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Tsann-Wang Yu

Abstract

Numerical models require parameterization of surface evaporation rates when a surface energy budget equation is included in the calculation of the surface temperature. Techniques for parameterizing surface moisture and evaporation are reviewed. Two methods in particular are investigated with observational data taken from the Wangara, the O'Neill, the Kerang and the Davis experiments. Method I is based on parameterization of the surface moisture with the surface evaporation computed through the flux-profile similarity relationships. Method II is based on a modified form of the Penman equation which requires a knowledge of solar radiation and soil heat flux to calculate surface evaporation. Comparisons of the results of these methods are made with observations. Method I fails to account for condensation which is often observed during the night. Method II is shown to be superior to Method I in the calculation of surface evaporation for both day and night.

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Tsann-Wang Yu

Abstract

Theories and formulations for determining heights of the stable atmospheric boundary layer are reviewed. The performance of prognostic and diagnostic equations for parameterizing the nocturnal boundary layer heights are evaluated with observational data taken from the Wangara field experiment. The observed boundary layer heights are correlated with the values specified by the prognostic and diagnostic equations. It is concluded that use of the prognostic equations is very unsatisfactory. The diagnostic formulas are found to be appropriate, however, only during slightly and extremely stable conditions. None of the diagnostic formulas appears useful for other stability classes.

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Tsann-wang Yu

Abstract

A technique is presented to deduce wind direction from satellite microwave measurements of wind speed information. The technique, based on simple Ekman boundary layer dynamics, makes use of surface pressure fields routinely analyzed at the National Meteorological Center. To demonstrate its application, a three-day sample of altimeter and scatterometer wind speed data, taken from the Seasat satellite, was used to deduce wind directions. The deduced wind vectors are presented and compared with the NMC 1000 mb wind analyses, and with the subjectively edited vector winds. It is suggested that the technique proposed in this study could be applied to the ocean surface wind speed measurements derived from a satellite altimetric mission to produce a more useful parameter, namely, the ocean surface wind vector. This technique can also be used to objectively resolve potential ambiguities in scatterometer wind directions.

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Tsann-Wang Yu

Abstract

No abstract available.

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Tsann-Wang Yu

Abstract

A one-dimensional version of the Techniques Development Laboratory boundary layer model has been adopted to simulate the O'Neill fifth period and the Wangara day 32 experiment data. Fourteen parameterization schemes for the vertical turbulent exchange processes in the atmospheric boundary layer are examined. These include 1) conventional K theories based on Blackadar's mixing length with wind shear and stability functions, 2) O'Brien K formula with time-varying mixed layer heights governed by the surface heat and momentum fluxes based on the work of Deardorff, and 3) a turbulent energy closure model.

Numerical results were compared to observations. In general, it was found that all the models perform well in the simulation of the Wangara day 32 data but less satisfactorily in the O'Neill fifth-period simulation. The most satisfactory simulation in wind speeds was reproduced by the turbulent energy model. For temperatures the O'Brien K formula topped by the mixed layer heights performs the best, particularly during the convective hours.

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Tsann-Wang Yu
and
Norman Y. Wagner

Abstract

Surface wind data taken from 19 km offshore to 14 km inland during several days of onshore wind occurrence were reduced and analysed. The mean kinetic enemy per unit mass and its changes normal to the coastline were computed directly from the wind data. Analysis of these data shows two well defined regimes of diurnal variation in wind speed. The marine air has a nighttime speed maximum and a daytime minimum. As the air moves inland, the speed distribution becomes bimodal with the primary maximum occurring in the daytime and the secondary maximum at night.

As expected Intuitively and predicted by theory, the speed changes most abruptly near the change in surface roughness (the coastline). Also as predicted by theory, complete dynamic equilibrium with the new lower boundary is not achieved until the air is ∼5–12 km downwind from the coastline.

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Tsann-Wang Yu
and
Ronald D. Mcpherson

Abstract

Two days of global scatterometer-derived oceanic surface winds during the period of 0000 GMT 16 July-0000 GMT 18 July 1978 from the SEASAT-A satellite are used in the NMC's global data assimilation and forecast experiments to gain a preliminary appreciation of the impact of this dataset. The NMC's global data assimilation system used in this study is described. The nature of the scatterometer winds and their error characteristics are discussed.

Two parallel 48-hour data assimilation experiments are conducted: one including scatterometer wind data (SASS), the other without (NCSASS). After 48 hours of assimilation, large differences have evolved between SASS and NOSASS analyses due to the scatterometer winds. Comparison of the analyses with the operational analysis generated by the Australian Bureau of Meteorology suggests that the influence of scatterometer winds was beneficial in the Southern Hemisphere.

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Xin-Zhong Liang
,
Min Xu
,
Xing Yuan
,
Tiejun Ling
,
Hyun I. Choi
,
Feng Zhang
,
Ligang Chen
,
Shuyan Liu
,
Shenjian Su
,
Fengxue Qiao
,
Yuxiang He
,
Julian X. L. Wang
,
Kenneth E. Kunkel
,
Wei Gao
,
Everette Joseph
,
Vernon Morris
,
Tsann-Wang Yu
,
Jimy Dudhia
, and
John Michalakes

The CWRF is developed as a climate extension of the Weather Research and Forecasting model (WRF) by incorporating numerous improvements in the representation of physical processes and integration of external (top, surface, lateral) forcings that are crucial to climate scales, including interactions between land, atmosphere, and ocean; convection and microphysics; and cloud, aerosol, and radiation; and system consistency throughout all process modules. This extension inherits all WRF functionalities for numerical weather prediction while enhancing the capability for climate modeling. As such, CWRF can be applied seamlessly to weather forecast and climate prediction. The CWRF is built with a comprehensive ensemble of alternative parameterization schemes for each of the key physical processes, including surface (land, ocean), planetary boundary layer, cumulus (deep, shallow), microphysics, cloud, aerosol, and radiation, and their interactions. This facilitates the use of an optimized physics ensemble approach to improve weather or climate prediction along with a reliable uncertainty estimate. The CWRF also emphasizes the societal service capability to provide impactrelevant information by coupling with detailed models of terrestrial hydrology, coastal ocean, crop growth, air quality, and a recently expanded interactive water quality and ecosystem model.

This study provides a general CWRF description and basic skill evaluation based on a continuous integration for the period 1979– 2009 as compared with that of WRF, using a 30-km grid spacing over a domain that includes the contiguous United States plus southern Canada and northern Mexico. In addition to advantages of greater application capability, CWRF improves performance in radiation and terrestrial hydrology over WRF and other regional models. Precipitation simulation, however, remains a challenge for all of the tested models.

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