Multiscale simulation of the urban wind environment under typhoon weather conditions

Accurate simulations of planetary boundary layer(PBL)winds in urban areas require combining meteorological knowledge and fine-grained geometrical information.Computational fluid dynamics(CFD)is widely used to assess pedestrian wind comfort and wind disasters in planning resilient cities.However,the CFD-predicted PBL is highly affected by the inflow boundaries.Wind profiles under extreme weather conditions,such as tropical cyclones,can hardly be determined,and associated uniform logarithmic or power law expressions have not been obtained.In this study,urban wind flow over mountainous terrain was simulated using a one-way nested simulation approach between mesoscale and microscale models.The inflow wind speed,turbulence scalars,and potential temperature in the CFD code are sustained by the numerical weather prediction(NWP)model.Methodologies considering typhoon weather conditions were examined to improve the numerical accuracy in determining mesoscale typhoon structures and pedestrian-level wind conditions.The numerical errors were quantified in mesoscale and microscale formulations.A new tendency assimilation was proposed by incorporating local-scale observations into the CFD domain.This approach entailed empirical mode decomposition to quantify the mean wind speed differences between the observations and NWP results,which were then extrapolated to NWP-CFD nested interfaces via multiplication by the spatial correlation coefficient.The numerical performance was validated against both on-site observations for meteorological purposes and wind profiles retrieved from the experimental LiDAR of the landfalling typhoon Haima.The simulated wind field exhibited an increased accuracy in the local urban area.More specifically,the index of agreement in wind speeds was improved from 0.28 to 0.72,and the mean absolute errors were reduced from 5.46 m/s to 1.89 m/s.

Computational fluid dynamics simulation of the wind flow over an airport terminal building

Turbulence in the wake generated by wind flow over buildings or obstacles may produce complex flow patterns in downstream areas.Examples include the recirculating flow and wind deficit areas behind an airport terminal building and their potential impacts on the aircraft landing on nearby runways.A computational fluid dynamics(CFD) simulation of the wind flow over an airport terminal building was performed in this study of the effect of the building wake on landing aircraft.Under normal meteorological conditions,the studied airport terminal building causes limited effects on landing aircraft because most of the aircraft have already landed before entering the turbulent wake region.By simulating the approach of a tropical cyclone,additional CFD sensitivity tests were performed to study the impacts of building wake under extreme meteorological conditions.It was found that,in a narrow range of prevalent wind directions with wind speeds larger than a certain threshold value,a substantial drop in wind speed(>3.6 m/s) along the glide path of aircraft was observed in the building wake.Our CFD results also showed that under the most critical situation,a drop in wind speed as large as 6.4 m/s occurred right at the touchdown point of landing aircraft on the runway,an effect which may have a significant impact on aircraft operations.This study indicated that a comprehensive analysis of the potential impacts of building wake on aircraft operations should be carried out for airport terminals and associated buildings in airfields to ensure safe aviation operation under all meteorological conditions and to facilitate implementation of precautionary measures.

Performance of LIDAR- and radar-based turbulence intensity measurement in comparison with anemometer-based turbulence intensity estimation based on aircraft data for a typical case of terrain-induced turbulence in association with a typhoon

The Hong Kong Observatory (HKO) provides low-level turbulence alerting service for the Hong Kong International Airport (HKIA) through the windshear and turbulence warning system (WTWS). In the WTWS, turbulence intensities along the flight paths of the airport are estimated based upon correlation equations established between the surface anemometer data and the turbulence data from research aircraft before the opening of the airport. The research aircraft data are not available on day-to-day basis. The remote sensing meteorological instruments, such as the Doppler light detection and ranging (LIDAR) and radar, may be used to provide direct measurements of turbulence intensities over the runway corridors. The performances of LIDAR- and radar-based turbulence intensity data are studied in this paper based on actual turbulence intensity measurements made on 423 commercial jets for a typical case of terrain-induced turbulence in association with a typhoon. It turns out that, with the tuning of the relative operating characteristic (ROC) curve between hit rate and false alarm rate, the LIDAR-based turbulence intensity measurement performs better than the anemometer-based estimation of WTWS for turbulence intensity at moderate level or above. On the other hand, the radar-based measurement does not perform as well when compared with WTWS. By combining LIDAR- and radar-based measurements, the performance is slightly better than WTWS, mainly as a result of contribution from LIDAR-based measurement. As a result, the LIDAR-based turbulence intensity measurement could be used to replace anemometer-based estimate for non-rainy weather conditions. Further enhancements of radar-based turbulence intensity measurement in rain would be necessary.

Improving the wind and temperature measurements of an airborne meteorological measuring system

The Aircraft Integrated Meteorological Measuring System 20 Hz(AIMMS-20) has been used by the Hong Kong Observatory(HKO),China in data collection for tropical cyclone situations over the South China Sea and windshear and turbulence measurement at the Hong Kong International Airport(HKIA).This paper discusses possible methods for further enhancing the quality of the wind and temperature measurements from the system.For wind measurement,the enhancement methods include:error modelling of the accelerometer(e.g.,bias offset and cross-axis rate sensitivity),global positioning system(GPS) phase lag consideration,better representation of the inertial measurement unit(IMU) velocity based on the GPS velocities and considering their location differences,consideration of the slower update of GPS velocity,and wing flexure.For temperature measurement,the methods include the consideration of the temperature sensor response and the sensor housing response.The results of typical flights using AIMMS-20 show that the accuracy of the wind and temperature data could be improved by 20%-30%.Though the discussion in the present paper is related mainly to a specific meteorological measuring system on a particular aircraft,the techniques so employed should be a useful reference for similar systems installed on other aircraft.