INTRODUCTION

An appropriate understanding and prediction of Earth’s energy and water cycles is fundamental for a sustainable development of the Earth system. However, our understanding of the distribution of heat and water in the atmosphere still shows critical gaps on all temporal and spatial scales [1-3], which is mainly due to a lack of accurate measurements of TD profiles with high vertical and temporal resolution, especially in the lower troposphere [4]. Accurate, high temporal and spatial resolution observations of TD profiles in the lower troposphere from the surface to the interfacial layer at the top of the atmospheric boundary layer (ABL) are essential for improving weather forecasting [e.g., [5], [6]] and re-analyses [[7]], and understanding the Earth system. Furthermore, these measurements are of primary importance to improve parameterizations of land-surface and turbulent transport processes in the ABL, conducting to a substantial improvement of the representation of clouds and precipitation and the prediction of extreme events,. This is essential for advanced climate projections [[8], [9]]. More specifically, global scale measurements of 3-dimensional TD profiles would have a dramatic impact on our system understanding in four key research areas [[10]]:

Progress in these areas would not only contribute to weather and climate research and forecasting, but also to other related disciplines such as soil, hydrological, and agricultural sciences, in addition to human health and air quality applications.

Observational requirements to be fulfilled by networks of satellite and ground-based remote sensors, with a specific focus on the lower troposphere, have been identified considering four primary application fields: (1) monitoring, (2) verification and calibration, (3) data assimilation and (4) process studies [[10]]. For assessing climate trends, high quality water vapour and temperature measurements are required not only at the Earth’s surface, but also throughout the troposphere and stratosphere. This is only partly addressed by the current passive observing systems, as in fact, because of the intrinsic limitations in the inversion of the radiative transfer equation, these systems cannot provide the quality data required to progress in the above areas. This is primarily due to the lack in vertical resolution (>1km), which is insufficient to retrieve vertical gradients in water vapour and temperature profiles, temperature inversions, entrainment processes, etc., especially in the lower troposphere over land, where surface effects strongly limit infrared sounding approaches.

Observational requirements for lower tropospheric water vapour and temperature profiling impose a vertical resolution of ~100 m (i.e. sufficiently high to allow resolving the temperature and moisture gradients), with the bias affecting water vapour mixing ratio and temperature measurements in each single vertical range bin not exceeding 5 % and 0.5 K, respectively, and the random uncertainty not exceeding 10 % and 1 K, respectively. Similar requirements were identified within the World Meteorological Organization Integrated Global Observing Systems [12-14], the World Climate Research Program (WCRP) and the Global Climate Observing System (GCOS) [15]. These observational requirements cannot be met by passive remote sensing techniques (neither in the infrared nor in the microwave region) and can only be fulfilled by a new active remote sensing system in space based on the lidar technique [16,17]. Combining vibrational and rotational Raman backscatter signals, simultaneous measurements of water vapour and temperature profiles and a variety of derived variables are possible with unprecedented vertical and horizontal resolution (see more details in the next section), especially from the surface to the lower troposphere, this being the key concept of the mission Atmospheric Thermodynamics LidAr in Space – ATLAS. The instrument is based on the experience and know-how gained with the development and operation of several existing ground-based instruments [18-31] and airborne instruments [[32]2-34]. On-going and planned lidar missions, such as CALIPSO, CATS, ADM-Aeolus, EarthCARE, have demonstrated the applicability of laser-based active remote sensing in space. ATLAS would complement these missions, being the first space mission based on the Raman lidar technology. Additionally, the impact of ATLAS on weather and climate research can be further enhanced by exploiting synergies with passive instruments on other space platforms, such as IASI-A/B, CrIS and AIRS [35-37] and GNSS occultation.

2.

MISSION CONCEPT

The core element of the proposed Earth Explorer mission is a nadir-viewing water vapour and temperature Raman lidar system. In the present mission concept, the Raman lidar is conceived and designed to collect four primary lidar signals: the water vapour vibrational Raman signal, P_H2O(z), the high- and low-quantum number O₂-N₂ rotational Raman signals, P_loJ(z) and P_HiJ(z), and the elastic backscatter signal at the laser wavelength λ₀, P_λ0(z). The direct calculation of atmospheric temperature is obtained from the rotational Raman backscattered signals through the expression [19]:

where, more specifically, P_LoJ(z) and P_HiJ(z) are the low (LoJ) and high (HiJ) quantum number rotational Raman backscatter signals in the anti-Stokes branch at wavelengths λ_LoJ and λ_HiJ, respectively, received from the scattering volume at altitude z, and a and b are two calibration constants. The direct calculation of the water vapour mixing ratio is obtained from Raman backscattered signals through the following equation [38]:

where P_H2O(z) is the water vapour vibrational Raman lidar signal at at wavelength λ_H2O, while P_ref(z) is a temperature-independent reference signal obtained from a linear combination of the two temperature sensitive rotational Raman lidar signals P_LoJ (z) and P_HiJ (z), K is a calibration constant [38], ΔTrs(z) is a differential transmission term, which accounts for the different atmospheric transmission by molecules and aerosols at λ_H2O and λ_LoJ/HiJ.

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 ultraviolet, 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 the Raman technique. With a laser power of up to 250 W in the ultraviolet and a telescope with a 4 m primary mirror, the above specified observational requirements are 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 another being 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 those in previous lidar missions, but stability and optical quality demands are significantly relaxed (no astronomic quality required) 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).

An assessment of the specifications of the different lidar sub-systems has been performed with an analytical simulation model for space-borne Raman lidar systems developed at Università della Basilicata [16, 17]. 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 is 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. Figure 1 illustrates the vertical profiles of measurement uncertainty for water-vapour mixing ratio, Δx_H2O(z)/x_H2O(z), and of temperature, ΔT, considering the tropical and mid-latitude summer atmospheric models. Considering a vertical resolution of 200 m, values of Δx_H2O(z)/x_H2O(z) for the tropical atmosphere 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 Δx_H2O(z)/x_H2O(z) 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 ΔT 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.

Figure 1.

Vertical profiles of Δx_H2O(z)/x_H2O(z) (panel a) and ΔΤ (panel b) for the tropical (continuous lines) and midlatitude summer (dashed lines) reference models. Red lines represent night-time conditions, while blue lines represent daytime conditions. Thin lines represent a horizontal resolution of 20 km, while bold lines represent a horizontal resolution of 50 km.