Present-day satellite-based global observations are for limited to scatterometer-derived surface winds obtained over oceans, and atmospheric motion vector (AMV) winds computed by cross-correlating features from a series of images of cloud and water vapor structure. Although covering large portions of the earth, neither scatterometer nor AMV wind observations provide important information on shear and other aspects of the vertical wind profile. Several configurations of Doppler lidar instruments, which can measure the full wind profile, have been investigated over more than three decades for potential deployment on space-based platforms. Doppler lidar instrument development coupled with Observing System Simulation Experiments (OSSEs) have demonstrated the improvement in forecasting skill that could be gained by adding space-based Doppler lidar winds to the current observational data set and assimilating them into numerical forecast models. The OSSEs have shown, in general, that wind observations from jet-stream heights in the middle and upper troposphere provide the most impact for improving medium range forecasting.
Although measurement of winds from space using Doppler lidar is challenging, progress on lidar technology has progressed to the point where such observations are technologically feasible. Lidars make use of atmospheric aerosols and molecules to scatter back optical energy to the receiver. This backscattered energy is collected and analyzed to compute the Doppler shift imparted by movement of the scatterers and produce an estimate of the atmospheric winds. Many modern surface-based Doppler lidar instruments used for, e.g., atmospheric research, and wind energy applications, are designed to measure winds by analyzing the signal backscattered by clouds and other atmospheric aerosol particles such as dust or pollutants. Such instruments work well near the surface where sufficient aerosol particles are present and not too distant from the measuring system. For full-troposphere, space-based measurements, however, the capability to also utilize atmospheric molecules as scatterers is also needed in order to probe non-turbid clear regions of the globe. Much recent Doppler lidar progress relating to space-based measurements has been associated with development and demonstration of technology that can provide winds from energy backscattered by atmospheric molecules and to design instruments that can utilize both molecules and aerosols as scatterers.
A major step forward in space-based observations will be the scheduled launch of the European Space Agency’s Aeolus wind mission in late 2017 or early 2018. Aeolus utilizes a laser transmitter operating in the ultraviolet (UV) spectral region and two receivers to obtain measurements from both atmospheric aerosols and molecules. Because only a single line of sight perpendicular to the satellite motion will be provided by Aeolus, the mission will not directly measure the full horizontal wind vector. The satellite is designed to operate in a dusk/dawn polar orbit at a height of 408 km above earth, although lower orbits are currently being considered to take advantage of a solar activity minimum coincident with the scheduled mission time frame. Data will be downlinked once per orbit to ground stations at Svalbard, Norway and other locations for use in operational forecasting. The raw lidar returns will be analyzed to produce radial wind vectors, which with then undergo a series of processing steps prior to assimilation into analysis and forecast models. Forecast centers from several countries will assimilate the Aeolus observations into their forecast systems and assess the improvements gained due to addition of the wind measurements.
The US is also studying potential satellite instruments for measuring global winds. The 2007 NRC Decadal Survey laid out a winds instrument as one of seventeen missions aimed at renewing US investment in earth observations for the upcoming ten years. Different concepts have been put forth in the US to address the NRC-identified need, including a “hybrid” instrument incorporating both UV and infrared (IR) laser sources and receivers, instruments designed for operation at high aerosol portions of the globe, and UV instruments similar to Aeolus but utilizing different receiver technology for analyzing returns from aerosols. All US designs extend the Aeolus concept by providing measurements from two lines of sight in order to obtain the full wind vector. Recently, airborne campaigns to demonstrate technology suitable for potential space-based instruments were carried out; on both cases the preliminary data showed generally good agreement with measurements from dropsondes deployed from the aircraft.
Although progress has been slow, making available space-based Doppler wind observations continues as a measurement objective that promises to have a significant impact on operational forecasting and research for both weather and climate. The launch of Aeolus, combined with progress in the US and elsewhere on Doppler wind technology, will hopefully enable the next steps toward the eventual goal of one or more Doppler instruments providing global wind measurements to the international forecasting community.