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Robert S. Allan

Abstract

Interactions between monsoon and teleconnection circulations Over the greater Australasian region 35°N–35°S, 85°E–175°E, are observed in 850 and 200 mb monthly mean velocity potential (¯χ) and streamfunction (¯ψ) fields during the peak of the Southern Hemisphere summers (January and February 1973–77) prior to the Winter Monsoon Experiment (WMONEX). Aspect of the spatial and temporal Australasian monsoon and teleconnection systems are presented through a closer examination of the January 1973–75 sub-period, using vertical crow sections of monthly mean divergence and monthly mean surface streamfunction (¯ψsfc) fields.

Most importantly this study suggests that 1arge-scale Southern Oscillation/walker Circulation (SO/WC) teleconnection patterns are associated with variations in components of the Asian monsoon system. Interannual and intraseasonal fluctuations in the Near Equatorial Troughs of the Northern (NETNH) and Southern (NETSH) Hemispheres appear to be related to large-scale atmospheric circulation changes. Over Australia the southward displacement of a strong section of the NETSH is observed during the “wet” summers of 1974 and 1976. It is suggested that, at least in the lower levels, the Australian monsoon circulation in 1974 was almost entirely contained and maintained within the Southern Hemisphere. In the equatorial western Pacific, fluctuations in the NETNH broadly coincide with northeast trade wind variability and possibly the El Niño cycles of 1972–73, 1975 and 1976.

At 200 mb, circulation patterns suggest that changes in anticyclonic maxima (¯ψ minima) in the Australian region are associated with teleconnection overturnings. In the 1973 summer, 200 mb velocity potential fields appear to be synonymous with a SO/WC “breakdown” and the development of a strong El Niño phenomenon.

The variable structure of the SO/WC overturning which would accompany some of the above events is observed in a series of vertical cross sections of monthly mean divergence between the surface and 200 mb levels during January 1973–75. Evidence is presented to show that the concept of the SO/WC as a simple large-scale vertical cell which fluctuates in magnitude and direction may be something of a simplification.

In January 1973–75, ¯ψsfc fields suggest that NETNH activity in the Asian sector is not homogeneous between the surface and 850 mb levels. The extent of cross-equatorial Bow into Australia seems to be related to the coupling between the NETNH, and the cyclonic activity over the northwest of the continent. In fact, variability in this flow may result from either a low-level, quasi-permanent heat low or deeper and more structured NETSH conditions being dominant in this region. Rainfall and monsoonal activity in Australia appear to be related to extremes in this phenomenon.

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Allan H. Murphy
,
Wu-ron Hsu
,
Robert L. Winkler
, and
Daniel S. Wilks

Abstract

This paper summarizes the results of an experiment in which National Weather Service forecasters formulated probabilistic quantitative precipitation forecasts (QPFs) during a 17-month period in 1981–82. These forecasts expressed the likelihood that certain threshold amounts of precipitation would be equaled or exceeded in 12-hour periods at four locations in Texas. The forecasters had no previous experience in quantifying the uncertainty in such forecasts, but they did receive feedback regarding their collective performance at the end of the first year of the experiment. In the evaluation of the experimental results, particular attention is focused on three issues: 1) the reliability and skill of the subjective QPFs; 2) the effects of feedback and experience on the quality of these forecasts; and 3) the relative performance of the subjective probabilistic QPFs and objective probabilistic QPFs produced by the model output statistics system.

The subjective probabilistic QPFs possess positive skill, although they exhibit considerable overforecasting for larger precipitation amounts. Moreover, the feedback provided to the forecasters evidently contributed to modest increases in the reliability and skill of their forecasts. In this regard, the quality of the subjective and objective QPFs is generally comparable in the first year of the experiment. However, after the receipt of the feedback, the skill of the subjective forecasts exceeded the skill of the objective forecasts. These results are considered to be encouraging regarding the ability of forecasters to formulate reliable and skillful probabilistic QPFS, but more extensive experiments should be undertaken to investigate this and related issues in greater detail.

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