Marcus Thatcher

Using a Scale-Selective Filter for Dynamical Downscaling with the Conformal Cubic Atmospheric Model

Abstract This article examines dynamical downscaling with a scale-selective filter in the Conformal Cubic Atmospheric Model (CCAM). In this study, 1D and 2D scale-selective filters have been implemented using a convolution-based scheme, since a convolution can be readily evaluated in terms of CCAM’s native conformal cubic coordinates. The downscaling accuracy of 1D and 2D scale-selective filters is evaluated after downscaling NCEP Global Forecast System analyses for 2006 from 200-km resolution to 60-km resolution over Australia. The 1D scale-selective filter scheme was found to downscale the analyses with similar accuracy to a 2D filter but required significantly fewer computations. The 1D and 2D scale-selective filters were also found to downscale the analyses more accurately than a far-field nudging scheme (i.e., analogous to a boundary-value nudging approach). It is concluded that when the model is required to reproduce the host model behavior above a specified length scale then the use of an appropria...

Projections of rapidly rising surface temperatures over Africa under low mitigation

An analysis of observed trends in African annual-average near-surface temperatures over the last five decades reveals drastic increases, particularly over parts of the subtropics and central tropical Africa. Over these regions, temperatures have been rising at more than twice the global rate of temperature increase. An ensemble of high-resolution downscalings, obtained using a single regional climate model forced with the sea-surface temperatures and sea-ice fields of an ensemble of global circulation model (GCM) simulations, is shown to realistically represent the relatively strong temperature increases observed in subtropical southern and northern Africa. The amplitudes of warming are generally underestimated, however. Further warming is projected to occur during the 21st century, with plausible increases of 4?6 ?C over the subtropics and 3?5 ?C over the tropics by the end of the century relative to present-day climate under the A2 (a low mitigation) scenario of the Special Report on Emission Scenarios. High impact climate events such as heat-wave days and high fire-danger days are consistently projected to increase drastically in their frequency of occurrence. General decreases in soil-moisture availability are projected, even for regions where increases in rainfall are plausible, due to enhanced levels of evaporation. The regional dowscalings presented here, and recent GCM projections obtained for Africa, indicate that African annual-averaged temperatures may plausibly rise at about 1.5 times the global rate of temperature increase in the subtropics, and at a somewhat lower rate in the tropics. These projected increases although drastic, may be conservative given the model underestimations of observed temperature trends. The relatively strong rate of warming over Africa, in combination with the associated increases in extreme temperature events, may be key factors to consider when interpreting the suitability of global mitigation targets in terms of African climate change and climate change adaptation in Africa.

Simulating Australian Urban Climate in a Mesoscale Atmospheric Numerical Model

We develop an urban canopy scheme coupled to a mesoscale atmospheric numerical model and evaluate the simulated climate of an Australian city. The urban canopy scheme is based on the Town Energy Budget approach, but is modified to efficiently represent the predominately suburban component of Australian cities in regional climate simulations. Energy conservation is improved by adding a simple model of air-conditioning to prevent the urban parametrization acting as an energy sink during the Australian summer. In-canyon vegetation for suburban areas is represented by a big-leaf model, but with a largely reduced set of prognostic variables compared to previous approaches. Although we have used a recirculation/venting based parametrization of in-canyon turbulent heat fluxes that employs two canyon wall energy budgets, we avoid using a fixed canyon orientation by averaging the canyon fluxes after integrating over 180° of possible canyon orientations. The urban canopy scheme is evaluated by simulating the climate for Melbourne, Australia after coupling it to The Air Pollution Model. The combined system was found to predict a realistic climatology of air temperatures and winds when compared with observations from Environmental Protection Authority monitoring stations. The model also produced a plausible partitioning of the urban energy budget when compared to urban flux-tower studies. Overall, the urban canyon parametrization appears to have reasonable potential for studying present and predicting changes in future Australian urban climates in regional climate simulations.

A Technique for Dynamically Downscaling Daily-Averaged GCM Datasets Using the Conformal Cubic Atmospheric Model

Abstract In this paper the authors dynamically downscale daily-averaged general circulation model (GCM) datasets over Australia using the Conformal Cubic Atmospheric Model (CCAM). The technique can take advantage of the wider range of Coupled Model Intercomparison Project phase 3 (CMIP3) daily-averaged GCM datasets than is available using 3-hourly datasets. The daily-averaged host GCM atmospheric data are fitted to a time interpolation formula and then differentiated in time to produce a first-order estimate of the atmosphere at 0000 UTC on each simulation day. The processed GCM data are forced into CCAM using a scale-selective filter with an 18° radius. Since this procedure is unable to account for the diurnal cycle, the forcing data are only applied to winds and air temperatures once per day between 800 and 100 hPa. Lateral boundary conditions are not required since CCAM employs a variable-resolution global grid. The technique is evaluated by downscaling daily-averaged 2.5° NCEP reanalyses over Australi...

Assessment of Urban Heat Island and Mitigation by Urban Green Coverage

Urban heat island (UHI) is a growing threat to human well-being and poses increasing pressure on urban utility infrastructure, especially during summer months. This study examined the UHI in Melbourne using remote sensing imagery from MODIS to derive land surface temperature (LST) for the summer of 2009. Then, the potential of urban green coverage in reducing extreme summer temperatures in Melbourne was investigated using an urban climate model for 2009 and for projected 2050 and 2090 future climates. Modeling results showed that the average summer daily maximum (ASDM) temperature differences between Melbourne CBD, suburbs and rural areas were in the range of 0.5–2.0 °C. It was also found that despite the projected climate warming in 2050 and 2090, the cooling benefit in terms of the reduction in the average summer daily maximum temperature due to various urban forms and vegetation schemes remains similar to that estimated for 2009. Thus, the cooling benefit due to various urban forms and green schemes in future climates can be reasonably projected based on the benefits identified with the present-day climate.

An improved parameterisation of ozone dry deposition to the ocean and its impact in a global climate–chemistry model

Schemes used to parameterise ozone dry deposition velocity at the oceanic surface mainly differ in terms of how the dominant term of surface resistance is parameterised. We examine three such schemes and test them in a global climate–chemistry model that incorporates meteorological nudging and monthly-varying reactive-gas emissions. The default scheme invokes the commonly used assumption that the water surface resistance is constant. The other two schemes, named the one-layer and two-layer reactivity schemes, include the simultaneous influence on the water surface resistance of ozone solubility in water, waterside molecular diffusion and turbulent transfer, and a firstorder chemical reaction of ozone with dissolved iodide. Unlike the one-layer scheme, the two-layer scheme can indirectly control the degree of interaction between chemical reaction and turbulent transfer through the specification of a surface reactive layer thickness. A comparison is made of the modelled deposition velocity dependencies on sea surface temperature (SST) and wind speed with recently reported cruise-based observations. The default scheme overestimates the observed deposition velocities by a factor of 2–4 when the chemical reaction is slow (e.g. under colder SSTs in the Southern Ocean). The default scheme has almost no temperature, wind speed, or latitudinal variations in contrast with the observations. The one-layer scheme provides noticeably better variations, but it overestimates deposition velocity by a factor of 2–3 due to an enhancement of the interaction between chemical reaction and turbulent transfer. The two-layer scheme with a surface reactive layer thickness specification of 2.5 μm, which is approximately equal to the reaction-diffusive length scale of the ozone–iodide reaction, is able to simulate the field measurements most closely with respect to absolute values as well as SST and wind-speed dependence. The annual global oceanic deposition of ozone determined using this scheme is approximately half of the original oceanic deposition obtained using the default scheme, and it corresponds to a 10 % decrease in the original estimate of the total global ozone deposition. The previously reported modelled estimate of oceanic deposition is roughly one-third of total deposition and with this new parameterisation it is reduced to 12 % of the modelled total global ozone deposition. Deposition parameterisation influences the predicted atmospheric ozone mixing ratios, especially in the Southern Hemisphere. For the latitudes 45–70 S, the two-layer scheme improves the prediction of ozone observed at an altitude of 1 km by 7 % and that within the altitude range 1–6 km by 5 % compared to the default scheme.

Constructing weather data for building simulation considering urban heat island

This study proposed a method to construct urban hourly weather data by adopting the ‘morphing’ approach considering urban heat islands. The method starts with the surrounding rural weather as the ‘baseline climate’ and an Urban Canopy Model is used to provide the differences in the monthly average values of major climate parameters due to urban heat islands. The ‘baseline climate’ is then morphed to generate the urban hourly weather file for building simulations. It was demonstrated that the constructed hourly weather data agree well with the observations. When used for building thermal simulations, the differences between the proposed approach and the observation in the total energy requirement of space heating and cooling and heat stress risk index were within 5% and 1.0℃, respectively. Practical applications: The proposed approach will allow academics and building engineers to construct realistic urban hourly weather data to analyse the impacts of urban heat islands on energy requirements and thermal stress for different urban planning and design options.

Bias and variance correction of sea surface temperatures used for dynamical downscaling

Bias correction is a widely used method to deal with the deficiencies of climate models in representing the current climate. While it is mainly applied to prepare the output of global or regional climate models (GCMs/RCMs) for climate impact assessment, it has also been used recently to correct GCM output before it is downscaled by RCMs. For most RCMs, 3-D atmospheric fields as well as sea surface temperatures (SSTs) should be corrected in order to create forcing fields. The global stretched grid, conformal-cubic atmospheric model (CCAM), is able to run regional simulations with SST-only forcing. Therefore, only the monthly SSTs obtained from the GCM need to be corrected. In previous studies, the climatological bias was removed, while the bias in the temporal variability was still present. In this study, a simple method for correction of the mean and variance is proposed. The impact of the bias correction is tested using global even-grid CCAM simulations forced with raw and corrected SSTs from ACCESS1.0. Results indicate an improved precipitation pattern in the tropics for all seasons using corrected SSTs. There is also a slight improvement in the precipitation pattern in December–February and March–May and in the response to the El Nino–Southern Oscillation due to the additional variance correction.

A New Approach for Coupled Regional Climate Modeling Using More than 10,000 Cores

This paper describes an alternative method for coupling atmosphere-ocean regional climate models that communicates momentum, radiation, heat and moisture fluxes between the atmosphere and ocean every time-step, while scaling to more than 10,000 cores. The approach is based on the reversibly staggered grid, which possesses excellent dispersive properties for modeling the geophysical fluid dynamics of both the atmosphere and the ocean. Since a common reversibly staggered grid can be used for both atmosphere and ocean models, we can eliminate the coupling overhead associated with message passing and improve simulation timings. We have constructed a prototype of a reversibly staggered, atmosphere-ocean coupled regional climate model based on the Conformal Cubic Atmospheric Model, which employs a global variable resolution cube-based grid to model the regional climate without lateral boundary conditions. With some optimization, the single precision, semi-implicit, semi-Lagrangian prototype model achieved 5 simulation years per day at a global 13 km resolution using 13,824 cores. This result is competitive with state-of-the-art Global Climate Models than can use more than 100,000 cores for comparable timings, making CCAM well suited for regional modeling.