Aerosol direct radiative impact over the INDOEX area based on passive and active remote sensing

Abstract

This study is dedicated to the direct radiative impact assessment of the pollution aerosol particles during the Indian Ocean Experiment (INDOEX). We use here an instrumental synergy based on active and passive ground‐based (Goa University, India), airborne (Mystere 20 research aircraft), and spaceborne (Meteosat‐5) devices. An original method based on ground‐based active actinic flux measurements is developed to assess the atmospheric, columnar aerosol single scattering albedo. This parameter has been found between 0.88 and 0.93 (at 440 nm) with an absolute uncertainty of 0.04 during the operating period from 11 to 23 March 1999. We have also assessed this parameter off the West Indian coast by comparing simultaneous airborne lidar and flux measurements with radiative transfer simulations. The value retrieved for the atmospheric column is close to 0.83 ± 0.05 (at 523 nm) in agreement with the coastal value. The horizontal and vertical extent of the aerosol plume is investigated using airborne lidar and Meteosat‐5 satellite. Using the Meteosat‐5‐derived aerosol optical thickness, we have estimated the regional extent of the shortwave aerosol direct radiative forcing. The vertical profile of the aerosol extinction coefficient derived from lidar sounding is used to assess the atmospheric shortwave heating rate induced by the aerosol layer. For an aerosol optical thickness of 0.6 (±0.12), as it has been observed in the aerosol plume in late March, the top of atmosphere direct aerosol forcing is −17 (±5) W m−2. The surface forcing is between 2.5 and 4.5 higher than the top of atmosphere forcing. This difference leads to a significant heating of the three lowest kilometers of the atmosphere by the aerosols. For an aerosol optical thickness of 0.6, the atmospheric heating rate induced by aerosols is between 0.8 and 1.2 Kd−1 depending on the aerosol single scattering albedo value.