Objective Comparison of High Resolution Far- and Mid-infrared Spectral Observations for Atmospheric Retrievals

Aronne Merrelli University of Wisconsin
David Turner National Oceanic and Atmospheric Administration

Category: Radiation

Working Group: Cloud Life Cycle

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Preliminary results using the measured climatology from the TWP Darwin site. We used LBLRTM to model the infrared spectra and compute Jacobian matrices at the mean state. The results shown are from a simple linear retrieval, which directly uses the mean state Jacobian to estimate the profile.

High spectral resolution observations of the atmospheric emission spectrum in the infrared have proven utility for atmospheric thermodynamic state retrievals. Current and future operational satellite instruments (AIRS, IASI, CrIS) produce these measurements in the mid-infrared (MIR) portion of the spectrum (roughly 4–15 μm) on a routine basis. In comparison, the far-infrared (FIR) portion of the spectrum (wavelengths longer than 15 μm) is poorly observed, mainly due to sensor limitations in this wavelength region. In this study we are investigating the differences between retrievals based on MIR and FIR measurements to understand how FIR measurements will improve our ability to sense the atmosphere. Water vapor, liquid water, and ice all have different absorption properties in the MIR and FIR spectral regions. For example, the FIR contains the rotational absorption band of water vapor, with no significant interference from minor atmospheric constituents, while the MIR contains the vibrational-rotational absorption band of water vapor, along with interference from minor atmospheric constituents such as methane. We are developing a modeling framework to simulate high spectral resolution observations and retrievals using both FIR and MIR spectra, using an optimal estimation approach (C. Rodgers, 2000). Our initial work focuses on clear-sky atmospheres, but the performance in cloudy conditions, especially ice clouds, is the primary objective of our research due to the markedly different absorption characteristics of ice and liquid in the FIR and MIR. The simulation and retrieval framework is based on climatological priors generated from radiosonde profiles collected at the ARM ground sites. The retrieval algorithm is applied consistently on the simulated observations from both spectral regions. With this approach, we can compare the available information content in the MIR and FIR for temperature and water vapor retrievals. Although the high altitude temperature retrievals will be very similar for both spectral regions (each contains half of the carbon dioxide absorption band at 15 μm), the water vapor retrievals will have different information due to the different absorption bands.

This poster will be displayed at ASR Science Team Meeting.

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