ATOMMS: A Next-Generation Occultation System for Earth

I am leading an effort to develop a next-generation cm and mm wavelength satellite-to-satellite occultation measurement system that we refer to as the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS) with funding previously from NASA and presently from NSF. This new class of active, orbiting, limb-viewing spectrometer measurements is essentially a cross between GPS occultations and the microwave limb sounder (MLS).

Rationale

Assessing and quantifying our changing climate requires a precise, accurate, all-weather global observing system that determines the atmospheric climate state revealing the important scales of both its slow evolution and its more rapid variations that reveal the processes at work. GPS RO has many powerful features for monitoring climate but some key limitations as well. In particular, interpreting the GPS RO observations is limited by the wet-dry ambiguity inherent to tropospheric refractivity in regions of the troposphere warmer than 240K. Resolving the wet-dry ambiguity requires combining the GPS-derived refractivity with other information. However, other satellite observations are generally not spatially and temporally coincident with the GPS measurements and, more importantly, do not contain vertical information equivalent to that of the GPS RO observations. While data assimilation in theory solves this problem, the state estimate produced by data assimilation relies on an atmospheric model. This creates a rather odd and ambiguous situation because, despite our uncertainty in the realism of weather/climate models, we are using them to estimate the atmospheric state to understand climate and how it is changing.

When you don’t understand something, you measure it.

In my opinion, unambiguously answering basic questions about our changing climate and climate processes requires a global, all-weather measurement system that is independent of models and initial assumptions.

ATOMMS Capability Summary

The ATOMMS observations will yield a quantum step in performance relative to passive observations in terms of vertical resolution, precision and accuracy of moisture, ozone, temperature and pressure measurements in both clear and cloudy conditions. Kursinski et al. (2002) estimated that typical precisions of the ~200 m vertical resolution temperature, geopotential height and moisture profiles will be ~0.4 K, 10 m and 1-3% respectively extending from near the surface to the mesopause (ionosphere effects at mm-wavelengths are negligible). 1-3% precision ozone profiles will extend from the upper troposphere into the mesosphere. With additional signal frequencies, other trace constituents such as water isotopes, can be measured in the upper troposphere and above with similar performance. The system will also measure cloud liquid water and ice content. Based on preliminary estimates for cloudy conditions, we anticipate performance will be no more than a factor of two worse than clear sky performance.

The accuracy of the moisture and ozone profiles will be at least as good as the individual profile precision and may be one to two orders of magnitude better depending on our spectroscopic knowledge (which we will refine from orbit during the mission). The system is self-calibrating and experiences no drift because the signal source is viewed either immediately before or after each occultation.

In my view the importance of such a system lies in its unique capabilities for defining climate, monitoring changes in climate and creating unique constraints needed for determining processes. In May 2005 I submitted a white paper describing the importance of such a system to the National Research Council Decadal Study for NASA and NOAA (NRC White paper).

Estimating the Accuracy of ATOMMS

Kursinski et al. (2002a) developed a first order assessment of the performance of a combined 22 and 183 GHz system in clear conditions.

Temperature profiling

The ATOMMS RO measurements at much shorter wavelengths have several very important advantages over GPS RO:

1. The high ATOMMS frequencies are insensitive to the ionosphere eliminating an important GPS RO error source.

2. As a result, ATOMMS temperature profiles will extend up to the thermosphere,

3. The absorption information removes the need for an independent estimate of temperature to start the hydrostatic integral creating temperature estimate fully independent of other measurements or models.

4. The absorption information allow very accurate ATOMMS temperature profiles to extend down the surface at high latitudes and to ~3 km in the tropics.

Separating the effects of liquid water and water vapor.

Kursinski et al. (2002b) demonstrated that in the presence of spherically symmetric clouds, ATOMMS can profile water vapor, temperature, pressure and liquid water clouds almost as well as under clear conditions. Clouds are of course notoriously inhomogeneous and clumpy. With NSF funding, we are developing and assessing a promising retrieval approach that separates the liquid water and vapor effects in the presence of inhomogeneous liquid water clouds. A manuscript describing and assessing this approach is underway.

Scintillations

The short wavelengths of ATOMMS are more sensitive to turbulence than GPS resulting in phase and amplitude scintillations (the twinkling of a star). We are estimating the scintillations associated with turbulence and assessing and mitigating their impact on the ATOMMS occultations.

Spectroscopy

Interpreting the extremely high precision of the ATOMMS measurements pushes the limits of present microwave spectroscopy. We are working with researchers at JPL to refine the spectroscopy. Angel Oarola, my graduate student, has refined the existing spectroscopic models describing the dielectric properties of liquid water and has submitted a manuscript describing his model. We are using the model to evaluate ATOMMS.

ATOMMS Instrument Development

I am working with Professor Chris Walker in the UA Astronomy Department developing the instrument design and developing a prototype.

ATOMMS mission concept

Ultimately a constellation of approximately a dozen small satellites carrying ATOMSS instrumentation would provide long-term, all-weather characterization of the thermodynamic state of Earth’s troposphere and middle atmosphere fully sampling the diurnal cycle each day with more than 1,000 cm and mm-wave occultations and 6,000 to 8,000 GPS occultations precisely profiling the near-surface environment through the middle atmosphere and ionosphere. The unique coverage and performance of this system will fill many critical needs defined for a global climate observing system. A suite of orbiting platforms consisting of ATOMMS and complementary passive systems with higher horizontal resolution and more continuous coverage, together with particulate-sensitive active observations characterizing aerosols, clouds and precipitation will be a dramatically more powerful global observing system than exists today for understanding, monitoring and forecasting Earth’s climate and enhance existing and planned weather.

The cost of putting such a constellation of transmitters and receivers into orbit is non-trivial, probably ~$400M. Given the escalating cost of NPOESS and the ~$250M daily cost of ongoing foreign wars, this is certainly easily achievable. The real question is, “Are climate change and the related policy decisions sufficiently important to justify a true climate-quality global observing system?” The answer in our opinion is an obvious YES and the problem then becomes getting ATOMMS into orbit ASAP to begin creating its long-term climate record.

Brief ATOMMS History

In 1997, several of us at JPL and University of Arizona began exploring how frequencies significantly higher than GPS could be used measure water vapor via its attenuation due to its 22 GHz water line and continuum spectral signatures. Initial results were presented at the Fall AGU meeting that year. In 1998, several of us wrote two large proposals at JPL involving University of Arizona researchers as well. The first, the Atmospheric Moisture and Ocean Reflection Experiment (AMORE) responded to the NASA ESSP AO. The second, the Active Tropospheric Ozone and Moisture Sensor (ATOMS) was in response to the NASA Instrument Incubator Program (IIP) AO. AMORE was to have been piggybacked on the COSMIC mission satellites yielding a combined mission entitled COSMIC-AMORE. While AMORE was very highly ranked scientifically, it was also ranked as technically immature and not selected. Funding for ATOMS (which was selected) allowed us to further develop the high frequency occultation capability.

In 2001 William Folkner and I developed the Mars Atmospheric Constellation Observatory (MACO) mission concept using the same idea at Mars (see MACO). In 2002, a European group headed by Gottfried Kirchengast and Per Hoeg took the atmospheric side of AMORE and proposed it to ESA where it proved too expensive to fit within the ESA budget.