Sheng, Jinyu
Permanent URI for this collectionhttps://hdl.handle.net/10222/22175
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Item Open Access An Observational Study of the Energy-Transfer between the Seasonal Mean Flow and Transient Eddies(1991-03) SHENG, J.; DEROME, J.The large-scale energetics of the Northern Hemisphere atmospheric motions are computed in the frequency domain using a 5-year data set from the ECMWF operational analyses. The geographical distributions of kinetic energy (KE) for the fast transient (periods shorter than 10 days) assume the shape of elongated bands indicating the structure of the storm tracks. The slow transients (periods longer than 10 days) exhibit local maxima of KE over the eastern regions of the major oceans. The slow transient available potential energy (APE), on the other hand, displays maxima over the American and Asian continents. Both baroclinic and nonlinear conversions are responsible for the maintenance of low-frequency disturbances when hemispherically-integrated quantities are considered. Low-frequency disturbances over the land are maintained primarily through baroclinic energy conversions, while those over the oceans are maintained primarily through a barotropic energy transfer from the seasonal mean flow and through a nonlinear energy transfer from high frequency eddies. With the flow separated in terms of three frequency "bands", namely, the seasonal mean, the low- and the high-frequency eddies, the energy cycle is a rather simple one. During summer, (i) the APE flows from lower to higher frequency bands, (ii) the baroclinic conversion transforms APE to KE for all frequency bands, and finally, (iii) KE flows from higher to lower frequencies (including the time-mean flow). For the winter season, the energy flows in the same direction, except for one transfer, the exchange of KE between the time-mean flow and the slow transients. The direction is from the time-mean flow to the slow transients, which is presumably due to the time-mean motions being barotropically unstable during the winter.Item Open Access GCM experiments on changes in atmospheric predictability associated with the PNA pattern and tropical SST anomalies(2002-08) Sheng, J.Based on results from a simple three-level quasi-geostrophic model, Lin and Derome suggested that atmospheric predictability is influenced by the Pacific/North American (PNA) pattern. In the present study, predictability experiments are conducted with the Canadian Centre for Climate Modelling and Analysis general circulation model (CCCma GCM). A 47-yr integration of the GCM with specified sea surface temperature (SST) for the years 1948-94 is first performed. Forecasts are initiated whenever the PNA pattern is in a strong positive or strong negative phase during this simulation. For each forecast, an ensemble of six initial conditions is generated with small random perturbations. Forecasts initiated when the PNA is in its positive phase have smaller growth rates of ensemble standard deviation than forecasts initiated when the PNA is in its negative phase. Regional characteristics of the prediction spread are also examined. Similar experiments are conducted to determine the relationship between atmospheric predictability and SST anomalies in the tropical Pacific. Forecasts initiated when tropical SST anomalies are positive have smaller growth rates of ensemble standard deviation than forecasts initiated when tropical SST anomalies are negative. However, cases with positive tropical SST anomalies but without a strong PNA pattern show a similar prediction spread to cases with negative SST anomalies. The results suggest that, in comparison to the PNA pattern, the influence of tropical SST anomalies is only secondary. A set of three-layer diagnostic equations is used to analyze the GCM results. It is speculated that the transient eddies have a stronger influence on the circulation anomalies (and therefore reduce the atmospheric predictability more) in the negative PNA phase than in the positive PNA phase.