Anu Dudhia

Abstract:

Carbon tetrafluoride (CF4 or FC-14) is produced mainly as a byproduct of the manufacture of aluminum. CF4 does not harm the stratospheric ozone layer, but it is a powerful greenhouse gas due to its absorption characteristics in the infrared. It is an inert tracer and has a very long lifetime in the stratosphere (probably many thousands of years). Thus it is expected to be uniformly mixed both geographically and vertically around the globe. Measurements of the carbon tetrafluoride reveal that it is essentially independent of altitude up to at least 50 km (e.g. ATMOS observations). The MIPAS experiment, onboard Envisat satellite, launched on 1st March 2002, is a high resolution Fourier Transform Spectrometer observing infrared limb emission spectra. From these measurements, profiles of atmospheric pressure, temperature and several species can be retrieved. MIPAS spectra contain also information on carbon tetrafluoride. In the present study we use Envisat-MIPAS to retrieve carbon tetrafluoride and we quantify its concentration as a function of altitude in the altitude range 6-68 km. Here we present new measurements of CF4 mixing ratios at high altitude, where few measurements have been made in the past.

Abstract:

MIPAS is a Fourier transform interferometer on Envisat, observing the infra-red thermal self emission of the atmosphere. We investigate the trend in the observed concentrations of CFC-11 (CCl3F) and CFC-12 (CCl 2F2) throughout the life of the instrument. From September 2002 to March 2004, the nominal 'high resolution' (0.025 cm -1) reprocessed L1B data is used. New work was performed to enable the use of the 'reduced resolution' (0.0625cm-1) data now being taken so the trend calculation could be extended to early 2006. CFCs are implicated in the catalytic destruction of ozone and the polar 'ozone holes'. They are controlled by the Montreal protocol, and this work provides some means of quantifying its effect. The long time series from a single satellite instrument is useful for tracking the evolution of the CFC stratospheric loading, and checking the accuracy of simulations. The profiles of these species were determined using 'MORSE', developed at Oxford. It uses optimal estimation. Resulting profiles were combined a posteriori and three-day means calculated on a monthly basis. These means were used to determine the trends in various zonal latitude bands and globally.

Abstract:

The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument has been operating on-board the ENVISAT satellite since March 2002. In the first two years, it acquired in a nearly continuous manner high resolution (0.025 cm-1 unapodized) emission spectra of the Earth's atmosphere at limb in the middle infrared region. This paper describes the level 2 near real-time (NRT) and offline (OL) ES A processors that have been used to derive level 2 geophysical products from the calibrated and geolocated level 1b spectra. The design of the code and the analysis methodology have been driven by the requirements for NRT processing. This paper reviews the performance of the optimized retrieval strategy that has been implemented to achieve these requirements and provides estimated error budgets for the target products: pressure, temperature, O3, H2O, CH4, HNO 3, N2O and NO2, in the altitude measurement range from 6 to 68 km. From application to real MIPAS data, it was found that no change was needed in the developed code although an external algorithm was introduced to identify clouds with high opacity and to exclude affected spectra from the analysis. In addition, a number of updates were made to the set-up parameters and to auxiliary data. In particular, a new version of the MIPAS dedicated spectroscopic database was used and, in the OL analysis, the retrieval range was extended to reduce errors due to uncertainties in extrapolation of the profile outside the retrieval range and more stringent convergence criteria were implemented. A statistical analysis on the χ2 values obtained in one year of measurements shows good agreement with the a priori estimate of the forward model errors. On the basis of the first two years of MIPAS measurements the estimates of the forward model and instrument errors are in general found to be conservative with excellent performance demonstrated for frequency calibration. It is noted that the total retrieval error is limited by forward model errors which make effectless a further reduction of random errors. However, such a reduction is within the capabilities of MIPAS measurements, which contain many more spectral signatures of the target species than what has currently been used. Further work is needed to reduce the amplitude of the forward model errors, so that the random error and the total error budget can be reduced accordingly. The importance of the Averaging kernels for a full characterization of the target products is underlined and the equations are provided for their practical applications.

Abstract:

The Michelson Interferometer for Passive Atmospheric Sounding is a polar orbiting high resolution mid-infrared emission limb sounder with a nominal vertical resolution of 3 km. Work to extend the list of routinely retrieved species led to the examination of ammonia, NH3. In this paper, we explore the feasibility of upper tropospheric retrievals around 6-km altitude, discussing appropriate methods. We demonstrate a weak north to south gradient outside the error bars, using co-addition to improve signal-to-noise and compare these first results with a model. (c) 2006 Published by Elsevier Ltd on behalf of COSPAR.