Major Advances Foreseen in Humidity Profiling From the Water Vapour Lidar Experiment in Space (Wales)

Abstract

The first active humidity profiling system in space is being developed by the European Space Agency (ESA) in the horizon of 2010. Data quality in terms of vertical resolution, accuracy, low bias, and vertical extent is expected to be unsurpassed. There are pressing needs for high-quality water vapor observations to fulfill societal and scientific requirements in environmental understanding and operational weather prediction. At one level, these needs arise from requirements for more reliable information about the earth's climate system (Houghton et al. 1996(Houghton et al. ,2001) and closely related needs for improved weather forecasts (Weckwerth et al. 1999;Koch et al. 1997;Bell and Hammon 1989 (GEWEX 1998a,b; WCRP 2000).Data obtained from ground-based and airborne differential absorption lidars (DIALs) have proven the ability of the DIAL technique to meet these needs in terms of good vertical resolution, high precision, and low bias (Browell and Ismail 1995;Wulfmeyer and Bosenberg 1998;Ehret et al. 1999; Bruneau et al. 2001a,b). We argue that a spaceborne DIAL in the 2008-12 time frame would be a major advance, by achieving global coverage together with the high quality offered from the active remote sensing technique (Browell et al. 1998 The objective of this paper is not to report on technical solutions, but rather to show that the WALES mission will be of substantial benefit to both NWP and climate research. It is well recognized that water vapor effects dominate the earth's climate. Water vapor's radiative properties make it the most important greenhouse gas, and it plays a direct role in other major atmospheric processes including the formation of clouds and precipitation, surface and chemical processes, and dynamics (see Fig. 1). Microphysical processes involving atmospheric aerosol and leading to the formation and development of cloud and precipitation are strongly dependent on the water vapor distribution. Atmospheric water vapor has both direct and indirect effects on the radiation budget of the atmosphere (Elliot and Gaffen 1995). Climate sensitivity to increasing C02 is primarily dependent on water vapor feedback mechanisms taking place in the middle and upper troposphere (Sinha and Harries 1995). Furthermore, atmospheric dynamics at all space scales is strongly dependent on the variability of the atmospheric humidity field. The water vapor distribution is determined by the hydrological cycle. Water vapor is much less well mixed than other greenhouse gases because of its strong links with temperature and because of the interactions between clouds, convection, and atmospheric transport, especially in the upper troposphere and lower stratosphere. Because of these characteristics, water vapor has high variability in space and time. Together with its large dynamic range, this variability represents a major challenge for its observation.The main terrestrial sources of humidity data are currently land and ship synoptic reports, buoy data, and radiosonde soundings. Space-based humidity estimates are typi...

Emerging Technologies and Synergies for Airborne and Space-Based Measurements of Water Vapor Profiles

“…In 2007, the annual global cloud-free fraction was 53.5%, suggesting that the retrieved water vapor profiles of a space-based DIAL could extend down to the surface for over half of the total retrieved profiles and thereby substantially add value to the next generation of atmospheric remote sensors targeting tropospheric dynamics Surv Geophys ( ) 38:1445Geophys ( -1482Geophys ( 1451 Atmospheric Sounding Interferometer). The results of the study indicated that for a fixed error threshold, the WALES water vapor DIAL concept under study would extend water vapor measurements to higher altitudes than for IASI, with improved vertical resolution near the surface (Gerard et al 2004). Di Girolamo et al (2008) also carried out an end-toend simulation of a WALES like lidar to assess the systematic and random error performance under a wide range of atmospheric conditions.…”

“…Unfortunately, WALES was not selected for further development due to cost and technical risk associated with the laser under consideration; however, several feasibility studies were carried out and demonstrated the utility of such measurements for weather and climate applications. A concept study by Gerard et al (2004) assessed the performance of a space-based water vapor DIAL relative to passive infrared measurements such as from IASI (Infrared Fig. 3 Global map of annual mean CALIPSO-derived cloud-free fraction.…”

“…The results confirmed the capability of a space-based water vapor DIAL of accurately measuring the water vapor and aerosol structure from the UTLS down to the surface with better than ± 5% bias and ± 20% random error. The measurement requirements resulting from the space-borne WALES mission study (Gerard et al 2004;DiGirolamo et al 2008) are shown in , is used to define the water vapor field that is ingested into the WALES lidar simulator. Thin and thick clouds in the NWP model are depicted by the brown and black feature masks, respectively.…”

Water vapor differential absorption lidar measurements using a diode-pumped all-solid-state laser at 935 nm

“…Since the differential absorption lidar (DIAL) technique applied from satellites shows high potential to meet those requirements [7][8][9] there is need for an exploratory satellite mission demonstrating high-vertical resolution water vapor profiles by active remote sensing [5]. In recent years several lidar missions have thus been proposed to measure water vapor profiles from space [10][11][12][13].…”

Diode laser frequency stabilisation for water-vapour differential absorption sensing

“…It is based on the comparison of the lidar signals at two wavelengths, one coinciding with a water-vapour absorption line of adequate strength and the second sufficiently far away from any line not to undergo substantial absorption. The quality of the data obtained from ground-based [5,6] and airborne [7][8][9][10] instruments in terms of vertical resolution, precision and accuracy indicates the feasibility of a space-borne instrument [11], from which major advances in humidity profiling with global coverage are expected [12]. While ground-based water-vapour DIALs operate at two wavelengths only (one on-line and the other off-line), space-borne instruments may operate at a set of different on-line wavelengths corresponding to absorption lines of different strengths, thus providing different penetration depths in the atmosphere.…”