25th JSC Session

Microwave remote sensing measurements have played an important role as a probe into understanding the chemistry and physics of the Earth's atmosphere, and their change caused by increased human activities. Sub-millimeter (submm)-wave technology can be said to be still in its infancy compared with spectroscopic techniques in other frequency regions, such as optical and infrared.

At present, a new generation of instruments toward the higher frequency measurements, submm wave and THz region is emerging in response to the increased demands imposed by near-future sensing technology to fulfill increased accuracy and precision. In general, detection sensitivity can be higher in submm wave region due to the stronger line intensity than that in the millimeter region, making submm wave observation more advantageous in identifying molecular species in the Earth's atmosphere. State-of-the-art detector systems will be implemented in these remote sensing instruments, giving superb sensitivity, and a number of different species can be monitored simultaneously; this is essential in determining the chemical interaction schemes occurring in the atmosphere of our own planet.

The UARS/MLS (Upper Atmosphere Research Satellite/Microwave Limb Sounder) was the first satellite in millimeter-wave region to monitor molecules in the Earth's atmosphere. The Odin/SMR (Sub-Millimeter Radiometer) launched in February 2001 is the first satellite in the submm region. The AURA/MLS will be launched in June 2004, which will probe standard molecules and will aim at a THz line of OH. JEM/SMILES, planned to be launched in 2008, will be equipped with an SIS (Super-conductor Insulator Super-conductor) receiver system. It will be more sensitive (by a factor of 6-20) than standard Schottky-Barrier-Diode receivers, thanks to updated super-sensitive detection technology. This new instrument will open up new horizons to observe species, which have been very difficult to measure otherwise due to their weak spectroscopic line intensities, low concentration, or rapidly changing altitude dependence that makes a long accumulation of their signal impractical. Retrieval of meaningful information from the atmospheric measurements critically depends on the accuracy of the laboratory spectroscopic measurements.

The aims of the workshop were: (i) to discuss the accuracies of molecular parameters required and, in particular, the discrepancies often found among the data from various groups; (ii) the parameterisation of the atmospheric foreign continuum, which dominates in atmospheric measurements, to derive the accurate abundance of the water vapour from upper troposphere to lower stratosphere. This second topic includes the measurements of the foreign continuum from the millimeter to far-infrared regions, its simulation based on the recent models, the observations from space to derive Upper Tropospheric Humidity (UTH), and the parameterisation of the foreign continuum on the basis of the theory of the molecular collision complex in the atmosphere.
We believe it is very important that the atmospheric community from North America, Europe and Japan got together to discuss the accuracy requirements imposed upon the experimental data and to sort out the sources of discrepancies often found among the data obtained by various groups. In October 2001, a similar meeting was held under the sponsorship of NASA/JPL in San Diego, USA [1]. This workshop was not exactly intended to be a follow up of the San Diego meeting, but we hoped it would come up with some positive notes to solve outstanding problems, such as more consistent and accurate determination of the temperature dependence of the pressure broadening coefficients. Background continuum in the submm -wave region that is not well characterized was also one of the major topics. This is of common interest in both atmospheric and astronomical observations. It was very rewarding to have had contributions in this workshop from both sides, as well as from detailed theories.
The workshop was attended by 43 researchers in the areas of atmospheric remote sensing, atmospheric opacity for astronomy and molecular spectroscopy.

Discussions

1. Microwave Spectroscopy

A long list of tropospheric and stratospheric molecules (H₂O, HDO, O₃, O₃ isotopes, HCl, CO, N₂O, HNO₃, HCN, H₂CO, CH₃CN, H₂O₂, ClO, HOCl, BrO, HOBr, HO₂, SO₂, OH, etc.) are monitored by current and up-coming submm radiometers (e.g., MLS, ASUR, Odin/SMR, B-SMILES, JEM/SMILES). The accuracy of molecular parameters and discrepancies of the data from various groups were discussed in this session. F. DeLucia, B.J. Drouin, G. Wlodarczak, G. Cazzoli, and T. Amao presented the current status and some of the outstanding problems about the systematic errors. They carried out precise rotational linewidth measurements, but statistically significant discrepancies persist among those groups. Inter-comparison of microwave spectral lineshape measurements should be made to resolve this issue.

From the discussions that followed the oral and poster presentations, it was generally agreed that the accuracy of the spectroscopic parameter seemed to be good for the broadening parameters but the temperature dependences turned out to be much more problematic, presumably due to the systematic errors on the temperature measurements.

2. Accuracy required from submm remote sensing observations

This session was addressed to discuss the accuracy of the molecular parameters required from the current satellite submm-wave remote sensing mission. C. Verdes pointed out that the quantity measured by the satellites contains implicit information on the atmospheric state (e.g., molecular species volume mixing ratio profiles, temperature profile). An uncertainty in the spectroscopic parameters will lead to a systematic retrieval error. Therefore, a thorough and careful investigation into the current accuracy of the spectroscopic parameters and their impact on the retrieval are necessary. She gave the requirements for accuracy of the spectroscopic parameter error from the retrieval results from a MASTER study in the 300 GHz region. J. Urban showed the overview of Odin/SMR measurements and retrieval. He also discussed the required accuracy of pressure broadening parameter of H₂O, HDO, and H₂¹⁸O from the sensitivity study of the Odin/SMR measurements. P. Hartogh gave a talk on the future of submm wave limb emission sounders. The Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory (HSO) and the German Receiver for Astronomy at THz frequencies (GREAT) on the Stratospheric Observatory for Infrared Astronomy (SOFIA) are two new microwave instruments covering submm wave bands between 500-1900 GHz (HIFI) and 1600-5000 GHz (GREAT). In preparation of the anticipated operation of the instruments in 2005 (GREAT) and 2007 (HIFI), they have performed microwave radiative transfer and retrieval calculations for a number of molecules with the main emphasis on water vapour. The boundary conditions of the modeling were briefly described and some results including modeled spectra and required observation times were presented. J. Inatani, who is the PI for the SMILES instrument team, presented the Current Status of JEM/SMILES. Y. Kasai discussed the accuracy of O₃ isotope measured by ASUR and SMILES, both using high sensitivity super-conductive SIS receivers.

From the discussions following the oral and poster presentations, the spectroscopic requirements that were deemed to be important for atmospheric sensing are: (1) Uncertainty of the pressure broadening parameter g0 (broadening coefficient at 296K) should be less than 3 % for the molecules, such as ozone; (2) The retrieval result is not very sensitive with temperature dependence, as when compared with g0. (Figure 1)

Figure 1. Retrieval case study for Odin/SMR observations of H2O-16 (left) and HDO (right) taken on September 12, 2002 around the equator using two bands centred at 488.9 and 490.4 GHz, respectively. Top: spectral measurements (thin black lines) and fits (thick gray lines) for tangent altitude of 20, 30, 40, and 50km. Bottom: retrieved profiles with error bars. Thick error bars indicate the error due to intrinsic receiver noise, thin error bars represent the total retrieval error including also the smoothing error due to the limited altitude resolution of measurement. A priori profiles and errors are also plotted. Corresponding averaging kernel functions indicating altitude ranges and resolution (FWHM) are shown as well. (J. Urban and Odin/SMR team, Observatoire de Bordeaux)

This session was addressed to the atmospheric continuum in submm and far-infrared region. The final goal of this discussion was to derive accurate humidity from upper troposphere to lower stratosphere, for example UTH, from global observations. There are three different perspectives: (1) The recent measurements of the atmospheric background continuum from the millimeter to far-infrared regions; (2) The modelling of the atmospheric continuum by theory of the inter-molecular interaction; (3) How to derive UTH from the continuum measurements of the satellite microwave limb emission, and how important it is in atmospheric sciences?

(1) The atmospheric background continuum is of common interest to atmospheric scientists and astronomers; this component is better known as atmospheric opacity for astronomers. They have studied opacity of the atmosphere in order to observe the emission from molecules in the inter-stellar molecular clouds; Two astronomers, S. Matsushita and J.R. Pardo, gave a talk about Fourier Transform Spectrometer (FTS) measurements of atmospheric opacity and comparisons with the recent models;

(2) The atmospheric continuum is also of interest to the molecular physicists as an edge of the spectrum line shape due to a molecular collision complex formation; R.H. Tipping and Q. Ma presented theoretical research to explain the atmospheric continuum from the microwave to far-infrared region;

(3) The measurements of the atmospheric continuum by satellite microwave limb emission inform us about the humidity of the upper troposphere; W.G. Read presented a talk on the Measurement of UTH from the UARS/MLS and N. Eguchi discussed Intraseasonal variations of water vapour and cirrus clouds in the tropical upper troposphere using the UTH derived by W.G. Read et al.

From the discussions following oral presentation, it can be concluded that there is still large uncertainty between the observation and the model parameterisation about (even more than) 20 % in the submm and far-infrared region. We should continue to exchange information between the theory and observations to derived more accurate humidity from upper troposphere to lower stratosphere with high altitude resolution (Figure 2).

We wish to thank the participants for their enthusiasm during the workshop. We gratefully acknowledge Prof. Watanabe, Dean of Faculty of Science, Ibaraki University. The financial support was almost entirely provided by the SMILES Mission team of Communications Research Laboratory (CRL). We express our sincere gratitude to all who contributed their best efforts to make this workshop a great success, in particular, Dr. Manabe, SMILES Group leader in CRL.

[1] Workshop on Laboratory Spectroscopy Needs for Atmospheric Sensing, San Diego, California, 23-26 October 2001 Chair: Michael Kurylo, NASA / NIST, Organizer: K. Jucks, SAO/Harvard, and B. Sen, NASA/JPL, http://atmoschem.jpl.nasa.gov/

[2] Proceedings of the “International Workshop on Critical Evaluation of mm-/submm-wave Spectroscopic Data for Atmospheric Observations”, CRL Conference Publication, January 2004.

`Stratospheric` `Processes` `And their` `Role in` `ClimateA project of the` `World Climate Research Programme`

Report on the "International Workshop on Critical Evaluation of millimeter-/sub-millimeter-wave Spectroscopic Data for Atmospheric Observations”
Ibaraki University, Mito, Japan 29-30 January 2004

Introduction

Discussions

1. Microwave Spectroscopy

2. Accuracy required from submm remote sensing observations

Stratospheric Processes And their Role in Climate A project of the World Climate Research Programme

Report on the "International Workshop on Critical Evaluation of millimeter-/sub-millimeter-wave Spectroscopic Data for Atmospheric Observations” Ibaraki University, Mito, Japan 29-30 January 2004

Introduction

Discussions

1. Microwave Spectroscopy

2. Accuracy required from submm remote sensing observations

`Stratospheric` `Processes` `And their` `Role in` `ClimateA project of the` `World Climate Research Programme`

Report on the "International Workshop on Critical Evaluation of millimeter-/sub-millimeter-wave Spectroscopic Data for Atmospheric Observations”
Ibaraki University, Mito, Japan 29-30 January 2004