Radiative and Dynamic Responses to Absorbing Aerosols over Pre-monsoon South Asia

 

Authors

Yan Feng — Argonne National Laboratory
V. Rao Kotamarthi — Argonne National Laboratory
Chun Zhao — Pacific Northwest National Laboratory

Category

Absorbing Aerosol

Description

Modeling studies have shown that aerosols especially the absorbing component can affect regional energy balance and the Indian summer monsoon. However, aerosols in climate simulations are often under-estimated compared with column-integrated aerosol optical depth (AOD) observations for this region. Here we examine the subsequent radiative and dynamic responses due to the uncertainty in aerosol absorption simulated with the 12-km WRF-Chem model for the pre-monsoon month, March, 2012. By doubling aerosol extinction in the model layers below 860hPa calculated in the control run, the differences in AOD and vertical profiles from satellite and ground-based observations are significantly reduced. Assuming that aerosol absorption is doubled proportionally with the simulated single scattering albedo, the atmosphere warming is increased from +6.3 W m-2 to 9.3 W m-2 on monthly and regional mean basis. It heats the lower troposphere below 3km by as much as 0.8 K day-1, and enhances the radiative heating of this layer by 50%. The net solar radiation at surface is reduced from -9.3 W m-2 to -14.2 W m-2. Over the land, surface cooling by aerosols is enhanced by -0.25 to -0.5 K, and results in more stabilization of the boundary layer (BL), lower BL height, and reduction in evaporation and sensible heat flux. Consequently, the aerosol effect on meridional temperature gradient and lower-troposphere moisture inflow from ocean to land is strengthened; water vapor and cloud fraction between 6N to 20N are increased by 2%-6% in March, when simulated aerosol extinctions are adjusted close to the observations. In contrast, assuming that the underestimation of aerosol extinction in the control run is due to scattering only, direct and semi-direct radiative feedbacks lead to more substantial aerosol cooling of the land surface and reduced warming in the atmosphere. The estimated dynamic and moisture response to aerosol radiative forcing is thus weakened compared with the control run. The major inference from this study is that aerosol impact on the monsoon-related cloud and precipitation could be different by a few percent with the same AOD predictions close to observations if aerosol absorption is not constrained. The magnitude of this uncertainty is comparable to the estimated absolute aerosol effect.

Lead PI

V. Rao Kotamarthi — Argonne National Laboratory