S. Schweitzer

Greenhouse gas profiling by infrared‐laser and microwave occultation in cloudy air: Results from end‐to‐end simulations

The new mission concept of microwave and infrared-laser occultation between Low Earth Orbit satellites (LMIO) is capable to provide accurate, consistent, and long-term stable measurements of many essential climate variables. These include temperature, humidity, key greenhouse gases (GHGs) such as carbon dioxide and methane, and line of sight wind speed, all with focus on profiling the upper troposphere and lower stratosphere. The GHG retrieval performance from LMIO data was so far analyzed under clear-air conditions only, without clouds and scintillations from turbulence. Here we present and evaluate an algorithm, built into an already published clear-air algorithm, which copes with cloud and scintillation influences on the infrared-laser transmission profiles used for GHG retrieval. We find that very thin ice clouds fractionally extinct the infrared-laser signals, thicker but broken ice clouds block them over limited altitude ranges, and liquid water clouds generally block them so that their cloud top altitudes typically constitute the limit to tropospheric penetration of profiles. The advanced algorithm penetrates through broken cloudiness. It achieves this by producing a cloud flagging profile from cloud-perturbed infrared-laser signals, which then enables bridging of transmission profile gaps via interpolation. Evaluating the retrieval performance with quasi-realistic end-to-end simulations, including high-resolution cloud data and scintillations from turbulence, we find a small increase only of GHG retrieval RMS errors due to broken-cloud scenes and the profiles remain essentially unbiased as in clear air. These results are encouraging for future LMIO implementation, indicating that GHG profiles can be retrieved through broken cloudiness, maximizing upper troposphere coverage.

Assessing the relations between spectral sensitivity and integrated water vapor for NDSA processing applied to a radio link between two LEO satellites

The NDSA (Normalized Differential Spectral Attenuation) approach is based on the conversion of a spectral parameter (the spectral sensitivity S) derived from power measurements, into the total content of water vapor (IWV, Integrated Water Vapor) along the propagation path between the two LEO satellites, through pre-determined IWV-S relations. This paper shows how some problems concerning the relationships between IWV (Integrated Water Vapor) and S could be overcome. In fact, two basic problems affected the reliability of such empirical IWV-S relations found so far: the first was the fact that the accuracy of the radiosonde data used to derive them was not uniformly distributed in the northern and southern hemisphere; the second was the limited amount of radiosonde data available at the highest altitudes (above 10 km), and their scarce reliability. Furthermore, the problem of correcting for the presence of liquid water needed to be considered. Here we present the results of a global scale analysis of the IWV-S relations made utilizing the ECMWF global atmospheric model. S and IWV were simulated and computed at all altitudes from 0 to 20 km, obtaining IWV-S relations for 17, 19, 21, 179 and 182 GHz. Also, the correction of IWV estimates by the presence of liquid water is shown to be effective by using an additional frequency around 30 GHz.