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Limited observations hinder understanding of turbulent characteristics in mountainous terrain resulting from heating or cooling of slopes, wind, vertical motions, and heat or moisture advection, which disperse aerosols and other pollutants over the region. In this study, the 1290 MHz radar wind profiler data are utilized to compute the boundary layer height (BLH), the refractive index structure constant (Cn2), and the energy dissipation rate (ɛ) over the central Himalayan site for the period of November 2011 to March 2012, from the intense Ganges Valley Aerosol Experiment (GVAX) field measurements. The radar wind profiler (RWP) based estimation of BLH and ɛ is validated against the radiosonde, representing the effectiveness of the datasets for further investigation. The strong seasonal variation of log Cn2 and log ɛ, with average values of ≈ -12 m−2/3 and -2 m2 s−3, respectively, is associated with the mountain-induced local circulations and stability in the atmospheric boundary layer. The weak stratification during weak flow is found to be responsible for deep mixing, particularly in the nocturnal boundary layer in spring. Furthermore, the level of cloud cover significantly impacts the strength of turbulence, with the highest cloud cover resulting in a substantial increase in log Cn2 (approximately -11 m−2/3) due to intense updraft and downdraft motions compared to clear skies. Additionally, the distribution of aerosol loading across the site, coupled with the behavior of BLH, atmospheric stability, and orographic-induced circulations, implies distinctive seasonal mechanisms for transporting aerosols toward the mountains. This study offers valuable insights into the diurnal and seasonal patterns of turbulent mixing and the mechanisms behind the transport of pollutants through boundary layer processes over the region. [ABSTRACT FROM AUTHOR]

This study characterizes the variability in different parameters using high-frequency ground-based instruments, i.e., ceilometer and Micro Rain Radar (MRR), on a diurnal scale installed at the Indian Institute of Technology Bhubaneswar, Odisha, India. The study evaluates cloud base height (CBH) and boundary layer height (BLH) to understand their role in influencing convection from liquid water content (LWC), rain rate, radar reflectivity, and fall velocity over a tropical location for different seasons on a diurnal scale. The data retrieved from these surface-based instruments are segregated into four different seasons, i.e., pre-monsoon, monsoon, post-monsoon, and winter, for 2019 and 2020. It is noted that the atmospheric processes influencing convection and associated rainfall over the study location differ for different seasons. The frequency distribution of different types of clouds depending on their vertical levels, i.e., low-level clouds (LLC), mid-level clouds (MLC), and high-level clouds (HLC), during different seasons revealed a high occurrence of low- to mid-level clouds in pre-monsoon and post-monsoon seasons, mid-level clouds in the monsoon season, and low-level clouds in the winter season. The results also illustrate that the rainfall during different seasons largely depends on the types of clouds, i.e., LLC, MLC, and HLC, as detected from the CBH layers, which contribute to convection and rainfall over the study location. It is also revealed that the boundary layer processes in convection and rainfall influence the evolution of BLH during different seasons. Further, the study compares model skills in verifying boundary layer height and rainfall against surface-based observations. Besides characterizing the diurnal variability in these parameters on a daily scale, the study also elucidates the relationship between the cloud types and BLH. The results indicate that LLC occurrences were positively related to BLH during the pre-monsoon and monsoon seasons in 2019 and 2020. In contrast, a negative correlation is observed for cloud types MLC and HLC with BLH for the rest of the seasons of 2019 and 2020. Also, during post-monsoon and monsoon, distinct signatures of deep convection are noted over the study region, suggesting the influence of the boundary layer on MLC and HLC. The results from radar reflectivity and fall velocity during the monsoon and post-monsoon season support the influence of MLC and HLC on deep convection over the region. The diurnal variation in the observed parameters on a seasonal scale will help in understanding the role of various atmospheric parameters and processes in convection and associated rainfall over a tropical location and can provide necessary verification skills for weather and climate regional models against observations. [ABSTRACT FROM AUTHOR]

The present study focused on the temporal variability, meteorological influences, potential sources, and long-range transport of atmospheric aerosols over two contrasting environments during 2011–2013. We have chosen Agartala (AGR) city in Northeast India as one of our sites representing the rural-continental environment and Patiala (PTA) as an urban site in Northwest India. The seasonal averaged equivalent black carbon (eBC) concentration in AGR ranges from 1.55 to 38.11 µg/m3 with an average value of 9.87 ± 8.17 µg/m3, whereas, at an urban location, PTA value ranges from 1.30 to 15.57 µg/m3 with an average value of 7.83 ± 3.51 µg/m3. The annual average eBC concentration over AGR was observed to be ~ 3 times higher than PTA. Two diurnal peaks (morning and evening) in eBC have been observed at both sites but were observed to be more prominent at AGR than at PTA. Spectral aerosol optical depth (AOD) has been observed to be in the range from 0.33 ± 0.09 (post-monsoon) to 0.85 ± 0.22 (winter) at AGR and 0.47 ± 0.04 (pre-monsoon) to 0.74 ± 0.09 (post-monsoon) at PTA. The concentration of eBC and its diurnal and seasonal variation indicates the primary sources of eBC as local sources, synoptic meteorology, planetary boundary layer (PBL) dynamics, and distant transportation of aerosols. The wintertime higher values of eBC at AGR than at PTA are linked with the transportation of eBC from the Indo-Gangetic Plain (IGP). Furthermore, it is evident that eBC aerosols are transported from local and regional sources, which is supported by concentration-weighted trajectory (CWT) analysis results. [ABSTRACT FROM AUTHOR]

In this study, we utilize a novel approach to solve the Ekman equations for eddy-viscosity profiles in the stable boundary-layer. By doing so, a well-known expression for the stable boundary-layer height by Zilitinkevich (Boundary-Layer Meteorology, 1972, Vol. 3, 141–145) is rediscovered. [ABSTRACT FROM AUTHOR]

This study assesses the characteristics of atmospheric boundary layer (ABL) between Facility for Airborne Atmospheric Measurements (FAAM) aircraft observations and NCMRWF Unified Model (NCUM) forecasts on 07 July 2016. On this day the flight was operated over monsoon trough region. Two model forecasts data are utilized (global-17 km; regional-4 km) at a lead time of day-1 to day-3. The chosen variables are boundary layer height (Zi) in km, wind speed (WS) in m s−1, vertical velocity (w) in m s−1, water vapor mixing ratio (r) in g kg−1, and temperature (T) in oC. Special emphasis is given to the evolution of the ABL and peak of low-level jet (LLJ). Results suggest that both the models have noticed warm biases for T, wet biases for r, and negative biases for w. On the other hand, WS found to have negative (positive) biases during the straight level (profile) runs. Point-by-point correlations suggest that T, and r are in good agreement (R > = 0.94), and moderate agreement (ranges from 0.66 to 0.77) for WS, whereas the vertical velocities are poorly predicted (R < 0.2). Strikingly, the correlations (biases) are dropped (increased) with lead time in two models. Small improvements in the statistics (R, bias) are noticed when the model resolution changes from 17 to 4 km. Both models can identify the observed evolution of the ABL; however, Zi values are slightly lower in models compared to observations. Interestingly, the Zi values are slightly dropped (100–200 m) from day-1 to day-3 forecast in two models. The peak of LLJ follows clear time variation, and its height is decreased with forecast lead time. Wind speeds below (above) the ABL height increases (decreases) with lead time, respectively. By considering all flight points across the flight track, the mean Zi is 100–500 m higher in the global model compared to the regional model. The noticed differences between two model ABL heights are discussed in the light of soil moisture conditions, fluxes, temperature advection, and static stability. Overall, a good correspondence is seen between FAAM observations and model forecasts, whereas the regional forecasts have better agreements in predicting ABL characteristics (10–20%). [ABSTRACT FROM AUTHOR]

The spatiotemporal distribution and characterization of aerosol optical properties in the north of Vietnam were investigated extensively using the long-term measurements obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) (from 2010 to 2019) and two Aerosol Robotic Network (AERONET) stations (Nghia Do, Hanoi City, from 2010 to 2019 and Son La from 2012 to 2019) located in fast-developing urban areas. This study also analyzed the tendency of AOD over different wavelengths as well as the influences of meteorological parameters and boundary layer height (BLH) on AOD and Ångström exponent (AE). The annual mean AOD500 at Nghia Do and Son La stations were 0.81 and 0.78, respectively. Our results revealed the existence of particles emitted from anthropogenic activities in Hanoi and Son La with the dominance of fine particles (more than 90% of the AE440–870 were larger than 1). Besides, a high percentage of AE440–870 larger than or approximately 1.5 during the dry and transition months in Son La indicated the existence of biomass-burning aerosol particles due to forest fires and burning of agriculture residuals. The classification results for aerosol types showed the presence of both biomass burning and urban/industrial aerosol types at Nghia Do and Son La sites. Among the investigated meteorological parameters, surface solar radiation expressed a significant correlation with AE440–870 in all three seasons at the two sites due to the secondary formation of fine particles induced by the high solar radiation condition. The impacts of meteorological parameters and BLH on AOD were not observed simultaneously during three seasons at Nghia Do and Son La stations. [ABSTRACT FROM AUTHOR]

Traditional observation methods of the boundary layer are susceptible to external factors. In particular, detection data are relatively scarce in special areas such as plateaus and oceans. However, occultation observation data have the unique advantage of global coverage, and the boundary layer information of the occultation profile is rich. Based on the machine learning algorithm, this paper combines GPS occultation technology to improve the algorithm and constructs the boundary layer height simulation model based on the actual situation and analyzes its functional structure one by one. Moreover, in order to verify the validity of the simulation model, a simulation analysis is performed in combination with actual data. In addition, the performance of the model is verified from multiple aspects, and the results of the research are calculated by mathematical statistics, and the corresponding statistical diagram is drawn. Through experimental analysis, it can be known that the model constructed in this paper can effectively simulate the boundary height and has certain practical effects. [ABSTRACT FROM AUTHOR]

Although coherent Doppler wind lidar (CDWL) is promising in detecting boundary layer height (BLH), differences between BLH results are observed when different CDWL measurements are used as tracers. Here, a robust solution for BLH detections with CDWL is proposed and demonstrated: mixed layer height (MLH) is retrieved best from turbulent kinetic energy dissipation rate (TKEDR), while stable boundary layer height (SBLH) and residual layer height (RLH) can be retrieved from carrier-to-noise ratio (CNR). To study the cause of the BLH differences, an intercomparison experiment is designed with two identical CDWLs, where only one is equipped with a stability control subsystem. During the experiment, it is found that the CNR could be distorted by instrument instability because the coupling efficiency from free-space to the polarization-maintaining fiber of the telescope is sensitive to the surrounding environment. In the ML, a bias up to 2.13 km of the MLH from CNR is found, which is caused by the CNR deviation. In contrast, the MLH from TKEDR is robust as long as the accuracy of wind is guaranteed. In the SBL (RL), the CNR is found capable to retrieve SBLH and RLH simultaneously and robustly. This solution is tested during an observation period over one month. Statistical analysis shows that the root-mean-square errors (RMSE) in the MLH, SBLH, and RLH are 0.28 km, 0.23 km, and 0.24 km, respectively. [ABSTRACT FROM AUTHOR]

Brahmaputra Valley of northeast India is relatively pristine, with low local emission levels, but is susceptible to long-range transport from west Asian deserts. Hourly measurements of aerosol particulate matter (PM2.5 and PM10) and black carbon (BC) over a period of 1 year (June 2013–May 2014) over Tezpur station (26.63° N, 92.77° E), located in the Brahmaputra Valley, display annual mean PM2.5 and BC of 42 ± 25 µg m−3 and 6 ± 1.5 µg m−3, respectively, indicating that Tezpur enjoys lower pollution levels than other northeast stations. The lowest concentrations of PM2.5 and PM10 (~ 14.7 and 24 µg m−3, respectively) are observed during the monsoon season (JJAS). The highest values (~ 91 and 114 µg m−3) are observed during the winter (DJF) due to local atmospheric conditions and some additional transport from the west Asian deserts and the Indo-Gangetic Basin (IGB). Local emissions seem to be mainly responsible for moderate values (~ 24 and 42 µg m−3) of PM during the post-monsoon (ON) season with weak winds. Coarse particles (diameter > 2.5 µm) are much higher (approx. 47% of PM10) in the pre-monsoon than in the other seasons due to the long-range transport of dust particles from the IGB and west Asian deserts as well as sea salt particles from the Bay of Bengal. WRF-Chem model simulation underestimated the concentration of PM2.5 and PM10 as compared to daily mean observations. The variation in the "model–observation" time series for relative humidity (RH) and boundary layer height suggests that underestimation of PM10 is due mainly to the underestimation of RH and overestimation of the ventilation coefficient by the model during winter and pre-monsoon seasons. [ABSTRACT FROM AUTHOR]

Taiyuan frequently experiences heavy PM2.5 pollution in winter under unfavorable meteorological conditions. To understand how the meteorological factors influence the pollution in Taiyuan, this study involved a systematic analysis for a continuous period from November 2016 to January 2017, using near-surface meteorological observations, radiosonde soundings, PM2.5 measurements, and three-dimension numerical simulation, in combination with backward trajectory calculations. The results show that PM2.5 concentration positively correlates with surface temperature and relative humidity and anti-correlates with near-surface wind speed and boundary layer height (BLH). The low BLH is often associated with a strong thermal inversion layer capping over. In addition to the high local emissions, it is found that under certain synoptic conditions, the southwesterly and southerly winds could bring pollutants from Linfen to Taiyuan, leading to a near-surface PM2.5 concentration higher than 200 μg m−3. Another pollution enhancing issue is due to the semi-closed basin of Taiyuan affecting the planetary boundary layer (PBL): the surrounding mountains favor the formation of a cold air pool in the basin, which inhibits vertical exchanges of heat, flux, and momentum between PBL and the free troposphere, resulting in stagnant conditions and poor air quality in Taiyuan. These findings can be utilized to improve the understanding of PM2.5 pollution in Taiyuan, to enhance the accuracy of forecasting pollution, and to provide scientific support for policy makers to mitigate the pollution. [ABSTRACT FROM AUTHOR]

Vertically exploring the characteristics of the typhoon boundary layer (TBL) plays an important role in recognizing typhoon structure. The boundary layer radial direction and tangential wind characteristics of Typhoon Usagi based on the observational data of three boundary layer wind profiler stations along the route of Typhoon Usagi (No. 1319) and by combining with sounding data. The results show that: (1) maximum tangential wind appears in the vicinity of the eye area of Usagi, and it basically maintains a height of around 1 800 m when Usagi keeps a strong typhoon level, with the rapidly decreasing strength of Usagi after it lands, the speed of the maximum tangential wind and its vertical range both decrease; (2) the height of the maximum tangential wind is close to that of the inflow layer top of the typhoon, and is greater than that of the boundary layer estimated on the basis of Richardson number or potential temperature gradient, while the height of mixed layer judged on the basis of the signal-to-noise ratio (SNR) or its gradient is usually low; (3) the the boundary layer height can reach higher than 2 100 m before Usagi lands. When the typhoon level or above is achieved, the boundary layer height observed by various stations does not change much, basically staying at between 1 200 and 1 600 m. With the decreasing strength of Usagi after its landfall, the boundary layer height rapidly drops. [ABSTRACT FROM AUTHOR]

Based on a comprehensive analysis on Sonic Anemometer and gradient data, wind profile radar (WPR) and GPS sounding data of March-August 2008 from the boundary layer (BL) tower observation system at Dali on the southeastern edge of Tibetan Plateau (TP), it is found that the strengths of turbulent kinetic energy (TKE), buoyancy term and shear term depend on vegetation cover in association with local stability and thermodynamic condition. Strong kinetic turbulence appears when near surface layer in neutral condition with the large contribution from shear term. In an unstable condition within near surface layer, the atmospheric turbulent motion is mainly thermal turbulence, as buoyancy term is obviously larger than shear term. Under a stable condition the intermittent turbulence is accompanied by weak shear and buoyancy term, and TKE is significantly less than neutral or instable condition. The study also presents that the buoyancy term contribution at Nyingchi station in the southern slopes of the TP large topography in spring is significantly larger than that at Dali over the southeastern TP edge, reflecting that the thermal turbulence makes an important contribution to convection activity in the southern slopes of TP. Dali station is located in complex terrain with mountain and valley leading to larger kinetic turbulence. From the perspective of interaction of turbulence-convection in different scales, the study revealed that the height of convective boundary layer (CBL) could reach up to 1500-2000 m. TKE, shear term, and buoyancy term in near surface layer have the notable correlations with BL height and local vertical motion. The daytime thermodynamic turbulence effect of heat flux and buoyancy term has an obvious impact on the height of CBL, whereas mechanical turbulence only exerts a less impact. Mechanical turbulence in near surface layer has a significant impact on vertical motion especially in the forenoon with impacting height of 2500-3000 m. The peaks in diurnal variations of shear term and buoyancy term correspond to the high instable periods, especially in summer forenoon. Our observation analysis characterized the convection activity triggered by TKE source and their interaction in the southeastern TP edge. [ABSTRACT FROM AUTHOR]

We detected a severe haze process in Guangzhou area with lidar and microwave radiometer, performed an inversion to get boundary layer height by wavelet covariance transform, and analyzed the correlation between meteorological factors of boundary layer and visibility from the perspective of dynamical and thermodynamic structures. Our results indicate that the boundary layer height shows significant daily changes, consistent with ground visibility variation. During the cleaning process, the boundary layer height exceeded 1 km; during severe haze, the height was only 500 m. Temperature gradient of 50-100 m, which was 30 h lag, was remarkably correlated with visibility, with the correlation coefficient of 0.77. High layer visibility (255 m) and low layer stability were significantly anticorrelation, and the maximum anticorrelation coefficient was up to −0.76 in cleaning days and −0.49 in haze days. In the related boundary layer meteorological factors, surface ventilation coefficient was linearly correlated with ground visibility, with the greatest correlation coefficient of 0.88. The correlation coefficients of boundary layer height, ground wind velocity, relative humidity and ground visibility were 0.76, 0.67, and −0.77, respectively. There was a strong correlation between different meteorological factors. The dominant meteorological factor during this haze process was surface ventilation coefficient. In the area without boundary layer height sounding, ground visibility and wind velocity could be used to estimate boundary layer height. [ABSTRACT FROM AUTHOR]

The planetary boundary layer (PBL), which directly interacts with the underlying surface, differs significantly in its nature from the low-turbulent stably stratified free atmosphere. Fluctuations of the Earth's surface heat balance immediately affect the PBL and assimilate there owing to the effective mechanism of turbulent heat transfer. In this case the upper boundary of the PBL plays the role of a cover, preventing the direct penetration of thermal effects and contaminants into an overlying atmospheric layer. In view of this, air pollution is especially dangerous when associated with shallow PBL. In addition, local peculiarities of climate change are mainly determined by the PBL height due to the high sensitivity of thin stably stratified PBLs to the thermal effects. Deep convective PBLs are not very sensitive to weak thermal effects, but they significantly affect the formation of convective cloudiness and the climate system as a whole by means of the turbulent entrainment of the thermal energy, humidity, aerosols, and other admixtures through the upper boundary. The PBL height and turbulent entrainment must be calculated when simulating and forecasting air pollution, abnormal frosts and heat, and other hazardous phenomena. In this paper we discuss the state-of-the-art knowledge in the area of PBL height simulation and suggest a new model of turbulent entrainment for convective PBLs. [ABSTRACT FROM AUTHOR]

Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z i, during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, φ m, has been found to be a function of both z/L and z i /L. At constant z/L, φ m decreases with decreasing boundary layer height. Since φ m is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m−2 or about 1% and the mean surface stress increase by 1.4 mN m−2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m−2 and an increase of the mean annual surface stress by 2.6 mN m−2. [ABSTRACT FROM AUTHOR]