Terrain Effects Research Articles

Wind tunnel test on the aerodynamic admittance of a rail vehicle in crosswinds

The aerodynamic admittance of a rail vehicle was investigated by wind tunnel test. Aerodynamic force was measured in the cases of three typical railway structures, including on the flat ground, above an embankment, and on a bridge, under two turbulent flow fields. First of all, three-component aerodynamic coefficients of the vehicle on each structure were obtained under uniform flow with respect to three wind attack angles and four wind direction angles. Secondly, the aerodynamic forces on the vehicle and the corresponding wind speed were evaluated to establish an aerodynamic admittance function of the vehicle. The aerodynamic admittance of the rail vehicle approximated a constant value in the low-frequency domain, but decreased with the reduced frequency increasing. The effects of the different reduced frequencies on the drag are greater than the lift admittance of the vehicle, while moment admittance stays steady-state. Finally, in order to reflect the unsteady characteristics of the buffeting force on the vehicle, the aerodynamic admittance functions of the vehicle were fitted to the expression of the frequency response function of a mass-spring-damping system, which was then verified. Furthermore, the effects of flat terrain and mountainous terrain were investigated, revealing that the influence of turbulence intensity on aerodynamic admittance is significant.

Construction and application of comprehensive drought monitoring model considering the influence of terrain factors: a case study of southwest Yunnan, China.

Droughts in winter and spring are one of the most prominent natural disasters in the Yunnan Province in China. They occur frequently, with long durations and have a wide range of damage, which has a serious impact on social and economic development, as well as agricultural production and, therefore, strongly impacts the lives of the people living in the region. The traditional drought monitoring model does not take terrain into consideration, thereby affecting the comparative nature of results, as baseline conditions are not the same. Therefore, this study proposed a comprehensive drought monitoring model considering the influence of terrain factors to improve the evaluation effect. Firstly, based on NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) and Tropical Rainfall Measurement Mission (TRMM 3B43) data, vegetation condition index (VCI), temperature condition index (TCI), precipitation condition index (TRCI), and three terrain factors ground elevation (DEM), slope (SLOPE), aspect (ASPECT) were selected as model parameters. Then, a comprehensive drought monitoring model without considering terrain factors (Model A) and a comprehensive drought monitoring model of considering terrain factors (Model B) were constructed by using multiple linear regression models. Finally, the effects of the two models were evaluated by using standardized precipitation evapotranspiration index (SPEI) in southwest Yunnan Province, China, and model B was used to analyze the drought in winter and spring in the study area from 2008 to 2019. The results showed that (1) the correlation coefficient of model B was higher than that of model A in winter and spring and the standard error of model B was lower than that of model A. (2) The grade consistency rate of Model A and SPEI was 0.92 in winter and 0.33 in spring; the grade consistency between model B and SPEI was 0.83 in winter and 0.75 in spring, and therefore the monitoring effect of model B was more stable. (3) There were periodic droughts during the study period, and the degree of drought in spring was less than in winter. Medium and severe droughts were observed in winter. Thus, this study concluded that the effect of terrain has an important influence on the evaluation of droughts. The comprehensive drought monitoring model which considers topographic factors can effectively identify the occurrence of drought, and therefore provide significant input with regards to disaster prevention and mitigation policies in southwest Yunnan.

Dynamic PSF-based jitter compensation and quality improvement for push-broom optical images considering terrain relief and the TDI effect.

Platform attitude jitter is inevitable during a satellite flight. The jitter reduces the quality of push-broom optical images, resulting in geometric deformation and image blur. This paper proposes an approach that offers jitter effect compensation and quality improvement for push-broom optical images based on dynamic point spread function (PSF) estimation and iterative image restoration considering the effect of terrain relief and time delay integration (TDI). First, the attitude jitter is precisely detected and estimated by considering the terrain relief. The jitter information is then used to establish the dynamic PSF of each image line considering the changing jitter values and TDI effect. Finally, based on the constructed PSF, the image is improved by iterative restoration using the optimal-window Richardson-Lucy algorithm. The method is validated by both simulation and real data experiments. In the simulation experiment, we apply jitter with different amplitudes and frequencies to generate the degraded images and then restore the image using the proposed restoration method. The results show that the proposed method can effectively restore images affected by jitter. In addition, real data experiments are carried out with multispectral remote sensing images from the ZY-3 satellite. The results show that in addition to the improvement of the radiometric quality, the geometric quality is also significantly improved in both the across-track and the along-track directions. The experimental results validated that the proposed method outperformed other methods without considering the terrain and TDI effect.

Characteristics of Greening along Altitudinal Gradients on the Qinghai–Tibet Plateau Based on Time-Series Landsat Images

The Qinghai–Tibet Plateau (QTP) is ecologically fragile and is especially sensitive to climate change. Previous studies have shown that the vegetation on the QTP is undergoing overall greening with variations along altitudinal gradients. However, the mechanisms that cause the differences in the spatiotemporal patterns of vegetation greening among different types of terrain and vegetation have not received sufficient attention. Therefore, in this study, we used a Landsat NDVI time-series for the period 1992–2020 and climate data to observe the effects of terrain and vegetation types on the spatiotemporal patterns in vegetation greening on the QTP and to analyze the factors driving this greening using the geographical detector and the velocity of the vertical movement of vegetation greenness isolines. The results showed the following: (1) The vertical movement of the vegetation greenness isolines was affected by the temperature and precipitation at all elevations. The precipitation had a more substantial effect than the temperature below 3000 m. In contrast, above 3000 m, the temperature had a greater effect than the precipitation. (2) The velocity of the vertical movement of the vegetation greenness isolines of woody plants was higher than that of herbaceous plants. (3) The influence of slope on the vertical movement of vegetation greenness isolines was more significant than that of the aspect. The results of this study provided details of the spatiotemporal differences in vegetation greening between different types of terrain and vegetation at a 30-m scale as well as of the underlying factors driving this greening. These results will help to support ecological protection policies on the QTP.

Evaluation of ultra-high degree gravity field models: a case study of Eastern Tibetan Plateau and Sichuan Province

This study validates the ultra-high-degree gravity field models in terms of the internal error estimate and the external precision. Internal error estimate is evaluated by geoid error degree variance and cumulative geoid height errors. The evaluation of the external precision is carried out using observed ground gravity data sets in Qinghai-Tibet Plateau and Sichuan Basin of mainland China. The results show that the geoid degree error is at the millimeter level, and the accumulated geoid error is at the centimeter level, and SGG-UGM-2 has the highest precision in terms of geoid errors. However, in terms of gravity anomaly, the GECO model has the highest precision of 37.080 mGal in the Qinghai-Tibet Plateau, and after terrain correction, its precision can reach 28.907 mGal, an improvement of 22%. In the Sichuan Basin, EGM2008 performs best with a precision of 7.202 mGal; the precision of EGM2008 becomes 6.648 mGal after terrain correction. These results mean that the terrain correction must be considered in the area where topography varies largely, while when the terrain is relatively flat, the effect of terrain on gravity can be ignored.

Identification of large-scale atmospheric structures under different stability conditions using Dynamic Mode Decomposition

We investigate the characteristic of large-scale coherent motions over a large horizontal domain using the Dynamic Mode Decomposition (DMD) spectral analysis algorithm applied on measurements from two long-range pulsed lidars. We show the results and advantages of this methodology on six cases representative of three thermal stratification conditions at two heights relevant for wind energy: near-neutral, unstable and stable stratification at 50m and 200m above ground level. For these cases the DMD algorithm show three types of structures: streaks near the surface for near-neutral for neutral stratification, large-scale convective rolls for the unstable cases and sheet-like rotational patches for stable conditions. The DMD algorithm also shows the stationary effects of the terrain on the flow at 50m above ground level, within the atmospheric surface layer. The possibility of isolating terrain effects from coherent motions makes DMD attractive for studying complex atmospheric flow phenomena as well as to have more realistic input for wind farm flow simulations.

Modeling the view-angle dependence of the gap fraction in subtropical forests by using terrestrial laser scanning

Determining how the view angle relates to the directional gap fraction (DGF) of natural forests with varying canopy structure and terrain is beneficial for assessing the radiation transfer and understory growth. However, it remains challenging to determine how DGF depends on the view angle due to the limitation to obtain the DGF at arbitrary view angles. This study verifies a point number-based method based on terrestrial laser scanning (TLS) data to estimate the DGF at arbitrary view angles for subtropical forests on either sloped or flat terrain. We then explore how the vertical DGF in the non-nadir direction relates to that in the nadir direction, and analyze the effects of forest canopy structure and terrain on the relationship. Finally, we quantify the dependence of DGF on both view zenith angle (VZA) and view azimuthal angle (VAA). The results show that the TLS-based method captures over 67% of the variations (root mean square error≤0.12) of the optically measured DGF. The vertical DGF in the non-nadir direction is related by a power function model to that in the nadir direction. Sloped terrain weakens the dependence of DGF on the VZA especially when VZA is large than 60° in this study, and the low forest density, low average tree height, and small crown diameter strengthen the dependence of DGF on the VZA. The DGF depends on the VZA more than the VAA for forests on flat terrain. However, the VAA also plays a key role in DGF estimation for forests on sloped terrain. This study proves that TLS is a useful tool to acquire reference data of DGF which can help to understand aerial and satellite estimation analyses in subtropical forested areas.

A Study on Seismic Response of RCC Buildings on Hill Slopes Using STAAD.Pro

Abstract: The majority of India's hilly regions are prone to earthquakes. A building on a steep slope is distinct from other structures. That is to say, structures that are to be constructed on hilly terrain have a higher risk factor of falling prey to seismic activities as compared to their counterparts built on a rather plainer terrain. The numerous floors/storeys of such a structure step back towards the hill slope, and buildings may also have setbacks. As such, the column of a hill structure sits at different heights on the angle of the terrain; its analysis differs from that of buildings on level ground. The current study looked at G+3 and G+4 structures with different slope angles, such as 00 , 7.50 , 150 , 22.50 , and 300 . Both Step back and Step back & set back types of building configurations have been studied in this paper. The earthquake forces are calculated according to IS: 18932002; the structures are situated in seismic zone IV, with a damping ratio of 5%. Linear Static and Linear Dynamic methods were employed to conduct the seismic study. To investigate the influence of shifting column heights in the ground level due to sloping ground, a 3-dimensional analytical model of building plan was created and the same was studied using the structural analysis application "STAAD.Pro." To quantify the effects of diverse sloping terrain, response parameters like top storey displacement, base shear, shear in bottom storey column, and time period were thoroughly studied. It has been discovered that short columns on the elevated side of the terrain/slope experience a greater shear force as compared to columns of increased height on the lower side of the terrain. Under earthquake stresses, Step back & Setback structures showed better resilience to seismic forces as compared to Step back buildings. Step back setback buildings have substantially lower base shear and top floor displacement than setback buildings on sloping land. Keywords: Earthquake, Slope, STAAD.Pro, Step back, Step back & set back

Investigation on Geometry Computation of Spaceborne GNSS-R Altimetry over Topography: Modeling and Validation

The spaceborne Global Navigation Satellite Systems Reflectometry (GNSS-R) offers versatile Earth surface observation. While the accuracy of the computed geometry, required for the implementation of the technique, degrades when Earth’s surface topography is complicated, previous studies ignored the effects of the local terrain surrounding the ideal specular point at a suppositional Earth reference surface. The surface slope and its aspect have been confirmed that it can lead to geolocation-related errors in the traditional radar altimetry, which will be even more intensified in tilt observations. In this study, the effect of large-scale slope on the spaceborne GNSS-R technique is investigated. We propose a new geometry computation strategy based on the property of ellipsoid to carry out forward and inverse calculations of path geometries. Moreover, it can be extended to calculate unusual reflected paths over versatile Earth’s topography by taking the surface slope and aspects into account. A simulation considering the slope effects demonstrates potential errors as large as meters to tens kilometers in geolocation and height estimations in the grazing observation condition over slopes. For validation, a single track over the Greenland surface received by the TechDemoSat 1 (TDS-1) satellite with a slope range from 0% to 1% was processed and analyzed. The results show that using the TanDEM-X 90 m Digital Elevation Model (DEM) as a reference, a slope of 0.6% at an elevation angle of 54 degrees can result in a geolocation inaccuracy of 10 km and a height error of 50 m. The proposed method in this study greatly reduces the standard deviation of geolocations of specular points from 4758 m to 367 m, and height retrievals from 28 m to 5.8 m. Applications associated with topography slopes, e.g., cryosphere could benefit from this method.

A Numerical Study for Tropical Cyclone Atsani (2020) Past Offshore of Southern Taiwan under Topographic Influences

Tropical Cyclone Atsani occurred in late October 2020 and moved westward offshore south of Taiwan. During its offshore passage, the cyclone deflects northward as it closes to the southern end of Taiwan. A global model MPAS at a multi-resolution of 60-15-3-km is applied to explore the track responses of Atsani and identify the topographic effects of the Central Mountain Range (CMR) on the cyclone circulation and the associated track deflection. With a 3-km resolution targeted at the Taiwan area, the cyclone track deflection can be reasonably simulated, with more sensitivity to physics schemes and dynamic vortex initialization and less sensitivity to initial environmental perturbations. When the Taiwan terrain is removed, the cyclone indeed deflects more northward earlier, in particular for simulations with a stronger cyclone that tends to generate stronger east-west wind asymmetry in the absence of the terrain. Idealized simulations with a regional model WRF at 3-km resolution are also utilized to contrast the track deflection of different departing cyclones, similar to the real case. It was found that northward deflection will be induced near south of the CMR-like terrain for both stronger and weaker westbound cyclones departing at different latitudes south of the terrain. We have explained why a further northward track at earlier stages is induced in the absence of the terrain effects in regard to model initial states. In both real and idealized cases, the track deflection of the cyclone moving around the terrain is dominated by the wavenumber-one horizontal potential vorticity (PV) advection that is somewhat offset by both vertical PV advection and differential diabatic heating.

Examining Terrain Effects on the Evolution of Precipitation and Vorticity of Typhoon Fanapi (2010) after Departing the Central Mountain Range on Taiwan

Abstract Typhoon Fanapi (2010) made landfall in Hualien in Taiwan on 0100 UTC 19 September 2010 and left Taiwan on 1200 UTC 19 September 2010, producing heavy rainfall and floods. Fanapi’s eyewall was disrupted by the Central Mountain Range (CMR) and reorganized after leaving the CMR. High-resolution simulations (nested down to 1-km horizontal grid size) using the Advanced Research Weather Research and Forecast (WRF) model, one simulation using the full terrain (CTL) and another set of simulations where the terrain on Taiwan was removed, were analyzed. Precipitation areas were classified into different sub-regions by a convective-stratiform separation algorithm to assess the impact of precipitation structure on Fanapi’s eyewall evolution. The percentage of deep convection increased from 9% to 20% when Fanapi underwent an eyewall reorganization process while departing the CMR. In the absence of terrain, moderate convection occupied most of the convective regions during the period when Fanapi moved across Taiwan Island. The low-level total vorticity stretching within the convective, stratiform and weak echo regions in the no-terrain experiment were of similar magnitudes, but the total vorticity stretching within the convective region at low levels was dominant in the CTL experiment. Total vorticity stretching in the region of deep convection increased after eyewall reorganization, and later became stronger than that in the moderate convection region. In the absence of the CMR, total vorticity stretching in moderate convection dominated. The total vorticity stretching within the deep convective region in the CTL experiment played an essential role in the reorganization of Fanapi’s eyewall through a bottom-up process.

Analyzing the Effects of a Basin on Atmospheric Environment Relevant to Optical Turbulence

The performance of adaptive optics (AO) systems are highly dependent upon optical turbulence. Thus, it is necessary to have the appropriate knowledge of the spatiotemporal characteristics of optical turbulence strength. In this paper, the spatiotemporal distribution of meteorological parameters (wind and temperature) and optical turbulence parameters (turbulence strength, temperature gradient, and wind shear) derived from pulsed coherent Doppler lidar, a microwave radiometer, and ERA5 reanalysis data are investigated, and the results show that the meteorological parameters in a basin develop independently, while the external influence will increase above the basin. By fitting radiosonde data, an existing parameterized model was improved to be more in line with the evolutionary properties of local optical turbulence. The development of temperature gradient and wind shear is influenced by the basin, which ultimately leads to an optical turbulence vertical profile that is discrepant at different altitude layers. The results indicate that temperature gradient plays a dominant role in turbulence generation below 2 km, and wind shear increases its impact significantly above 2 km. Furthermore, the optical turbulence parameters (outer scale, turbulence diffusion coefficient, and turbulence energy dissipation rate) and optical turbulence strength have good consistency, which might be derived from the combined effect of terrain and complex environment. Finally, the integrated parameters for astronomy and optical telecommunication were derived from optical turbulence strength profiles. An appropriate knowledge of optical turbulence is essential for improving the performance of adaptive optics systems and astronomical site selection.

The GPM-DPR Blind Zone Effect on Satellite-Based Radar Estimation of Precipitation over the Andes from a Ground-Based Ka-band Profiler Perspective

Abstract A vertically pointing Ka-band radar (Metek MIRA-35C) installed at the Instituto Geofísico del Perú, Atmospheric Microphysics and Radiation Laboratory (LAMAR) Huancayo Observatory, which is located at an elevation of 3.3 km MSL in the Andes Mountains of Peru, is used to investigate the effects of terrain on satellite-based precipitation measurement in the Andes. We compare the vertical structure of precipitation observed by the MIRA-35C with Ka-band radar measurements from the Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement (GPM) mission core satellite using an approach based on Taylor’s hypothesis of frozen turbulence that attempts to reduce the impact of spatiotemporal offsets between these two radar measurements. From 3 April 2014 to 20 May 2018, the DPR measured precipitation near LAMAR during 15 of its 157 coincident overpasses. There were six simultaneous observations with MIRA-35C. We found that the average of the DPR’s lowest clutter-free bin is 1.62 km AGL, but the presence of precipitation worsens the situation, causing a 0.4-km-deeper algorithm-detected blind zone for the DPR at the Huancayo Observatory. In the study area, the depth of the clutter layer observed with DPR often extends above the melting layer but can be highly variable, extending even as high as 5 km AGL. These results suggest that DPR estimates of stratiform precipitation over the Andes Mountains are likely underestimated because of the terrain effects on the satellite measurements and problems in its blind zone detection algorithms, highlighting the difficulty in estimating precipitation in mountainous terrain from spaceborne radar.

室蘭公園の開設から消失に至る過程に対する社会文化的観点からの史的考察

This study examined the historical significance of Muroran Park in Hokkaido, Japan, from a sociocultural perspective, based on related literature. The study focused on the following five aspects: (1) effect of urban areas, terrain, and park ideas on park establishment; (2) background to getting permission for free loan of the park site; (3) features of Seiroku Honda’s Muroran Park Plan and comparison with Kushiro Park; (4) efforts of the Muroran municipal administration and citizens to improve the park up until World War II; and (5) the desolation and disappearance of the park during and after the war. The factors that influenced the opening of the park were social opinions about an urban park, administrative policies related to urban parks, topographic features of the city, governance of Hokkaido over Muroran town, and the urban area of the industrial port city. Park improvements by the municipality (which was positively influenced by the Muroran Park Plan), donations by citizens, and the national policy promoting physical fitness contributed to the renovation of the park. Confiscation of the park by the Ministry of the Army at the World War II, changes in social conditions surrounding park administration after the war, and the enactment of the Urban Park Law led to the regenerated Muroran Park abandoned.

The forward modeling of 3D gravity and magnetic potential fields in space-wavenumber domains based on an integral method

With the help of the horizontal 2D Fourier transform, 3D convolutions can be transformed into vertical 1D integrations of different wavenumbers. It has already been proven that the Fourier domain method can greatly improve the efficiency of numerical simulations of 3D gravity and magnetic anomalies. Based on the aforementioned strategy, we have developed an efficient 1D integration algorithm in the space-wavenumber domain, which is combined with a fast continuation algorithm along the z-axis for high-efficiency calculations of nonconstant density or magnetization anomalies in rugged terrains. The calculation accuracy and efficiency of four 1D vertical integration methods (i.e., rectangular integrals, quadratic interpolations, Gaussian integrals, and cubic spline interpolations) are compared by using complex models (variable density distribution model and topographic model). The results indicate that, among the four integration methods, the quadratic interpolation method combined with a fast continuation algorithm achieves the best accuracy and efficiency. Compared with other Fourier domain algorithms, the proposed best 1D integration method in space-wavenumber domains achieves good numerical performance in calculating fields on planar and undulating terrains. Moreover, the proposed method is used to calculate the terrain effect on an airborne gravity and magnetic data set for realistic topography modeling. The results indicate that the new algorithm is suitable for the high-efficiency calculation of large-scale complicated 3D models.

Terrain Effects on the 13 April 2018 Mountainburg, Arkansas EF2 Tornado

Storm-scale interactions with rough terrain are complex. Terrain has been theorized to impact the strength of low-level mesocyclones. Surface roughness and modifications of the surrounding environment also may impact tornadogenesis or tornado intensity. The Mountainburg, Arkansas EF2 tornado on 13 April 2018 traveled along a path with minor variations in intensity and elevation throughout most of the nearly 19-km (11.8 mi) damage path as the storm moved along a river valley. A detailed damage survey showed that the tornado then made an abrupt ascent of more than 200 m (656 ft) in the last 2 km (1.2 mi) before dissipating. By examining model soundings and conducting a detailed terrain analysis, this study examines what role terrain may have had in channeling the momentum surge and enhancing the low-level vorticity to influence tornadogenesis. Other storm-scale factors are investigated to determine their potential impact on the demise of the tornado. The differential reflectivity column is studied to determine if the updraft was weakening. The relative position of the tornado and mesocyclone also are examined as the tornado ascended the terrain and dissipated to determine whether the change in elevation impacted the overall strength of the storm and to evaluate whether the storm was undergoing a traditional occlusion cycle. Finally, a large-eddy simulation model is used to explore physical changes in a tornado encountering terrain similar to the Mountainburg, Arkansas, tornado near its demise.

CALIOP-Based Quantification of Central Asian Dust Transport

Central Asia is one of the most important sources of mineral saline dust worldwide. A comprehensive understanding of Central Asian dust transport is essential for evaluating its impacts on human health, ecological safety, weather and climate. This study first puts forward an observation-based climatology of Central Asian dust transport flux by using the 3-D dust detection of Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP). The seasonal difference of transport flux and downstream contribution are evaluated and compared with those of the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). Central Asian dust can be transported not only southward in summer under the effect of the South Asian summer monsoon, but also eastward in other seasons under the control of the westerly jet. Additionally, the transport of Central Asian dust across the Pamir Plateau to the Tibetan Plateau is also non-negligible, especially during spring (with a transport flux rate of 150 kg m−1 day−1). The annual CALIOP-based downstream contribution of Central Asian dust to South Asian (164.01 Tg) is 2.1 times that to East Asia (78.36 Tg). This can be attributed to the blocking effect of the higher terrain between Central and East Asia. Additionally, the downstream contributions to South and East Asia from MERRA-2 are only 0.36 and 0.84 times that of CALIOP, respectively. This difference implies the overestimation of the wet and dry depositions of the model, especially in the low latitude zone. The quantification of the Central Asian dust transport allows a better understanding of the Central Asian dust cycle, and supports the calibration/validation of aerosol-related modules of regional and global climate models.

Characteristics and Causes of Extremely Persistent Heavy Rainfall of Tropical Cyclone In-Fa (2021)

The characteristics and causes of the persistent precipitation of an extreme-rainfall tropical cyclone (TC), In-Fa, in 2021 are studied. It is shown that the extremity of In-Fa’s precipitation was mainly due to two aspects: massively accumulated quantity and an extremely long impact time. The heavy precipitation in Zhejiang resulted from the accumulation of very long but moderate precipitation, while that in Jiangsu resulted from the coaction of both long duration and strong intensity. The weak steering flow brought about by the large scale environment and the long continuation of the TC’s circulation were the two most important background conditions for the extremely long duration of heavy rainfall in Zhejiang and Jiangsu. Continuous energy input through the transportation of warmer and wetter air resulted in the persistence of In-Fa’s circulation. The terrain effect under the continuous northeasterly/northerly airflow caused by the slow movement of the TC was the major influencing factor for the extreme precipitation in Zhejiang. The convergence brought about by the TC’s vortex during landfall played another role in Zhejiang’s heavy rainfall. On the one hand, the terrain led to the development of low-level vertical circulation, resulting in convergence and updraft in the windward side of Siming Mountain; on the other hand, the terrain also provided for the vertical transport of water vapor. The main factors for the extreme precipitation in Jiangsu were the long continuation of low-level jets that caused persistent low-level convergence and the development of a mesoscale rainband. The convergence zone was located in the western front and evolved with the changing of the jets. The high-energy/high-humidity conditions and their consistency with the location of the convergence provided favorable conditions for the triggering and development of convection.

Decomposition of water vapor flux divergence and its application to a blizzard event over Ili Valley in Central Asia during 30 Nov to 1 Dec 2018

Central Asia is experiencing an increase in the occurrence of extreme flood events while still featuring in scarce precipitation. These extreme events are governed by weather systems of various scales as well as complex terrain effects. It is therefore difficult to analyze and forecast precipitation with traditional methods. In this study, an approach is designed by decomposing water vapor flux divergence (Qvall) under model's terrain following coordinates following the Helmholtz theorem. The traditionally used but important variable for depicting anomalous precipitation, Qvall, is partitioned into three components, i.e., flow divergence (Qvdiv), moisture advections by large-scale motion (QvL) and by sub-synoptic motion (QvS). Applications to a blizzard event occurred over Ili Valley during 30 Nov to 1 Dec 2018 is performed as an example. The eastward water vapor flux brings moisture from Balkhash Lake to Ili Valley during the entire precipitation event, while Qvall fails to indicate the initiation and weakening of precipitation. Temporal and spatial evolutions of the three decomposed components are analyzed with comparison to precipitation on both windward slope and lee side. Qvdiv with dominant magnitude always converges no matter precipitation intensifies or weakens, and therefore leads to false moisture aggregation signals in Qvall. The other two components, QvL and QvS, converge prior to precipitation and diverge or weaken before the event ends. Besides, QvL performs better than QvS on windward slope and shows moisture transport from upper levels while QvS achieves better on lee side. Future applications of this flow decomposition approach may thus extend to improving precipitation predictions with negligible computational costs over complex topography in other places around the world.

Impact of deep basin terrain on PM2.5 distribution and its seasonality over the Sichuan Basin, Southwest China

The terrain effect on atmospheric environment is poorly understood in particular for the polluted region with underlying complex topography. Therefore, this study targeted the Sichuan Basin (SCB), a deep basin with severe PM2.5 pollution enclosed by the eastern Tibetan Plateau (TP), Yunnan-Guizhou Plateaus (YGP) and mountains over Southwest China, and we investigated the terrain effect on seasonal PM2.5 distribution and the meteorological mechanism based on the WRF-Chem simulation with stuffing the basin topography. It is characterized that the three-dimensional distribution of topography-induced PM2.5 concentrations over the SCB with the seasonal shift of regional PM2.5 averages from approximately 30 μg m−3 in summer to 90 μg m−3 in winter at surface layer and from summertime 10 μg m−3 to wintertime 30 μg m−3 in the lower free troposphere. Such basin-forced PM2.5 changes presented the vertically monotonical declines concentrated within the lower troposphere below 3.6 km in spring, 2.3 km in summer, 2.6 km in autumn and 4.8 km in winter. Impacts of deep basin aggravated PM2.5 accumulation within the SCB and transport toward the surrounding plateaus contributing approximately 50–90% to PM2.5 levels over the regions of eastern TP and northern YGP. In the SCB, atmospheric thermal structure in the lower troposphere could build a vertical convergence layer between the boundary layer and free troposphere, acting as a lid inhibiting air diffusion, which was regulated by the terrain effects on interactions of westerlies and Asian monsoons, especially the wintertime strong warm lid deteriorating air pollution in the SCB. Furthermore, warm and humid air conditions within the basin prompted sulfur oxidation ratio by +0.02 and nitrogen oxidation ratio by +0.22 effectively producing the secondary PM2.5 in atmospheric environment.