91̽»¨

Abstract:

MIPAS is a limb viewing fourier transform spectrometer operating in the infra-red. It scans the tangent altitude range 6-68 km at a vertical resolution of approximately 3km. CO has various natural and anthropogenic sources including forest fires and industry. In the troposphere, CO is the main sink of the principal oxidising agent - the hydroxyl radical - and so reduces its capacity for the removal of other atmospheric pollutants. In the stratosphere, its intermediate lifetime makes it useful as a tracer for stratospheric motions and processes of tropospheric-stratospheric exchange. However, the retrieval of CO from a limb sounding IR instrument is complicated because of non-LTE (non-local thermodynamic equilibrium) effects. Ordinarily, the internal vibrational energy level populations are controlled by collisions between molecules and follow the translational (kinetic) energy distribution of the ambient gas (LTE). However, if collisions are infrequent, radiative processes can lead to a non-Boltzmann distribution of the internal energy level populations (non-LTE). CO is strongly affected by non-LTE in the IR down to around 40km and modelling these processes is difficult. These effects are especially problematic in the limb viewing geometry due to the long path length viewed through the upper atmosphere. However, using a joint CO, vibrational temperature (Tv) retrieval it is possible to estimate the non-LTE effects without modelling the energy level populations. Instead, we retrieve a parameterisation of the non-LTE effects, Tv, directly from the emission spectra.

Abstract:

MIPAS has been making measurements of atmospheric infrared emission spectra since July 2002, giving a global dataset spanning almost 5 years -potentially long enough to start making assessments of any trends in stratospheric temperature and composition. However, any analysis is complicated by the fact that MI-PAS operated until March 2004 with a spectral sampling of 0.025 cm -1 then, after a lengthy interruption, resumed operations in January 2005 sampling at 0.0625 cm-1. The nominal scan pattern was also changed: from 17 spectra per limb scan for the high resolution to 27 spectra per limb scan for the reduced resolution. ESA-processed L2 data for the reduced resolution have only recently started to become available, so for this study retrievals for 15 selected days throughout the mission have been performed using the 91̽»¨ retrieval algorithm MORSE. With such a limited dataset, and significant inter-annual variability, it is not possible to conclude unambiguously whether the results show evidence of either a bias between the two modes of MIPAS operation or evidence of an atmospheric trend.

Abstract:

A linear algorithm has been developed to retrieve temperature, pressure and composition at lower mesospheric altitudes using the limb radiances from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). The algorithm is based on the assumption that at mesospheric heights, due to the thin optical path, the retrieval can be treated linearly. This avoids re-running the radiative transfer model and hence avoids the high CPU cost. A comparison against the operational level 2 (L2) products is given.

Abstract:

After 2 years of quasi continuously operations (from July 2002 to March 2004), MIPAS on ENVISAT was stopped due to problems in the mirror drive of the interferometer. Operations with reduced spectral resolution and a new measurement scenario were resumed in January 2005. Significant modifications were performed in the ESA operational processor in both the algorithms and the auxiliary data. Performances evaluated on the basis of the first set of available MIPAS measurements in the new operation mode processed with the ESA operational processor are discussed in this paper. The new measurements are characterised by an improved vertical and horizontal resolution and a reduced standard deviation. The analysis of χ-test statistics indicate that larger mean χ2- values are found in the new operation mode, especially for O3, CH4 and N2O.

Abstract:

The deuterium content of water vapour in a given air mass is sensitive to its temperature and condensation history. Isotopic measurements therefore have the potential to shed light on the transport of air and water vapour into the stratosphere. Previous measurements of the isotopic composition in the upper troposphere and stratosphere have been sparse in terms of both spatial and temporal coverage. Presented here are retrievals of the deuterium content of water vapour (HDO or delta D) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite instrument. These retrievals offer the first global scale coverage of the isotopic composition of water vapour in this altitude region and span a time period of almost two years. The spatial coverage and the time span of the dataset offer previously unattainable insight into the mean seasonal and spatial distributions of the isotopic composition of water vapour in the upper troposphere and stratosphere. Measurements of HDO are extremely challenging due to low sensitivity in the spectra at low temperatures and water vapour amounts. Nonetheless, the data show a number of interesting results. Zonal mean profiles show the greatest depletion in delta D in the tropical upper troposphere, with decreasing depletion with altitude in the stratosphere due to the influence of methane oxidation. Seasonal zonal means also show a strong depletion in the southern polar spring at around 300 mbar, which is thought to be related to the occurrence of polar stratospheric clouds and dehydration events in the polar vortex. Geographically, the regions and time periods where the greatest depletions are observed in the tropical upper troposphere are those associated with strong convective activity. Results confirm that temporal variability is central to the transport of water vapour into the tropical tropopause layer (TTL) and stratosphere. The data presented here show an annual cycle in delta D in the TTL which is tied to temperature and suggests that this signature propagates upwards into the stratosphere. The data also show a number of points where extremely dry air is associated with relatively enriched delta D values. We postulate that these points are evidence of ice lofting. Copyright (C) 2007 Royal Meteorological Society.