Vertically resolved separation of dust and other aerosol types by a new lidar depolarization method.

This paper developed a new retrieval framework of external mixing of the dust and non-dust aerosol to predict the lidar ratio of the external mixing aerosols and to separate the contributions of non-spherical aerosols by using different depolarization ratios among dust, sea salt, smoke, and polluted aerosols. The detailed sensitivity tests and case study with the new method showed that reliable dust information could be retrieved even without prior information about the non-dust aerosol types. This new method is suitable for global dust retrievals with satellite observations, which is critical for better understanding global dust transportation and for model improvements.

Simulation study on light propagation in an isotropic turbulence field of the mixed layer.

Water tank experiments and numerical simulations are employed to investigate the characteristics of light propagation in the convective boundary layer (CBL). The CBL, namely the mixed layer (ML), was simulated in the water tank. A laser beam was set to horizontally go through the water tank, and the image of two-dimensional (2D) light intensity fluctuation formed on the receiving plate perpendicular to the light path was recorded by CCD. The spatial spectra of both horizontal and vertical light intensity fluctuations were analyzed, and the vertical distribution profile of the scintillation index (SI) in the ML was obtained. The experimental results indicate that 2D light intensity fluctuation was isotropically distributed in the cross section perpendicular to the light beam in the ML. Based on the measured temperature fluctuations along the light path at different heights, together with the relationship between temperature and refractive index, the refractive index fluctuation spectra and the corresponding turbulence parameters were derived. The obtained parameters were applied in a numerical model to simulate light propagation in the isotropic turbulence field. The calculated results successfully reproduce the characteristics of light intensity fluctuation observed in the experiments.

Simulation study on light propagation in an anisotropic turbulence field of entrainment zone.

The convective atmospheric boundary layer was modeled in the water tank. In the entrainment zone (EZ), which is at the top of the convective boundary layer (CBL), the turbulence is anisotropic. An anisotropy coefficient was introduced in the presented anisotropic turbulence model. A laser beam was set to horizontally go through the EZ modeled in the water tank. The image of two-dimensional (2D) light intensity fluctuation was formed on the receiving plate perpendicular to the light path and was recorded by the CCD. The spatial spectra of both horizontal and vertical light intensity fluctuations were analyzed. Results indicate that the light intensity fluctuation in the EZ exhibits strong anisotropic characteristics. Numerical simulation shows there is a linear relationship between the anisotropy coefficients and the ratio of horizontal to vertical fluctuation spectra peak wavelength. By using the measured temperature fluctuations along the light path at different heights, together with the relationship between temperature and refractive index, the one-dimensional (1D) refractive index fluctuation spectra were derived. The anisotropy coefficients were estimated from the 2D light intensity fluctuation spectra modeled by the water tank. Then the turbulence parameters can be obtained using the 1D refractive index fluctuation spectra and the corresponding anisotropy coefficients. These parameters were used in numerical simulation of light propagation. The results of numerical simulations show this approach can reproduce the anisotropic features of light intensity fluctuations in the EZ modeled by the water tank experiment.

Meteorological conditions conducive to PM2.5 pollution in 2016/2017 in the Western Yangtze River Delta, China.

Cities in Anhui province in the western Yangtze River Delta (YRD), China experienced more PM2.5 pollution days in the winter of 2016/2017 (Dec 2016 to Feb 2017) than in the previous two winters under conditions of emission deductions. By employing back-trajectory-clustering analysis together with daily air quality index (AQI) data from 2015 to 2017, routine and reanalysis meteorological data, and some climate indices, we investigated the transport paths, large-scale vertical motion and related climate background conducive to PM2.5 pollution in Anhui province. We obtained 5 air-mass paths affecting Anhui province in winter; among them, the slow-moving air-masses from the northeast and northwest often led to PM2.5 pollution. Thus, they belong to adverse transport paths, which accounted for approximately 52% in northern Anhui and 62% in central Anhui. Compared with winter 2015/2016, the proportions of adverse transport paths in winter 2016/2017 increased 13% in Hefei (central site), 3% in Suzhou (northern site), and 9% in Chizhou (southern site); correspondingly, east winds increased, and north winds weakened in the boundary layer, which favoured the accumulation of pollutants in Anhui. The processes of pollution and cleaning in Anhui were also closely related to vertical motion of the middle troposphere (500 hPa), and the sinking (ascending) corresponding to the aggravation (mitigation) of pollution. Compared with the winter of 2015/2016, the percentage of downward vertical velocity at 500 hPa exceeding 0.2 Pa/s increased evidently in the winter of 2016/2017. Thus, the vertical velocity at 500 hPa can be used as an important factor for air quality prediction in winter. The interannual changes in transport conditions are related to changes in the Asia zonal and meridional circulations and may further be ascribed to the thermal and dynamic conditions in the Tropical Ocean.

Recent Trends of Summer Convective and Stratiform Precipitation in Mid-Eastern China.

Many studies have reported on the trends of precipitation in Mid-Eastern China (EC). However, the trends of convective and stratiform precipitation are still unknown. Here, we examine the trends of summer convective and stratiform precipitation in EC from 2002 to 2012 on the basis of the TRMM observations. Results revealed that the rain frequency (RF) for both convective and stratiform precipitation increased in majority regions of Southern EC (SEC), but decreased in Northwest part of Northern EC (NEC). The decreasing rate of RF for stratiform precipitation in NEC is twice as much as that for convective precipitation, while the increase of convective precipitation in SEC is more evident than stratiform precipitation. The rain rate (RR) exhibited a decreasing trend in most portions of EC for both convective and stratiform precipitation. In SEC, neither PW nor WVT has good ability in explaining the precipitation variability. However, in NEC, PW is closely correlated to convective RF and WVT is more closely related to stratiform RF.