Introduction

The Active Temperature, Ozone and Moisture Microwave Spectrometer (ATOMMS) is a natural extension of the GPS radio occultation (RO) concept that uses at least two frequency bands approximately 10 and 100 times higher than GPS frequencies to probe the 22 GHz and 183 GHz water lines and 184 GHz and 195 GHz ozone lines. A third band between 500 GHz and 600 GHz would also be quite useful for profiling H2O and its isotopes and initializing the hydrostatic integral.

Serious work on the ATOMMS concept began in 1998 as the Atmospheric Moisture and Ocean Reflection Experiment (AMORE) (which included the 22 GHz

Institute of Atmospheric Physics, University of Arizona, Tucson, Arizona, USA e-mail: [email protected]

portion of the ATOMMS occultation concept) was proposed to NASA as an Earth System Science Pathfinder (ESSP) mission. While AMORE was technically too immature for selection, the same year NASA did fund the Atmospheric Temperature, Ozone and Moisture Sounder (ATOMS), a joint effort between the University of Arizona and JPL that included both the 22 GHz and 183-195 GHz occultation concepts via its Instrument Incubator Program (IIP). AMORE and ATOMS also triggered parallel research and proposal efforts in Europe such as ACE+ and ACCURATE. In the US, the National Science Foundation (NSF) has funded the ongoing development and refinement of the ATOMMS concept since 2001 including funding in 2007 the ATOMMS aircraft to aircraft occultation demonstration that is summarized at the end of this paper. We also note the Mars Atmospheric Climate Observatory (MACO) mission concept that is focused on the Martian hydrological cycle using water occultation observations at 183 GHz supplemented by other observations. MACO was developed with seed funding from NASA in 2000 for the Mars Scout opportunity in 2002 (Kursinski et al. 2004b). While it was not selected for a Phase A study for the second Mars Scout opportunity in 2006 because it was viewed as too high risk, MACO has been recognized as a revolutionary concept for quantitatively determining the Martian hydrological cycle and climate and will hopefully become a real mission in the future.

ATOMMS is designed to address key open questions about climate such as, "Is the upper troposphere warming faster than the lower troposphere and the surface", "Where is the transition between tropospheric warming and stratospheric cooling", and the closely related question: "How are lapse rates adjusting to the changes in vertical heating and dynamical feedbacks associated with climate change".

ATOMMS has a niche in the upper troposphere/lower stratosphere (UTLS) regime where water vapor and ozone are very important radiatively. Our ability to measure vertically resolved water vapor in the upper troposphere under all sky conditions has been close to nil. Existing observational techniques have very different types of uncertainties, errors, and resolutions and the comparisons have not agreed very well, problems that can be resolved by ATOMMS. ATOMMS can answer fundamental open questions on basic behavior and trends in the UTLS regime.

In this context, it is important to note that the problem of deriving of atmospheric temperature and constituent profiles from satellite measurements of radiance emitted by the atmosphere and surface is mathematically ill-posed (e.g., Huang et al. 2002) A continuum of profiles exists that are consistent with the radiance measurements and selecting a "unique" profile from this continuum requires imposing additional constraints and assumptions that have their own implicit climatology. From the standpoint of determining unambiguously how our climate is changing and evaluating model realism and predictive skill, this situation is quite poor because the imposed climatological assumptions imbedded in the "unique" profile solutions are inseparable from true climatological behavior.

Analyses produced by Numerical Weather Prediction (NWP) centers are quite powerful. However, in a climate context, the analyses are inherently ambiguous because they suffer from the same problem in that the analyzed state estimate is based in part on atmospheric models that contain unknown errors. In terms of evaluating and improving climate model realism and accuracy, which is a critical goal of climate change research, analyses are incestuous because they are not independent of the quantity, the model, which is being assessed. With this in mind, an overarching goal of ATOMMS has been to create an observing system capable of estimating several key climate state variables and determine how the climate system is truly evolving, independently of atmospheric models.

ATOMMS was conceived to overcome the wet-dry ambiguity that limits direct interpretation of the GPS RO refractivity profiles in the warmer (> 240 K) regions of the troposphere because the wet and dry contributions to refractivity cannot be separated using refractivity measurements alone. By measuring both bending and absorption along each occultation signal path, each ATOMMS occultation provides sufficient information to resolve the wet-dry ambiguity and simultaneously determine temperature, pressure, and moisture directly from the observations themselves.

Based on our assessment described below, ATOMMS will profile water vapor, temperature, and pressure from near the surface to the mesopause and ozone through the middle atmosphere down into the upper troposphere. It will do so in both clear and cloudy conditions to provide the first unbiased characterization of the troposphere. With the proper choice of frequencies, it will also determine water isotopes and winds in the middle atmosphere (Kursinski et al. 2004a). Occultations are extremely well suited for observing certain scales of behavior and ATOMMS will determine telltale signature of processes that must be represented accurately in models. ATOMMS' four orders of magnitude lower sensitivity to the ionosphere vs. GPS means the ATOMMS profiles are essentially insensitive to the solar and diurnal cycles in the ionosphere (unlike GPS) and will extend to the mesopause or higher depending on the choice of signal frequencies as discussed below. ATOMMS will also determine any subtle systematic GPS RO ionospheric errors over the solar and diurnal cycles.

ATOMMS will measure behavior at important scales of variability including in and below clouds thus avoiding biases that result for sensors that are incapable of probing below clouds. ATOMMS will monitor trends and variability and separate behavior in the free troposphere from that in the planetary boundary layer (PBL). The ATOMMS information will be used to understand the processes controlling moisture in troposphere and stratosphere and coupling to clouds and precipitation and in general improve physical model representations for future weather and climate predictions.

In this paper, we summarize several important aspects of ATOMMS that have evolved significantly since OPAC-1. We have developed a new approach to the hydrostatic upper boundary condition derived directly from the ATOMMS observations. Our understanding of the impact of the effects of turbulence and how to mitigate them has improved dramatically since OPAC-1. We also summarize a new approach that we have developed to derive atmospheric profiles in the presence of inhomogeneous liquid water. We conclude with a summary of our planned ATOMMS aircraft-aircraft occultation demonstration that represents a major step towards an orbiting ATOMMS observing system.

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