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Wenbo Tang, Pak Wai Chan, and George Haller

Abstract

Using observational data from coherent Doppler light detection and ranging (lidar) systems situated at the Hong Kong International Airport (HKIA), the authors extract Lagrangian coherent structures (LCS) intersecting the flight path of landing aircraft. They study the time evolution of LCS and compare them with onboard wind shear and altitude data collected during airplane approaches. Their results show good correlation between LCS extracted from the lidar data and updrafts and downdrafts experienced by landing aircraft. Overall, LCS analysis shows promise as a robust real-time tool to detect unsteady flow structures that impact airplane traffic.

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Wenbo Tang, Pak Wai Chan, and George Haller

Abstract

The accurate real-time detection of turbulent airflow patterns near airports is important for safety and comfort in commercial aviation. In this paper, a method is developed to identify Lagrangian coherent structures (LCS) from horizontal lidar scans at Hong Kong International Airport (HKIA) in China. LCS are distinguished frame-independent material structures that create localized attraction, repulsion, or high shear of nearby trajectories in the flow. As such, they are the fundamental structures behind airflow patterns such as updrafts, downdrafts, and wind shear. Based on a recently developed finite-domain–finite-time Lyapunov exponent (FDFTLE) algorithm from Tang et al. and on new Lagrangian diagnostics presented in this paper that are pertinent to the extracted FDFTLE ridges, the authors differentiate LCS extracted from lidar data. It is found that these LCS derived from horizontal lidar scans compare well to convergence and divergence suggested by vertical slice scans. At HKIA, horizontal scans are predominant: they cover much bigger azimuthal ranges as compared with only two azimuthal angles from the vertical scans. LCS extracted from horizontal scans are thus advantageous in providing organizing turbulence structures over the entire observational domain as compared with a single line along the vertical scan direction. In Part II of this study, the authors will analyze the evolution of LCS and their impacts on landing aircraft based on recorded flight data.

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Kai-Kwong Hon and Pak-Wai Chan

Abstract

Low-level turbulence [rapid headwind fluctuations below 1600 ft (500 m)] poses potential safety hazards to landing/departing aircraft and is capable of disrupting air traffic. Timely, accurate alerts of low-level turbulence require reliable determination of its intensity, quantified by an internationally adopted aircraft-independent metric [cube root of the eddy dissipation rate (EDR1/3)], which cannot be directly measured but only inferred from observational data. In this paper, a large-scale survey of terrain-induced low-level turbulence intensity around the Hong Kong International Airport (HKIA) during tropical cyclone (TC) passage is presented, utilizing EDR1/3 values determined from multiple remote sensing and in situ sources, including the scanning Doppler lidar, the terminal Doppler weather radar (TDWR), a high-resolution anemometer, and the operational Windshear and Turbulence Warning System (WTWS) at HKIA. Over a 18 720-min study period spanning five TC cases between 2010 and 2012, ground-based EDR1/3 was computed using a variety of first-principle and empirical methods and was shown to demonstrate a strong linear correlation with airborne values determined from quick access recorder (QAR) data of over 350 landing flights. Spatiotemporal features as experienced on board aircraft were also reproduced by the lidar- and TDWR-derived profiles. Positive skill could be extracted from threshold-based alerting of low-level turbulence events by considering each ground-based source individually, while a combination of lidar and TDWR alerts demonstrated enhanced performance and hence the potential value of complementary surveillance under clear-air and in rain conditions. This study serves to establish the ability of ground-based instruments in correlating with airborne EDR1/3 and the performance of threshold-based alerting algorithms for turbulence events, contributing toward improvements in turbulence-alerting techniques for the aviation community.

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Hossein Amini Kafiabad, Pak Wai Chan, and George Haller

Abstract

Recent studies have shown that aerial disturbances affecting landing aircraft have a coherent signature in the Lagrangian aerial particle dynamics inferred from ground-based lidar scans. Specifically, attracting Lagrangian coherent structures (LCSs) mark the intersection of localized material upwelling within the cone of the lidar scan. This study tests the detection power of LCSs on historical landing data and corresponding pilot reports of disturbances from Hong Kong International Airport. The results show that a specific LCS indicator, the gradient of the finite-time Lyapunov exponent (FTLE) field along the landing path, is a highly efficient marker of turbulent upwellings. In particular, in the spring season, projected FTLE gradients closely approach the efficiency of the wind shear alert system currently in operation at the airport, even though the latter system relies on multiple sources of data beyond those used in this study. This shows significant potential for the operational use of FTLE gradients in the real-time detection of aerial disturbances over airports.

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Soo-Hyun Kim, Hye-Yeong Chun, and Pak Wai Chan

Abstract

Two indicators of turbulence—the eddy dissipation rate (EDR) and derived equivalent vertical gust velocity (DEVG)—are calculated using aircraft observations from Hong Kong–based airlines, whose aircraft included Boeing and Airbus models, for 39 months from February 2011 to April 2014. Characteristics of the two turbulence indicators that were calculated at 1-min intervals from the flight data are investigated. For Boeing and Airbus aircraft, there are large seasonal variations in the 90th and 99th percentiles of EDR and DEVG, whereas there are relatively small seasonal variations in the medians of EDR and DEVG. For the turbulence encounters estimated from EDR and DEVG, the authors compute their correlations for each level of turbulence and each type of aircraft. Strong correlations (larger than 0.7) occurred for all levels of turbulence encounters for Boeing aircraft, whereas relatively weak correlations (less than 0.5) occurred for Airbus aircraft. This difference is due to the different characteristics of recorded Boeing and Airbus aircraft data (the number of decimals and data sampling frequency). Based on correlation analyses, the authors construct the best-fit curves using mean EDR values for each DEVG bin and mean DEVG values for each EDR bin and obtain relationships between EDR and DEVG for Boeing and Airbus aircraft. The EDR and DEVG-derived EDR for moderate-or-greater-level turbulence are generally similar for Boeing aircraft.

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Brent Knutson, Wenbo Tang, and Pak Wai Chan

Abstract

The operational light detection and ranging (lidar) data from the Hong Kong International Airport (HKIA) in China are assimilated in the six-nest, high-resolution Weather Research and Forecasting (WRF) Model. The existing radar data assimilation schemes in the WRF data assimilation (WRFDA) package have been adapted to accommodate the high temporal frequency and spatial resolution of the lidar observations. The weather data are then used to produce Lagrangian coherent structures to detect atmospheric hazards for flights. The coherent structures obtained from the various datasets are contrasted against flight data measured on aircraft. It is found that both WRF and WRFDA produce coherent structures that are more distinguishable than those obtained from two-dimensional retrieval, which may improve the detection of true wind shear hazards.

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Xiaoxue Wang, Yuguo Li, Kai Wang, Xinyan Yang, and Pak Wai Chan

Abstract

The atmospheric boundary exhibits an obvious diurnal cycle. The daily cycle of climate variation has a significant effect on urban airflow, and an understanding of it is very important for city-scale environmental control. A new and simple daily cycle temperature boundary condition for simulations of urban airflows with computational fluid dynamics (CFD) is described herein. An analytical surface temperature formula was obtained after simplifying longwave radiation and sensible heat flux terms. The formula provides a reasonably good prediction of ground surface temperatures on sunny days without adding much complexity. The accuracy of the prediction of daily surface temperature variations in homogeneous soils was evaluated with a benchmark experiment and with weather station data. The new boundary condition was implemented in the commercial CFD software Fluent for a city-scale model. The implementation demonstrates the importance of including diurnal temperature profiles and atmospheric boundary conditions in CFD simulations of urban plumes in which an ideal urban heat island circulation occurs.

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Dan Wu, Fuqing Zhang, Xiaomin Chen, Alexander Ryzhkov, Kun Zhao, Matthew R. Kumjian, Xingchao Chen, and Pak-Wai Chan

Abstract

Cloud microphysics significantly impact tropical cyclone precipitation. A prior polarimetric radar observational study by Wu et al. (2018) revealed the ice-phase microphysical processes as the dominant microphysics mechanisms responsible for the heavy precipitation in the outer rainband of Typhoon Nida (2016). To assess the model performance regarding microphysics, three double-moment microphysics schemes (i.e., Thompson, Morrison, and WDM6) are evaluated by performing a set of simulations of the same case. While these simulations capture the outer rainband’s general structure, microphysics in the outer rainbands are strikingly different from the observations. This discrepancy is primarily attributed to different microphysics parameterizations in these schemes, rather than the differences in large-scale environments due to cloud-environment interactions. An interesting finding in this study is that the surface rain rate or liquid water content is inversely proportional to the simulated mean raindrop sizes. The mass-weighted raindrop diameters are overestimated in the Morrison and Thompson schemes and underestimated in the WDM6 scheme, while the former two schemes produce lower liquid water content than WDM6. Compared with the observed ice water content based on a new polarimetric radar retrieval method, the ice water content above the environmental 0 °C level in all simulations is highly underestimated, especially at heights above 12 km MSL where large concentrations of small ice particles are typically prevalent. This finding suggests that the improper treatment of ice-phase processes is potentially an important error source in these microphysics schemes. Another error source identified in the WDM6 scheme is overactive warm-rain processes that produce excessive concentrations of smaller raindrops.

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Jian Huang, Zhongshui Zou, Qingcun Zeng, Peiliang Li, Jinbao Song, Lin Wu, Jun A. Zhang, Shuiqing Li, and Pak-wai Chan

Abstract

The turbulent structure within the marine atmospheric boundary layer is investigated based on four levels of observations at a fixed marine platform. During and before a cold front, the ocean surface is dominated by wind sea and swell waves, respectively, affording the opportunity to test the theory recently proposed in laboratory experiments or for flat land surfaces. The results reveal that the velocity spectra follow a k −1 law within the intermediate wavenumber (k) range immediately below inertial subrange during the cold front. A logarithmic height dependence of the horizontal velocity variances is also observed above the height of 20 m, while the vertical velocity variances increase with increasing height following a power law of 2/3. These features confirm the attached eddy model and the “top-down model” of turbulence over the ocean surface. However, the behavior of velocity variances under swell conditions is much different from those during the cold front, although a remarkable k −1 law can be observed in the velocity spectra. The quadrant analysis of the momentum flux also shows a significantly different result for the two conditions.

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Yihong Duan, Qilin Wan, Jian Huang, Kun Zhao, Hui Yu, Yuqing Wang, Dajun Zhao, Jianing Feng, Jie Tang, Peiyan Chen, Xiaoqin Lu, Yuan Wang, Jianyin Liang, Liguang Wu, Xiaopeng Cui, Jing Xu, and Pak-Wai Chan

Abstract

Landfalling tropical cyclones (TCs) often experience drastic changes in their motion, intensity, and structure due to complex multiscale interactions among atmospheric processes and among the coastal ocean, land, and atmosphere. Because of the lack of comprehensive data and low capability of numerical models, understanding of and ability to predict landfalling TCs are still limited. A 10-yr key research project on landfalling TCs was initiated and launched in 2009 in China. The project has been jointly supported by the China Ministry of Science and Technology, China Meteorological Administration (CMA), Ministry of Education, and Chinese Academy of Sciences. Its mission is to enhance understanding of landfalling TC processes and improve forecasting skills on track, intensity, and distributions of strong winds and precipitation in landfalling TCs. This article provides an overview of the project, together with highlights of some new findings and new technical developments, as well as planned future efforts.

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