Atmospheric influences on infrared-laser signals used for occultation measurements between Low Earth Orbit satellites

Abstract

Abstract. LEO-LEO infrared-laser occultation (LIO) is a new occultation technique between Low Earth Orbit (LEO)satellites, which applies signals in the short wave infrared spectral range (SWIR) within 2 µm to 2.5 µm. It is part of the LEO-LEO microwave and infrared-laser occultation (LMIO) method that enables to retrieve thermodynamic profiles (pressure, temperature, humidity) and altitude levels from microwave signals and profiles of greenhouse gases and further variables such as line-of-sight wind speed from simultaneously measured LIO signals. Due to the novelty of the LMIO method, detailed knowledge of atmospheric influences on LIO signals and of their suitability for accurate trace species retrieval did not yet exist. Here we discuss these influences, assessing effects from refraction, trace species absorption, aerosol extinction and Rayleigh scattering in detail, and addressing clouds, turbulence, wind, scattered solar radiation and terrestrial thermal radiation as well. We show that the influence of refractive defocusing, foreign species absorption, aerosols and turbulence is observable, but can be rendered small to negligible by use of the differential transmission principle with a close frequency spacing of LIO absorption and reference signals within 0.5 %. The influences of Rayleigh scattering and terrestrial thermal radiation are found negligible. Cloud-scattered solar radiation can be observable under bright-day conditions, but this influence can be made negligible by a close time spacing (within 5 ms) of interleaved laser-pulse and background signals. Cloud extinction loss generally blocks SWIR signals, except very thin or sub-visible cirrus clouds, which can be addressed by retrieving a cloud layering profile and exploiting it in the trace species retrieval. Wind can have a small influence on the trace species absorption, which can be made negligible by using a simultaneously retrieved or a moderately accurate

Profiling wind and greenhouse gases by infrared-laser occultation: results from end-to-end simulations in windy air

“…The thermodynamic state variables, including temperature, pressure, and humidity, are obtained from the LMO-based MW retrieval of EGOPS (Schweitzer et al, 2011a). These variables, in combination with the altitude level information, are the basis for the LIO-based IR retrieval of xEGOPS (Proschek et al, 2011).…”

“…Both the MW and IR-laser signals undergo refraction, defocusing, and absorption loss (Kursinski et al, 2000). The IR-laser signals were treated to be additionally influenced by aerosol extinction (medium aerosol load), Rayleigh scattering, and scintillations (Schweitzer et al, 2011a).…”

“…In this way the transmission purely due to an individual target GHG becomes known with only negligible influence of foreign species. A so-called absorptive Abel transform (Kursinski et al, 2002;Schweitzer et al, 2011a) is then used to derive the target species absorption coefficient on the IR altitude grid. Finally, the target species absorption coefficient and a modeled absorption cross section of the target species (via RFM based on initial/background target species profile and p and T profiles from LMO) are used to calculate the retrieved GHG VMR profile.…”

“…Schweitzer et al (2011a) have analyzed these clear air influences in detail and Proschek et al (2011) have studied the GHG retrieval performance under these conditions. Recently Proschek et al (2014) have also carefully studied the influences of clouds and how retrievals successfully perform over broken cloudiness.…”

Greenhouse gas measurements over a 144 km open path in the Canary Islands

“…The laser radiation is absorbed by molecules in the atmosphere and is subject to a number of "broadband" effects, such as aerosol and Rayleigh scattering, atmospheric scintillation and cloud absorption, that have weaker wavelength dependence than sharp molecular absorption lines (Schweitzer et al, 2011a). The influence of these broadband effects can largely be cancelled by making a differential measurement with two laser beams: one tuned to the peak absorption of a suitable vibration-rotation line and the other to a nearby "reference" wavelength subject only to broadband effects .…”