Retrieval of minor constituents in a cloudy atmosphere with remote-sensing millimetre-wave measurements

Abstract

ABSTRACT:The presence of clouds in remote-sensing measurements is often the cause of information loss about the minor atmospheric constituent profiles that are the goal of the measurement. Millimetre-and submillimetre-wave sounding can be used to abate the problems of cloud contamination. A quantitative assessment of this possibility is provided with the systematic simulation of the retrieval of minor atmospheric constituents in the presence of ice cirrus clouds of different density and particle size. The simulations are made for the particular case of the MARSCHALS instrument which is a heterodyne limb-sounding spectrometer that measures the atmospheric thermal emission in the millimetre-and submillimetre-wave regions operating on board a stratospheric aircraft. Four types of measuring conditions are identified: clouds that do not modify the observation, clouds that are adequately described with an atmospheric continuum absorption model, clouds for which an error is introduced when the limited vertical resolution of the sounding does not resolve the sharp discontinuity of the cloud, and clouds which require the modelling of scattering effects for a correct retrieval. When a rigorous modelling of the cloud is performed, the retrieval of minor atmospheric constituent is marginally affected by the presence of the cloud. We find that in the worst case the cloud causes a loss, with respect to clear-sky conditions, of two degrees of freedom from about 90 and of 7% of information content relative to climatological information.

Technical Note: Measurement of the tropical UTLS composition in presence of clouds using millimetre-wave heterodyne spectroscopy

“…The largest absorption observed in Fig. 12 around scan 12 corresponds to clouds that in the retrieval study of Del Bianco et al (2007) were located close to the boundary between case 2 and 3.…”

“…So it is important to develop instruments that can avoid the attenuation of the cirrus clouds that are often present in the UTLS. Fortunately, the optical depth of cirrus clouds is not the same at all wavelengths (Warren, 1984): the relatively low extinction coefficient of ice particles at longer wavelengths (far infrared and millimetre wave spectral interval) can be exploited to measure the UTLS region also in presence of cirrus clouds (Del Bianco et al, 2007).…”

The Far‐infrared Earth

“…Turning to much longer wavelengths, in the millimeter wavelength region, a very useful region of high transparency is observed even for the most dense ice cirrus clouds. In a case study performed for the Millimetre‐wave Airborne Receiver for Spectroscopic Characterisation of Atmospheric Limb‐Sounding instrument [ Oldfield et al , 2001], which observes the atmospheric emission in the millimeter wave region with a spectral resolution of 200 MHz, Del Bianco et al [2007] have quantified the loss of information caused by ice clouds when making atmospheric composition measurements with this instrument. They showed that the effect of most clouds is adequately described with a model of atmospheric continuum absorption.…”

GBB-Nadir and KLIMA: Two Full Physics Codes for the Computation of the Infrared Spectrum of the Planetary Radiation Escaping to Space

“…KLIMA is an extension of a previous code called MARC (millimetre-wave atmospheric-retrieval code) [37] specifically developed for the millimetre-wave spectral region and limb geometry. The updated version of this code, KLIMA, can be used in various geometrical configurations (limb [38,39], zenith [40] and nadir [23,[41][42][43][44]) and spectral bands (from millimetre and sub-millimetre wave [37] to near-infrared [45]). Finally, the KLIMA code was also used to simulate the nadir and limb radiances for a Martian atmosphere in the context of some internal tests related to the planetary Fourier spectrometer (PFS) instrument on board of Mars express (MEx) [11] and compared with the simulations obtained using the code described by Ignatiev et al [20].…”