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Wu-Ron Hsu and Wen-Yih Sun

Abstract

A primitive equation numerical model has been used to study the Mei-Yu system that occurred in the middle of May 1987. Although cumulus heating was not included in one of the experiments, all of the major features of a Mei-Yu system other than deep convection are reproduced in that experiment. These features are a cloud band along the coastline in southern China; a weak temperature gradient, a strong moisture gradient, and a strong wind shear across the cloud band; and a supergeostrophic low-level jet (LLJ) with an accompanying secondary circulation.

When the moist air coming from the south encounters the migrating high pressure system in the north, it turns to the east-northeast and becomes saturated. The resulting stratiform cloud covers all of southern China and the pressure trough at the 850-hPa level deepens by 1.5 hPa due to the latent heat released. It induces convergent motion in the lower level and generates a direct, cross-frontal secondary circulation that helps keep the low-level wind supergeostrophic. Without such large-scale latent heating, the low-level wind would be weaker; the amount of moisture that can be transported to the frontal zone would also be drastically reduced. Once the LLJ forms. it may trigger heavy precipitation in the later stages of the Mei-Yu system. Our study suggests that cumulus heating may not have played a critical role in the formation of the LLJ, at least for the particular Mei-Yu system under consideration.

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Wen-Yih Sun and Wu-Ron Hsu

Abstract

A cold air outbreak over the East China Sea was simulated by using a high-resolution mesoscale planetary boundary layer model developed at Purdue University. The model includes the major physical processes such as surface fluxes of heat and moisture, condensation and evaporation, and vertical turbulent transfer of heat, moisture, and momentum.

The distribution of specific humidity during the passage of the front reveals that moisture is pushed upward along the frontal surface as observed. The cold dry air behind the cold front is quickly modified by strong convection over the warm water surface, especially over the Kuroshio. A cloud-free region exists near the coast where the air is very cold and dry. As air moves over the water and undergoes modification, a layer of stratocumulus forms downstream from the cloud-free region. The depth of the convective boundary layer increases toward the Kuroshio. In the horizontal momentum equations, the diffusion term almost balances the ageostrophic force in the convective layer. The numerical results compare favorably to the AMTEX '74 observation.

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Wen-Yih Sun, Jiun-Dar Chern, Ching-Chi Wu, and Wu-Ron Hsu

Abstract

Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show:

(a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast.

(b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast.

(c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiwan.

(d) The Froude number is an important parameter to estimate the blocking effect of the central mountain range; however, the flow pattern also depends on other parameters, such as the shape of mountains, the terrain of the surrounding areas, and other meteorological parameters.

(e) The budget study of the vorticity equation shows that stretching, tilting, and friction are important for the formation of lee vortices in our results.

These results may provide some physical explanations for the observed mesolow and cyclonic flow in the southeast and northwest of Taiwan during the late spring and early summer—a transitional period of the winter and summer monsoons in Taiwan.

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Allan H. Murphy, Richard W. Katz, Robert L. Winkler, and Wu-Ron Hsu

Abstract

The purposes of this paper are to describe a dynamic model for repetitive decision‐making in the cost–loss ratio situation and to present some theoretical and numerical results related to the optimal use and economic value of weather forecasts within the framework of the model. This model involves the same actions and events as the standard (i.e., static) cost–loss ratio situation, but the former (unlike the latter) is dynamic in the sense that it possesses characteristics (e.g., decisions, events) that are related over time. We assume that the decision maker wants to choose the sequence of actions over an n‐occasion time period that minimizes the total expected expense. A computational technique known as stochastic dynamic programming is employed to determine this optimal policy and the total expected expense.

Three types of weather information are considered in studying the value of forecasts in this context: 1) climatological information; 2) perfect information; and 3) imperfect forecasts. Climatological and perfect information represent lower and upper bounds, respectively, on the quality of all imperfect forecasts, with the latter considered here to be categorical forecasts properly calibrated according to their past performance. Theoretical results are presented regarding the form of the optimal policy and the relationship among the total expected expenses for these three types of information. In addition, quality/value relationships for imperfect forecasts are described.

Numerical results are derived from the dynamic model for specific values of the model parameters. These results include the optimal policy and the economic value of perfect and imperfect forecasts for various time horizons, climatological probabilities, and values of the cost–loss ratio. The relationship between the accuracy and value of imperfect forecasts also is examined.

Several possible extensions of this dynamic model are briefly discussed, including decision‐making problems involving more actions and/or events, more complex structures of the costs and losses, and more general forms of imperfect forecasts (e.g., probability forecasts).

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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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Bao-Fong Jeng, Hway-Jen Chen, Shwu-Ching Lin, Tzay-Ming Leou, Melinda S. Peng, Simon W. Chang, Wu-Ron Hsu, and C.-P. Chang

Abstract

The Central Weather Bureau (CWB) in Taipei, Republic of China has entered the era of operational numerical weather prediction with the complete online operations of a Global Forecast System (GFS) and the Limited-Area Forecast Systems (LAFS). A brief description of the Regional Forecast System (RFS) and the Mesoscale Forecast System (MFS) of the LAFS are presented in this paper. The RFS has a horizontal resolution of 90 km, depends on the GFS for boundary values, and produces forecast up to 48 h over the eastern parts of Asia and the northwestern Pacific Ocean. The MFS has a resolution of 45 km, uses RFS analysis and forecast as initial and boundary conditions, and produces 24-h forecasts for Taiwan and its immediate vicinity. Model configurations, numerics, physical parameterizations, performance statistics, and two significant weather cases of the two forecast systems are discussed. Future improvements and new plans will also be given.

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