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# Simulating Aerosol Optical Depth and Direct Radiative Effects over the Tibetan Plateau with a High-Resolution CAS FGOALS-f3 Model

• Current global climate models cannot resolve the complex topography over the Tibetan Plateau (TP) due to their coarse resolution. This study investigates the impacts of horizontal resolution on simulating aerosol and its direct radiative effect (DRE) over the TP by applying two horizontal resolutions of about 100 km and 25 km to the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere Land System (CAS FGOALS-f3) over a 10-year period. Compared to the AErosol RObotic NETwork observations, a high-resolution model (HRM) can better reproduce the spatial distribution and seasonal cycles of aerosol optical depth (AOD) compared to a low-resolution model (LRM). The HRM bias and RMSE of AOD decreased by 0.08 and 0.12, and the correlation coefficient increased by 0.22 compared to the LRM. An LRM is not sufficient to reproduce the aerosol variations associated with fine-scale topographic forcing, such as in the eastern marginal region of the TP. The difference between hydrophilic aerosols in an HRM and LRM is caused by the divergence of the simulated relative humidity (RH). More reasonable distributions and variations of RH are conducive to simulating hydrophilic aerosols. An increase of the 10-m wind speed in winter by an HRM leads to increased dust emissions. The simulated aerosol DREs at the top of the atmosphere (TOA) and at the surface by the HRM are –0.76 W m–2 and –8.72 W m–2 over the TP, respectively. Both resolution models can capture the key feature that dust TOA DRE transitions from positive in spring to negative in the other seasons.
摘要: 目前全球气溶胶气候耦合模式分辨率普遍较低，无法解决青藏高原地区复杂的地形问题。本研究利用中国科学院大气物理研究所自主研发的全球气溶胶气候耦合模式CAS FGOALS-f3中水平分辨率为100 km和25 km的两个版本，研究了水平分辨率对模拟青藏高原上空气溶胶及其直接辐射效应的影响。与气溶胶地基观测相比，水平分辨率为25 km的高分辨率模式比低分辨率模式能更好地再现气溶胶光学厚度的空间分布和季节周期。与低分辨率模式相比，高分辨率模式模拟得到的气溶胶光学厚度与地基观测的偏差和均方根误差分别降低了0.08和0.12，相关系数增加了0.13。低分辨率模式不足以重现与青藏高原复杂地形强迫有关的气溶胶变化，尤其在青藏高原的东部边缘区域。高分辨率模式和低分辨率模式中亲水性气溶胶光学特性模拟的差异是由模式中相对湿度的差异造成的。高分辨率模式中更合理的相对湿度分布和变化有助于模拟亲水性气溶胶的吸湿增长。在冬季，高分辨率模式中10米风速的增大会导致沙尘排放量的增加。利用高分辨率模式定量计算得到青藏高原地区大气顶和地表的气溶胶直接辐射效应分别为–0.76 W m–2 和–8.72 W m–2 。高分辨率模式和低分辨率模式均可以模拟得到青藏高原地区沙尘气溶直接辐射效应从春季的正值向其他季节的负值转变的关键特征。
• Figure 1.  Spatial distributions of terrain height from the low-resolution model (LRM: ~100 km) (top row) and high-resolution model (HRM: ~25 km) (bottom row). The white line represents the boundary of the Tibetan Plateau. The red dots represent the seven sites in AERONET, and the red triangle represents the Lhasa site.

Figure 2.  Spatial distributions of annual mean aerosol optical depth from the simulations with LRM and HRM for the 2005–14 period. The circles represent site observations from AERONET. The value in the top-right of each subgraph is the average within the region of the black line.

Figure 3.  The annually-averaged aerosol optical depth of dust, carbonaceous, and sulfate from the simulations with HRM and LRM during the 2005–14 period. The average of each aerosol component over the Tibetan Plateau is labeled at the top-right of each subgraph. DU: dust; CA: carbonaceous; SU: sulfate.

Figure 4.  Comparison of the different component seasonal mean AODs over the Tibetan Plateau between the HRM (a) and LRM (b). The scatter diagrams represent the AERONET AOD against the HRM AOD (c) and LRM AOD (d).

Figure 5.  Monthly aerosol surface mass concentrations from available measurements and simulations at the Lhasa site. Scatterplot of observations vs. HRM (a) and LRM (b). The time series of dust surface mass concentrations simulated in HRM and LRM are compared with the observations from Apr. 2007 to Nov. 2007. The shaded area represents the standard deviation of the observations.

Figure 6.  Spatial distributions of relative humidity (RH) at 500 hPa (units: %) from the HRM (a), LRM (b), ERA5 dataset (c), and the difference between HRM and LRM (d) during the 2005–14 period. The comparison of the seasonal mean RH at 500 hPa (e) over the Tibetan Plateau among the HRM, LRM, and ERA5.

Figure 7.  Differential spatial distributions of 10-m wind speed (units: m s–1) between the HRM and LRM in four seasons. The vectors represent the difference in the wind field at 500 hPa.

Figure 8.  Spatial distributions of dust, carbonaceous, sulfate, and all component aerosol direct radiative effect (units: W m–2) at the top of the atmosphere (TOA) for the 2005–14 period from the simulations in the HRM and LRM.

Figure 9.  Comparisons of the seasonal mean different component aerosol direct radiative effects (units: W m–2) at the TOA (a–b) and the surface (c–d) over the Tibetan Plateau between the HRM and LRM.

Figure 10.  Same as Fig. 8, but for the aerosol direct radiative effect (units: W m–2) at the surface.

Figure 11.  Comparisons of the vertical structure of the 550 nm aerosol extinction coefficients (km–1) over the Tibetan Plateau between the HRM and LRM.

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## Manuscript History

Manuscript revised: 11 March 2022
Manuscript accepted: 20 March 2022
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Simulating Aerosol Optical Depth and Direct Radiative Effects over the Tibetan Plateau with a High-Resolution CAS FGOALS-f3 Model

###### Corresponding author: Tie DAI, daitie@mail.iap.ac.cn;
• 1. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 2. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing 210044, China
• 3. University of Chinese Academy of Sciences, Beijing 100049, China
• 4. International Center for Climate and Environment Science, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 5. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China

Abstract: Current global climate models cannot resolve the complex topography over the Tibetan Plateau (TP) due to their coarse resolution. This study investigates the impacts of horizontal resolution on simulating aerosol and its direct radiative effect (DRE) over the TP by applying two horizontal resolutions of about 100 km and 25 km to the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere Land System (CAS FGOALS-f3) over a 10-year period. Compared to the AErosol RObotic NETwork observations, a high-resolution model (HRM) can better reproduce the spatial distribution and seasonal cycles of aerosol optical depth (AOD) compared to a low-resolution model (LRM). The HRM bias and RMSE of AOD decreased by 0.08 and 0.12, and the correlation coefficient increased by 0.22 compared to the LRM. An LRM is not sufficient to reproduce the aerosol variations associated with fine-scale topographic forcing, such as in the eastern marginal region of the TP. The difference between hydrophilic aerosols in an HRM and LRM is caused by the divergence of the simulated relative humidity (RH). More reasonable distributions and variations of RH are conducive to simulating hydrophilic aerosols. An increase of the 10-m wind speed in winter by an HRM leads to increased dust emissions. The simulated aerosol DREs at the top of the atmosphere (TOA) and at the surface by the HRM are –0.76 W m–2 and –8.72 W m–2 over the TP, respectively. Both resolution models can capture the key feature that dust TOA DRE transitions from positive in spring to negative in the other seasons.

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