Planetary Boundary Layer Meteorology Research Articles

The effect of BC on aerosol–boundary layer feedback: potential implications for urban pollution episodes

Abstract. Beijing suffers from poor air quality, particularly during wintertime haze episodes when concentrations of PM2.5 (particulate matter with a diameter < 2.5 µm) can peak at > 400 µg m−3. Black carbon (BC), an aerosol which strongly absorbs solar radiation, can make up to 10 % of PM2.5 in Beijing. BC is of interest due to its climatic and health impacts. BC has also been found to impact planetary boundary layer (PBL) meteorology. Through interacting with radiation and altering the thermal profile of the lower atmosphere, BC can either suppress or enhance PBL development depending on the properties and altitude of the BC layer. Previous research assessing the impact of BC on PBL meteorology has been investigated through the use of regional models, which are limited both by resolution and the chosen boundary layer schemes. In this work, we apply a high-resolution model (UCLALES-SALSA) that couples an aerosol and radiative transfer model with large-eddy simulation (LES) to quantify the impact of BC at different altitudes on PBL dynamics using conditions from a specific haze episode which occurred from 1–4 December 2016 in Beijing. Results presented in this paper quantify the heating rate of BC at various altitudes to be between 0.01 and 0.016 K/h per µg/m3 of BC, increasing with altitude but decreasing around PBL top. Through utilising a high-resolution model which explicitly calculates turbulent dynamics, this paper showcases the impact of BC on PBL dynamics both within and above the PBL. These results show that BC within the PBL increases maximum PBL height by 0.4 % but that the same loading of BC above the PBL can suppress PBL height by 6.5 %. Furthermore, when BC is present throughout the column, the impact of BC suppressing PBL development is further maximised, with BC causing a 17 % decrease in maximum PBL height compared to only scattering aerosols. Assessing the impact of these opposite effects, in this paper, we present a mechanism through which BC may play a prominent role in the intensity and longevity of Beijing's pollution episodes.

Time-lapse photography and its teaching usefulness in planetary boundary layer meteorology

Time-lapse photography and its teaching usefulness in planetary boundary layer meteorology

Assessing surface air pressure sensing using 118 GHz O2-absorption radar system

This study assesses surface air pressure remote sensing using a differential absorption radar at the 118 GHz O2 line. The considered system would have, at least two, closely-spaced frequency channels. Environmental impacts like surface reflections and absorptions from water vapor and other gases, and liquid water clouds on the radar signals of these channels are very similar, but the difference in O2 absorption is substantial. The surface pressure could be retrieved from the radar measured differential absorption optical depth of the atmosphere. The operational frequencies of the radar system are optimally selected, and the transmitted powers of the paired stronger and weaker O2 absorption channels are carefully distributed to balance the signal-to-noise ratios of these channels. Simulations of radar signals and pressure retrievals show that with an airborne system cruising at 10 km altitude, surface pressure measurements could achieve 1 mb accuracy within 1-s data processing. These pressure measurements would provide urgently needed supports in improving severe and general weather forecasts and gain unique knowledge on planetary boundary layer meteorology. Other advantages of this kind of high frequency radars would be their higher spatial resolution and increased potentials of small- or CubeSat applications.

Multilevel Validation of Doppler Wind Lidar by the 325 m Meteorological Tower in the Planetary Boundary Layer of Beijing

The high-frequency monitoring of three-dimensional wind fields is crucial in planetary boundary layer meteorology. Doppler wind lidar and meteorological towers are the most important instruments for site observations of three-dimensional wind fields. This study systematically investigated and compared the performances of three wind measurement instruments: A Doppler wind lidar (Windcube 100s), cup anemometer/wind vane and sonic wind anemometer mounted on the 325 m meteorological tower in the polluted urban city of Beijing. The horizontal wind speed measurements of the Doppler wind lidar closely matched those of the cup anemometer and the sonic wind anemometer with high coefficients of determination (R2: 0.79–0.96 and 0.90–0.97, respectively). Moreover, the results also showed good agreement between the three measurements of the prevailing horizontal wind direction. Conversely, there were weak correlations between the vertical wind speeds of the Doppler wind lidar and sonic wind anemometer with low coefficients of determination (R2: 0.30–0.46). With increasing temporal scale, the consistency in the vertical wind increased. In addition, the Doppler wind lidar seemed to correlate better with the sonic wind anemometer at heights exceeding 300 m (R2: 0.48–0.77). Note that there was a remarkable difference between the Doppler wind lidar and 325 m meteorological tower observations as the aerosol concentrations changed rapidly. Different wind measurement instruments have unique advantages and are thus irreplaceable. The Doppler wind lidar is better at measuring larger turbulent eddies.

A multi-meteorological comparison for episodes of PM10 concentrations in the Berlin agglomeration area in Germany with the LOTOS-EUROS CTM

Particulate matter (PM) remains as one of the most relevant air-quality concerns in urban environments. The Berlin agglomeration area is still affected by exceedances of the daily limit value of the PM concentration, especially during wintertime PM episodes. In this study, we present test-case studies with the LOTOS-EUROS CTM to improve the representation of PM episodes in the Berlin agglomeration area. A variety of simulations were compared for two winter episodes characterized by cold stagnant conditions, using different meteorological input data (from the European Centre for Medium Weather Forecast (ECMWF) and the Consortium for Small-Scale Modelling-Climate Limited-area Modelling (COSMO-CLM)) and horizontal and vertical resolutions of the LOTOS-EUROS CTM. The LOTOS-EUROS CTM indicates too high mixing from the planetary boundary layer (PBL) to higher layers, leading to an underestimation of the PM mass concentration in the Berlin agglomeration. As major impact factor the mixing layer height (MLH) can be identified. Through applying the COSMO-CLM model the meteorological representation of the PBL and MLH can significantly be improved, whereas sensitivity studies only exhibit a small variation of the PBL meteorology and did not further improve the MLH. As the MLHs of both models are underestimated compared to observations and their derivation is questionable, we advise not to use this quantity any longer in CTMs. By contrast, applying a multi-level approach excluding the MLH, provides a considerable increase in the total PM mass concentration amount. The redistribution and increased nitrate and ammonium concentration can be mentioned as the main culprit. However, the best-fit simulations were obtained for the multi-level configuration fed by COSMO-CLM input data, additionally representing a more realistic urban increment.

Dust-induced radiative feedbacks in north China: A dust storm episode modeling study using WRF-Chem

Abstract Radiative forcing of dust aerosol and the radiative feedbacks on the planetary boundary layer (PBL) in North China during a typical Asian dust storm in the early April of 2011 was investigated by an online coupled meteorology-chemistry-aerosol model WRF-Chem. Dust-induced daily mean radiative forcing (RF) at the ground surface and in the atmosphere were estimated to be −21.1 W m−2 and 12.7 W m−2, respectively, over Gobi desert, and −13.1 W m−2 and 4.8 W m−2, respectively, in downwind region over the North China Plain (NCP). Comparatively, radiative perturbation on short-wave radiation was approximately twice that on long-wave radiation in magnitude. In the daytime, when solar radiation dominated, the surface cooling and atmospheric heating due to dust increased PBL stability, leading to reductions of PBL height (PBLH) about 90 m and decreases in wind speed up to 0.4 m s−1. On the contrary, the radiative forcing in terrestrial radiation caused an opposite response at night, especially in the downwind region. Although dust emission was repressed by weakened wind speed during daytime, the elevated PBLH along with larger deflation at night lifted more dust particles to higher altitude (by up to 75 m in average), which prolonged dust residence time in the atmosphere and further intensified dust loading in downwind areas. Taking dust radiative feedbacks into consideration notably narrowed gaps between model-predicted air temperature vertical profiles with corresponding observations, suggesting a significant importance of dust-radiation interaction in PBL meteorology during dust storms.

Mesoscale modelling study of the interactions between aerosols and PBL meteorology during a haze episode in China Jing–Jin–Ji and its near surrounding region – Part 2: Aerosols' radiative feedback effects

Abstract. Two model experiments, namely a control (CTL) experiment without aerosol–radiation feedbacks and a experiment with online aerosol–radiation (RAD) interactions, were designed to study the radiative feedback on regional radiation budgets, planetary boundary layer (PBL) meteorology and haze formation due to aerosols during haze episodes over Jing–Jin–Ji, China, and its near surroundings (3JNS region of China: Beijing, Tianjin, Hebei, East Shanxi, West Shandong and North Henan) with a two-way atmospheric chemical transport model. The impact of aerosols on solar radiation reaching Earth's surface, outgoing long-wave emission at the top of the atmosphere, air temperature, PBL turbulence diffusion, PBL height, wind speeds, air pressure pattern and PM2.5 has been studied focusing on a haze episode during the period from 7 to 11 July 2008. The results show that the mean solar radiation flux that reaches the ground decreases by about 15% in 3JNS and 20 to 25%in the region with the highest aerosol optical depth during the haze episode. The fact that aerosol cools the PBL atmosphere but warms the atmosphere above it leads to a more stable atmospheric stratification over the region, which causes a decrease in turbulence diffusion of about 52% and a decrease in the PBL height of about 33%. This consequently forms a positive feedback on the particle concentration within the PBL and the surface as well as the haze formation. Additionally, aerosol direct radiative forcing (DRF) increases PBL wind speed by about 9% and weakens the subtropical high by about 14 hPa, which aids the collapse of haze pollution and results in a negative feedback to the haze episode. The synthetic impacts from the two opposite feedbacks result in about a 14% increase in surface PM2.5. However, the persistence time of both high PM2.5 and haze pollution is not affected by the aerosol DRF. On the contrary over offshore China, aerosols heat the PBL atmosphere and cause unstable atmospheric stratification, but the impact and its feedback on the planetary boundary layer height, turbulence diffusion and wind is weak, with the exception of the evident impacts on the subtropical high.

Mesoscale modeling study of the interactions between aerosols and PBL meteorology during a haze episode in Jing–Jin–Ji (China) and its nearby surrounding region – Part 1: Aerosol distributions and meteorological features

Abstract. The urbanized region ofJing(Beijing)-Jin(Tianjin)-Ji (alias of Hebei province) and its nearby surrounding region (3JNS) is becoming China's most polluted area by haze, exceeding even the Yangtze and Pearl river deltas. Aside from pollutant emission, the meteorology of the planetary boundary layer (PBL) is the most important factor affecting haze pollution. Focusing on July 2008, the aerosol optical properties and PBL meteorology features closely related to haze formation were simulated in the 3JNS region using an online atmospheric chemical transport model. The relationship between regional PBL meteorology, PM2.5, and haze is discussed. Model results accurately simulated the aerosol optical depth (AOD), single scattering albedo (SSA) and asymmetry parameter (ASY), validated by comparison with observations from the MODerate Resolution Imaging Spectroradiometer (MODIS), the China Aerosol Remote Sensing NETwork (CARSNET) and the Aerosol Robotic NETwork (AERONET). Modeled PBL wind speeds showed reasonable agreement with those from the National Centers for Environmental Prediction (NCEP) Reanalysis 2. A monthly mean AOD value as high as 1.2 was found from both model and observations, with a daily mean larger than 2.0 during haze episodes in the 3JNS region. Modeled and observed SSA values of 0.90–0.96 and ASY values of 0.72–0.74 demonstrated the high scattering characteristic of summer aerosols in this region. PBL wind speeds from modeled and NCEP data both showed a reversing trend of PM2.5 variation, illustrating the importance of the "PBL window shadow" in haze formation. Turbulence diffusion and PBL height had opposite phases to surface PM2.5, indicating that lower PBL height and weaker PBL turbulence diffusion are essential to haze formation. It is noted that homogeneous air pressure does not occur at the surface, but at an 850–950 hPa height during the haze episode. The momentum transmitting downward of the cold air from above the PBL to the low PBL and surface lead to an increase in surface wind speeds and haze dispersal.