Modeling radiative impact of aerosols over south Asia constrained by observations of vertical distribution

 

Authors

Yan Feng — Argonne National Laboratory
V. Rao Kotamarthi — Argonne National Laboratory
Richard L. Coulter — Argonne National Laboratory

Category

Aerosol-Cloud-Radiation Interactions

Description

It has been suggested that high aerosol loadings in south Asia may impact the regional energy balance and hydrological cycle significantly. However, most of the aerosol and climate simulations in this region are global climate model (GCM) studies, which generally under-predict compared with satellite-observed aerosol optical depth (AOD). Also there is a lack of quantification of aerosol vertical distributions in those large-scale models. In the present study, we will address two questions: (1) does the under-prediction in AOD exist uniformly in height, and (2) how do the discrepancies between the simulated and observed AOD affect the radiative balance and heating in the atmosphere? We will present 12-km Weather Research and Forecasting coupled with chemistry (WRF-Chem) simulations and evaluations of aerosol and cloud properties from data gathered during the Ganges Valley Aerosol Experiment (GVAX). While the regional model simulates the spatial distribution and seasonal cycle of AOD similar to observations, the modeled zonal mean (0–25°N, 60–95°E) AOD is nearly a factor of 2 lower than the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. Comparisons with the micropulse lidar (MPL) measurements at the mountain-top site (Nainital: 29°N, 79°E, 1900 m) and a valley site (Kanpur: 26°N, 80°E, 120 m) indicate that aerosol extinction is significantly under-predicted from the surface to 2 km and over-predicted at higher altitudes. This bias in aerosol extinction profile can lead to an increase in the planetary boundary-layer (PBL) radiative heating, more than what is due to a larger column AOD increase. Sensitivity studies are conducted to examine the impact on the radiative balance and cloud properties, by increasing aerosol extinction uniformly in height (Experiment I) and below 2 km only (Experiment II) by a factor of 2 at each time step, with the same single-scattering albedo. The calculated monthly mean AOD in Experiment II agrees the best with the MODIS AOD and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) aerosol extinction profiles, compared to the control run and the Experiment I. Radiative forcing of aerosols increases from -4.1 W m-2 in the control run to -6 W m-2 in Experiment II at the top of atmosphere and from -9.7 W m-2 to -14.3 W m-2 at the surface. Uniformly increasing aerosol extinction (Experiment I) enhances the aerosol perturbation on equivalent potential temperature profile as in the control run. But increasing aerosol extinction below 2 km only (Experiment II) leads to different responses in lapse rate. The impact on predicted cloud cover depends on the combination of the feedback in lapse rate and turbulence intensity.