Planetary Boundary Layer Structure Research Articles

Using EUREC4A/ATOMIC field campaign data to improve trade wind regimes in the Community Atmosphere Model

Abstract. Improving the prediction of clouds in shallow-cumulus regimes via turbulence parameterization in the planetary boundary layer (PBL) will likely increase the global skill of global climate models (GCMs) because this cloud regime is common over tropical oceans where low-cloud fraction has a large impact on Earth's radiative budget. This study attempts to improve the prediction of PBL structure in tropical trade wind regimes in the Community Atmosphere Model (CAM) by updating its formulation of momentum flux in CLUBB (Cloud Layers Unified by Binormals), which currently does not by default allow for upgradient momentum fluxes. Hindcast CAM output from custom CLUBB configurations which permit countergradient momentum fluxes are compared to in situ observations from weather balloons collected during the ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte and Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (EUREC4A/ATOMIC) field campaign in the tropical Atlantic in early 2020. Comparing a version with CAM–CLUBB with a prognostic treatment of momentum fluxes results in vertical profiles that better match large-eddy simulation results. Countergradient fluxes are frequently simulated between 950 and 850 hPa over the EUREC4A/ATOMIC period in CAM–CLUBB. Further modification to the planetary boundary layer (PBL) parameterization by implementing a more generalized calculation of the turbulent length scale reduces model bias and root mean squared error (RMSE) relative to sounding data when coupled with the prognostic momentum configuration. Benefits are also seen in the diurnal cycle, although more systematic model errors persist. A cursory budget analysis suggests the buoyant production of momentum fluxes, both above and below the jet maximum, significantly contributes to the frequency and depth of countergradient vertical momentum fluxes in the study region. This paper provides evidence that higher-order turbulence parameterizations may offer pathways for improving the simulation of trade wind regimes in global models, particularly when evaluated in a process study framework.

Characteristics of Advection Fog at Qingdao Liuting International Airport

The advection fog characteristics at Qingdao Liuting International Airport during 2000–2022 are studied based on surface observation, sounding and reanalysis data. Surface observation data show that there were two types of fog: evaporation fog (EF) dominated by northwesterly wind in winter and cooling fog (CF) dominated by southeasterly wind in spring and summer. CF is thicker than EF due to different planetary boundary layer (PBL) structures. For EF, the middle and low troposphere are affected by dry and cold air, while CF is affected by warm and moist air below 850 hPa. When EF formed, downdrafts and a positive vertical gradient of the pseudo-equivalent potential temperature indicate stable PBL, surface heat flux is upward from sea to atmosphere and surface wind diverges near the air–sea interface. When CF formed, these characteristics are reversed. Fog is significantly affected by sea–land–atmosphere interactions. The moisture source is mainly from surface fluxes released by the Yellow Sea in the case of EF, while it is from moist air at low latitudes and local land transpiration in the case of CF. The difference in temperature between the sea surface and surface air changes from the range of 0–8 K for EF but from −4–0 K for CF.

The Effects of Planetary Boundary Layer Features on Air Pollution Based on ERA5 Data in East China

The planetary boundary layer (PBL) structure and its evolution can significantly affect air pollution. Here, the PBL’s characteristics and their association with air pollution were analyzed in Hefei, east China, using ERA5 reanalysis data, weather observations and air pollutant measurements from 2016 to 2021. In the near-surface level, air pollution was directly influenced by ground meteorological conditions, and high PM2.5 was normally related to weak wind speed, northwest wind anomalies, low temperature and high relative humidity. Moreover, in the trajectory analysis, air masses from the north and the northwest with short length played an important role in the high PM2.5 with pollutant transport within the PBL. Furthermore, high PM2.5 showed a tight dependence on PBL stratification. There was high temperature and relative humidity and low wind speed and PBL height within all PBL altitudes in the polluted condition. Notably, vertical wind shear (VWS) and temperature gradient tended to be much weaker below 900 hPa, which created a deeply stable stratification that acted as a cap to upward-moving air. Such a PBL structure facilitated more stable stratification and enhanced the generation of air pollution. Finally, the stable stratification in the PBL was related to the special synoptic configuration for the high PM2.5 conditions, which included the block situation at the high level, the southerly wind anomalies at the middle level and the wild range of the uniform pressure field at the near-ground level. Therefore, air pollutant concentrations were regulated by ground factors, PBL structure and the synoptic situation. Our results provide a precise understanding of the role of PBL features in air pollution, which contributes to improving the assimilation method of the atmospheric chemistry model in east China.

Influence of surface layer schemes on tropical cyclone Hato (2017) intensity

Three surface layer schemes (MYNN, ETA, and MM5) in the Weather Research and Forecasting (WRF 4.1) model were selected to conduct simulation experiments for tropical cyclone (TC) Hato (2017). The influence of different surface layer schemes on TC intensity was compared. Although the planetary boundary layer and land surface schemes were the same, surface layer schemes minimally affected TC tracks but significantly affected TC intensity. The highest correlation coefficient and smallest bias and root mean square error between the simulated TC intensity in the ETA and observations. The TC intensity simulated using ETA was the strongest and closest to the observations. The surface latent heat fluxes simulated using ETA at the time of the TC landing were 1200–1500 W/m2, and is 800–1200 W/m2 by MM5 and MYNN, respectively. The surface sensible heat flux simulated using ETA was 400–800 W/m2 and that simulated using MM5 and MYNN was 200–400 W/m2. The equivalent potential temperature and radial wind simulated using ETA were significantly higher than those simulated using MM5 and MYNN. The larger the energy transmitted from the surface to the tropical cyclone, the stronger the TC. The dynamic structural characteristics were consistent with the tropical cyclone intensity. The ETA indicated the strongest unstable planetary boundary layer structure, stronger convergence at the lower layer, divergence at the upper layer, vertical transport, and absolute angular momentum, and induced the formation of the strongest TC. The comparison results revealed that the surface fluxes from different surface layer schemes can affect the dynamic and thermal structures of the planetary boundary layer owing to the different stability functions. The surface fluxes were also affected by the temperature and wind in the planetary boundary layer. This interaction between the surface layer and planetary boundary layer affects the eye structure and secondary circulation of the TC and TC intensity.

The feedback effects of aerosols from different sources on the urban boundary layer in Beijing China

The interaction of aerosols and the planetary boundary layer (PBL) plays an important role in deteriorating urban air quality. Aerosols from different sources may have different effects on regulating PBL structures owing to their distinctive dominant compositions and vertical distributions. To characterize the complex feedback of aerosols on PBL over the Beijing megacity, multiple approaches, including in situ observations in the autumn and winter of 2016–2019, backward trajectory clusters, and large-eddy simulations, were adopted. The results revealed notable distinctions in aerosol properties, vertical distributions and thermal stratifications among three types of air masses from the West Siberian Plain (Type-1), Central Siberian Plateau (Type-2) and Mongolian Plateau (Type-3). Low loadings of 0.28 ± 0.26 and 0.15 ± 0.08 of aerosol optical depth (AOD) appeared in the Type-1 and Type-2, accompanied by cool and less stable stratification, with a large part (80%) of aerosols concentrated below 1500 m. For Type-3, the AOD and single scattering albedo (SSA) were as high as 0.75 ± 0.54 and 0.91 ± 0.05, demonstrating severe pollution levels of abundant scattering aerosols. Eighty percent of the aerosols were constrained within a lower height of 1150 m owing to the warmer and more stable environment. Large-eddy simulations revealed that aerosols consistently suppressed the daytime convective boundary layer regardless of their origins, with the PBL height (PBLH) decreasing from 1120 m (Type-1), 1160 m (Type-2) and 820 m (Type-3) in the ideal clean scenarios to 980 m, 1100 m and 600 m, respectively, under polluted conditions. Therefore, the promotion of absorbing aerosols below the residual layer on PBL could be greatly hindered by the suppression effects generated by both absorbing aerosols in the upper temperature inversion layer and scattering aerosols. Moreover, the results indicated the possible complexities of aerosol-PBL interactions under future emission-reduction scenarios and in other urban regions.

Impact of thermal structure of planetary boundary layer on aerosol pollution over urban regions in Northeast China

This study investigated the characteristics of aerosol pollution in three provincial capitals (Shenyang, Changchun, and Harbin) in Northeast China and the impact of the thermal structure of the planetary boundary layer (PBL) using surface air quality monitoring data and meteorological observations and sounding data from January 1 to January 20, 2020. The number of pollution days in Shenyang, Changchun, and Harbin were 13, 17, and 19, respectively, and the number of heavy pollution days was 4, 3, and 13, respectively. Harbin suffered the heaviest and longest pollution with the smallest diurnal variation between daytime low and nighttime high PM2.5. The atmosphere in Harbin was more stable than that in Shenyang and Changchun, and PM2.5 concentration in the three cities tended to increase with decreasing planetary boundary layer height (PBLH). The lower winter air temperature in Northeast China led to a shallower PBL and weaker ventilation coefficient, which in turn led to an increase in PM2.5 concentration, indicating that air temperature is the key meteorological factor affecting local pollution in Northeast China. The analysis of two typical pollution events showed that a warm advection aloft suppressed the development of the PBL in Shenyang. In contrast, the suppressed effect of warm advection on the PBL was weakened in Harbin, which originally had a low PBLH due to cold weather. Moreover, the daytime convective boundary layer in Harbin developed 1–2 h later than that in Shenyang; the delayed development of PBL during the daytime in Harbin was also conducive to poor air quality.

Simultaneous Assimilation of Planetary Boundary Layer Observations from Radar and All-Sky Satellite Observations to Improve Forecasts of Convection Initiation

Abstract Accurate predictions of the location and timing of convection initiation (CI) remain a challenge, even in high-resolution convection allowing models (CAMs). Many of the processes necessary for daytime CI are rooted in the planetary boundary layer (PBL), which numerical models struggle to accurately predict. To improve ensemble forecasts of the PBL and subsequent CI forecasts in CAM ensembles, we explore the use of underused data from both the GOES-16 satellite and the national network of WSR-88D radars. The GOES-16 satellite provides observations of brightness temperature (BT) to better analyze cloud structures, while the WSR-88D radars provide PBL height estimates and clear-air radial wind velocity observations to better analyze PBL structures. The CAM uses the Advanced Research Weather Research and Forecasting (WRF-ARW) model at 3-km horizontal grid spacing. The ensemble consists of 40 members and observations are assimilated using the Gridpoint Statistical Interpolation (GSI) Ensemble Kalman Filter (EnKF) system. To evaluate the influence of each observation type on CI, conventional, WSR-88D, and GOES-16 observations are assimilated separately and jointly over a 4-h period and the resulting ensemble analyses and forecasts are compared with available observations for a CI event on 18 May 2018. Results show that the addition of the WSR-88D and GOES-16 observations improves the CI forecasts out several hours in terms of timing and location for this case.

Impacts of regional wind circulations on aerosol pollution and planetary boundary layer structure in Metro Manila, Philippines

Fine particulate matter (PM2.5) concentrations in Metro Manila, Philippines have consistently exceeded the guideline values set by the World Health Organization (WHO). Although there has been much progress in understanding the components and sources of PM2.5, limited research has been done on the influence of meteorological factors. In particular, the influence of the planetary boundary layer height (PBLH) on PM2.5 concentration has not been studied due to inadequate observations. From January 2019–June 2020, measurements from a High Spectral Resolution Lidar (HSRL) filled this gap and allowed for PBLH estimation and aerosol typing. This paper investigates the roles of PBLH and regional and local wind circulations on the temporal evolution of aerosol pollution. Results show that daytime and nighttime PBLH variability is associated with solar heating and radiative cooling, respectively. Cloud-free conditions during the dry season yield a higher PBL growth rate than during the wet season when lower daytime and elevated nighttime PBLH are observed. Lower PM2.5 levels are generally observed during daytime when PBLH is at its maximum. However, the PBLH has a significant inverse correlation with PM2.5 only in the months of December-January-February. We find that horizontal directional wind shear between synoptic and mesoscale circulations confounds the PM2.5 - PBLH relationship by creating stagnant conditions conducive to aerosol accumulation. The lower 20% of PM2.5 concentrations occur during the prevalence of strong monsoon winds. On the other hand, the upper 80% are found during the occurrence of compound mesoscale winds (i.e., sea/land/lake/valley/mountain breezes and channeling monsoon winds). In addition, mountain breeze is found to be associated with lifting of aerosols, resulting in multi-layering within the PBL. The findings in the present study emphasize the role of complex topography and mesoscale scale winds arising from the landscape on aerosol pollution variability.

Land-sea difference of the planetary boundary layer structure and its influence on PM2.5 — Observation and numerical simulation

A comprehensive set of observations were collected along a sea-coast-inland area. According to these observations, the planetary boundary layer heights (PBLH) during winter and summer for sampling locations in sea, coast, and inland areas were 737 m, 483 m and 372 m, and 450 m, 645 m and 646 m, respectively. Little seasonal difference was observed for the daily variation of sensible heat flux over the sea, with the maximum of 55 W/m2 at 12:00 in winter and 27 W/m2 at 13:00 in summer. The duration of sea breeze was ∼10 h in summer and only 3 h in winter, extended to inland area ∼ 50 km and upward 920 m vertically. PM2.5 at coastal area was about 5 μg/m3 higher than inland during summer afternoon. Over the sea, PM2.5 mainly concentrated below 200 m in winter, increased with height at night and decreased with height in the morning below 300 m in summer. A typical land-sea breeze episode was analyzed with observed and simulated result. According to the observed data, the sea breeze extended to inland ∼50 km and upward 300 m vertically. From the simulating result, there was a clear temperature gradient between sea and land from surface to 400 m, when influenced by the sea breeze, the wind and temperature profiles showed different characteristics, and forming a high concentration center of PM2.5 at 300 m. These results provide insights into the structure of land-sea planetary boundary layer, and provide support for the prediction of heavy pollution episode.

The dominant role of aerosol-cloud interactions in aerosol-boundary layer feedback: Case studies in three megacities in China

Interactions between aerosols and meteorology have received increasing attention in recent decades. Through interactions with radiation, aerosols involve in thermodynamic processes and cause cloud adjustment, referred to as the direct and semi-direct effects respectively. They also involve in cloud microphysical processes by severing as cloud condensation nuclei or ice nuclei, referred to as indirect effect. Aerosol direct effect is found to potentially exacerbate air quality by stabilizing the planetary boundary layer (PBL). However, their impacts through the interaction with clouds, including semi-direct and indirect effect remain unclear. In this study, we conducted model simulations to evaluate the direct, semi-direct and indirect effects of aerosols in PBL structure and surface PM2.5 concentration during three heavy haze events under overcast conditions. Overall, the aerosol-PBL feedback results in a 22%–36% decrease of PBL height and 5%–28% increase of PM2.5 concentration. The indirect effect always has the largest impact on PBL and PM2.5 pollution, accounting for 59%–84% of the changes. The semi-direct effect is the weakest on average, although it can exceed the direct effect at certain times and locations. Black carbon aerosols play the vital role in both the direct and semi-direct effects. Our findings promote the understanding of heavy haze formation, and highlight the dominant role of aerosol-cloud interaction in the feedback process of aerosols to PBL structure and air quality.

Regional PM2.5 pollution confined by atmospheric internal boundaries in the North China Plain: boundary layer structures and numerical simulation

Abstract. This study reveals mesoscale planetary boundary layer (PBL) structures under various pollution categories during autumn and winter in the North China Plain (NCP). The role of the atmospheric internal boundaries (AIBs, referring to the discontinuity of meteorological conditions in the lateral direction) in regulating PBL structures and shaping the PM2.5 pollution patterns is emphasized. The Weather Research and Forecast (WRF) model is used to display the three-dimensional meteorological fields, and its performance is evaluated by surface observations and intensive soundings. The evaluation demonstrates that the model reasonably captures the mesoscale processes and the corresponding PBL structures. Based on the reliable simulations, three typical pollution cases are analyzed. Case 1 and case 2 represent the two main modes of the wind shear category pollution, which is featured with airflow convergence line/zone as AIB, and thus is dominated by dynamical effect. Case 1 presents the west–southwest wind shear mode associated with a trough convergence belt. The convergent airflow layer is comparable to the vertical scale of the PBL, allowing PM2.5 transport to form a high pollution area. Case 2 exhibits another mode with south–north wind shear. A “lying Y-shaped” convergence zone is formed with a thickness of about 3000 m, extending beyond the PBL. It defines a clear edge between the southern polluted air mass and the clean air in the north. Case 3 represents the topographic obstruction category, which is characterized by a cold-air damming AIB in front of the mountains. The PBL at the foothills is thermally stable and dynamically stagnant due to the capping inversion and the convergent winds. It is in sharp contrast to the well-mixed/ventilated PBL in the southern plains, especially in the afternoon. At night, this meteorological discontinuity becomes less pronounced. The diurnal variation of the PBL thermal and dynamical structure causes the pollutants to concentrate at the foot of the mountains during the daytime and locally accumulate throughout the entire plain in the evening. These results provide a more complete mesoscale view of the PBL structure and highlight its spatial heterogeneity, which promotes the understanding of air pollution at the regional scale.

Dust-planetary boundary layer interactions amplified by entrainment and advections

Mineral dust contributes to more than half of the global aerosol loading. However, the radiative impacts of dust aerosols on planetary boundary layer (PBL) structure have not been explored sufficiently. During a typical dust storm event over Tarim Basin, dust aerosols exhibit a well-mixed distribution during the daytime in spite of a shallow layer of dust particles accumulated at higher altitudes. By contrast, nocturnal dust plumes are located near the surface due to stable stratification. We demonstrate that these differentiated vertical distributions determine the spatial heterogeneity of dust loading, radiative fluxes and PBL height variations. Dust aerosols cause daytime PBL suppression and nighttime PBL promotion through modulating surface and atmospheric radiative budgets. Specifically, dust-induced cooling effect within PBL directly inhibits the daytime PBL development. PBL suppression effect is then amplified by entrainment processes resulting in excessively low PBL height, especially for dust particles below but near the PBL top. Dust plumes weaken updrafts from PBL and downdrafts of the free atmosphere, which further reduce the entrainment mixing through attenuating horizontal and vertical advection, and eventually amplify PBL suppression. At night, near-surface dust aerosols stimulate a warm and unstable lower atmosphere, generate warm advection heating and promote the PBL development. Our study highlights the importance of specifying entrainment parameters and nighttime advection activities in quantifying the dust-PBL interactions.

Online vertical measurement of air pollutants: Development of a monitoring platform on a skyscraper and its application in Shanghai

In this study, an online monitoring platform for vertical measurement of air pollutants was established atop a tall building (623 m), namely, the Shanghai Tower. Moreover, air pollutants such as CO, NO2, O3 and particles with an aerodynamic diameter of 2.5 μm or smaller (PM2.5) were measured vertically at nine different heights. Based on continuous measurement data from June to November 2019, the seasonal and daily variation characteristics of the vertical profiles of these air pollutants in Shanghai were analyzed. The results indicated that CO, NO2 and PM2.5 generally showed a decreasing trend from the near ground to a height of 105 m, and the NO2 concentration continued to decrease to less than 20 μg m−3 above 600 m. The PM2.5 concentration fluctuated with height. However, the O3 concentration increased with height in general and the increase was more significant at higher altitudes in summer, with an increase rate of 1.28 μg m−3/10 m above 415 m. CO and PM2.5 tended to be impacted by regional transport in higher altitudes in autumn. Besides, a typical air pollution event in summer was analyzed to elucidate the detailed evolution process of the vertical concentration distribution. The obtained vertical observation data clearly revealed the impact of planetary boundary layer (PBL) structure that the sudden decline of planet boundary layer height (PBLH) by nearly 2/3 in an hour led to the explosive growth of O3 at all heights, especially at 105–255 m. Our study demonstrates that the established vertical online monitoring platform plays an important role in comprehensively capturing the vertical structure of urban air quality and explaining the air pollution evolution processes.

Synoptic condition and boundary layer structure regulate PM2.5 pollution in the Huaihe River Basin, China

Huaihe River Basin is an open plain area in eastern China, experiencing frequent heavy PM2.5 pollution events during the last decades. The physical mechanisms underlying the pollution have not yet been well understood. In this study, we investigated the variations of PM2.5 pollution in the Huaihe River Basin and its associated meteorological processes. Long-term PM2.5 measurements from 2015 to 2019, as well as multi-scale meteorological observations, reanalysis data, and air quality simulation data were analyzed. As a regional mean, the PM2.5 concentration shows the highest pollution level in winter and lowest in summer, influenced by both the annual cycle of emission rate and the variations of planetary boundary layer (PBL) structure. Wintertime PM2.5 pollution episodes showed synchronous variation in multiple cities in this region, characterized by weak winds, humid, and strong thermal stratifications within the PBL. Two synoptic patterns associated with winter heavy regional pollution were identified. One was characterized by the elevated warm advection from the southwest, which can suppress the development of the PBL. Another was controlled by a high pressure in the west, causing effective transport of pollutants from the north to the basin. The alternating occurrence of these two synoptic modes is critical to the frequent regional pollution in the Huaihe River Basin.

Land-Sea Difference of the Planetary Boundary Layer Structure and its Influence on Pm2.5 — Observation and Numerical Simulation

Land-Sea Difference of the Planetary Boundary Layer Structure and its Influence on Pm2.5 — Observation and Numerical Simulation

Simulating the meteorology during persistent Wintertime Thermal Inversions over urban areas. The case of Madrid

Persistent wintertime inversions over cities are the most critical conditions for air quality, and also one of the most challenging situations to simulate with a meteorological model. In this study, the ability of the meteorological Weather Research and Forecasting (WRF) model, coupled with the multilayer urban canopy parameterization BEP-BEM (Building Effect Parameterization - Building Energy Model), to simulate the evolution of the Planetary Boundary Layer (PBL) structure over the city of Madrid, Spain, during a multiday inversion episode, is assessed. The model results are evaluated against airport soundings, fourteen meteorological stations within and around the urban area, and remotely sensed surface temperatures. The study indicates that the PBL structure is determined by the interaction between the urban and rural heat and momentum fluxes, the topography, and the downward turbulent transport of heat. The best air temperature spatial distribution is obtained when the 6th order horizontal filter is applied only to wind and not to temperature, and when the soil moisture is reduced to 25% of the initial value provided by the global scale model. However, the comparison against satellite surface temperature data indicates that with such a dry soil the model underestimates the surface temperatures during the night and overestimates them during the day in rural areas. This compensating error points to a likely deficiency in the PBL and surface schemes during the persistent inversions. In urban areas, the simulations with the urban canopy parameterization tend to overestimate the nocturnal surface and air temperatures, while they reproduce wind speed correctly. Finally, a new methodology, based on a comparison of the differences between maximum and minimum temperatures for couples of stations, to assess the model's capability to reproduce the spatial variability of air temperature, is introduced, and the results show that the use of the multilayer urban canopy scheme and the adjustment of the soil moisture are both needed to improve the reproduction of such spatial distribution.

Understanding the Major Impact of Planetary Boundary Layer Schemes on Simulation of Vertical Wind Structure

The structure and evolution of the atmospheric planetary boundary layer (PBL) plays an important role in the physical and chemical processes of cloud–radiation interaction, vertical mixing and pollutant transport in the atmosphere. The PBL parameterization scheme describes the vertical transport of atmospheric momentum, heat, water vapor and other physical quantities in the boundary layer. The accuracy of wind field simulation and prediction is one of the most significant parameters in the field of atmospheric science and wind energy. Limited by the observation data, there are few studies on wind energy development. A 3D Doppler wind LiDAR (DWL) providing the high-vertical-resolution wind data over the urban complex underlying surface in February 2018 was employed to systematically evaluate the accuracy of vertical wind field simulation for the first time. 11 PBL schemes of the Weather Research and Forecasting Model (WRF) were employed in simulation. The model results were evaluated in groups separated by weather (sunny days, hazy days and windy days), observation height layers of wind field, and various observation wind speeds. Among these factors, the simulation accuracy is most closely related to the observation height layers of wind field. The simulation is fairly accurate at a height of 1000–2000 m, as most of the relative mean biases for wind speed and wind direction are less than 20% and 6% respectively. Below 1000 m, the wind speed and direction biases are about 30–150% m·s−1 and 6–30%, respectively. Moreover, when the observed wind speed was lower than 5 m·s−1, the biases were usually large, and the wind speed relative mean bias reaches up to 50–300%. In addition, the accuracy of the simulated wind profile is better in the range of 10–15 m·s−1 than other speed ranges, and is better above 1000 m than below 1000 m in the boundary layer. We see that the WRF boundary layer schemes have different applicabilities to different weather conditions. The WRF boundary layer schemes have significant differences in wind field simulations, with larger error under the complex topographies. A PBL scheme is not likely to maintain its advantages in the long term under different conditions including altitude and weather conditions.

The influence of stagnant and transport types weather on heavy pollution in the Yangtze-Huaihe valley, China

The ambient atmospheric PM2.5 concentrations in Anhui Province, China, which links the Yangtze River Delta region, China's fastest growing economy area, with the Beijing-Tianjin-Hebei (BTH) region, China's most polluted region, are influenced not only by local emissions, but also by changes in regional circulation. During the period 2013–2017, when China adopted a series of pollution abatement measures, there were still occasional pollution episodes with significant increases in PM2.5 concentrations. PM2.5 rise instead during the period 2013–2017 in Anhui (the Center of the Yangtze-Huaihe, YH), when pollution emissions continued to decrease? What is the controlling mechanism behind these? By analyzing elements such as ground-based PM2.5 concentration and the planetary boundary layer (PBL) structure affecting it as well as larger scale circulation, combined with the analysis of a parameterized index, one can find that aerosol pollution in the YH region can usually be classified into three types. (1) There is a short-term transport stage (TS) in the initial stage of pollution, then as the pollutant concentrations increase, the PBL height decreases, the temperature inversion is gradually formed or strengthened, the wind speed decreases and the relative humidity of the lower layer increases, forming a two-way feedback mechanism in the cumulative stage (CS). (2) Pollutant concentrations will not drop rapidly in the later stage of CS, while a short-term TS will occur again. (3) The explosive rise (ER) events are mainly affected by transportation in the YH. The first of these types tends to be accompanied by the emergence and maintenance of heavy pollution periods (HPEs), and some phases is accompanied by explosive rises (ERs) in PM2.5 that at least double in a short period of time. To sum up, deterioration of meteorological conditions explaining approximately 68% to the increase in PM2.5 in the ER.

Planetary Boundary-Layer Structure at an Inland Urban Site under Sea Breeze Penetration

We evaluated the performance of the Weather Research and Forecasting Model in simulating the boundary-layer structure at an urban site in Seoul on two clear summer days against observations made using a ceilometer, a Doppler wind lidar, and a microwave radiometer. The planetary boundary-layer height (PBLH) was estimated from two different methods using observations: the ceilometer-based method (CBM) and bulk Richardson number method (BRM). The maximum PBLH was well captured by the model but PBLH was underestimated in the late afternoon and evening. To examine the cause for underestimation of PBLH in the evening, we compared the wind and virtual potential temperature structure between the simulation and observations. The model captured the timing of the sea breeze well, but it considerably overestimated the intensity of the simulated sea breeze, resulting in an overestimated cold advection by a sea breeze. The effect of the overestimated cold advection on evening PBLH was greater when the sea breeze arrived in the late afternoon and hence the vertical extent of the sea breeze was shallow. This study underscores the importance of accurately simulating the sea breeze for PBLH prediction in the evening, over the inland urban areas under sea breeze penetration.

Spring Planetary Boundary Layer Structure and Corresponding Cloud Characteristics Under Different Prevailing Wind Directions Over the Kuroshio Sea Surface Temperature Front in the East China Sea

AbstractThe planetary boundary layer (PBL) structure and corresponding stratocumulus (Sc) clouds under different prevailing wind directions over the East China Sea were studied using CloudSat and sounding observations from March–April 2007–2017. The dominant factors affecting the PBL structure and atmospheric response mechanism associated with the Kuroshio sea surface temperature front (SSTF) differed under the three prevailing wind directions. When northeasterly (NE) wind prevails, the opposite spatially high‐pass‐filtered meteorological parameters between the warm tongue and cold pool indicate that the PBL structure is predominantly determined by the SSTF via the sea level pressure adjustment mechanism. However, when northwesterly (NW) and southeasterly (SE) winds prevail, a downward kinetic energy tongue extends from the free atmosphere to the sea surface, indicating that the dynamic PBL structure is predominantly controlled by atmospheric circulation. The (minor) difference in relative humidity between the cold and warm sides indicates that the thermodynamic PBL structure is dominated by the SSTF through the vertical mixing mechanism (by atmospheric circulation) under NW (SE) prevailing wind. Completely different PBL structures lead to different temperature inversion (TI) and Sc cloud characteristics under the three prevailing wind directions. TI mainly occurs as low tropospheric inversion over the warm tongue, while the frequency of near‐surface inversion is large over the cold pool under NE and SE prevailing wind directions. The Sc cloud fraction over the warm tongue is two times larger than that over the cold pool under NE prevailing wind and is the largest (smallest) when NW (SE) wind prevails.