John L. McGregor

Regional climate modelling

SummaryRegional climate modelling is becoming increasingly popular. The most common technique employs high resolution limited-area models to economically produce detaited climatologies for selected regions. A short review is presented of the underlying principles, recent simulations limitations of the method and future prospects.

Regional Climate Model Intercomparison Project for Asia

Improving the simulation of regional climate change is one of the high-priority areas of climate study because regional information is needed for climate change impact assessments. Such information is especially important for the region covered by the East Asian monsoon where there is high variability in both space and time. To this end, the Regional Climate Model Intercomparison Project (RMIP) for Asia has been established to evaluate and improve regional climate model (RCM) simulations of the monsoon climate. RMIP operates under joint support of the Asia–Pacific Network for Global Change Research (APN), the Global Change System for Analysis, Research and Training (START), the Chinese Academy of Sciences, and several projects of participating nations. The project currently involves 10 research groups from Australia, China, Japan, South Korea, and the United States, as well as scientists from India, Italy, Mongolia, North Korea, and Russia. RMIP has three simulation phases: March 1997–August 1998, which c...

Project to Intercompare Regional Climate Simulations (PIRCS): Description and initial results

The first simulation experiment and output archives of the Project to Intercompare Regional Climate Simulations (PIRCS) is described. Initial results from simulations of the summer 1988 drought over the central United States indicate that limited-area models forced by large-scale information at the lateral boundaries reproduce bulk temporal and spatial characteristics of meteorological fields. In particular, the 500 hPa height field time average and temporal variability are generally well simulated by all participating models. Model simulations of precipitation episodes vary depending on the scale of the dynamical forcing. Organized synoptic-scale precipitation systems are simulated deterministically in that precipitation occurs at close to the same time and location as observed (although amounts may vary from observations). Episodes of mesoscale and convective precipitation are represented in a more stochastic sense, with less precise agreement in temporal and spatial patterns. Simulated surface energy fluxes show broad similarity with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations in their temporal evolution and time average diurnal cycle. Intermodel differences in midday Bowen ratio tend to be closely associated with precipitation differences. Differences in daily maximum temperatures also are linked to Bowen ratio differences, indicating strong local, surface influence on this field. Although some models have bias with respect to FIFE observations, all tend to reproduce the synoptic variability of observed daily maximum and minimum temperatures. Results also reveal the advantage of an intercomparison in exposing common tendencies of models despite their differences in convective and surface parameterizations and different methods of assimilating lateral boundary conditions.

An Updated Description of the Conformal-Cubic Atmospheric Model

An updated description is presented for the quasi-uniform Conformal-Cubic Atmospheric Model. The model achieves high efficiency as a result of using semi-Lagrangian, semi-implicit time differencing. A reversible staggering treatment for the wind components provides very good dispersion characteristics. An MPI methodology is employed that allows the model to run efficiently on multiple processors. The physical parameterizations for the model are briefly described, and results are shown for the Held-Suarez test, the Aqua-Planet Experiment and an AMIP simulation having 125 km resolution. Antarctic snow accumulation is also shown from a shorter simulation having 50 km resolution.

Economical Determination of Departure Points for Semi-Lagrangian Models

Abstract An Eulerian procedure that avoids both interpolation and iteration is proposed for determining the departure points of trajectories. It is applicable to semi-Lagrangian models formulated either on the plane or on the sphere. The technique can achieve a high degree of accuracy; it is also simpler and more economical than other schemes, especially when applied on the sphere. The technique is applied to the cone advection test on the plane, as well as to a “Gaussian hill” problem on a rotating sphere.

The CSIRO Conformal-Cubic Atmospheric GCM

Global atmospheric models are usually formulated upon latitude-longitude grids. Near the poles, these grids have disproportionately high resolution, which may severely constrain the time step of integration or require special filtering. Advection problems may also occur near the poles of such grids. Quite recently, a global conformal-cubic grid was devised by (1996). This grid avoids the disadvantages of latitude-longitude grids, but does require careful selection of numerical techniques to account for the eight vertices of the grid (McGregor, 1996).

Investigating the haze transport from 1997 biomass burning in Southeast Asia: its impact upon Singapore

Abstract The 1997 Indonesia forest fires was an environmental disaster of exceptional proportions. Such a disaster caused massive transboundary air pollution and indiscriminate destruction of biodiversity in the world. The immediate consequence of the fires was the production of large amounts of haze in the region, causing visibility and health problems within Southeast Asia. Furthermore, fires of these magnitudes are potential contributors to global warming and climate change due to the emission of large amounts of greenhouse gases and other pyrogenic products.The long-range transport of fire-related haze in the region is investigated using trajectories from the CSIRO Division of Atmospheric Research Limited Area Model (DARLAM). Emission scenarios were constructed for hotspot areas in Sumatra and Kalimantan for the months of September and October 1997 to determine the period and fire locations most critical to Singapore. This study also examines some transport issues raised from field observations. Results show that fires in the coastal areas of southeast Sumatra and southwest Kalimantan can be potential contributors to transboundary air pollution in Singapore. Singapore was directly affected by haze from these areas whereas Kuala Lumpur was heavily affected by the haze coming from Sumatra. In most cases, Singapore was more affected by fires from Kalimantan than was Kuala Lumpur. This was mainly a result of the shifting of monsoons. The transition of monsoons resulted in weaker low-level winds and shifted convergence zones near to the southeast of Peninsular Malaysia. In addition to severe drought and massive fire activity in 1997, the timing of the monsoon transition has a strong influence on haze transport in the region.

Semi-Lagrangian Advection on Conformal-Cubic Grids

Abstract It has been demonstrated by McGregor that semi-Lagrangian advection techniques may be efficiently applied to a cubic gnomonic grid on the sphere. Despite the nonorthogonal nature of that grid, the accuracy is superior to that of conventional latitude–longitude grids. The present paper demonstrates even greater accuracy by applying similar techniques to the related conformal-cubic grid devised by Rancic et al.; an important new feature is a simple iterative technique for the inverse calculation of grid coordinates. Advection over the vertices of the grid exhibits none of the problems that occur over the poles of a latitude–longitude grid. A stretched grid configuration is also presented showing further improvements. It is finally shown that the departure points may be interpolated onto a B-grid version and advection performed simply on the staggered grid without loss of accuracy.

January and July Climate Simulations over the Australian Region Using a Limited-Area Model

Abstract High-resolution climate simulations are presented for January and July over the Australian region using a limited-area model nested within a GCM. One new aspect of these simulations is that the model domain extends to tropical regions both north and south of the equator. Objective measures of skill are used to assess the quality of the model simulations, and the performance of the model is verified over various subregions of the model domain. Poleward of tropical regions, the nested model climatologies produced are generally superior to those produced by the GCM, and compare reasonably well to observations over the Australian continent at the regional scale; in particular the simulations of precipitation and screen temperature are improved.

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...