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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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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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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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Zhengyu Liu, S. G. H. Philander, and R. C. Pacanowski

Abstract

Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coast in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.

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Chester S. Gardner, Marcus S. Miller, and C. H. Liu

Abstract

During 13 nights of Rayleigh lidar measurements at Urbana Illinois in 1984–86, thirty-six quasi-monochromatic gravity waves were observed in the 35–50 km altitude region of the stratosphere. The characteristics of the waves are compared with other lidar and radar measurements of gravity waves and with theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (λ2) ranged from 2 to 11.5 km and the measured vertical phase velocities (c z) ranged from 10 to 85 cm s−1. The vertical wavelengths and vertical phase velocities were used to infer observed wave periods (T ob) which ranged from 100 to 1000 min and horizontal wavelengths (λx) which ranged firm 70 to 2000 km. There may be errors, in the inferred values of the horizontal wavelengths because they were calculated by assuming that the observed period inferred the intrinsic period. Dominant wave activity was found at vertical wavelengths between 2–4 km and 7–10 km. No significant seasonal variations were evident in the observed parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with T ob, which is consistent with recent sodium lidar studies of quasi-monochromatic waves near the mesopause. An average amplitude growth length of 20.9 km for the rms wind perturbations was estimated from the data. Kinetic energy density associated with the waves decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects.

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Jiabin Liu, Inez Y. Fung, and John C. H. Chiang

Abstract

Rainbands that migrate northward from spring to summer are persistent features of the East Asian summer monsoon. This study employs a machine learning algorithm to identify individual East Asian rainbands from May to August in the 6-hourly ERA-Interim reanalysis product and captures rainband events during these months for the period 1979–2018. The median duration of rainband events at any location in East Asia is 12 h, and the centroids of these rainbands move northward continuously from approximately 28°N in late May to approximately 33°N in July, instead of making jumps between quasi-stationary periods. Whereas the length and overall area of the rainbands grow monotonically from May to June, the intensity of the rainfall within the rainband dips slightly in early June before it peaks in late June. We find that extratropical northerly winds on all pressure levels over East China are the most important anomalous flow accompanying the rainband events. The anomalous northerlies augment climatological background northerlies in bringing low moist static energy air and thus generate the front associated with the rainband. Persistent lower-tropospheric southerly winds bring in moisture that feeds the rainband and are enhanced a few days prior to rainband events, but they are not directly tied to the actual rainband formation. The background northerlies could originate as part of the Rossby waves resulting from the jet stream interaction with the Tibetan Plateau. The ageostrophic circulation in the jet entrance region peaks in May and weakens in June and July and does not prove to be critical to the formation of the rainbands.

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C. H. Liu, G. Dester, S. J. Franke, J. Röttger, and C-J. Pan

Abstract

A novel extension of the spaced antenna method for VHF/UHF radar and wind profiler applications is introduced in this paper. It is proposed that instead of pointing the spaced antenna beams vertically, off-vertical oblique configuration should be used. It will be, shown that this technique can be used to obtain horizontal wind at independent “points” in the atmosphere without making assumptions about homogeneity or variation of the wind field between these points. Good experimental confirmations are achieved by comparing the results with those derived from the conventional spaced antenna method and the Doppler method. Applications of the proposed technique in measuring kinematic properties of the wind field such as divergence, vorticity, etc., in the atmosphere and the accuracy of these measurements are discussed.

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Abigail L. S. Swann, Inez Y. Fung, Yuwei Liu, and John C. H. Chiang

Abstract

In the mid-Holocene, the climate of northern Africa was characterized by wetter conditions than present, as evidenced by higher paleolake levels and pollen assemblages of savannah vegetation suggesting a wetter, greener Sahara. Previous modeling studies have struggled to simulate sufficient amounts of precipitation when considering orbital forcing alone, with limited improvement from considering the effects of local grasslands. Here it is proposed that remote forcing from expanded forest cover in Eurasia relative to today is capable of shifting the intertropical convergence zone northward, resulting in an enhancement in precipitation over northern Africa approximately 6000 years ago greater than that resulting from orbital forcing and local vegetation alone. It is demonstrated that the remote and local forcing of atmospheric circulation by vegetation can lead to different dynamical patterns with consequences for precipitation across the globe. These ecoclimate teleconnections represent the linkages between the land surface in different regions of the globe and by inference show that proxy records of plant cover represent not only the response of vegetation to local climate but also that vegetation’s influence on global climate patterns.

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Xiangyu Ao, C. S. B. Grimmond, H. C. Ward, A. M. Gabey, Jianguo Tan, Xiu-Qun Yang, Dongwei Liu, Xing Zhi, Hongya Liu, and Ning Zhang

Abstract

The Surface Urban Energy and Water Balance Scheme (SUEWS) is used to investigate the impact of anthropogenic heat flux Q F and irrigation on surface energy balance partitioning in a central business district of Shanghai. Diurnal profiles of Q F are carefully derived based on city-specific hourly electricity consumption data, hourly traffic data, and dynamic population density. The Q F is estimated to be largest in summer (mean daily peak 236 W m−2). When Q F is omitted, the SUEWS sensible heat flux Q H reproduces the observed diurnal pattern generally well, but the magnitude is underestimated compared to observations for all seasons. When Q F is included, the Q H estimates are improved in spring, summer, and autumn but are poorer in winter, indicating winter Q F is overestimated. Inclusion of Q F has little influence on the simulated latent heat flux Q E but improves the storage heat flux estimates except in winter. Irrigation, both amount and frequency, has a large impact on Q E. When irrigation is not considered, the simulated Q E is underestimated for all seasons. The mean summer daytime Q E is largely overestimated compared to observations under continuous irrigation conditions. Model results are improved when irrigation occurs with a 3-day frequency, especially in summer. Results are consistent with observed monthly outdoor water use. This study highlights the importance of appropriately including Q F and irrigation in urban land surface models—terms not generally considered in many previous studies.

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