Planetary Boundary Layer Processes Research Articles

Observed Characteristics of Planetary Boundary Layer Processes and Associated Convection over a Tropical Location on the East Coast of India

Observed Characteristics of Planetary Boundary Layer Processes and Associated Convection over a Tropical Location on the East Coast of India

Impact of PBL schemes on multiscale WRF modeling over complex terrain, Part I: Mesoscale simulations

This study investigates the performance of two commonly used planetary boundary layer (PBL) schemes, the local turbulence closure (Mellor-Yamada-Janjić: MYJ) and the nonlocal turbulence closure (Yonsei University: YSU), over the tropical coastal region of Shenzhen, China, using the Weather Research and Forecasting (WRF) model. The simulation results are evaluated by comparing them to various measurements. Additionally, we also investigate the performance of the PBL schemes over different land cover types and their ability to reproduce the physical relationships between several pairs of interrelated near-surface atmospheric parameters. The results show that both PBL schemes underestimate temperature and PBL height and overestimate wind speed and turbulent fluxes throughout the whole day, but this can be improved by increasing horizontal resolution. We find that the overestimation of wind speed and turbulent heat fluxes is mainly caused by the strong mechanical mixing in the model. Some improvements are required such as more accurate surface morphology parameters and momentum transport coefficients. Overall, the YSU scheme outperforms the MYJ scheme at all horizontal resolutions over complex terrain in terms of near-surface variables, PBL vertical structures, and the internal relations between turbulent heat fluxes and atmospheric instability. However, the difference between the two PBL schemes, varying over different land covers and with time periods and altitudes within the PBL, is generally greater over the land than over the sea for thermal-related variables (especially during daytime), while for variables associated with water vapor is the opposite. This highlights the importance of the underlying surface for the performance of PBL schemes and the need to select appropriate PBL schemes for simulating the evolution of PBL processes over different land covers.

Fine scale simulation of sea salt aerosol under strong wind: WRF sensitivity test on PBL schemes and LES under Typhoon Hagibis

Identifying which factors among the planetary boundary layer (PBL) scheme, grid resolution, and source or sink terms dominantly influence the sea salt aerosol (SSA) spatial distribution is necessary. The dependence of SSA transport on PBL processes was investigated using a numerical weather prediction (NWP) model with a grid scale of ∼143 m. Size-segregated SSA transport during Typhoon Hagibis, which made landfall in Japan in 2019, was simulated using the Weather Research and Forecasting (WRF) model. The PBL schemes of Yonsei University (YSU), scale-aware YSU (Shin and Hong, 2015's scheme, SH) and Mellor-Yamada Nakanishi-Niino (MYNN), and large-eddy simulation (LES) subjected to turbulence transition enhancement via a cell perturbation method were tested to evaluate the reproducibility of observed values of wind speed, surface pressure, relative humidity, and temperature at meteorology masts. The results showed that wind speed was most influenced by the selection of the PBL scheme and LES. The distance from the coastline, as determined by the wind direction during the most intense typhoon stage, was correlated with a decrease in the time averaged SSA concentration when the storm moved inland. Despite the simulated SSA emission scheme's high sensitivity to wind speed (nearly 3rd power), the near-surface average concentration was less sensitive to the PBL schemes and LES selection than wind speed. However, the maximum arrival of the SSA differed by ∼20 % during the most vigorous stage of the typhoon. Budget analysis of the SSA transport equation highlighted that the advection term and gravity settling were most dependent on the PBL schemes and LES. The impacts of both factors on SSA transport were not limited to near-surface tendencies but were in a broader range over the atmosphere, up to 1–2 km. These results indicated that the vertical structure of the SSA field over the boundary layer depended on the PBL schemes and LES, thereby modulating the inland arrival of the SSA.

Surf, Turf, and Above the Earth: Unmet Needs for Coastal Air Quality Science in the Planetary Boundary Layer (PBL)

AbstractCoastal areas are some of the most densely populated and economically important regions in the world. As such, protecting the health of the human population and ecosystems at the coastal interface and understanding the impacts of environmental stressors such as air pollutants provides wide‐ranging benefits. Air quality (AQ) processes within coastal regions have been studied using ground and space‐based platforms, with intensive field campaigns focused on addressing key science questions that are typically partitioned into either direct atmospheric effects (e.g., anthropogenic emissions creating air pollution) or indirect processes and feedback loops (e.g., terrestrial/marine biogenic processes modifying atmospheric properties). The atmospheric planetary boundary layer (PBL) and its depth (or height) connect land, air, and the water surface via many pathways, especially with transport and exchange processes tied to the complexities of the coastal interface. We still cannot accurately characterize—through field, aircraft, or space‐based observations—the spatial and temporal PBL variability and processes within the PBL that couple together coastal dynamics and air quality. Several upcoming geostationary and polar‐orbiting satellite missions are likely to make significant progress in characterizing these air/land/water interactions over the next decade. Here, we present a framework of the current understanding of the PBL's role in coastal regions, primarily regarding air quality and atmospheric deposition, to motivate future concerted efforts from ground‐ and space‐based platforms to achieve a holistic understanding of the coastal interface.

The Global Representativeness of Fair‐Weather Atmospheric Electricity Parameters From the Coastal Station Maitri, Antarctica

AbstractAtmospheric electricity parameters (AEP) measurements from Antarctica predominantly feature either the potential gradient (PG) and/or air‐Earth current (AEC) density. We report for the first time simultaneous measurements of the bipolar ions concentration/conductivity, PG, and AEC density. AEP measurements were carried out at Maitri (70.8°S, 11.8°E) from December 2018 to November 2019. We formulated a few criteria, irrespective of the weather conditions, to select the electrically quiet days and some additional criteria based on the conductivity measurements to discern globally representative data (GRD) from such days. The measurements of the PG and AEC density over the Antarctic plateau demonstrated the diurnal curves similar to the Carnegie pattern, which represents the global thunderstorms and electrified shower clouds (ESCs) occurring on different continents and oceans, we regard the data having such trend as GRD. We found significant variability in the concentration of small bipolar ions/conductivity in the austral summer which in turn affects GRD. However, the concentration of bipolar ions is nearly consistent at ∼250 negative ions cm−3 and ∼300 positive ions cm−3 in winter and enhances the probability of GRD. Such differences can arise out of the prevalent planetary boundary layer processes in the two seasons. When the PG varied between ∼50 Vm−1 and ∼150 Vm−1 and the maximum range of conductivity variations was ∼0.2 × 10−14 ℧ m−1, the AEPs represented the signatures of the global thunderstorm and ESC activities.

Improving the Wind Power Density Forecast in the Middle- and High-Latitude Regions of China by Selecting the Relatively Optimal Planetary Boundary Layer Schemes

As the power generation mode with the lowest carbon emissions, wind power generation plays an indispensable role in achieving the goal of carbon neutralization. To optimize the wind power density (WPD), forecasting is crucial to improve wind power utilization and power system stability. However, because near-surface wind is characterized by notable randomness, diversity, intermittence, and uncontrollability, accurately forecasting the WPD on wind farms remains a challenging task. In this study, we attempted to improve the WPD forecast in the middle- and high-latitude regions of China (wind energy resources are abundant there) by selecting the relatively optimal planetary boundary layer (PBL) scheme, as the PBL processes exert notable effects on the near-surface wind field directly. Based on a whole month in the summer (July 2021), seven PBL schemes were compared quantitatively by using the Weather Research and Forecasting (WRF) model for a total of 70 runs (for each run, the forecast period was 3 days). The results show that no PBL schemes could always show the best performance in forecasting all variables, and the forecast accuracy showed a notable dependence on the evolution of the weather systems. Among the seven PBL schemes, the Medium-Range Forecast (MRF) scheme showed the overall best performance in forecasting the 100 m wind speed, sea level pressure, and 2 m temperature, which ensured that it had the highest forecast skill for the WPD in the middle- and high-latitude regions of China. Further analyses indicate that the background conditions were also well forecasted by the MRF scheme (ranked first or second). This was a crucial reason why the WPD forecast was the best for the MRF scheme.

The influence of different parameterizations on diurnal cycle of land precipitation in CAS-ESM

The impacts of physical parameterizations on the diurnal cycle of precipitation (DCP) over land are investigated by comparing long-term simulations of the Chinese Academy of Sciences Earth System Model (CAS-ESM). Significant improvements in the DCP occur when the model replaces the Community Atmosphere Model version 4 (CAM4) physics package with the CAM5 package at both global and regional scales. Globally, the reduced amplitudes and delayed phases of DCP with the CAM5 package can be explained by the diurnal variations in convective available potential energy (CAPE). Regionally, the improvements in the DCP are caused by the decreased daytime convergence at 850 hPa, smaller vertically integrated water vapor transport, weakened vertical velocity, and smaller stratus cloud liquid water amount simulated by the CAM5 package. Besides, CAS-ESM with the CAM5 package produces the decreased amplitudes and delayed phases of the diurnal variation of the convective cloud, which is closely related to the planetary boundary layer (PBL) processes. CAS-ESM with the CAM5 package exhibits a significantly lower PBL height than with the CAM4 package for both direct model outputs and estimates with the bulk Richardson number method. The peaks of lifting condensation level (LCL) deficit (difference between PBL height and LCL height) occur later (∼1–3 h) and are lower with the CAM5 package than with the CAM4 package. Consequently, CAS-ESM with the CAM5 package decreases the probability of cloud formation and produces more realistic DCP. This study underscores the importance of PBL schemes to the simulated convective clouds and DCP.

The Effect of Surface Heating Heterogeneity on Boundary Layer Height and Its Dependence on Background Wind Speed

AbstractThe planetary boundary layer height (PBLH) is a fundamental variable of the planetary boundary layer (PBL). The effect of surface heterogeneity is challenging in the PBL parameterization for numerical weather and climate models. Here we use large eddy simulation data to investigate the impact of surface heating heterogeneity on the PBLH spatial variations and mean values over a domain of typical mesoscale model grid size. It is found that this impact depends on background wind condition. Variance decomposition and Fourier spectra both reveal that for the fully developed sheared convective PBL, surface heterogeneity contributes little to the spatial variation of PBLH, except for the case with heterogeneity scale of 14.4 km. This suggests that surface heterogeneity with length scales less than ∼10 km, which is typical for mesoscale modeling, does not induce significant subgrid spatial variation of PBLH. The domain‐averaged PBLH is found to generally increase as surface heterogeneity scale decreases, until reaching the scale on the order of PBLH. The findings can be used to inform the parameterization development of PBL processes over heterogeneous surface.

How Does Sea Surface Temperature Drive the Intertropical Convergence Zone in the Southern Indian Ocean?

Abstract The Indian Ocean has an intriguing intertropical convergence zone (ITCZ) south of the equator year-round, which remains largely unexplored. Here we investigate this Indian Ocean ITCZ and the mechanisms for its origin. With a weak semiannual cycle, this ITCZ peaks in January–February with the strongest rainfall and southernmost location and a northeast–southwest orientation from the Maritime Continent to Madagascar, reaches a minimum around May with a zonal orientation, grows until its secondary maximum around September with a northwest–southeast orientation, weakens slightly until December, and then regains its mature phase in January. During austral summer, the Indian Ocean ITCZ exists over maximum surface moist static energy (MSE), consistent with convective quasi-equilibrium theory. This relationship breaks up during boreal summer when the surface MSE maximizes in the northern monsoon region. The position and orientation of the Indian Ocean ITCZ can be simulated well in both a linear dynamical model and the state-of-the-art Community Atmosphere Model version 6 (CAM6) when driven by observed sea surface temperature (SST). To quantify the contributions of the planetary boundary layer (PBL) and free-atmosphere processes to this ITCZ, we homogenize the free-atmosphere diabatic heating over the Indian Ocean in CAM6. In response, the ITCZ weakens significantly, owing to a weakened circulation and deep convection. Therefore, in CAM6, the SST drives the Indian Ocean ITCZ directly through PBL processes and indirectly via free-atmosphere diabatic heating. Their contributions are comparable during most seasons, except during the austral summer when the free-atmosphere diabatic heating dominates the mature-phase ITCZ. Significance Statement The intertropical convergence zone (ITCZ) is the globe-encircling band where trade winds converge and strong rainfall occurs in the tropics. Its rains provide life-supporting water to billions of people. Its associated latent heating invigorates the tropical atmospheric circulation and influences climate and weather across the planet. The ITCZ is located north of the equator in most tropical oceans, except in the Indian Ocean where it sits south of the equator year-around. In contrast to the well-known northern ITCZs, the origin of the southern ITCZ in the Indian Ocean remains unknown. This work provides the first explanation for how ocean surface temperature works together with processes in the lower and upper atmosphere to shape the unique ITCZ in the Indian Ocean.

Observation and Simulation of Low-Level Jet Impacts on 3D Urban Heat Islands in Beijing: A Case Study

Abstract In this study, we focused on the impacts of the planetary boundary layer (PBL) low-level jet (LLJ) on the horizontal distribution, vertical development, and 3D structure of urban heat island (UHI). Observational datasets were collected from 224 automatic weather stations (AWSs), and an intensive sounding experiment was conducted in Beijing from 28 August to 2 September 2016. Three-dimensional simulations were operated by the Weather Research and Forecasting (WRF) Model. The results show the following: Ri was smaller than 0.25 at both urban and suburban stations near the surface when the LLJ was present. Through turbulent mixing, the LLJ extended the horizontal distribution of the canopy UHI downwind and increased the total UHI area by approximately 1 × 103 km2. The temperature lapse rate in the urban area was 0.7°C (100 m)−1 with the LLJ, twice that in the absence of an LLJ. The jet enhanced the vertical mixing above the urban area, accompanied by a near-surface TKE up to 0.52 m2 s−2, elevating the vertical UHI development height to 200 m. The LLJ is capable of increasing the temperature of the downwind urban area by a maximum of 8.5°C h−1 through warm advection. The temperature advection in the upper air caused by the LLJ also tilted the 3D UHI structure as a plume. Results reproduced the process by which the LLJ affect the 3D UHI structure through turbulence and advection, and could also provide ideas regarding the influence of the LLJ in other PBL processes.

Variation of Ozone, Carbon Monoxide, and Oxides of Nitrogen at Bengaluru, India

The present study analyses the continuous in-situ observations of surface ozone (O3), carbon monoxide (CO), and nitrogen oxides (NOX) conducted in an urban location, Bengaluru, India, during the year 2019 (January to December). The seasonal concentration of O3 fluctuated with the highest concentrations in the summer (39.6 ppbv) and winter (40.4 ppbv) and the lowest concentrations during the monsoon (16.8 ppbv). The seasonal mixing ratio of CO showed the highest value in post-monsoon (1.71 ppmv) and lowest during monsoon (0.79 ppmv). The seasonal trend of NOX showed highest in winter (56.8 ppbv) and lowest in monsoon (22.5 ppbv). The monthly mixing ratios of O3, CO, and NOX showed distinct variability, which may be attributed to changing anthropogenic activities, planetary boundary layer processes, and local meteorology. O3 was significantly related to temperature but inversely associated with relative humidity and wind speed. The association between CO, NOX with relative humidity, temperature, wind speed showed discrete results. The (dO3/dt) in the morning and evening duration were about 5.0 ppbv/h and -4.1 ppbv/h respectively.

Impact of lidar data assimilation on planetary boundary layer wind and PM2.5 prediction in Taiwan

Accurate simulations of planetary boundary layer (PBL) processes in numerical weather models are vital for air quality predictions. Advanced remote sensing techniques, such as lidar detection and ranging (lidar), can provide aerosol information with high temporal and vertical resolutions in the PBL. In this study, a lidar data assimilation system was developed based on the Weather Research and Forecasting-Local Ensemble Transform Kalman Filter (WRF-LETKF) framework coupled with the Community Multiscale Air Quality (CMAQ) model. The objective was to investigate the impact of lidar data assimilation on PBL prediction and the subsequent influence on PM2.5 prediction for a high-air-pollution event.The fine particulate matter (PM2.5) profiles retrieved from two micropulse lidar observations in northern and central Taiwan were assimilated in the WRF-LETKF system. Three numerical experiments, BASE (with a nudging strategy), CTRL (with an ensemble framework), and LDA (with assimilation of lidar-retrieved PM2.5 profiles), were conducted for a high-air-pollution episode. The BASE simulation overestimates the wind speed, which also leads to PM2.5 underestimation. The CTRL and LDA simulations are able to improve the wind fields and enhance the PM2.5 accumulation. With a strong error correlation between the lidar-retrieved PM2.5 concentration and the wind fields, the LDA simulation effectively corrects the wind flow from the surface to the PBL top, which further adjusts the PM2.5 transport processes and leads to results that agree well with observations.

Recent Advances in Our Understanding of Tropical Cyclone Intensity Change Processes from Airborne Observations

Recent (past ~15 years) advances in our understanding of tropical cyclone (TC) intensity change processes using aircraft data are summarized here. The focus covers a variety of spatiotemporal scales, regions of the TC inner core, and stages of the TC lifecycle, from preformation to major hurricane status. Topics covered include (1) characterizing TC structure and its relationship to intensity change; (2) TC intensification in vertical shear; (3) planetary boundary layer (PBL) processes and air–sea interaction; (4) upper-level warm core structure and evolution; (5) genesis and development of weak TCs; and (6) secondary eyewall formation/eyewall replacement cycles (SEF/ERC). Gaps in our airborne observational capabilities are discussed, as are new observing technologies to address these gaps and future directions for airborne TC intensity change research.

Study of Antarctic Blowing Snow Storms Using MODIS and CALIOP Observations With a Machine Learning Model

AbstractAs a common phenomenon over Antarctica, blowing snow (BLSN), especially the large BLSN storms, play an important role in the Antarctic surface mass balance, radiation budget, and planetary boundary layer processes. This study presents the work on BLSN storm identification and analysis with observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite. Spectral analysis shows that BLSN identification is feasible with MODIS daytime data. A random forest machine learning model is developed and observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization are used for training. Model performance results show that machine‐learning based classification can achieve over 90% overall accuracy when classifying MODIS pixels into cloud, clear, and BLSN categories. The machine learning model is applied to MODIS observations during the month of October 2009 for BLSN storm analysis. Results show that the size of BLSN storms has a large spectrum and can reach hundreds of thousands km2. The MODIS based BLSN storm frequency map extends the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations coverage limit from 82°S to the South Pole. A BLSN storm belt, which extends from the South Pole region to the coastal area between 130°E and 160°E along the Transantarctic Mountains, provides a potential pathway of snow transport. These results are important in improving the understanding of BLSN impact on Antarctic surface mass balance and boundary layer processes.

Evaluation and improvement study of the Planetary Boundary-Layer schemes during a high PM2.5 episode in a core city of BTH region, China

Accurate depictions of planetary boundary layer (PBL) processes are important for both meteorological and air quality simulations. This study examines the sensitivity of the model performance of the Weather Research Forecasting model coupled with Chemistry (WRF-Chem) to five different PBL schemes and further to different turbulence parameters for the simulation of a winter haze episode in Tianjin, a core city of the Beijing-Tianjin-Hebei (BTH) region in China. To provide a direct and comprehensive evaluation of the PBL schemes, measurements from multiple instruments are employed, including both meteorological and air quality quantities from near-surface observations, vertical sounding measurements and ceilometer data. Moreover, the vertical distribution of the turbulent exchange coefficient is derived from sounding measurements and is utilized to evaluate the PBL schemes. The results suggest that the Mellor-Yamada-Janjic (MYJ) scheme is generally statistically superior to the other schemes when comparing observations. However, considerable model discrepancies still exist during certain stages of this haze episode, which are found to be predominantly due to the deficiency of MYJ in distinguishing the intensity of turbulent mixing between different pollution stages. To improve the model performance, this study further tests the impact of different closure parameters on the simulation of winter haze episode. In the MYJ scheme, the closure parameters play a key role in the turbulent mixing within the PBL and therefore in haze simulations. Sensitivity experiments with different MYJ parameters confirm this diagnosis and suggest that a larger Prandtl number (Pr), rather than the default value in the MYJ formulation, may be more applicable for haze simulations under stable atmospheric conditions.

Impacts of PBL schemes on PM2.5 simulation and their responses to aerosol-radiation feedback in GRAPES_CUACE model during severe haze episodes in Jing-Jin-Ji, China

Using the online-coupled Global/Regional Assimilation and Prediction System coupled with the Chinese Unified Atmospheric Chemistry Environment (GRAPES_CUACE) model, numerical experiments are conducted to compare the impacts of three planetary boundary layer (PBL) schemes—Medium Range Forecast (MRF), Yonsei University (YSU), and Mellor-Yamada-Janjic (MYJ)—on PM2.5 simulations and their responses to aerosol-radiation feedback (ARF), under different terrain and weather conditions. The baseline experiments without ARF with all three PBL schemes can reproduce the spatial and temporal distribution characteristics of surface PM2.5 on the whole and show that the model can capture the PM2.5 variation better in unstable conditions (clean days) than in stable conditions (pollution days). The three PBL schemes show differences in the simulations of meteorological factors, e.g., the MRF scheme produces the best 10-m wind speed, the MYJ scheme produces the best 2-m temperature, and the YSU scheme produces the lowest PBL height (PBLH) and vertical diffusivity. Consequently, there is no significant difference among the three PBL schemes in surface PM2.5 simulations due to the combined action of the surface and the PBL structure. However, all the baseline experiments massively underestimate PM2.5 during severe pollution days. The online computing of ARF in the three PBL schemes can substantially improve the underestimation of PM2.5 by reducing sensible heat fluxes, 2-m temperature, friction velocity, and the PBL height. More importantly, the response intensity of three PBL schemes to ARF is very different. For PM2.5 simulations, MYJ has the weakest response, whereas MRF and YSU have similar responses. During the heavy pollution days with more intense aerosol-radiation effect, the increase of PM2.5 induced by ARF in MRF and YSU is about twice that in MYJ, making the PM2.5 simulated by MRF and YSU more consistent with the observations. This result shows the ARF dependence on the PBL processes and suggests the importance of PBL-ARF interaction to the PM2.5 numerical prediction during haze episodes.

Different effects of latent heat in planetary boundary layer and cloud microphysical processes on Typhoon Sarika (2016)

Three simulation experiments were conducted on Typhoon (TC) “Sarika” (2016) using the WRF model, different effects of the latent heat in planetary boundary layer and cloud microphysical process on the TC were investigated. The control experiment well simulated the changes in TC track and intensity. The latent heat in planetary boundary layer or cloud microphysics process can affect the TC track and moving speed. Latent heat affects the TC strength by affecting the TC structure. Compared with the CTL experiment, both the NBL experiment and the NMP experiment show weakening in dynamics and thermodynamics characteristics of TC. Without the effect of latent heat, the TC cannot develop upwards and thus weakens in its intensity and reduces in precipitation; this weakening effect appears to be more obvious in the case of closing the latent heat in planetary boundary layer. The latent heat in planetary boundary layer mainly influences the generation and development of TC during the beginning stage, whereas the latent heat in cloud microphysical process is conducive to the strengthen and maintenance of TC in the mature stage. The latent heat energy of the cloud microphysical process in the TC core region is an order of magnitude larger than the surface enthalpy. But the latent heat release of cloud microphysical processes is not the most critical factor for TC enhancement, while the energy transfer of boundary layer processes is more important.

Multi-scale processes in severe haze events in China and their interactions with aerosols: Mechanisms and progresses

<p indent=0mm>Severe haze events occur frequently in China, characterized by exceedingly high concentration of fine particulate matter (smaller than <sc>2.5 µm,</sc> or PM_2.5). These extremes are caused by synthetic physical-chemical processes, including emissions, chemical formation, planetary boundary layer processes, regional circulation, weather and climate. These processes are multi-scales, ranging from nanometers to thousands of kilometers. The complex interplays among these processes make it more difficult to understand the formation of severe haze events, which also influences model development and weather forecast. Here, we review the contributions of dominant mechanisms in severe haze events, especially their roles in temporal oscillations of PM_2.5 concentrations, and discuss the interplays among these atmospheric multi-scale processes. Previous studies indicate that: (1) Secondary aerosols become the dominant components in aerosols. Heterogeneous aqueous reactions play important roles in gas-to-particle conversion, especially in the late haze period; (2) PM_2.5 owns extensive temporal oscillations (on daily, weekly, to monthly timescales), which are caused by multi-scale processes; (3) high aerosols in China have already influenced photochemistry, boundary layer, weather and climate processes. The complex interplays among aerosols and the above processes make it more difficult to unravel the causes, mechanisms, and trends for haze pollution. In the future, the following issues should be considered: cooperative observations of aerosols, gas pollutants, photochemistry, and meteorological variables should be strengthened, especially their vertical distribution in the troposphere; multidisciplinary research should be strengthened, especially among atmospheric physics, chemistry, weather and climate; and model simulations related to the interplays among aerosols and atmospheric physical and chemical processes should be strengthened.

Multi-nested WRF simulations for studying planetary boundary layer processes on the turbulence-permitting scale in a realistic mesoscale environment

The Weather Research and Forecasting (WRF) model was applied in a nested configuration from a 2.7 km convection-permitting domain via grey-zone resolutions of 900 m and 300 m down to the 100 m turbulence-permitting scale. Based on sensitivity studies, this approach was optimized to investigate the evolution of small-scale processes in the PBL for a clear sky case during the HOPE experiment in western Germany on 24 April 2013. The results were compared with theoretical and experimental findings from literature and high-resolution lidar observations collected during the campaign. Simulations with parameterized turbulence were able to capture the temporal evolution of the PBL height, but almost no internal structure was simulated in the boundary layer. Only the turbulence-permitting simulations were capable of reproducing the morning transition from the stable nighttime to the daytime convective boundary layer and the following break-up into turbulent eddies. Comparisons with lidar data showed that the turbulence-permitting simulations reproduced the observed turbulence statistics. Nevertheless, the potential temperature in the boundary layer was 1 K cooler than observed, caused by a lower surface temperature mixed upward by the turbulent eddies. The simulated PBL height was underestimated by 200 m, reflected in a well-captured profile of specific humidity up to a height of 900 m and an overly strong decrease of moisture above. The general shape of the variance profiles of potential temperature and specific humidity were captured by the model. However, the simulated variability throughout the boundary layer was lower and the different heights of the variance peaks indicated that the model may not fully capture the turbulent processes at the top of the boundary layer. Identifying those systematic differences between nested simulations and observations demonstrated the value of this model approach for process studies and parameterization tests.

Regional atmospheric pollutant transport mechanisms over the North China Plain driven by topography and planetary boundary layer processes

Comprehensive measurements were conducted in winter 2018 and combined with RMPAS-Chem model simulations to analyze the regional transport mechanisms of atmospheric pollutants over the North China Plain. The instruments used consisted of four Vaisala CL51 ceilometers for planetary boundary layer (PBL) heights and aerosol backscatter profiles, two wind profilers, one radiosonde for the profiles of meteorological variables, and an instrumented King-Air 350 aircraft for the profiles of atmospheric pollutants and meteorological variables. Additionally, observations from Environmental Protection Bureau stations were also analyzed, including hourly concentrations of surface PM2.5, SO2, NO2, CO, and O3. The results suggest that regional atmospheric pollutant transport is driven by a combination of topography and PBL processes. First, a mountain-induced vertical vortex forms over downwind regions; this elevates ground pollutants to form an elevated pollutant layer (EPL) at an altitude of 1.4–1.7 km. The EPL is then transported to Beijing via an enhanced southerly wind. Finally, the pollutants in the EPL are transported downward to the surface through PBL processes.