Distinguishing the provenance of fine-grained eolian dust over the Chinese Loess Plateau from a modelling perspective

The provenance of fine-grained eolian dust over East Asia is distinguished using a regional climate model. Five major source regions within China and Mongolia are considered: sandy lands in the northeastern China, deserts in the northern China, the Gobi deserts, the Taklimakan deserts in western China and deserts on the Tibet an Plateau. The contribution of each dust source is evaluated for the downwind eolian sediments in the Chinese Loess Plateau (CLP) and Japan Sea (JS). The results show that the adjoining northern and Gobi deserts dominate the dust depositions over CLP and over eastern China, although Taklimakan deserts are actually the largest emission source. On the marine deposits in JS, Taklimakan deserts exert a more significant role since the particles from Taklimakan tend to be raised into upper atmosphere and delivered for a longer distance. The agent for dust delivery also differs among different sources. Dust from northern deserts is dominantly carried by the local northwesterly winds in spring associated with the East Asian winter monsoon system and restricted in the low-level atmosphere, while the westerly winds in the mid- to high-level troposphere become of great importance and more likely to be responsible for the transport of Taklimakan dust.</p

Analytical studies of the cloud droplet spectral dispersion influence on the first indirect aerosol effect

Atmospheric aerosols (acting as cloud condensation nuclei) can enhance the cloud droplet number concentration and reduce the cloud droplet size, and in turn affect the cloud optical depth, as well as the cloud albedo, and thereby exert a radiative influence on climate (the first indirect aerosol effect). In this paper, based on various relationships between cloud droplet spectral dispersion (E &gt;) and cloud droplet number concentration (N (c)), we analytically derive the corresponding expressions of the cloud radiative forcing induced by changes in the cloud droplet number concentration. Further quantitative evaluation indicates that the cloud radiative forcing induced by aerosols for the different E &gt;-N (c) relationships varies from -29.1% to 25.2%, compared to the case without considering spectral dispersion (E &gt; = 0). Our results suggest that an accurate description of E &gt; - N (c) relationships helps to reduce the uncertainty of the first indirect aerosol effect and advances our scientific understanding of aerosol-cloud-radiation interactions.</p

Sensitivity analysis of modelled responses of vegetation dynamicson the Tibetan Plateau to doubled CO2 and associated climatechange

Increases in the atmospheric CO2 concentration affect both the global climate and plant metabolism, particularly for high-altitude ecosystems. Because of the limitations of field experiments, it is difficult to evaluate the responses of vegetation to CO2 increases and separate the effects of CO2 and associated climate change using direct observations at a regional scale. Here, we used the Community Earth System Model (CESM, version 1.0.4) to examine these effects. Initiated from bare ground, we simulated the vegetation composition and productivity under two CO2 concentrations (367 and 734 ppm) and associated climate conditions to separate the comparative contributions of doubled CO2 and CO2-induced climate change to the vegetation dynamics on the Tibetan Plateau (TP). The results revealed whether the individual effect of doubled CO2 and its induced climate change or their combined effects caused a decrease in the foliage projective cover (FPC) of C3 arctic grass on the TP. Both doubled CO2 and climate change had a positive effect on the FPC of the temperate and tropical tree plant functional types (PFTs) on the TP, but doubled CO2 led to FPC decreases of C4 grass and broadleaf deciduous shrubs, whereas the climate change resulted in FPC decrease in C3 non-arctic grass and boreal needleleaf evergreen trees. Although the combination of the doubled CO2 and associated climate change increased the area-averaged leaf area index (LAI), the effect of doubled CO2 on the LAI increase (95 %) was larger than the effect of CO2-induced climate change (5 %). Similarly, the simulated gross primary productivity (GPP) and net primary productivity (NPP) were primarily sensitive to the doubled CO2, compared with the CO2-induced climate change, which alone increased the regional GPP and NPP by 251.22 and 87.79 g C m(-2) year(-1), respectively. Regionally, the vegetation response was most noticeable in the south-eastern TP. Although both doubled CO2 and associated climate change had a generally positive effect on LAI, GPP and NPP, it should be noted that the climate change had a somewhat negative effect on the vegetation structure and productivity of the TP.</p

Sensitivity analysis of modelled responses of vegetation dynamicson the Tibetan Plateau to doubled CO2 and associated climatechange

Increases in the atmospheric CO2 concentration affect both the global climate and plant metabolism, particularly for high-altitude ecosystems. Because of the limitations of field experiments, it is difficult to evaluate the responses of vegetation to CO2 increases and separate the effects of CO2 and associated climate change using direct observations at a regional scale. Here, we used the Community Earth System Model (CESM, version 1.0.4) to examine these effects. Initiated from bare ground, we simulated the vegetation composition and productivity under two CO2 concentrations (367 and 734 ppm) and associated climate conditions to separate the comparative contributions of doubled CO2 and CO2-induced climate change to the vegetation dynamics on the Tibetan Plateau (TP). The results revealed whether the individual effect of doubled CO2 and its induced climate change or their combined effects caused a decrease in the foliage projective cover (FPC) of C3 arctic grass on the TP. Both doubled CO2 and climate change had a positive effect on the FPC of the temperate and tropical tree plant functional types (PFTs) on the TP, but doubled CO2 led to FPC decreases of C4 grass and broadleaf deciduous shrubs, whereas the climate change resulted in FPC decrease in C3 non-arctic grass and boreal needleleaf evergreen trees. Although the combination of the doubled CO2 and associated climate change increased the area-averaged leaf area index (LAI), the effect of doubled CO2 on the LAI increase (95 %) was larger than the effect of CO2-induced climate change (5 %). Similarly, the simulated gross primary productivity (GPP) and net primary productivity (NPP) were primarily sensitive to the doubled CO2, compared with the CO2-induced climate change, which alone increased the regional GPP and NPP by 251.22 and 87.79 g C m&minus;2 year&minus;1, respectively. Regionally, the vegetation response was most noticeable in the south-eastern TP. Although both doubled CO2 and associated climate change had a generally positive effect on LAI, GPP and NPP, it should be noted that the climate change had a somewhat negative effect on the vegetation structure and productivity of the TP</p

Variation in rainy season precipitation and associated water vapor transport over the Chinese Loess Plateau during 1961−2012

The variation in rainy season precipitation and its associated atmospheric water vapor transport during 1961 to 2012 over the Chinese Loess Plateau (LP) are examined by using precipitation data from 53 in situ stations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The results show that the rainy season (June to September, hereafter JJAS) precipitation over the LP exhibits strong interannual variability, and the whole LP becomes wetter or drier synchronously, while no significant long-term linear trend is observed. The climatological water vapor transport from the southern boundary, which originates from both the Indian and the East Asian monsoons, dominates the JJAS precipitation over the LP, although the moisture from the western boundary carried by the Westerlies plays a non-negligible role. Quantitatively, the water vapor from the western boundary is similar to 73% of that from the southern boundary. The interannual variability of JJAS precipitation mainly results from the change in water vapor transport from the southern boundary. A positive rainfall anomaly of 100 mm is associated with a positive water vapor transport of similar to 166 (23) kg m(-1) s(-1) from the southern (western) boundary. Both the composite analysis on moisture transport for the wet years and the regression fields of water vapor transport on the precipitation index of the LP feature a cyclone anomaly centered over the northeastern Indian Ocean and an anticyclone anomaly centered over the Pohai region, which strengthen the moisture transport along the southern boundary of the LP, leading to excessive rainfall.</p

Anti-phased response of northern and southern East Asian summer precipitation to ENSO modulation of orbital forcing

The timing of orbital-scale Asian monsoon changes, as a direct response of northern summer insolation or remarkably lagged by southern insolation, is still unclear. In particular, various monsoon records obtained in the East Asian monsoon region show distinct phase relationships, indicating additional forcing/feedback mechanisms. Here, monsoon proxies covering the past several precession cycles, either from cave stalagmites or from land/ocean deposits, are first reviewed to present the nearly inverse precipitation responses to the precession forcing between southern and northern East Asia. Modern meteorological observations show that, different modes of tropical Pacific sea surface temperature (SST) could lead to an out-of-phase interannual change in the East Asian summer precipitation. This ENSO influence is also found in the precession scale monsoon variability from the long-term transient modeling, which can explain the phase differences among monsoon proxies. At precession maxima, the East Asian summer monsoon strengthens, causing more precipitation in the north and less precipitation in the south. The SST-precipitation teleconnection is closely associated with a high pressure anomaly due to surface cooling over northwestern Pacific. Therefore, the timing of Asian paleo-monsoon might be significantly influenced by the &quot;internal&quot; ocean feedbacks and one can not expect all the monsoon proxies are consistently responded to the &quot;external&quot; insolation forcing.</p

Numerical study of aerosol effect on three types of cloudsand precipitation in Beijing area

Three types of rainfall (storm, moderate and slight rainfall) in the Beijing area were simulated by the Weather Research and Forecast (WRF3.2) model coupled with Milbrandt-two-moment cloud microphysics scheme, to explore the effect of aerosols on clouds and precipitation under continental and maritime aerosol scenarios. Results indicate that an increase of aerosols has various effects on clouds and precipitation. (1) The amount of surface precipitation is obviously affected. With an increase of aerosol concentration, the 48-hr total precipitation of storm and moderate rainfall decreased by 23% and 16.6%, respectively, and the 24-hr total precipitation of slight rainfall decreased by 14.0%. (2) The distribution of surface precipitation is also clearly affected. The average precipitation for a rain storm increases in most parts of western Beijing and decreases by more than 20 mm in most parts of eastern Beijing with increasing aerosol concentration. The average precipitation of moderate rainfall decreases by 0.1–5 mm in most parts of the Beijing area. The effect of increased aerosol concentration is weak for slight rainfall distribution in the study area. (3) With an increase of aerosol concentration, a narrower width and lower precipitation peak value are found in the storm rainfall, and its duration is prolonged for the high aerosol concentration. An earlier precipitation termination of moderate rainfall is found with increasing aerosol concentration. (4) The upper-air hydrometeors vary with aerosol concentration. For storm and moderate rainfall, significantly higher cloud water concentration and lower rain water were found under the continental aerosol scenario

Effects of Dimensionality on Simulated Large-Scale Convective Organization and Coupled Waves

Tropical multi-scale convective organization of the super-cluster kind and convectively coupled gravity waves are investigated by both two and three-dimensional cloud-system-resolving simulations. The experimental setup includes a constant-temperature ocean surface, constant and horizontally-uniform radiative cooling in the troposphere, and a uniform easterly background wind. The objective of this study is to quantify the impacts of dimensionality on the simulated large-scale convective patterns and associated gravity waves. Eastward propagating large-scale coherent precipitating convection occurs regardless of the spatial dimension. The convective organization has a horizontal wavenumber-one structure in the computational domain and travels at about 13-17 m s(-1) relative to the ground, equivalent to 19-23 m s(-1) relative to the environmental flow. However, the convectively-induced wave signature is much weaker in three dimensions than in two dimensions, as well as a faster translation and a smaller tilt of the vertical. Moreover, a two-dimensional framework generates additional organizational modes compared to the three-dimensional results, including a fast westward-moving system with a mean-flow-relative speed comparable to the eastward-moving wavenumber-1 counterpart and the quasi-stationary (relative to the background flow) higher wavenumber precipitating system. This does not necessarily imply that these additional modes are artifacts of two dimensionality.</p

A modeling study of the effects of aerosols on clouds and precipitation over East Asia

The National Center for Atmospheric Research Community Atmosphere Model (version 3.5) coupled with the Morrison-Gettelman two-moment cloud microphysics scheme is employed to simulate the aerosol effects on clouds and precipitation in two numerical experiments, one representing present-day conditions (year 2000) and the other the pre-industrial conditions (year 1750) over East Asia by considering both direct and indirect aerosol effects. To isolate the aerosol effects, we used the same set of boundary conditions and only altered the aerosol emissions in both experiments. The simulated results show that the cloud microphysical properties are markedly affected by the increase in aerosols, especially for the column cloud droplet number concentration (DNC), liquid water path (LWP), and the cloud droplet effective radius (DER). With increased aerosols, DNC and LWP have been increased by 137% and 28%, respectively, while DER is reduced by 20%. Precipitation rates in East Asia and East China are reduced by 5.8% and 13%, respectively, by both the aerosol&#39;s second indirect effect and the radiative forcing that enhanced atmospheric stability associated with the aerosol direct and first indirect effects. The significant reduction in summer precipitation in East Asia is also consistent with the weakening of the East Asian summer monsoon, resulting from the decreasing thermodynamic contrast between the Asian landmass and the surrounding oceans induced by the aerosol&#39;s radiative effects. The increase in aerosols reduces the surface net shortwave radiative flux over the East Asia landmass, which leads to the reduction of the land surface temperature. With minimal changes in the sea surface temperature, hence, the weakening of the East Asian summer monsoon further enhances the reduction of summer precipitation over East Asia.</p

Transient simulation of orbital-scale precipitation variation in monsoonal East Asia and arid central Asia during the last 150 ka

A long-term transient simulation is conducted using the Community Climate System Model version 3 and the orbital acceleration technique to analyze the impact of insolation change caused by the Earth&#39;s orbital forcing on precipitation in the monsoonal East Asia (EA) and arid central Asia (CA) over the past 150 ka. Our results show that annual precipitation in both EA and CA has strong signals of the 20 ka precessional cycles and varies in phase with the Northern Hemisphere (NH) summer insolation. Similar characteristics can also be observed from previously published oxygen isotope records of stalagmites near EA and CA. Composite analyses based on seven precessional cycles suggest that the increase (decrease) in the NH summer (winter) insolation enhances EA (CA) summer (winter) precipitation by modulation of the Asian monsoon (westerly) circulation in summer (winter). When the precession-induced NH summer insolation increases, the Asian summer monsoon circulation is enhanced and EA precipitation increases significantly. Meanwhile, the increase in the summer insolation at the precessional scale is accompanied by a decrease in the winter insolation, which causes dramatic cooling of the troposphere in the lower latitudes. Consequently, the CA winter precipitation increases due to the changes in the temperature gradient and the westerly circulation. Therefore, the responses of the Asian monsoon and westerly circulation to summer and winter insolation variations induced by the precessional cycles determine precipitation in the respective rainy seasons and are the primary cause leading to the synchronous variation patterns of annual precipitation in EA and CA at the orbital scale.</p