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A case of vortex/wave shedding at a mountain near Hong Kong International Airport (HKIA) in a tropical cyclone situation is studied in this paper. Compared with similar studies at HKIA in the past, this paper contains novel results by considering both actual observations and numerical simulation of the event, and by studying the shedding in a neutral boundary layer instead of a stably stratified atmosphere in the spring time. The vortex/wave shedding is clearly observed from the radial velocity imagery of Terminal Doppler Weather Radar (TDWR). The shedding period is about 7 min, which is consistent with previous results in the literature. The vortex/wave adjacent to the background strong southeasterly flow also leads to significant wind shear over the arrival runway corridor to the west of the north runway of HKIA. It may also be associated with the significant headwind changes encountered by an arriving aircraft that had to conduct missed approach due to the rapidly fluctuating winds. The shedding event is simulated by using the Regional Atmospheric Modelling System (RAMS) version 4.4 with a horizontal resolution down to 50 m. The model is found to capture the vortex/wave shedding successfully. The simulated shedding period is similar to actual observations, though the associated area of reverse flow is much smaller. As such, with faster computers, it may be possible to forecast the occurrence of significant wind shear associated with vortex/wave shedding in an area of complex terrain and provide earlier alert to the aviation weather forecasters. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

A weak waterspout was observed at the Hong Kong International Airport on the morning of June 8, 2020. This paper documents the meteorological observations of this weak system, which was well captured by surface wind sensors and a Doppler weather radar, and this study also investigated the capability of a high-resolution numerical weather prediction model to simulate the system in a near real-time configuration. Despite the short-lived nature of the vortex, the model effectively reproduced the occurrence of a weak vortex around the time the system was observed, although with positional errors of the order of a few kilometres. The simulated vertical cross-sections of the reflectivity and Doppler velocity were consistent with the actual radar observations and reflected the slanting nature of the micro-vortex. These results may serve as a useful reference for the study of weak waterspouts worldwide. [ABSTRACT FROM AUTHOR]

ABSTRACT Vortex/wave shedding is sometimes observed in the Hong Kong International Airport ( HKIA) region by such sophisticated instruments as Doppler LIght Detection And Ranging ( LIDAR) systems. During the past three years several vortex/wave shedding cases have been documented, often to the west of HKIA, downwind of the mountains which can block and disturb the prevailing airflow from the southeast. These shedding movements might trigger turbulence and wind shear, threatening aircraft landing from the west. This paper documents, for the first time, that such shedding can also occur at the runway corridor to the east of the airport's south runway, caused by mountains on the eastern side of Lantau Island. It calculates the shedding period and Froude number of the events, comparing these data with results reported in the literature. It also discusses, for the first time, the use of a computational fluid dynamics ( CFD) model to simulate these events, based on homogeneous initialization only and using the upper air ascent data closest in time to the events. The simulations were shown to successfully reproduce the main features of the shedding, pointing to the possibility of applying this kind of simulation method in real-time forecasts to provide early warning services for aircraft taking off from HKIA. [ABSTRACT FROM AUTHOR]

In the evening of 11 April 2011, vortex/wave shedding from a mountain to the south of the Hong Kong International Airport (HKIA) appeared to occur following the establishment of fresh to strong easterly airflow in Hong Kong. This paper documents the observations by the various meteorological instruments of the event and discusses the possibility of forecasting the vortex/wave shedding using a high resolution numerical weather prediction (NWP) model. The vortex/wave shedding was well observed by the Light Detection and Ranging (LIDAR) system at HKIA. The vortex/wave appeared as an area of reversed flow against the background east to southeasterly airflow in the airport area. It was detached from the mountain and moved to the northwest with the background southeasterly flow, with a gradual decrease of the spatial extent in the area of reversed flow. The vortex/wave brought about significant wind shear to both arrival flight paths to the west of HKIA, as evidenced from the headwind profile data measured by the LIDARs and the aircraft data. It had rather large vertical extent, reaching the top of the boundary layer, but did not extend downward to the surface, as shown from the vertical scans of the LIDAR and the surface wind observations. The shedding event is successfully captured by high-resolution NWP model with a spatial resolution of 50 m. The model output has a lead time of 6-7 h ahead of the event, indicating the possible occurrence of terrain-induced wind shear in the form of vortex/wave shedding from a mountain. As a result, the numerical simulation could be useful in giving an early alert to the aviation weather forecaster about the significant wind shear event, which could be hazardous to the landing aircraft. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

This paper reviews recent advances in weather forecasting capability in the United Kingdom and their implications for increasing the lead time with which flood warnings can be issued. The events of summer 2007 have highlighted the vulnerability of parts of the United Kingdom to flooding and the need for long lead time flood warnings to enable the protection of people and critical infrastructure. Historically, computer weather forecasting models have been unable to forecast at the scales of importance for flood warning, and so the warning processes have been forced to rely on measurements on the ground. Examples are presented to demonstrate that new forecasting technologies, currently being implemented, enable warnings to be produced much earlier, provided they are couched in probabilistic terms and interpreted appropriately. Crown Copyright © 2009. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd [ABSTRACT FROM AUTHOR]

A tornado/waterspout associated with a supercell thunderstorm was observed in the adjacent waters of southern China on the evening of 18 April 2019. The case was documented using surface weather observations and Doppler radar data. A weather station near the tornado/waterspout, recorded wind gusts corresponding to hurricane intensity for a few seconds, never observed before in the region. The Doppler velocity associated with the tornado/waterspout was analysed and the vertical profile of the velocity could be very useful for wind engineering applications in the region. Dual‐polarization observations also were analysed and exhibited some similar signatures documented in supercell tornadoes in other parts of the world. Predictability of the tornado/waterspout was studied using a numerical weather prediction model. It showed that the location and timing of the waterspout/tornado could be roughly reproduced, and the model might be used to give earlier alerting to the vessels operating in the south China coastal waters. [ABSTRACT FROM AUTHOR]

Numerous studies have elucidated that white heads and chalky rice are caused by hot, dry and strong winds at night. However, these studies have only used synoptic weather charts and near‐surface observation data to confirm the role of foehn in white head occurrences. This study used a high‐resolution three‐dimensional numerical weather prediction model to show that strong foehn effects induced a significant number of white heads in rice in Yamagata Prefecture, Japan. The model accurately predicted and represented the location and timing of the observed white heads, occurring only in a limited area (87.1 ha) on the southwest side of Mt. Chokai during the nighttime. Specifically, the simulated nocturnal FTP (transpiration forcing) continuously exceeded 30 at that location. This high FTP was caused by high temperature (>24°C), strong wind speed (>8 m s−1) and low humidity (<65%) caused by the strong foehn winds descending over the southwest slope of the mountain. In the other areas, FTP did not exceed 30 because either wind speed, water vapour deficit or both were insufficiently high. [ABSTRACT FROM AUTHOR]

The present study focussed on the formation of upslope fog. This fog causes chronic expressway closures near Beppu Bay (BB) in Oita Prefecture, Japan. A dense fog event, occurring on 3 and 4 May 2020, resulting from the passage of an extratropical cyclone through the southern Japanese islands, was simulated using the Weather Research and Forecasting (WRF) numerical model. The simulation results indicated that warm and humid air with a lifting condensation level less than 20 m flowed into a plain from BB. Simulated fog and poor visibility were more severe on mountain slope adjacent to BB than on the surroundings. Here, the westward horizontal flux of water vapour by the wind flow was prolonged at lower heights. Analyses of the simulated elements verified that the upslope fog was induced by moist adiabatic cooling of the air lifted by the upslope winds. At the mature stage of upslope fog, a simulated local vertical circulation generated in conjunction with the upper synoptic flow extended the dense fog into the upper atmosphere over BB. During the decaying stage of upslope fog on mountain, a fog covered on the entire BB. This was associated with weakening winds at lower heights. Sensitivity experiments for model boundary conditions supported the importance of mountain steepness and the rectangular shape of BB for the generation of upslope fog. Finally, an application possibility of dense fog forecasts was discussed by using a simulated difference in the equivalent potential temperature between the base of the mountain and Beppuwan expressway interchange located midslope. [ABSTRACT FROM AUTHOR]

The semiarid climate and rugged terrain in the interior west of the United States do not favour the development of bow echoes, a type of convective storm associated with intense, damaging winds. However, on 21 April 2011, a bow echo associated with a fast-moving midtropospheric perturbation formed across the Great Salt Lake (GSL) in Utah, producing damaging winds along its path. Intrigued by the rarity of this bow echo and the inability of the North American Mesoscale model (NAM) to forecast it, this event was studied by using available observations and conducted simulations with the Advanced Research Weather Research and Forecasting (WRF) model. Sensitivity to the microphysics schemes (MPSs), horizontal grid spacing, intensity of moisture content, and a physical lake model in the WRF model were examined. It was found that: (a) reduction in grid spacing from 12 and 4 km to 1 km along with improved depiction of low-level moisture substantially improved the bow echo simulation, (b) the presence of GSL did not impact bow echo development, and (c) the WRF model appeared to inherit a phase error in the passage of the midtropospheric perturbation from the NAM initial and lateral boundary conditions. The phase error resulted in a 1-2 h delay in the bow echo passage. These results highlight the difficulties in simulating such a bow echo event, and suggest similar challenges future faced by subsequent regional climate downscaling studies on future extreme weather in the western United States. [ABSTRACT FROM AUTHOR]

Southern peninsular India experiences widespread thunderstorm activities during the pre-monsoon season due to the presence of a north-south trough, an easterly trough, and low pressure areas over the surrounding Indian Seas. In March 2008, southern peninsular India received unusual heavy rains because of the interaction between a large amplitude easterly trough and a mid-latitude westerly trough during 12 to 14 March, and because of the formation of a low pressure system over the Arabian Sea during 21 to 24 March. Simulation of these two heavy rainfall events is attempted using the Weather Research and Forecasting ( WRF) model. The model's performance is studied in terms of the wind, pressure, movement of these synoptic scale systems, moisture and rainfall fields. Results indicate that this model has the capability to predict the movement of both easterly and westerly troughs and to simulate low pressure very well. The model is able to capture the areas of precipitation maxima in both the cases. The bias score is found to be more than 1 in precipitation thresholds up to 35 mm and less than 1 in precipitation thresholds above 35 mm. The maximum values of critical success index ( CSI) and equitable threat score ( ETS) are 0.49, 0.32 and 0.42, 0.13 in 24 and 48 h forecasts, respectively, in Case 1 and 0.69, 0.23 and 0.71, 0.19 in Case 2. The values of CSI and ETS drop considerably above the 30 mm rainfall threshold in both 24 and 48 h forecasts showing that the model performs better in lower rainfall thresholds. [ABSTRACT FROM AUTHOR]

In 2011, the Hong Kong Observatory ( HKO) and the Government Flying Service ( GFS) of the Hong Kong Government conducted reconnaissance data collection flights for two tropical cyclones over the South China Sea. It was the first time in the region that flight measurements of this kind had been carried out specifically for tropical cyclones. Horizontal wind velocity components, temperature and relative humidity at selected flight levels were recorded at the temporal resolution of 20 times per second to delineate the wind flow and other meteorological conditions. They were found to be consistent with other synoptic station data sparsely located over the sea areas, as well as in good agreement with the ocean surface wind data from the satellite microwave scatterometer. In this study, aircraft measurements for the two tropical cyclones in 2011 and one in July 2009 were assimilated into the HKO mesoscale Non- Hydrostatic Model ( NHM) using its three-dimensional variational data assimilation ( 3DVAR) to investigate the impact of the data on the model analysis and forecast. Model sensitivity experiments with and without the flight measurements were conducted. The flight observations were found to improve model analysis and forecasts of wind circulation, temperature and humidity in the vicinity of the tropical cyclones. Improvement in track forecast was obtained although an insignificant effect was seen in predicting the central pressure of the storms. The inclusion of flight measurements also improved the forecast of the surface wind distribution and precipitation associated with the tropical cyclones. [ABSTRACT FROM AUTHOR]

ABSTRACT A series of observation system simulation experiments ( OSSEs) and a real case study are conducted to investigate the application of the Doppler radar data assimilation technique for numerical model quantitative precipitation forecasts ( QPFs). A four-dimensional variational Doppler radar analysis system ( VDRAS) is adopted for all experiments. The first set of OSSEs demonstrates that when the background field contains the imperfect information predicted from a mesoscale model, the incorrect convective-scale perturbations in the background can result in spurious scattered precipitation. However, a smoothing procedure can be used to remove the fine structures from the primitive model output in order to avoid this over-prediction. Results from the second set of OSSEs indicate that the lack of low-elevation data owing to radar scan and/or beam blockage could significantly alter the retrieved low-level thermal and dynamical structures when a different number of data assimilation cycles is applied. These impacts could lower the rainfall forecast capability of the model. The third set of OSSEs shows that, when the rainwater is assimilated over a long assimilation window, the non-linearity embedded in the microphysical process could lead the minimization algorithm in a wrong direction, causing a further degradation of the rainfall prediction. However, using multiple short assimilation cycles produces better minimization and forecast results than those obtained with a single long cycle. A real case experiment based on data collected during Intensive Operation Period ( IOP) #8 of the 2008 Southwest Monsoon Experiment ( SoWMEX) is conducted to provide a verification of the conclusions obtained from OSSEs under a realistic framework. [ABSTRACT FROM AUTHOR]

The spatial distribution of the annual mean urban heat island UHI intensity was simulated applying empirical models based on datasets from urban areas of Szeged and Debrecen, using simple and easily determinable urban surface cover variables. These two cities are situated on the Alföld Great Hungarian Plain and have similar topographic and climatic conditions. Temperature field measurements were carried out, Landsat satellite images were evaluated, and then one and multiple variable models were constructed using linear regression techniques. The selected multipleparameter models were verified using independent datasets from three urban settlements. In order to obtain some impression of the mean UHI patterns in other cities with no temperature measurements available, the better model was extended to urban areas of four other cities situated in geographical environments similar to Szeged and Debrecen. The main shortcoming of typical empirical models, namely that they are often restricted to a specific location, is overcome by the obtained model since it is not entirely site but more region specific, and valid in a large and densely populated area with several settlements. Copyright © 2009 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Numerical simulation of particle trajectory and atmospheric dispersion has been performed for an airborne accidental release from a nuclear power plant site. A Longrange Particle transport and Dispersion Model LPDM based on a Lagrangian approach is developed and tested in this work. The Lagrangian transportdispersion model is directly coupled with an atmospheric prediction model, RAMS Regional Atmospheric Modeling System, to provide necessary meteorological fields in a threedimensional domain. An advantage of this direct coupling is that the meteorological data generated by RAMS can be used directly for trajectory calculations without storage, thus reducing the CPU time consumed in the data storage and retrieval. This effort was done to be able to use this directly coupled modelling system for realtime predictions in case of an accidental release from a potential site.The simulated Lagrangian trajectories were compared with those obtained using observed hourly weather data obtained from an onsite meteorological tower. The results indicated that this oneway coupling between LPDMRAMS provided almost identical trajectories when compared with those obtained using LPDM alone driven by hourly observed wind data. The comparison demonstrated the reliability of the RAMS meteorological predictions for the site under consideration. The comparison also indicated that LPDM run in a stand alone mode, with hourlyobserved wind data, could also be used for trajectory calculations over flat terrain.The model was developed on a parallel processing computer SGI workstation, ORIGIN 2000 computer with eight processors for use in realtime forecast mode. The computational time was about onethird of the simulation time, while using four processors. The model options need to be explored to reduce the computational time further and test its performance for realtime atmospheric dispersion applications. Copyright © 2009 Royal Meteorological Society [ABSTRACT FROM AUTHOR]