Abstract
A recent study of CMIP5 models (Kok et al., 2018) shows the direct climate feedback by dust aerosols is about -0.04 to +0.02 Wm-2K-1, which accounts for a significant fraction of the direct climate feedback by all aerosols. Dust is also a major source of ice nuclei for aerosol indirect effects and supplies the key nutrients (iron/phosphorus) to the open ocean. Therefore, quantification of dust life cycle and radiative effects in E3SMv1 has important implications on both the water cycle and biogeochemical simulations in response to climate change. Here we focused on evaluation of the simulated dust distributions for estimating dust direct radiative effects under the present-day conditions. The impact of increasing model resolution on dust simulations is also examined.
The model estimates of global dust AOD are similar independent of resolution between the low- and high- resolution E3SMv1 models (0.026 vs 0.03), since the dust emissions are tuned to match the observational constraint of global dust AOD at 0.03±0.01. Yet, there are substantial differences in the predicted dust AOD distributions depending on the model resolution, i.e., about 5 times differences over the Taklimakan desert. The changes in dust AOD are more than ±10% near the major dust source regions or downwind, resulting from local changes of dust emissions, dry and wet removal efficiencies due to increase of the resolved spatial scales. They could further affect the simulated regional energy balance, especially in the dust-influenced high latitudes where the climate sensitivity to dust forcing is large. Compared to the 10-year ground-based AERONET observations of AOD, the low-resolution E3SMv1 underpredicts the total AOD by about 31% averaged over the 247 AERONET sites. However, the underestimation of AOD is mainly due to aerosol predictions in Asia associated with the anthropogenic emissions (year 2000). Over the selected 14 ‘dusty’ AERONET sites, the modeled mean AOD (0.299) agrees well with the AERONET data (0.311), for a correlation coefficient of 0.91. The vertical distribution of dust is compared with the CALIPSO satellite-retrieved profiles, indicating an underestimation of dust vertical transport. This is related to the short lifetime of dust simulated by the E3SMv1 for less than 2 days, due to excessive dry and wet dust removals compared with other global climate models.
The estimated annual and global mean of dust direct radiative effect at the TOA is -0.077 Wm-2 in the default E3SMv1 low-resolution model. Sensitivity studies with improved dust optics in the shortwave (AERONET-based) and size distribution (Kok, 2011) suggest a stronger TOA dust cooling effect of -0.4 Wm-2 with a weaker atmospheric warming, which could affect the simulated atmospheric energy balance significantly with the v1 model. Our estimate lies in the lower end of global estimates of the present-day dust direct forcing ranging from -0.6 to +0.1 Wm-2. The high-resolution model estimates a less global cooling effect by about 10% with larger regional differences. This study shows that the magnitude and possibly the sign of dust direct radiative effects in v1 are subject to large uncertainties for water cycle simulations. It requires more than constraint of the global dust AOD. Future improvement are need for simulation of dust vertical profiles and longwave radiation.