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Steven J. Lambert

Abstract

A comparison is made of the divergent wind analyses of the National Meteorological Center (NMC) and those of the ECMWF/WMO dataset produced by the European Centre for Medium Range Weather Forecasts (ECMWF). Using a reliability criterion based on the degree of correlation between month-long time series of daily values, the utility of the analyses is assessed over the globe on seven pressure levels from 100 to 1000 mb for all Januarys and Julys between 1980 and 1986. The results indicate that from January 1983 onward, the analyses for the winter hemisphere midlatitudes (≈20°–70°) and for the summer hemisphere midlatitudes (≈35°–70°) should be considered reliable except in midtropospheric levels where the divergence is weak. The tropical (between about 20° latitude of the winter hemisphere and 35° latitude of the summer hemisphere) analyses are especially poor and fall far short of satisfying the reliability criterion.

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Steven J. Lambert

Abstract

The available potential energy–kinetic energy budget of the Canadian Climate Centre general circulation model for the months of January. April, July and October is presented in terms of the two-dimensional wave-number. Five years of model results are compared to calculations based on observations taken during the First GARP Global Experiment (FGGE).

Qualitatively, the simulated budget is realistic but a few of the magnitudes of the energies and conversions between them are not well simulated, arising in part from an excess of zonal available potential energy and zonal kinetic energy caused by the model's polar regions being too cold.

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Steven J. Lambert

Abstract

Using time series of simple statistics, the homogeneity of a long-term Northern Hemisphere 500-mb dataset is assessed. Several discontinuities coinciding with major analysis changes were found which indicate that care must be exercised in the use of this dataset in studies where homogeneous data are required.

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Steven J. Lambert

Abstract

The intraseasonal divergent and rotational kinetic energy (KE) budgets for the 30–60 day interval are calculated from the analyses of the European Centre for Medium Range Weather Forecasts/World Meteorological Organization (ECMWF/WMO) dataset, and from simulations by the Canadian Climate Centre (CCC) General Circulation Model (GCM). Comparison of the results indicates that the model simulates the rotational KE well but there are differences in the observed and simulated rotational KE budgets. In the observed budget, the most important term is the nonlinear interaction term indicating that the atmosphere draws on energy produced over a wide range of spatial and temporal scales to maintain the rotational KE maximum in the intraseasonal time scales. The model, on the other hand, uses a more direct approach in that it tends to maintain the simulated rotational KE maximum by using KE generated at spatial scales similar to those of the KE maximum.

Both the divergent KE and its budget are reasonably well simulated by the model. Most of the divergent KE in the intraseasonal time scales is a result of the Madden-Julian Oscillation (MJO). Even though the model simulates the intraseasonal divergent KE and its budget well, it has problems in simulating the MJO. The simulated MJO-like phenomenon in the model does not display well-organized spectral peaks concentrated in a single octave of the spectrum as seen in the observations, but displays much more random behavior with the variance spread out over a wide range of frequencies.

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Steven J. Lambert

Abstract

The extratropical winter cyclone climatologies for the Northern and Southern Hemispheres are presented for a control, or 1 × CO2 simulation, and an enhanced greenhouse warming, or 2 × CO2 simulation, using the second generation Canadian Climate Centre general circulation model. When compared to the control climatology, the 2 × C02 simulation exhibits a significant reduction in the total number of lows in both winter hemispheres. Although the total number of cyclones decreases, the frequency of intense cyclones increases, with this behavior being more significant in the Northern Hemisphere. Examination of the storm tracks in both simulations indicates that there is little change in their geographical positions with global warming.

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Steven J. Lambert

Abstract

The cyclone event climatology is presented for a five-year simulation by the Canadian Climate Centre general circulation model. Winter season results are given for the extratropical regions of both the Northern and the Southern hemispheres and the results are compared to an event climatology based on five years of observed data.

In the Northern Hemisphere, classification of the simulated and the observed 1000 mb lows as a function of central height shows that the model has a deficiency of weak cyclones and an excess of intense cyclones while the number of cyclones of medium intensity is well simulated. The geographical distribution of the cyclone events shows that those features of the observed climatology which are thermal in nature, such as a preference for lows to form and move over relatively warm underlying surfaces, are reasonably well simulated, but the topographical features such as lee cyclogenesis and the avoidance of high terrain by lows are poorly simulated.

The Southern Hemisphere also shows a deficiency of simulated cyclones although less pronounced than the Northern Hemisphere. The number of weak and strong cyclones are well simulated, and the paucity of Southern Hemisphere lows results from a deficiency of cyclones of moderate intensity. The observed climatology, which is nearly free of mountain influences, is well simulated and only a few minor problems are evident, such as a poor simulation of the winter cyclogenesis in the lee of the Andes and a tendency for model cyclones to be present over high terrain.

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Herschel L. Mitchell, Cécilien Charette, Steven J. Lambert, Jacques Hallé, and Clément Chouinard

Abstract

A global data assimilation system has been in operation at the Canadian Meteorological Centre (CMC) since March 1991 when it replaced the previous hemispheric system. This paper describes the system and presents an evaluation of its performance from several points of view, including the fit of the analyses and short-range forecasts to observations, the relative roles of various components of the system, the functioning of some specific subcomponents in a particular case, and the ability of the system to represent important as aspects of the mean monthly general circulation. This latter part of the evaluation includes comparisons with the corresponding statistics derived from the analyses of the National Meteorological Center.

The global data assimilation system is found to be functioning well, especially in extratropical regions with reasonable data coverage. Problems and weaknesses of the system are discussed.

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Herschel L. Mitchell, Clément Chouinard, Cécilien Charette, Richard Hogue, and Steven J. Lambert

Abstract

The improvement of analysis and data assimilation techniques can have a large impact, as shown here in the context of the Canadian global and regional data assimilation systems. Both of these systems utilize the same analysis component that was recently changed as follows: (a) a completely 3D algorithm replaced the previous split 3D scheme, which involved separate vertical and horizontal steps; (b) the assimilation of SATEM data was revised and is now done in terms of thicknesses over relatively thick layers; (c) an additional analysis level (at 925 hPa) was added and a derived temperature analysis replaced the former temperature analysis; (d) observation and forecast error statistics were revised; and (e) a correction procedure was introduced for certain types of radiosondes to offset the negative impact of solar and longwave radiation.

While many of these changes are interrelated, preventing a systematic evaluation of each in isolation, it is shown that the revised 3D algorithm eliminates a problem that sometimes occurred in areas of dense surface data, SATEM data have a large positive impact in the Southern Hemisphere, and the radiosonde bias-correction scheme very significantly reduces the geopotential height bias observed previously in the upper atmosphere over certain regions, such as western North America.

The overall evaluation of the analysis changes shows that in general the new analysis results in more accurate 6-h forecasts, with the largest improvements in the Tropics and especially in the Southern Hemisphere. In conjunction with these forecast gains, the evaluation of the general circulation statistics for August also show significant changes: the new analyses are more energetic, exhibiting a substantially stronger Hadley circulation and stronger zonal winds about Antarctica. The global forecasts from the revised analysis system consistently exhibit a significantly more rapid spinup of global precipitation as compared to the previous system.

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