Ice clouds play an important role in the energy budget of the atmosphere as well as in the hydrological cycle.
Currently cloud ice is one of the largest remaining uncertainties in climate models. Large discrepancies arise
from different assumptions on ice cloud properties, in particular on microphysics, which are not sufficiently
constrained by measurements. Passive sub-millimeter wave (SMM) techniques have the potential of providing
direct information on ice content and particle sizes with daily global coverage. Here we introduce a concept for a
compact 2-receiver SMM sensor and demonstrate its capabilities on measurements of ice content, mean particle
size, and cloud altitude.
A new generation of sub-millimeter-wave receivers employing sensitive SIS (Superconductor-Insulator-
Superconductor) detector technology will provide new opportunities for precise passive remote sensing observation of
minor constituents in atmosphere. Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) was
designed to be onbord the Japanese Experiment Module (JEM) on the International Space Station (ISS) as a
collaboration project of National Institute of Information and Communications Technology (NICT) and Japan Aerospace
Exploration Agency (JAXA). SMILES scheduled to be launch in September 11, 2009 by the H-II Transfer Vehicle
(HTV). Mission Objectives are: i) Space demonstration of superconductive mixer and 4-K mechanical cooler for the
submillimeter limb emission sounding, and ii) global observations of atmospheric minor constituents. JEM/SMILES will
allow to observe the atmospheric species such as O<sub>3</sub>, H<sup>35</sup>Cl, H<sup>37</sup> Cl, ClO, BrO, HOCl, HO<sub>2</sub>, and HNO<sub>3</sub>, CH<sub>3</sub>CN, and
Ozone isotope species with the precisions in a few to several tens percents from upper troposphere to the mesosphere.
We have estimated the observation capabilities of JEM/SMILES. This new technology may allow us to open new issues
in atmospheric science.
This work presents clear-sky simulations to study water vapor (H<sub>2</sub>O) retrieval from a nadir sounder operating in
the TeraHertz (THz) and Far-Infrared (FIR) spectral domains (100-500 cm<sup>-1</sup>). The THz/FIR retrieval is compared
with retrieval from the mid-InfraRed (IR) 7μm H<sub>2</sub>O band (1200-2000 cm<sup>-1</sup>). The THz/FIR observations
are more sensitive in the upper troposphere and lower stratosphere than the IR measurements. On the other
hand, the IR sounder has better performance in the lower troposphere. The retrieval error due to uncertainties
on the temperature profile are of the same order of magnitude in the THz/FIR and IR bands. No significant
retrieval errors from contaminating species have been found. The calculations for several atmospheric scenarios
show that retrieval performances are not only dependent on the H<sub>2</sub>O abundance but also on the temperature
gradient. Hence, sensitivity in the UT/LS layer, with a low temperature gradient, is poor. The combination of
FIR and IR merges the advantages of both bands, and allows to slightly decorrelate temperature and H<sub>2</sub>O VMR.
With wavelengths in the order of the size of typical ice cloud particles and therefore being sensitive to ice clouds,
the Terahertz (THz) region is expected to bear a high potential concerning measuring ice cloud properties, in
particular microphysical parameters. In this paper we give an introduction to the characteristics of atmospheric
THz radiation between 0-5THz (wavelengths >60 μm and wavenumber<170 cm<sup>-1</sup> respectively) as well as ice
cloud optical properties and cloud effects in the THz region. Using radiative transfer model simulations we
analyze the sensitivity of THz spectra to ice content and particle size. For tropical cases cloud effects in the
order of 0.1 K/(g/m<sup>2</sup>) are found. Assuming instrumental sensitivities of typically around 1K these effects allow
for detection of clouds with columnar ice content of 10 g/m<sup>2</sup>. It is demonstrated that submillimeter (SMM)
instruments are sensitive to particles with sizes larger than 100 μm, while THz observations potentially can
measure particles as small as 10 μm.