Greenhouse gas profiling by infrared-laser and microwave occultation: retrieval algorithm and demonstration results from end-to-end simulations

Abstract

Abstract. Measuring greenhouse gas (GHG) profiles with global coverage and high accuracy and vertical resolution in the upper troposphere and lower stratosphere (UTLS) is key for improved monitoring of GHG concentrations in the free atmosphere. In this respect a new satellite mission concept adding an infrared-laser part to the already well studied microwave occultation technique exploits the joint propagation of infrared-laser and microwave signals between Low Earth Orbit (LEO) satellites. This synergetic combination, referred to as LEO-LEO microwave and infrared-laser occultation (LMIO) method, enables to retrieve thermodynamic profiles (pressure, temperature, humidity) and accurate altitude levels from the microwave signals and GHG profiles from the simultaneously measured infrared-laser signals. However, due to the novelty of the LMIO method, a retrieval algorithm for GHG profiling is not yet available. Here we introduce such an algorithm for retrieving GHGs from LEO-LEO infraredlaser occultation (LIO) data, applied as a second step after retrieving thermodynamic profiles from LEO-LEO microwave occultation (LMO) data. We thoroughly describe the LIO retrieval algorithm and unveil the synergy with the LMOretrieved pressure, temperature, and altitude information. We furthermore demonstrate the effective independence of the GHG retrieval results from background (a priori) information in discussing demonstration results from LMIO end-toend simulations for a representative set of GHG profiles, including carbon dioxide (CO 2 ), water vapor (H 2 O), methane (CH 4 ), and ozone (O 3 ). The GHGs except for ozone are well Correspondence to: V. Proschek (veronika.proschek@uni-graz.at) retrieved throughout the UTLS, while ozone is well retrieved from about 10 km to 15 km upwards, since the ozone layer resides in the lower stratosphere. The GHG retrieval errors are generally smaller than 1 % to 3 % r.m.s., at a vertical resolution of about 1 km. The retrieved profiles also appear unbiased, which points to the climate benchmarking capability of the LMIO method. This performance, found here for clear-air atmospheric conditions, is unprecedented for vertical profiling of GHGs in the free atmosphere and encouraging for future LMIO implementation. Subsequent work will examine GHG retrievals in cloudy air, addressing retrieval performance when scanning through intermittent upper tropospheric cloudiness.

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

“…This together with a high signalto-noise ratio and a self-calibration step in the retrieval algorithm, which is intrinsic to the occultation method since the transmitted intensities during an occultation event are generally normalised with the unattenuated intensity measured at the top of the atmosphere, is the reason why the LIO retrieval results are expected to be very accurate and essentially free of measurement biases Schweitzer, 2007, 2011;Schweitzer, 2010;Proschek et al, 2011a). This means that the retrieval results are of high quality on a single-occultation-event basis, and therefore effectively independent of the specific LMIO mission that provides the measurements.…”

“…wind speed (besides the byproducts mentioned above; details on the trace species retrieval are given by Proschek et al, 2011a). The retrieval principle is based on differential transmission (Kursinski et al, 2002;Gorbunov and Kirchengast, 2007), alternatively also called differential absorption principle.…”

“…We also show the total impact of these atmospheric influences on the received signal intensity; this total influence is an important ingredient for power-budget calculations along the LEO-LEO intersatellite link and is needed to determine power requirements in an LIO mission design. Likewise it is an important input to generate simulated measurements subsequently used in retrieval processing in the frame of end-to-end simulations (Proschek et al, 2011a).…”

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

“…While the accuracy of this first demonstration experiment is not ideal, previous studies Proschek et al, 2011) indicate that greenhouse gas profiles for an ACCURATE mission are obtainable with < 1 to 4 % rms error (outside clouds; above 5 km; the goal for CO 2 is < 1 %). The sources of error contributing to the value of ± 15 ppm are expected to be significantly smaller for an ACCURATE mission than in this least-cost demonstration.…”

“…The detector offset error is a fixable issue (see Appendix A5), and significantly more accurate frequency knowledge and higher signal-to-noise ratios will be available from advanced instrumentation. Furthermore, accurate temperature, pressure and humidity will be determined from simultaneous microwave occultation measurements (Kirchengast and Schweitzer et al, 2011b), and a more accurate retrieval algorithm (Proschek et al, 2011) will be used to extract greenhouse gas concentrations from the infrared occultation measurements.…”