Ground-Based Sounders as a Solution to Infrared Sounding in Cloudy Environments

Infrared (IR) sounding of the atmosphere from space has been a feature of the operational observing system for decades. However, current space-based IR sounding methods lack sufficient information content to adequately resolve the structure of the boundary layer. Furthermore, IR sounding is frequently limited to the upper levels of the troposphere by the presence of clouds. Microwave sensors are able to profile below the cloud layer, but have worse vertical resolution than IR sounders.

The National Research Council has previously suggested the development of a nationwide network of ground-based thermodynamic profilers to supplement space-based sounders and improve observations of the planetary boundary layer. One of the instruments that could potentially be used in a future ground-based network of this type is the Atmospheric Emitted Radiance Interferometer (AERI). AERI measures downwelling IR radiance at approximately 1 cm^-1 resolution from 520 cm^-1 to 3000 cm^-1. This instrument is very similar to the space-based interferometers such as the Cross-track Infrared Sounder (CrIS) or the Infrared Atmospheric Sounding Interferometer (IASI) and has been deployed all over the world in a variety of climate regimes. The AERI has proven to be very effective at boundary layer observations, and the current retrieval algorithm, AERIoe, allows for the retrieval of thermodynamic variables up to cloud base.

Previous work has assessed information content of a synergy between the AERI and IASI instruments in clear sky scenes, revealing an increase in degrees of freedom and reduction in retrieval uncertainty throughout the troposphere, when compared to using one of the instruments individually. However, the benefits of a synergy between ground-based and space-based sounders have not been quantified in cloudy scenes. In theory, the ground-based sensor should provide information below the cloud layer, and the space-based sensor above, but quantifying that information and studying how that information is combined near the cloudy layer is important when considering the cost and potential gains for a theoretical ground-based network.

This study will work toward quantifying the gains in information from a synergy between the AERI and CrIS in an environment with an opaque cloud. The goal of this study is to quantify the increase in information able to be retrieved by a synergy of the two systems, as compared to using one of the instruments individually. The methodology of simulating IR radiative transfer in the presence of clouds will be presented as well.