Adm-aeolus - Esa's Wind Lidar Mission and Its Contribution to Numerical Weather Prediction

The European Space Agency (ESA) is developing a direct detection Doppler wind Lidar for the measuring of wind from space. The pulsed UV Lidar, with high spectral resolution capability, shall deliver horizontally projected single line-of-sight wind measurements at 24 vertical layers from each of its two channels; one molecular (clear air) and one particle (aerosol and cloud backscatter) channel. The instrument will measure the zonal wind component of the wind field in clear and particle-rich air (aerosol layers and transparent clouds), and down to the top of optically dense clouds. The required accuracy of the wind measurements, including representativeness errors, is 2 ms in the planetary boundary layer, 2-3 ms in the free atmosphere, and 3-5 ms in the lower stratosphere up to 30 km. The wind observations will be provided as spatial averages, continuously sampled along the satellite track. The satellite will fly in a polar dusk/dawn orbit, providing a global coverage of ~16 orbits per day. The measurements will be delivered near-real-time (NRT) for direct ingestion into operational numerical weather prediction (NWP) models. During the technical development of the ALADIN laser, changes to the mission measurement strategy had to be implemented in order to meet the strict user requirements on stability and measurement accuracy. This has led to changes in the spatial representativity of the data. As a result of these changes to the mission, new impact studies have been initiated to consolidate an optimized on-ground data processing and make best use of the Aeolus data in NWP assimilation systems. The various options for the on-ground processing of the continuously sampled data and the implications for the assimilation of the data will be presented. Earlier impact studies have shown that the largest impact of Aeolus is expected in regions with few other direct wind profile observation, e.g. over the oceans, in the Tropics and in the Southern Hemisphere. This is also expected to be the case for the new sampling strategy. Climate monitoring based on reanalysis data are expected to benefit from Aeolus observations through improvements of NWP analyses. One example is the detection of wind driven circulation changes in Arctic regions. Climate model processes involving wind dynamics, such as convectively coupled tropical waves, El Niño circulations and Monsoons, could be validated with tropical wind profiles from Aeolus. The Aeolus mission and the recent updates to its sampling, on-ground processing and data usage will be presented together with results from campaigns with the Aeolus airborne demonstrator (A2D). BACKGROUND AND MOTIVATION The European Space Agency’s (ESA’s) Living Planet Programme includes two types of complementary user driven missions: the research oriented Earth Explorer missions and the operational service oriented Earth Watch missions. Earth Explorer missions are divided into two classes, with Core missions being larger missions demonstrating the capabilities of new technologies addressing issues of wide scientific interest, and the Opportunity missions that are smaller in terms of cost to ESA. Both types of missions address the research objectives set out in the Living Planet Programme document (ESA, 1998), which describes the Agency’s strategy for Earth Observation in the coming decades. This has been extended in the ESA Earth Observation Strategy document (ESA, 1 Gert-Jan Marseille, Ad Stoffelen, Jos de Kloe (all KNMI), Linda Megner (MISU), Heiner Körnich (MISU/SMHI), Harald Schyberg (met.no) Lars Isaksen, Andras Horanyi, Saleh Abdalla and Michael Rennie (all ECMWF) 2006). All Earth Explorer missions are proposed, defined, evaluated and recommended by the scientific community. ESA’s second Earth Explorer Core mission, ADM-Aeolus, embarks a direct detection Doppler wind Lidar for the measuring of wind from space. The pulsed High Spectral Resolution Ultra Violet (UV) Lidar shall deliver horizontally projected single line-of-sight tropospheric and lower stratospheric wind profiles in clear and particle rich air (aerosol layers and transparent clouds) and down to the top of optically dense clouds. The measurements will be delivered near-real-time (NRT, within 3 hours) and quasi-real-time for the region close to the data downlink station (QRT, within 30 minutes), for direct processing and ingestion into operational numerical weather prediction (NWP) models. The motivation for the selection of the Aeolus mission was the need for more abundant direct wind profile measurements in the current Global Observing System (GOS), which is used e.g. by NWP models. In the current GOS, direct wind profile measurements are obtained from radiosondes, commercial aircraft ascends and descends and ground-based wind lidars and radars. The distribution of the measurements is, however, not homogenous, with most observations taken over land in the Northern Hemisphere. Winds can also be inferred from temperature soundings, which are abundant from satellites. However, the wind field can only be estimated from temperature measurements when the flow is in geostrophic balance, which means that only large-scale winds in the extra-tropics can be obtained. Air Motion Vectors also provide valuable wind observations from cloud and aerosol tracking. These measurements are, however, limited by the difficulty in performing accurate heightassignments. It is therefore expected that the Aeolus mission will largely contribute to the improvement of predictions of small-scales flows and forecasts in observation-sparse regions. The Aeolus mission and the recent updates to its measurement strategy and data processing are presented here together with results from NWP impact studies using simulated Aeolus data and results from campaigns with the Aeolus airborne demonstrator (A2D). More details can be found in the proceedings of earlier workshops of the IWWG, i.e. Ingmann et al (2004), Straume-Lindner et al (2006), Ingmann et al (2008), in the ADM-AEOLUS Science Report (ESA, 2008) and, more recently, in Straume-Lindner et al (2011). THE AEOLUS WIND MISSION Scientific motivation The current lack of homogenous sampling of the 3-dimensional wind field in large parts of the tropics and over the major oceans leads to major difficulties both in the studying of key processes in coupled climate systems and in the further improvement of NWP. It has been shown that direct wind profile measurements over the oceans and in the tropics are essential for improvements in short-range forecasts of severe weather (Marseille et al., 2008) and a correct representation of the dynamics in the tropics (Žagar, 2004). Also the WMO (2004) report emphasise that there is a need for more uniformly distributed wind profile measurements, in particular in the Tropical and Polar regions. In the 1980s, studies looked into which satellite-based remote sensing techniques are most suitable for global wind profiling, and it was demonstrated that an active optical system (lidar) could provide global measurements of the required accuracy (e.g. Menzies 1986, Baker et al. 1995). Recommendations from the scientific and NWP community therefore lead to the selection of the Aeolus space-based lidar as ESA’s second Earth Explorer Core mission in 1999. The primary aim of Aeolus is to provide global observations of vertical wind profiles from the surface trough-out the troposphere and lower stratosphere. Spin-off products from Aeolus will be optical properties profiles. Information on cloud/aerosol layers, optical densities, backscatter and extinction coefficients, lidar and scattering ratios can be obtained. These spin-off products could become useful for aerosol assimilation by NWP models, acting e.g. as a gap-filler between the dedicated CALIPSO and EarthCARE aerosol missions. However, because the optical properties products will be retrieved from backscattered light at one wavelength only with no information about its polarization, the distinction of clouds and aerosols will only rely on the Figure 1: The Aeolus orbit, pointing and sampling characteristics Figure 2: The Aeolus lidar measurement concept. The instruments emit and receive path is monostatic, but is shown skewed here for illustration purposes. The laser emits 355 nm frequency-narrow pulses at a frequency of 50 Hz, which are backscattered by molecules (Rayleigh scattering) and particles (Mie scattering) at various altitudes in the atmosphere (left panel). The movement of the molecules or particles along the laser line-of-sight causes a Doppler-shift of the emitted laser light, as illustrated in the right panel. The frequency shift is measured by the CCD detectors, allowing the estimation of the local wind speed. The backscattered laser light is also detected as a function of time, allowing the retrieval of wind profiles (left panel). The signals are time-averaged, resulting in layer averaged measurements from 250 m (near the surface) up to 2 km (in the stratosphere) Instrument’s high-spectral-resolution capability. Furthermore, the vertical and horizontal resolution of the optical products will be coarse as compared to dedicated aerosol lidars. Instrument and measurement concept ADM-Aeolus will embark a single instrument, namely the high spectral resolution Doppler wind lidar ALADIN (Atmospheric LAser Doppler INstrument). ALADIN is a pulsed UV lidar (355 nm, 50 Hz, circularly polarized). The instrument is measuring continuously along the track, as illustrated in Figure 1. Its high spectral resolution capability is the separate detection of the molecular (Rayleigh) and particle (Mie) backscattered signals in two channels. This makes it possible for Aeolus to deliver winds both in clear and (partly) cloudy conditions down to optically thick clouds. The height of the wind measurements in the atmosphere is calculated from the time it takes for the laser pulse to travel from the emitter to the backscatter altitude and back to the receive telescope (Figure 2). The backscatter signals are, furthermore, time-averaged resulting in layer-averaged measurements from 24 vertical bins per channel. The emitted laser pulse is frequency shifted and broadened by the motion of the scattering media before it re-enters the instrument and the instrument detectors. The frequency shift of the backscattered signal is proportional to the velocity of the scattering media along the instrument line-of-sight (LOS). The instrument is pointing perpendicular to the flight direction in order to remove any Doppler shift associated with the velocity of the spacecraft. After signal calibration and processing, the LOS wind speed can be retrieved and projected down to the horizontal LOS (HLOS). ADM-Aeolus will be launched in a sun-synchronous dawn-dusk orbit, with a descending equatorial crossing time at 6 am. A quasi-global coverage is achieved daily (by ~16 orbits, evenly distributed around the globe). The orbit is repeated afters 7 days (109 orbits). A detailed description of the instrument design and its operation can be found in (ESA, 2008).