Xiu-Qun Yang

Enhanced haze pollution by black carbon in megacities in China

Aerosol-planetary boundary layer (PBL) interactions have been found to enhance air pollution in megacities in China. We show that black carbon (BC) aerosols play the key role in modifying the PBL meteorology and hence enhancing the haze pollution. With model simulations and data analysis from various field observations in December 2013, we demonstrate that BC induces heating in the PBL, particularly in the upper PBL, and the resulting decreased surface heat flux substantially depresses the development of PBL and consequently enhances the occurrences of extreme haze pollution episodes. We define this process as the “dome effect” of BC and suggest an urgent need for reducing BC emissions as an efficient way to mitigate the extreme haze pollution in megacities of China.

Polluted dust promotes new particle formation and growth

Understanding new particle formation and their subsequent growth in the troposphere has a critical impact on our ability to predict atmospheric composition and global climate change. High pre-existing particle loadings have been thought to suppress the formation of new atmospheric aerosol particles due to high condensation and coagulation sinks. Here, based on field measurements at a mountain site in South China, we report, for the first time, in situ observational evidence on new particle formation and growth in remote ambient atmosphere during heavy dust episodes mixed with anthropogenic pollution. Both the formation and growth rates of particles in the diameter range 15–50 nm were enhanced during the dust episodes, indicating the influence of photo-induced, dust surface-mediated reactions and resulting condensable vapor production. This study provides unique in situ observations of heterogeneous photochemical processes inducing new particle formation and growth in the real atmosphere, and suggests an unexpected impact of mineral dust on climate and atmospheric chemistry.

Quantifying barotropic and baroclinic eddy feedbacks in the persistence of the Southern Annular Mode

Understanding the persistence of the Southern Annular Mode (SAM) is important for the intraseasonal and decadal predictability of SAM. Using the ERA-40 and ERA-Interim reanalysis data, this study introduces a new method to quantify the relative roles of barotropic and baroclinic eddy feedbacks in the SAM persistence. Through a hybrid Eulerian-Lagrangian Finite Amplitude Wave Activity diagnostic, it is found that (i) transient wave activity is important in driving the SAM, but it provides a negative feedback to the SAM persistence. (ii) Irreversible potential vorticity mixing, through barotropic processes in the upper troposphere, plays an important role in driving and sustaining the SAM variability. Particularly, following the poleward shift of the eddy-driven jet, the reduction/enhancement in effective diffusivity on the jets poleward/equatorward flank can be understood by a stronger/weaker zonal jet acting as a robust/leaky mixing barrier. (iii) Baroclinic eddy generation and vertical wave propagation mainly act to sustain the SAM variability.

The treatment of sublingual gland tumours

This study assessed the clinical and histological features and therapeutic efficacy of 25 cases of sublingual gland tumours from 1998 to 2008. There were 17 female patients and 8 male, the ratio of females to males was 2.1:1. The mean age was 48.6 years. 4 cases were benign tumours (16%). 21 cases were malignant sublingual gland tumours (84%) and of these, 18 were adenoid cystic carcinoma (86%). Adenoid cystic carcinoma was mainly of the histological type, and the other histological classifications included mucoepidermoid carcinoma, pleomorphic adenoma, myoepithelioma, oncocytoma and polymorphous low-grade adenocarcinoma. Sublingual gland tumours are rare and most are malignant. For malignant sublingual gland tumours, early diagnosis and aggressive surgical treatment, especially for tumours with nerve involvement, is the key to improving prognosis. Free radial forearm flap or pectoralis major myocutaneous flap are appropriate methods for mouth floor reconstruction. For benign sublingual gland tumours, the resection of tumour and sublingual gland is the preferred treatment.

A case study of urbanization impact on summer precipitation in the Greater Beijing Metropolitan Area: Urban heat island versus aerosol effects

Convection-resolving ensemble simulations using the WRF-Chem model coupled with a single-layer Urban Canopy Model are conducted to investigate the individual and combined impacts of land use and anthropogenic pollutant emissions from urbanization on a heavy rainfall event in the Greater Beijing Metropolitan Area (GBMA) in China. The simulation with the urbanization effect included generally captures the spatial pattern and temporal variation of the rainfall event. An improvement of precipitation is found in the experiment including aerosol effect on both clouds and radiation. The expanded urban land cover and increased aerosols have an opposite effect on precipitation processes, with the latter playing a more dominant role, leading to suppressed convection and rainfall over the upstream (northwest) area, and enhanced convection and more precipitation in the downstream (southeast) region of the GBMA. In addition, the influence of aerosol indirect effect is found to overwhelm that of direct effect on precipitation in this rainfall event. Increased aerosols lead to more cloud droplets with smaller size, which favor evaporative cooling and reduce updrafts and suppress convection over the upstream (northwest) region in the early stage of the rainfall event. As the rainfall system propagates southeastward, more latent heat is released due to the freezing of larger number of smaller cloud drops that are lofted above the freezing level, which is responsible for the increased updraft strength and convective invigoration over the downstream (southeast) area.

Delineating the Barotropic and Baroclinic Mechanisms in the Midlatitude Eddy-Driven Jet Response to Lower-Tropospheric Thermal Forcing

AbstractObservations and climate models have shown that the midlatitude eddy-driven jet can exhibit an evident latitudinal shift in response to lower-tropospheric thermal forcing (e.g., the tropical SST warming during El Nino or extratropical SST anomalies associated with the atmosphere–ocean–sea ice coupling). In addition to the direct thermal wind response, the eddy feedbacks—including baroclinic mechanisms, such as lower-level baroclinic eddy generation, and barotropic mechanisms, such as upper-level wave propagation and breaking—can all contribute to the atmospheric circulation response to lower-level thermal forcing, but their individual roles have not been well explained. In this study, using a nonlinear β-plane multilevel quasigeostrophic channel model, the mechanisms through which the lower-level thermal forcing induces the jet shift are investigated. By diagnosing the finite-amplitude wave activity budget, the baroclinic and barotropic eddy feedbacks to the lower-level thermal forcing are delinea...

Numerical experiments of the storm track sensitivity to oceanic frontal strength within the Kuroshio/Oyashio Extensions

The sensitivity of the North Pacific storm track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the storm track activity and meridional SST gradient; i.e., the storm track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the storm track intensification due to increased SST gradient, velocity-temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic-scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front-induced enhancement of storm track activity. In order to estimate the impact of the oceanic front on the maintenance of the near-surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near-surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross-frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

Long-term observation of air pollution-weather/climate interactions at the SORPES station: a review and outlook

This work presents an overall introduction to the Station for Observing Regional Processes of the Earth System–SORPES in Nanjing, East China, and gives an overview about main scientific findings in studies of air pollution-weather/climate interactions obtained since 2011. The main results summarized in this paper include overall characteristics of trace gases and aerosols, chemical transformation mechanisms for secondary pollutants like O3, HONO and secondary inorganic aerosols, and the air pollution–weather/climate interactions and feedbacks in mixed air pollution plumes from sources like fossil fuel combustion, biomass burning and dust storms. The future outlook of the development plan on instrumentation, networking and data-sharing for the SORPES station is also discussed.

An evaluation of dynamical downscaling of Central Plains summer precipitation using a WRF‐based regional climate model at a convection‐permitting 4 km resolution

A significant challenge with dynamical downscaling of climate simulations is the ability to accurately represent convection and precipitation. The use of convection-permitting resolutions avoids cumulus parameterization, which is known to be a large source of uncertainty. A regional climate model (RCM) based on the Weather Research and Forecasting model is configured with a 4 km grid spacing and applied to the U.S. Great Plains, a region characterized by many forms of weather and climate extremes. The 4 km RCM is evaluated by running it in a hindcast mode over the central U.S. region for a 10 year period, forced at the boundary by the 32 km North America Regional Reanalysis. The model is also run at a 25 km grid spacing, but with cumulus parameterization turned on for comparison. The 4 km run more successfully reproduces certain observed features of the Great Plains May-through-August precipitation. In particular, the magnitude of extreme precipitation and the diurnal cycle of precipitation over the Great Plains are better simulated. The 4 km run more realistically simulates the low-level jet and related atmospheric circulations that transport and redistribute moisture from Gulf of Mexico. The convection-permitting RCM may therefore produce better dynamical downscaling of future climate when nested within global model climate projections, especially for extreme precipitation magnitudes. The 4 km and 25 km simulations do share similar precipitation biases, including low biases over the central Great Plains and high biases over the Rockies. These biases appear linked to circulation biases in the simulations, but determining of the exact causes will require extensive, separate studies.

Anthropogenic aerosol effects on East Asian winter monsoon: The role of black carbon‐induced Tibetan Plateau warming

This study investigates anthropogenic aerosol effects on East Asian winter monsoon (EAWM) with Community Atmospheric Model version 5. In winter, the anthropogenic aerosol optical depth is the largest over southern East Asia and adjacent oceans. The associated EAWM change, however, is the most significant in northern East Asia, which is characterized by a significant surface cooling in northern East Asia and an acceleration of the jet stream around 40°N, indicating an intensification of the EAWM northern mode. Such an intensification is attributed to anthropogenic black carbon (BC) induced Tibetan Plateau (TP) warming. The BC is mostly transported from northern South Asia by wintertime westerly and southwesterly, and then deposited on snow, giving rise to a reduction of surface albedo and an increase of surface air temperature via the snow-albedo feedback. The TP warming increases meridional temperature gradient and lower-tropospheric baroclinicity over northern East Asia, leading to the jet stream acceleration around 40°N and the westward shift of East Asian major trough via the transient eddy-mean flow feedback. Such upper-tropospheric pattern favors more cold air outbreak, leading to a large surface cooling in northern East Asia. In southern East Asia, the effect of non-absorbing aerosols is dominant. The solar flux at surface is significantly reduced directly by scattering of non-absorbing aerosols, and indirectly by intensification of short wave cloud forcing. Accordingly, the surface air temperature in southern East Asia is reduced. The precipitation is also significantly reduced in South China and Indo-China Peninsula, where the aerosol indirect effect is the largest.