ADM-Aeolus ocean surface calibration and Level-2B processing

Abstract

Preparations for the European Space Agency’s (ESA) Atmospheric Dynamics Mission (ADM-Aeolus), which is scheduled for launch in 2011, are in full progress. The direct detection high spectral resolution Doppler Wind Lidar (DWL) satellite instrument will be the first to measure wind profiles from space, from the surface up to 30 km altitude [1] and [2]. To achieve this, a laser at 355 nm pulsed at 100 Hz is pointed towards the atmosphere. Backscattered light, both from molecules and aerosols, is detected by two independent spectrometers. A dual channel Fabry-Perot spectrometer to measure both sides of the Rayleigh (molecule) spectral peak, and a high resolution Fizeau spectrometer to measure the location of the Mie (aerosol) peak. In preparation of this mission, besides the actual construction of the instrument and satellite, a number of supporting activities have been initiated by ESA, including instrument simulation and ground processing software development, ground and air-borne measurement campaigns, and studies dedicated to specific issues such as calibration and sampling strategies. An important part of the preparations is to study the possibilities of calibrating the wind results by means of surface reflections. To estimate the effect of water motion due to waves, a simple wave model has been combined with a reflectivity model of the water surface. This includes specular reflection on smooth water surfaces and Lambertian reflection on foam caused by wind streaks and breaking waves. The effect of subsurface reflection is still being investigated. Using this model, the average net water movement that will be observed by the DWL instrument is estimated. Another part of the preparations includes the development of Level 2B (i.e., wind) processing software (L2Bp). This software will accept as input measurement data files (Level 1B) and uses an estimate of the atmospheric temperature and pressure profiles (from a numerical weather prediction model) to retrieve the wind profile from the spectrometer data. This software will be made available by ESA as source code, free of charge, to all interested users in the meteorological/research community, and may be used as standalone software, or integrated as subroutine in a larger system for use in scientific or operational applications. The working of the L2Bp will be illustrated at the conference.