Despite its many features, GPS RO is limited by its L-band wavelengths, selected to minimize GNSS signal interactions with the atmosphere. With funding from NSF, we have been developing a RO system that profiles the atmosphere via cm and mm wavelength absorption lines of water vapor and ozone in a satellite to satellite occultation geometry. This system, called the Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS), combines key features of GNSS RO and MLS, together with several additional features.
ATOMMS measures both the speed and absorption of light to profile water vapor, temperature, pressure simultaneously, which GNSS RO cannot do. Its profiles of water vapor, temperature and pressure heights will extend from near the surface into the mesosphere generally with ~1%, 0.4K and 10 m precision respectively and still better accuracy, with 100 m vertical resolution and a corresponding horizontal resolution of approximately 100 km. Ozone profiles will extend upward from the upper troposphere.
ATOMMS is designed as a doubly differential absorption system to eliminate drift and enable it to both see clouds as well as see thru them to deliver performance in clouds within a factor of 2 of that in clear skies. This all-weather sampling combined with its insensitivity to surface emissivity avoids the sampling biases that limit most existing satellite observations and records.
ATOMMS will profile slant liquid water through clouds as well as larger ice particles. Its range of wavelengths provide unique constraints on the raindrop particle size distribution and rain rates. It also profiles atmospheric turbulence via scintillations (“twinkling of a star”).
Line of sight wind profiles will extend upwards from the mid-stratosphere. Horizontal pressure gradients from ATOMMS unique pressure height profiling down to the surface (unlike GPS RO), will constrain the balanced portion of the winds.
ATOMMS combined vertical resolution and precision will profile atmospheric stability down to the surface from orbit (unlike GNSS RO) providing much needed information for predicting convection, precipitation and severe weather, particularly in remote regions of the globe. Thus ATOMMS is likely the closest capability yet to a global, sonde-like profiling capability from orbit, with significantly better absolute accuracy than sondes.
Furthermore, as an active spectrometer, ATOMMS will refine the line shape spectroscopy from orbit, as needed to extract the full potential accuracy in its absorption profiles as well as improve the accuracy derived from other remote sensing instruments using these same absorption lines.
With funding from NSF, we built and used prototype ATOMMS instrumentation on the ground to demonstrate some of its capabilities such as precisely measuring water vapor, cloud water, rainfall, turbulence and absorption line spectroscopy. In particular, water vapor was derived in clouds and rain with less than 1% ambiguity for optical depths up to 17, demonstrating ATOMMS's remarkable dynamic range.
The key challenge in implementing an orbiting ATOMMS capability is the need to place both transmitters and receivers in orbit, unlike GNSS RO which uses existing orbiting GNSS transmitters. Multiple satellites are needed to achieve the sampling densities needed for numerical weather prediction (NWP) and climate applications. A 12 satellite ATOMMS constellation would deliver 2,000 occultations per day, comparable to present day GNSS RO densities. 50 such satellites could provide 35,000 occultations per day. While previously considered outside the realm of possibility, very small GNSS RO receivers and cubesats being developed for GNSS RO constellations at a small fraction of the cost of JPSS opens analogous possibilities for ATOMMS. The key to an ATOMMS constellation is developing compact, low power transmitting and receiving ATOMMS instrumentation operating on small, dedicated microsatellites. We are pursuing such designs.
ATOMMS' most obvious application is climate monitoring as it was originally conceived to do. ATOMMS' unambiguous, high information content and its independence from climate models make it near-ideal for assessing climate models to improve them and reduce future uncertainty. As one NOAA scientist recently noted, if we lived in a sane world, ATOMMS would already be in orbit, creating the long term record needed to determine how climate is changing.
Based on GNSS RO's demonstrated impact on NWP and ATOMMS' substantially higher information content per occultation, ATOMMS promises significant NWP impact, at a minimum via bias-correcting other measurements and substantially more if a sufficient number of satellites can be placed into orbit.
At high latitudes, present satellite ambiguities associated with clouds and variable surface conditions has limited progress toward reducing the large spread in predicted ice melt. ATOMMS' unambiguous water vapor, temperature and cloud profiling down to the surface, in all-weather and surface conditions, will provide key constraints needed to answer open questions and reduce present model spread. In the upper troposphere and lower stratosphere, the precision and vertical resolution of ATOMMS temperature, water vapor and ozone profiles and ability to penetrate through clouds are well-suited to addressing key scientific questions there.
ATOMMS ability to profile down to the surface including turbulence offers a potential new set of observational constraints for understanding turbulent surface fluxes, particularly over the oceans, a key but poorly constrained variable in the global energy budget. ATOMMS quantitative global all weather information on temperature, water and winds promise much needed, detailed constraints to address several of the WCRP grand challenges tied to the hydrological cycle.
We will summarize the ATOMMS concept, performance and progress to date toward a versatile new global remote sensing system capable of delivering levels of performance from orbit approaching a global field campaign.