Rui Mao

Observed Holiday Aerosol Reduction and Temperature Cooling over East Asia

The air pollution in Chinese Spring Festival (CSF) period over eastern China was investigated using the long-term observations from 2001 to 2012 over 323 stations. The dominant feature of the pollutants around the CSF holidays is the significant reduction of concentration. During the 10day period around the CSF (but excluding the Lunar New Years Day, LNYD), PM10 experiences a reduction of −9.24%. In association with the aerosol reduction, temperature significantly drops over eastern China. From the third day before the LNYD to the second day after, the daily mean temperature anomaly is −0.81°C, and for no-rain days the anomaly is −0.85°C. The simultaneous anomalies of the daily maximum and minimum temperatures are −0.79°C and −0.82°C, respectively. From the third day to seventh day after the LNYD, the significant negative temperature anomalies move out of China, extending to a broad area from the South China Sea to the western North Pacific. Between the 8th and the 12th days, the significant temperature anomalies can still be found over 140°E–160°E and 15°N–25°N. The reduced downward longwave flux might play an important role in holiday cooling. The possible atmospheric feedback is discernable. The thermal and circulation configuration accompanying the cooling favors baroclinic interaction between upper and lower troposphere for the midlatitude cyclone. The anomalous cyclone becomes mature during the third to the seventh day after the LNYD and disappears 12 days later. The anomalous northern winds in association with the cyclone decrease the temperature and also help disperse the holiday aerosols over eastern China.

WITHDRAWN: Cause and predictability for the severe haze pollutions in downtown Beijing during November–December 2015

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The Influence of Vegetation Variation on Northeast Asian Dust Activity

In this study, we investigate the influence of vegetation variations on dust activity (dust load, dust transport in the troposphere, and dust weather frequency) over Northeast Asia during springtime. By using the Integrated Wind Erosion Modeling System, two model experiments are run over four-month periods, from February 1 to May 31, for each year from 1982 to 2006; one experiment uses the observed atmospheric conditions and vegetation (OBS), and the other uses the specified atmospheric conditions in 2006 and the observed vegetation (CTRL). Comparison of the two model experiments reveals that there are sensitive regions in southeastern Mongolia and central northern China, in which vegetation has a large potential to influence dust activity due to both the high dust emission rate and large variations in vegetation coverage. Over these sensitive regions, vegetation effectively lessens dust loads on interannual and interdecadal timescales; dust load is decreased by 2864 μg m−2 for an increment of 0.1 in the normalized difference vegetation index (NDVI). Vegetation increase in the sensitive areas also reduces two major branches of dust transports in the low troposphere; one stretches from eastern Mongolia to regions northeastward, and the other flows across the south of northeastern China to Korea. In addition to dust loads and transports, vegetation increase in the sensitive areas evidently decreases dust storm frequency and blowing dust frequency, but it exerts a weak influence on the floating dust frequency. In the sensitive regions, as NDVI increases by 0.1, dust storms, blowing dust, and floating dust decrease by 4.0 days/spring, 1.5 days/spring, and 0.2 days/spring, respectively. In summary, vegetation variations in southeastern Mongolia and central northern China have considerable impact on northeast Asian dust during springtime.

Numerical analysis for contribution of the Tibetan Plateau to dust aerosols in the atmosphere over the East Asia

Although the Tibetan Plateau is widely thought as a potential dust source to the atmosphere over East Asia, little is known about the temporal changes of Tibetan dust activities and Tibetan dust source strength. In this study, we address these two issues by analyzing dust storm frequencies and aerosol index through remote sensing data and by means of numerical simulation. The findings indicate that monthly dust profiles over the Tibetan Plateau vary significantly with time. Near the surface, dust concentration increases from October, reaches its maximum in February–March, and then decreases. In the middle to upper troposphere, dust concentration increases from January, reaches its maximum in May–June, and decreases thereafter. Although Tibetan dust sources are important contributors to dust in the atmosphere over the Tibetan Plateau, their contribution to dust in the troposphere over eastern China is weaker. The contribution of Tibetan dust sources to dust in the atmosphere over the Tibetan Plateau decreases sharply with height, from 69% at the surface, 40% in the lower troposphere, and 5% in the middle troposphere. Furthermore, the contribution shows seasonal changes, with dust sources at the surface at approximately 80% between November and May and 45% between June and September; in the middle and upper troposphere, dust sources are between 21% from February to March and less than 5% in the other months. Overall, dust aerosols originating from the Tibetan Plateau contribute to less than 10% of dust in East Asia.

Is the Antarctic oscillation trend during the recent decades unusual

Abstract The Antarctic oscillation (AAO) has been characterized by a persistently positive trend in summer (December–January–February, DJF) during the last 50 years. Thus, the question has arisen of whether the trend is unusual. By investigating five reconstructed historical AAO time series for the past 500 years, recurrences of similar and even stronger trends have been found, indicating that the recent DJF AAO trend is not unprecedented in a historical perspective. To estimate the possible roles played by greenhouse gases or/and ozone, an analysis for DJF AAO trends during the 1969–98 period was conducted using three multiple model ensembles derived from the projects of ‘The twentieth-century climate in coupled models’ (20C3M) and ‘Pre-industrial control experiment models’ (PICTL) of the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR4). The results show that the ozone depletion over Antarctica and global warming may play significant roles in the strengthening trend. Combining the simulations and reconstructions we emphasize that the AAO trend related to global warming may get much stronger when enhanced by low-frequency natural variability.

Numerical simulations of the effects of regional topography on haze pollution in Beijing

In addition to weather conditions and pollutant emissions, the degree to which topography influences the occurrence and development of haze pollution in downtown Beijing and the mechanisms that may be involved remain open questions. A series of atmospheric chemistry simulations are executed by using the online-coupled Weather Research and Forecasting with Chemistry (WRF-Chem) model for November-December 2015 with different hypothetical topographic height scenarios. The simulation results show that topography exerts an important influence on haze pollution in downtown Beijing, particularly the typical development of haze pollution. A possible mechanism that underlies the response of haze pollution to topography is that the mountains that surround Beijing tend to produce anomalous southerly winds, high relative humidity, low boundary layer heights, and sinking motion over most of Beijing. These conditions favor the formation and development of haze pollution in downtown Beijing. Furthermore, the reduction percentage in PM2.5 concentrations due to reduced terrain height in the southerly wind (S) mode is almost three times larger than that in the northerly wind (N) mode. In the context of the regional topography, the simple S and N modes represent useful indicators for haze prediction in Beijing to some extent, especially over medium to long time scales.

Anomalous holiday precipitation over southern China

The Chinese Spring Festival (CSF) is the most important festival in China. Officially, this holiday lasts approximately one week. Based on the long-term station observations from 1979 to 2012, this manuscript reports that during 10 the holidays, the precipitation over southern China (108°E-123°E and 21°N-33°N, 155 stations) has been significantly reduced. The precipitation frequency anomalies from the fourth day to the sixth day after Lunar New Year’s Day (i.e., days [+4, +6]) were found to decrease by -7.4%. At the same time, the daily precipitation amounts experienced a reduction of 0.62 mm d during days +2 to +5. The holiday precipitation anomalies are strongly linked to the relative humidity (RH) and cloud cover. The station observations of the RH showed an evident decrease from day +2 to +7, and a minimum 15 appeared on days [+4, +6], with a mean of -3.9%. The RH vertical profile displays a significant drying below approximately 800 hPa. Between 800 hPa and 1000 hPa, the mean RH is -3.9%. The observed station daytime low cloud cover (LCC) evidently decreased by -6.1% during days [+4, +6]. Meanwhile, the ERA-Interim daily LCC also shows a comparable reduction of -5.0%. The anomalous relative humidity is mainly caused by the decreased water vapor in the lower-middle troposphere. Evident negative specific humidity anomalies persist from day -3 to day +7 in the station 20 observations. The average specific humidity anomaly for days [+4, +6] is -0.73 g kg. When the precipitation days are excluded, the anomaly remains significant, being -0.46 g kg. A significant deficit of water vapor is observed in the lower troposphere below 700 hPa. Between 800 hPa and 1000 hPa, the mean specific humidity dropped by -0.70 g kg. This drier lower-middle troposphere is due to anomalous northerly winds, which are tightly related to the cyclonic circulation anomaly over northwestern Pacific. Authors have analyzed the concurrent holiday aerosol anomaly and their time-lag correlations 25 with the atmospheric circulation. And it is found that the East Asian aerosol concentrations during the CSF decrease evidently, the largest reduction occurring on days [-3, -1]. At the same time, a concurrent cooling is observed in the lower troposphere, which enhances the cold advection over East Asia. The strength of the anomalous cyclone on days [+4,+6] is statistically correlated with PM10 concentration over East China. Their strongest correlations occur when the PM10 leads by 9 to -6 days, with correlation coefficients varying from +0.52 to +0.57. The possible mechanism needs further clarification 30 by elaborate observation analysis and modeling.

Trends in the Frequency of High Relative Humidity over China: 1979-2012*

High relative humidity (HRH) is defined as a relative humidity of at least 80%, which is often associated with the occurrence of cloud layers. Thus, the frequency of HRH and its changes in the troposphere may be related to the occurrence frequency of cloud layers and their changes. In this study, trends in the frequency of HRH (defined as days with relative humidity

Simulation and causes of eastern Antarctica surface cooling related to ozone depletion during austral summer in FGOALS-s2

80%) over China from the surface to the midtroposphere (

Extreme drought event of 2009/2010 over southwestern China

400hPa) from 1979 to 2012 were analyzed using a homogenized humidity dataset for spring (March‐May), summer (June‐August), autumn (September‐November), and winter (December‐February). The results for the ground level indicate decreasing trends at most stations in southeastern China in spring and in northern Chinainsummer.Inthelowertroposphere(850and700hPa),moststationsoverChinaexhibitpositivetrends in summer, autumn, and winter. For the midtroposphere (500‐400hPa), increasing trends dominate over China in spring, summer, and autumn. Finally, six reanalysis datasets, the NCEP‐NCAR, NCEP‐DOE, CFSR, ERA-Interim, MERRA, and JRA-55 datasets, were compared with the observed increasing trends in HRH frequency in the low-to-middle troposphere. Similar increasing trends in HRH frequency in the reanalysis datasets and the homogenized humidity data are observed in certain seasons and for certain regions. These results are consistent with the increasing low-to-middle cloud amounts in recent decades.