Although still in the planning stages, a space-based Doppler wind lidar mission for global measurement of wind profiles offers an excellent example of a satellite research-to-operations mission. A research mission to focus on tropical and subtropical processes is currently being investigated. The Winds from the International Space Station for Climate Research (WISSCR) mission would deploy a Doppler lidar instrument on the International Space Station (ISS) for a 2-3 year study of tropical dynamical processes. In the tropics and subtropics the addition of wind fields to the observational data set should have a significant effect because the geostrophic relationship between the wind and mass fields is often not valid. Wind observations should also improve the quality of the reanalysis data sets widely used to investigate climate-related processes. As an example, significant differences currently exist between NCEP/NCAR and ECMWF reanalysis representations of the Hadley and Walker circulations, showing large discrepancies in the tropical divergent wind. Tropical regions are also a primary location for many important processes, including monsoons, tropical cyclones, Atlantic and Indian Ocean circulations, and pole-ward transport of tropical/subtropical air to middle latitudes. Each of these processes could be better characterized with improved observations of tropospheric and stratospheric winds.

Instrument and mission design studies have shown the feasibility of the WISSCR mission and developed a preliminary instrument design and mission concept. The instrument would include two lidar subsystems optimized for measuring winds from different atmospheric scatterers. This so-called hybrid configuration incorporates a near-infrared heterodyne subsystem for measuring winds where aerosols or clouds are abundant and a direct detection, ultraviolet Doppler subsystem designed for observations in aerosol-sparse regions where molecular scatter dominates. The two subsystems share pointing optics and telescopes.

Recently, in separate airborne campaigns, both heterodyne and direct detection lidar instruments have been demonstrated. Autonomous measurements from a NASA-Goddard direct detection system deployed on a NASA ER-2 aircraft showed good agreements with nearby sondes and corresponded well to model outputs used to predict instrument performance in space. A NASA Langley Research Center heterodyne lidar incorporating laser and receiver characteristics similar to those needed for space measurements was also demonstrated. Deployed on the NASA DC-8 aircraft, the instrument generated wind profiles from aerosols and clouds in the lower and mid troposphere that showed reasonable agreement with dropsonde values.

As part of a long-range plan to eventually include Doppler lidar measurements for operational use, mission studies for both demonstration and operational free-flyer missions have also been performed. The demonstration mission would deploy a hybrid instrument in a 400 km orbit to assess impacts on weather prediction and generate data for development and study of assimilation and analysis methodologies. The recent National Research Council Decadal Survey recommends a Doppler lidar demonstration mission as one of 17 priority missions to be carried out by NASA and NOAA over the next decade. A notional operational mission with the lidar deployed on a next-generation satellite in an 828 km orbit has also been studied and a preliminary design developed. The higher orbit of the operational satellite necessitates some changes and tradeoffs in instrument design.

In order to assess impact and develop the optimum design for proposed lidar winds missions, Observing System Simulation Experiments (OSSEs) have been performed. The OSSEs are aimed at assessing the added value of lidar winds to current and future observing system scenarios. OSSEs have generally shown significant impact of lidar winds on medium range forecasts even in regions where current observations are plentiful. In tropical, ocean, and Southern Hemisphere regions the lidar-measured winds provide additional impact because observations in these regions are more limited. OSSEs and other studies also indicate the value of lidar-measured winds for improving predictions of events such as mid-latitude cyclogenesis and tropical cyclone landfall location.