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Jianhua Sun and Sixiong Zhao

Abstract

This paper investigated the interactions between the synoptic patterns, quasi-stationary fronts, eastward-propagating cloud clusters from the Tibetan Plateau, surface conditions, and atmospheric stratification processes associated with a 20-day event of freezing precipitation over southern China from January to February 2008. It was found that the long duration of the freezing precipitation process was primarily caused by stationary and anomalous synoptic weather patterns such as a blocking high pressure system in the northern branch and a trough in the south branch of the westerlies, which resulted in the convergence of cold air from northern China and warm, moist air from the south. The cloud clusters over the Tibetan Plateau propagated eastward and showed noticeable impacts in the local areas when they moved over southern China during several similar cloud propagation processes from January to February 2008. An east–west-oriented quasi-stationary front system in southern China, which is rare during the Asian winter monsoon season, is responsible for producing freezing precipitation and snowstorms. A stronger horizontal gradient of the isolines of the pseudo-equivalent potential temperature and higher temperatures at the inversion layer in the western part of the front than that in its eastern part can be found. At the same time, low-level moisture convergence ahead of the front enhanced the formation, development, and persistence of freezing precipitation in the west part of the front. The thickness of the warm layer and the temperature inversion layer also modulated the intensity and duration of freezing rain and ice pellets. Temperature from about −1° to −3°C and weak winds were found to be favorable meteorological factors at the surface level for freezing precipitation. These analysis results are synthesized into a conceptual model that coherently describes the physics processes associated with the synoptic features and quasi-stationary front system as well as the atmospheric stratification process during the freezing precipitation event.

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Howard B. Bluestein

Abstract

During the evening of 4 May 2007, a large, powerful tornado devastated Greensburg, Kansas. The synoptic and mesoscale environments of the parent supercell that spawned this and other tornadoes are described from operational data. The formation and early evolution of this long-track supercell, within the context of its larger-scale environment, are documented on the basis of Weather Surveillance Radar-1988 Doppler (WSR-88D) data and mobile Doppler radar data. The storm produced tornadoes cyclically for about 30 min before producing a large, long-lived tornado. It is shown that in order to have forecasted the severe weather locations and times accurately, it would have been necessary to have predicted 1) the localized formation of an isolated convective storm near/east of a dryline, 2) the subsequent splitting and resplitting of the storm several times, 3) the growth of a new storm along the right-rear flank of an existing storm, and 4) the transition from the cyclic production of small tornadoes to the production of one, large, long-track tornado. It is therefore suggested that both extreme sensitivity to initial conditions associated with storm formation and the uncertainty of storm behavior made it unusually difficult to forecast this event accurately.

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Huijun Wang and Ke Fan

Abstract

A new scheme is developed to improve the seasonal prediction of summer precipitation in the East Asian and western Pacific region. The scheme is applied to the Development of a European Multimodel Ensemble System for Seasonal to Interannual Prediction (DEMETER) results. The new scheme is designed to consider both model predictions and observed spatial patterns of historical “analog years.” In this paper, the anomaly pattern correlation coefficient (ACC) between the prediction and the observation, as well as the root-mean-square error, is used to measure the prediction skill. For the prediction of summer precipitation in East Asia and the western Pacific (0°–40°N, 80°–130°E), the prediction skill for the six model ensemble hindcasts for the years of 1979–2001 was increased to 0.22 by using the new scheme from 0.12 for the original scheme. All models were initiated in May and were composed of nine member predictions, and all showed improvement when applying the new scheme. The skill levels of the predictions for the six models increased from 0.08, 0.08, 0.01, 0.14, −0.07, and 0.07 for the original scheme to 0.11, 0.14, 0.10, 0.22, 0.04, and 0.13, respectively, for the new scheme.

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