An "a-train" strategy for quantifying direct climate forcing by anthropogenic aerosols

This document outlines a practical strategy for achieving an observationally based quantification of direct climate forcing by anthropogenic aerosols. The strategy involves a four-step program for shifting the current assumption-laden estimates to an increasingly empirical basis using satellite observations coordinated with suborbital remote and in situ measurements and with chemical transport models. Conceptually, the problem is framed as a need for complete global mapping of four parameters: clear-sky aerosol optical depth σ radiative efficiency per unit optical depth E, fine-mode fraction of optical depth f_f, and the anthropogenic fraction of the fine mode f_af. The first three parameters can be retrived from satellites, but correlative, suborbital measurements are required for quantifying the aerosol properties that control E, for validating the retrieval of f_f and for partitioning fine-mode σ between natural and anthropogenic components. The satellite focus is on the "A-Train," a constellation of six spacecraft that will fly in informtion from about 2005 to 2008. Key satellite instruments for this report are the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radian Energy System (CERES) radiometers on Aqua the Onzone Monitoring Instrument (OMI) radiometer on the Polarization and anistropy of reflectances for Atmospheric Sciences coupled with obervations from a lidar (PARASOL), and the Cloud and Aeroso lide with othogonal Polarization (CALIOP) lidar on the Cloud-Aerosol Lidar on the Cloud-Aerosol Lidar and Infared Pathfinder Satellite Obervations (CALIPSO). This strategy is offered as an initial framework-subject to improvement over time-for scientists around the world to participate the A-Train opportunity. It is a specific implementations of the Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) program, presented earlier in this journal, which identified the integration of diverse data as the central challenge to progress in quantifying global-scale aerosol effects. By designing a strategy around this need for integration, we develop recommendations for both satellite data interpretation and correlative suborbital activities that represent, in many respects, departures from current practice.