As defined in the 2007 Decadal Survey's recommended CLARREO mission (Anderson, 2007), observations of climate are needed to monitor both the radiative energy (im)balance (short wave in and IR out) and the evolution of the climate state. In addition, detailed observations are needed to constrain key processes at work in the climate system. To determine and improve the realism and predictive skill of climate models, observations must provide these capabilities as independently from the models as possible. This is challenging because satellite observations typically do not contain sufficient information to determine the climate state uniquely and unambiguously (e.g. Rodgers, 2000). One of the great contributions of NWP has been the generation of analyses that in some sense optimally combine the information from observations with model forecasts to estimate the system state. The problem, from a climate standpoint, is that some portion of the model climatology, whose realism needs to be assessed, is imbedded within the analyses.

To address these fundamental observational needs as they relate to water vapor, ozone and temperature, we have developing a new remote sensing technique that combines features of GPS Radio Occultations (RO) and NASA's Microwave Limb Sounder (MLS). The technique, referred to as the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS), uses satellite-to-satellite occultations to actively probe the atmospheric absorption lines that MLS probes passively (Kursinski et al., 2002).

ATOMMS overcomes several key limitations of GPSRO such as directly and simultaneously profiling water and temperature independent of external information and reducing the sensitivity to the ionosphere by 4 orders of magnitude and eliminating leakage of subtle solar cycle ionospheric signatures into the lower atmosphere profiles. Probing via occultation offers several key advantages over passive emission including

• an order of magnitude better vertical resolution (100-200 m vs. 2-3 km),

• simple and unique retrievals,

• very high SNR and the precision needed to capture variability & signatures of processes,

• all-weather sampling that eliminates clear sky-only biases

• self-calibration that eliminates long term drift.

Our estimates of typical precisions of the ATOMMS observations of temperature, geopotential height and moisture profiles are ~0.4 K, 10 m and 1-3% respectively extending from the lower troposphere to the mesopause. In the tropics, the very high accuracy profiles extend down to ~3 km whereas in cold high latitude winter conditions, the high accuracy extends right to the surface. With averaging, these individual profile errors should decrease by at least an order of magnitude. Similar performance for ozone profiles will extend from the upper troposphere into the mesosphere. Performance in cloudy conditions will be within a factor of 2 of clear sky performance. With additional signal frequencies, other trace constituents such as water isotopes can be measured in the upper troposphere and above with similar performance.

NSF has funded the initial development and demonstration of the ATOMMS concept and performance through its Major Research Instrumentation (MRI) program. The first demonstration is planned for the spring of 2009 and will use two high altitude NASA WB-57F aircraft. The instrument is being built at the University of Arizona. The instrument is implemented as two halves with one residing in one plane and the other in the other plane. Essentially one half is the transmitter and the other the receiver. The two WB-57F aircraft used each carries a precisely pointable nose that has been developed to image debris falling from the Space Shuttle during launch. For the demonstrations, we will replace the existing optical imaging system with the ATOMMS microwave instrument.

ATOMMS will act as a planetary-scale scintillometer providing very sensitive vertical profiles of turbulence globally. A key objective of the ATOMMS aircraft-to-aircraft demonstration is to profile the turbulence and evaluate its impact on the ATOMMS' retrieved water vapor, ozone temperature and geopotential profiles.

Ultimately we are working toward a microsatellite constellation that will provide full sampling of the diurnal cycle each orbit as the COSMIC GPSRO mission does. The unique coverage and performance of this system would fill many of the critical needs defined for a global climate observing system.

References:

Anderson, D. (2007) Decadal Survey CLARREO Workshop Report, available at nasascience.nasa.gov/earth-science/decadal-surveys/Decadal_Survey_CLARREO.pdf

Anderson J., et al., 2007, The climate benchmark constellation: A critical category of small satellite observations, http://map.nasa.gov/clarreo_materials.html.

Kursinski, E. R., et al., J. Atmos. Oceanic Technol., 19, 1897-1914, 2002.

Rodgers, C., Inverse methods for atmospheric sounding: Theory and Practice, World Scientific Publishing, 2000.