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V. C. Liu

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C. Y. Liu

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Paul C. Liu

Abstract

An empirical spectral equation for fetch-limited deep-water wind waveswas derived by applying similarity analysis to wind and wave data recorded at the Lake Michigan Research Tower near Muskegon, Mich., during the autumn of 1967. The field data indicates that both the equilibrium range coefficient, β in S(ω) = βg 2ω−5 and the dimensionless peak-frequency parameter, ω m U */g, vary with Fo, where Fo = gF/U * 2 is the dimensionless fetch parameter with respect to fetch F and friction velocity U *. The equation produces reasonably good results in estimating actual wave spectra, provided sufficient duration is achieved in the wind field. The equation also indicates that a fully-developed state will not be reached at a steady wind speed as the very low-frequency waves grow continuously with increasing fetch.

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C. Liu
,
Y. Liu
, and
H. Xu

Abstract

In this work, the forecast accuracy of a numerical weather prediction model is improved by emulating physical dissipation as suggested by the second law of thermodynamics, which controls the irreversible evolutionary direction of a many-body system like the atmosphere. The ability of the new physics-based scheme to improve model accuracy is demonstrated via the case of the one-dimensional viscous Burgers equation and the one-dimensional diffusion equation, as well as a series of numerical simulations of the well-known 1998 successive torrential rains along the Yangtze River valley and 365 continuous 24-h simulations during 2005–06 with decreased root-mean-square errors and improved forecasts in all of the simulations.

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Wei Liu
,
Zhengyu Liu
, and
Esther C. Brady

Abstract

This paper is concerned with the question: why do coupled general circulation models (CGCM) seem to be biased toward a monostable Atlantic meridional overturning circulation (AMOC)? In particular, the authors investigate whether the monostable behavior of the CGCMs is caused by a bias of model surface climatology. First observational literature is reviewed, and it is suggested that the AMOC is likely to be bistable in the real world in the past and present. Then the stability of the AMOC in the NCAR Community Climate System Model, version 3 (CCSM3) is studied by comparing the present-day control simulation (without flux adjustment) with a sensitivity experiment with flux adjustment. It is found that the monostable AMOC in the control simulation is altered to a bistable AMOC in the flux-adjustment experiment because a reduction of the surface salinity biases in the tropical and northern North Atlantic leads to a reduction of the bias of freshwater transport in the Atlantic. In particular, the tropical bias associated with the double ITCZ reduces salinity in the upper South Atlantic Ocean and, in turn, the AMOC freshwater export, which tends to overstabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. This conclusion is consistent with a further analysis of the stability indicator of two groups of IPCC Fourth Assessment Report (AR4) CGCMs: one without and the other with flux adjustment. Because the tropical bias is a common feature among all CGCMs without flux adjustment, the authors propose that the surface climate bias, notably the tropical bias in the Atlantic, may contribute significantly to the monostability of AMOC behavior in current CGCMs.

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C. H. Liu
and
D. Y. C. Leung

Abstract

A three-dimensional second-order closure meteorological and pollutant dispersion model is developed, and the computed results are evaluated. A finite-element method is used to solve the governing equations because of its versatility in handling variable-resolution meshes and complex geometries. The one-dimensional version of this model is used to simulate a 24-h diurnal cycle for a horizontally homogeneous atmospheric boundary layer in neutral, stable, and unstable stratifications. The simulated turbulence fields under a convective boundary layer act as the background turbulence for simulating cases of three-dimensional pollutant dispersion from elevated point sources. The simulated turbulence and pollutant distribution compared well with experimental observations and with other numerical models, ensuring the validity of the adopted mathematical formulation as well as the developed model. The computed results provide an overview of turbulence structures in different atmospheric stabilities and are helpful to enhance understanding of the characteristics of air pollutant dispersion, such as plume rise and descent in a convective boundary layer. The current study suggests the need for an insightful and practical numerical model to perform air-quality analysis, one that is capable of overcoming the weaknesses of traditional Gaussian plume and k-theory dispersion models.

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C. H. Liu
and
D. Y. C. Leung

Abstract

A three-dimensional mesoscale meteorological model was developed based on second-moment closure equations that were solved by the finite-element method. This paper aims to evaluate the performance of the model under flat terrain and horizontally homogeneous atmospheric boundary layer conditions. The one-dimensional version of this model was tested against field measurements, a water tank experiment, and another numerical model. It showed several interesting behaviors of the atmospheric boundary layer under stable and unstable flows that are of primary interest for environmental studies.

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Rachel C. Zelinsky
,
Chidong Zhang
, and
Chuntao Liu

Abstract

Understanding convective initiation of the Madden–Julian oscillation (MJO) remains an unmet challenge. MJO initiation has been perceived as a process starting from a convectively suppressed large-scale condition with gradual growth of shallow convection to congestus and to deep convective and stratiform systems that cover a large-scale area. During the DYNAMO field campaign over the Indian Ocean, MJO initiation was observed to start from an existing intertropical convergence zone (ITCZ) south of the equator. This raises a question of what possible role the ITCZ may play in convective initiation of the MJO. This study addresses this question through analysis of satellite observations of precipitation and a global reanalysis product. By setting several criteria, MJO and ITCZ events were objectively identified and grouped according to whether MJO initiation was immediately preceded by an ITCZ. The results demonstrate that an ITCZ is neither a necessary nor sufficient condition for convective initiation of the MJO. Nonetheless, evolution of the large-scale circulation, moisture, and convective characteristics during MJO initiation can be different with and without a preexisting ITCZ. Convective growth begins gradually before and during MJO initiation when there is a preexisting ITCZ whereas it is abrupt and slightly delayed without a preexisting ITCZ. Such differences are presumably related to the existing large-scale moist condition of the ITCZ. The results from this study suggest that there are multiple mechanisms for convective initiation of the MJO, which should be considered in theoretical understanding of the MJO.

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Kin Sik Liu
and
Johnny C. L. Chan

Abstract

Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960–2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998–2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.

The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multidecadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200- and 850-hPa horizontal zonal winds (10°–17.5°N, 150°E–180°) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880-gpm line of the June–October 500-hPa geopotential height (0°–40°N, 100°E–180°), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a stronger-than-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998–2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.

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Y. Liu
and
J. C. H. Chiang

Abstract

Previous modeling and paleoclimate studies have suggested that cooling originating from the extratropical North Atlantic can abruptly weaken the Eurasian and North African monsoons. The climatic signature includes a widespread cooling over the Eurasian and North African continents and an associated increase to surface pressure. It is explored whether such coordinated changes are similarly exhibited in the observed twentieth-century climate, in particular with the well-documented shift of Sahel rainfall during the 1960s. Surface temperature, sea level pressure, and precipitation changes are analyzed using combined principal component analysis (CPCA). The leading mode exhibits a monotonic shift in the 1960s, and the transition is associated with a relative cooling and pressure increase over the interior Eurasia and North Africa, and rainfall reduction over the Sahel, South Asia, and East Asia. The local circulation changes suggest that the rainfall shift results from the regional response of the summer monsoons to these continental-wide changes. A similar CPCA analysis of atmospheric general circulation model (AGCM) simulations forced by twentieth-century-observed forcings shows similar results, suggesting that origins of the climate shift reside in the sea surface temperature changes, specifically over the extratropical North Atlantic. Finally, an AGCM forced with extratropical North Atlantic cooling appears to simulate these climate impacts, at least qualitatively. The result herein shows that the observed climate signature of the 1960s abrupt shift in Eurasian and North African climate is consistent with the influence of the abrupt high-latitude North Atlantic cooling that occurred in the late 1960s. A definitive causal relationship remains to be shown, and mechanisms elucidated.

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