response in the slow-external component. © 2016 Springer-Verlag Berlin Heidelberg

Luo S.,Beijing Normal University | Luo S.,Collaboration for Australian Weather and Climate Research | Sun Z.,Collaboration for Australian Weather and Climate Research | Zheng X.,Beijing Normal University | And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2016

Radiation field and cloud properties over the Southern Ocean area generated by the Australian Community Climate and Earth System Simulator (ACCESS) are evaluated using multiple-satellite products from the Fast Longwave And Shortwave radiative Fluxes (FLASHFlux) project and NASA/GEWEX surface radiation budget (SRB) data. The cloud properties are also evaluated using the observational simulator package COSP, a synthetic brightness temperature model (SBTM) and cloud liquid-water path data (UWisc) from the University of Wisconsin satellite retrievals. All of these evaluations are focused on the Southern Ocean area in an effort to understand the reasons behind the short-wave radiation biases at the surface. It is found that the model overestimates the high-level cloud fraction and frequency of occurrence of small ice-water content and underestimates the middle and low-level cloud fraction and water content. In order to improve the modelled radiation fields over the Southern Ocean area, two main modifications have been made to the physical schemes in the ACCESS model. Firstly the autoconversion rate at which the cloud water is converted into rain and the accretion rate in the warm rain scheme have been modified, which increases the cloud liquid-water content in warm cloud layers. Secondly, the scheme which determines the fraction of supercooled liquid water in mixed-phase clouds in the parametrization of cloud optical properties has been changed to use one derived from CALIPSO data which provides larger liquid cloud fractions and thus higher optical depths than the default scheme. Sensitivity tests of these two schemes in ACCESS climate runs have shown that applying either can lead to a reduction of the solar radiation reaching the surface and reduce the short-wave radiation biases. © 2016. Royal Meteorological Society.

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