The Atmospheric Thermodynamics LidAr in Space—ATLAS

An in-depth understanding and advanced prediction of the Earth’s temperature and water vapour fields is fundamental for a sustainable development of the Earth system. However, our understanding of the water and energy cycles still shows critical gaps on all temporal and spatial scales. This is mainly due to a lack of accurate high vertical resolution measurements of water vapour and temperature profiles - hereafter called thermodynamic (TD) profiles - with high temporal-spatial resolution, especially in the lower troposphere [1]. These observational gaps can be addressed with a new active remote sensing system in space based on the lidar technique. Combining vibrational and rotational Raman backscatter signals [2, 3], simultaneous measurements of water vapour and temperature profiles and a variety of derived variables are possible with unprecedented vertical and horizontal resolution, especially from the surface to the lower troposphere. This is the key concept of the Atmospheric Thermodynamics LidAr in Space – ATLAS, which was recently submitted (March 2018) to the European Space Agency in response to the Call for Earth Explorer-10 Mission Ideas in the Frame of the ESA Earth Observation Envelope Programme. The instrument is based on the experience and know-how gained with the development and operation of several existing ground-based instruments and airborne instruments.

In addition to the temperature and water vapour mixing ratio profile, among the independently measured further products: the particle backscatter and extinction coefficient profiles in the UV, allowing the determination of the optical properties of aerosol layers and the geometric and optical properties of clouds, thus complementing similar measurements performed with EarthCARE; the profile of relative humidity, the real daytime PBL depth over land and the oceans directly obtained from the temperature profile, as well as atmospheric stability parameters like buoyancy, convective available potential energy (CAPE) and convective inhibition (CIN).

Recent advances in solid-state laser, large-aperture telescope, and detector technologies allow achieving a new performance level from space with this technique. With a laser power of up to 250 W in the ultraviolet (UV) and a telescope with a 4 m primary mirror, the above specified observational requirements will be reached, thus realizing a breakthrough in earth system sciences. Different candidates for the laser transmitter of ATLAS are available, one of them being a frequency-tripled, diode-laser pumped Nd:YAG laser and the other one a frequency-doubled alexandrite laser. The use of a new generation of pump chambers and diode lasers results in a wall-plug efficiency of > 5 %. The receiver will consist of a large-aperture telescope with a diameter of 4 m. Such a telescope would be larger than the ones of previous lidar missions, but stability and optical quality demands are significantly relaxed (no astronomic quality needed) and the receiver technology is comparatively simple and very rugged. Simulations indicate an overall electrical power consumption for ATLAS of ~ 6000 W (5000 W for the laser and 1000 W for the remaining sub-systems). The rough estimate for ATLAS’ weight is ~ 600 kg (~ 350 kg for the telescope and ~ 250 kg for the other sub-systems). We foresee the use of a Vega-C launcher, which should be completely operative by the time of ATLAS launch. A launch by 2027/28 is considered certainly feasible.

An assessment of the specifications of the different lidar sub-systems has been performed with the analytical simulation model for space-borne Raman lidar systems developed at Università della Basilicata [2,3]. The expected performance was simulated under a variety of environmental and climate scenarios using different atmospheric reference models, which cover various climatic regions and seasons as well as a variety of solar illumination conditions. In our present simulations the payload is hosted on a frozen dusk/dawn low-Earth sun-synchronous orbit at an altitude of 450 km (inclination ~97 degrees). The horizontal spatial coverage domain will be global, i.e. from tropical to sub-polar regions. With daily overpasses at 6/18 h local time, it is possible to capture the TD states before and after the daytime development of the PBL, which are very important for data assimilation in weather forecast models. The figure below illustrates the vertical profiles of the water-vapour mixing ratio uncertainty, , and of temperature uncertainty, DT, for the tropical and mid-latitude summer atmospheric reference models. For the tropical atmosphere values of are in the range 2-20 % and 3-30 % up to 5.5 km for a horizontal resolution of 50 and 20 km, respectively, while for the mid-latitude summer atmosphere values of are in the range 2-20 % and 3-30 % up to 4 km for a horizontal resolution of 50 and 20 km, respectively. For both the tropical and mid-latitude summer atmosphere values of DT are in the range 0.4-1 K and 0.7-1.2 K up to 18 km for a horizontal resolution of 50 and 20 km, respectively. More results from the simulations and more aspects of the proposed mission will be illustrated at the conference.

References

1. Wulfmeyer, V., R. M. Hardesty, D. D. Turner, A. Behrendt, M. P. Cadeddu, P. Di Girolamo, P. Schlüssel, J. Van Baelen, and F. Zus. “A review of the remote sensing of lower tropospheric thermodynamic profiles and its indispensable role for the understanding and the simulation of water and energy cycles,” Review of Geophysics, 53, 819–895, doi:10.1002/2014RG000476, 2015.
2. Di Girolamo, A. Behrendt and V. Wulfmeyer: “Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman Lidar and of relative humidity in combination with differential absorption Lidar: performance simulations,” Applied Optics, 45, 2474-2494 (2006).
3. P. Di Girolamo, A. Behrendt and V. Wulfmeyer: “Space-borne profiling of atmospheric thermodynamic variables with Raman lidar: Performance simulations,” Optics Express, 26(7), 7955-7964 2018.