MIPAS Orbital Retrieval using Sequential Estimation (MORSE)

01MAY03

Introduction

Since MIPAS is one of the core instruments, ESA have undertaken to process and distribute MIPAS data operationally. This includes both atmospheric spectra (L1 data) and derived profiles of atmospheric pressure, temperature and concentrations of CH4, H2O, HNO3, N2O, NO2 and O3 (L2 data). The ESA processing algorithms were designed before launch so are intentionally relatively simple and robust. However the software is complex and not publicly available.

In addition to the ESA L2 products, MIPAS has the potential to retrieve numerous additional species or parameters, or the same species using more sophisticated algorithms, and a number of laboratories around Europe have been developing their own codes to do this, coordinated under the EU Framework V project AMIL2DA). However, these codes are generally designed only for local use by experts and often require large, dedicated machines.

The aims of this project are

1. to generate a retrieval algorithm MORSE which can either replicate or improve upon the ESA L2 processing, and
2. to make the software available for NERC users to download and install using only basic computing facilities and background knowledge.

The goal is to provide the resources to allow a postgraduate student or postdoctoral researcher working independently to perform their own retrievals from MIPAS spectra while leaving sufficient time to concentrate mainly on the scientific aspects of the results rather than the details of the processing.

Schedule

The project is split into 4 phases, each of 6 months, with software releases at the end of each phase. The start date is 1st January 2004.

6months - Phase I - MORSE v1 (1st July 2004)

12months - Phase II - MORSE v2 (1st January 2005)

18months - Phase III - MORSE v3 (1st July 2005)

24months - Phase IV - MORSE v4 (1st January 2006)

Science Case

These comments are based on the current state of the ESA processor and it should be noted that upgrades are anticipated in the time scale of the order of a year.

Phase I

MORSE v1 Features:

• Retrieval of single profile from nominal MIPAS scan
• ESA or user-supplied microwindows
• Optimal estimation retrieval

Reprocessing

ESA's processing is fully automated so that if retrievals fail for some reason, e.g., due to convergence or cloud contamination problems, the profiles are simply unavailable until possibly the next reprocessing run. The code will allow a user to hand-generate missing data, e.g., critical to a particular campaign, using the same spectral data as the ESA processor (although not an identical retrieval algorithm).

Aims: recover missing ESA profiles where L1 data is available

Microwindow selection

The ESA retrieval uses a fixed set of microwindows and spectroscopic data. A more flexible approach would allow the sensitivity to spectroscopic parameters to be assessed, e.g. assessing the impact of using a different set of line parameters for a particular molecule

Aims: spectroscopic validation

Improved ESA Products

ESA retrieve profiles of pT and key-species in near-real-time using a limited set of microwindows - typically only 2 or 3 per species. Additional accuracy can be obtained using more microwindows, at the expense of additional cpu time.

Aims: improved accuracy of standard products

Lower altitude limit

ESA's current processing is limited to 12km and above, ignoring spectra from 6 and 9km in order to minimise problems with clouds and convergence issues caused by large tropospheric water vapour. The option will be available to retrieve at these altitudes where clouds do not occur.

Aims: Upper-troposphere/lower stratosphere retrievals

Additional Products

Many more species can be retrieved, albeit with lower signal/noise or under enhanced conditions, using essentially the same algorithm e.g. CFCs, N2O5, Sulphur budget (SO2, SF6, OCS), Chlorine budget (ClONO2, HOCl, CCl4, ClO), NH3, HCN, C2H6 and H2O2

Aims: additional species

Retrieval algorithm

There are several possible methods for inverting a set of radiance measurements to retrieve a profile. ESA use the least squares fit approach which has its advantages in terms of being unbiased and maximising vertical resolution but suffers from instabilities and forces a specific set of retrieval levels to match the available measurements. It is intended to provide alternative approaches based on optimal estimation or regularisation in order to improve the robustness and flexibility of the retrieval.

Aims: more uniform retrievals with well-defined averaging kernels

Phase II

MORSE v2 Features:

• Simultaneous retrievals of multiple species
• Flexible retrieval/measurement spacing
• Choice of optimal estimation or regularisation

Retrieval sequence

A limitation on the ESA processing is that first pressure and temperature are retrieved, followed by each species individually. This necessitates the use of microwindows where the emission is primarily from a single species. Allowing for joint retrievals (e.g., pT plus H2O simultaneousl) allows use of spectral intervals where there is overlap between different molecules and should improve retrievals of the marginal species.

Aims: improved accuracy

Special Modes

MIPAS has a nominal scan mode of 17 elevation steps from 68-6km but also a number of so-called "special modes" where the scan pattern, and sometimes spectral resolution, are changed, e.g, concentrating on the UTLS region and improving the horizontal sampling. These special modes will be operated for a few days each during the first part of 2003 but ESA currently plan only on processing these data to L1, ie generating spectra but not retrieving profiles. The intention is for the science community to establish the usefulness of such observations so processing of this data will be included here.

Aims: retrievals of profiles during MIPAS Special Modes (eg polar vortex, strat/trop exchange, high horizontal resolution)

Retrieval algorithm

There are several possible methods for inverting a set of radiance measurements to retrieve a profile. ESA use the least squares fit approach which has its advantages in terms of being unbiased and maximising vertical resolution but suffers from instabilities and forces a specific set of retrieval levels to match the available measurements. It is intended to provide alternative approaches based on optimal estimation or regularisation in order to improve the robustness and flexibility of the retrieval.

Aims: more uniform retrievals with well-defined averaging kernels, eg for data assimilation studies

Phase III

MORSE v3 Features:

• Choice of continuum constraints
• Handling of horizontal gradients
• Orbital processing
• Retrieval of vibrational temperature and isotopic ratios

Isotopic ratios

Isotopic ratios give additional information on the history and source of atmospheric species. With MIPAS spectral resolution it is possible to isolate spectral lines from individual isotopes. Although these are generally much weaker than the main isotopic lines (through a lower concentration) a retrieval formulated in terms of determining the ratio of minor/main isotopes rather than the absolute concentration should remove many sources of inaccuracy.

Aims: Water vapour isotope distributions are the most likely target.

Vibrational Temperatures

The ESA retrieval assume local thermodynamic equilibrium, i.e. that all spectral emissions are characterised by the Planck function for the local temperature. At high altitudes, or short wavelengths, especially during day-time conditions the emissions of particular molecules deviate from this and are typically characterised by an internal `vibrational' temperature. The proposed retrieval will either allow vibrational temperature profiles to be externally supplied or retrieved directly. This may allow the retrieval of CO.

Aims: CO retrievals, non-LTE modelling

Continuum constraints

The ESA retrieval is built around the approach of simultaneously fitting an independent atmospheric continuum profile in each microwindow, effectively the molecular signature becomes the difference between the line peak and the continuum background rather than the zero radiance level. This is an advantage for molecules with distinct emission line features, e.g., in eliminating contributions from aerosol or thin clouds, but restricts the sensitivity to retrieving heavy molecules such as N2O5 and the CFCs. Two alternatives will be provided: either using broader microwindows or retrieving continuum absorption independently, e.g. from adjacent spectral regions, which is a useful product in itself.

Aims: cloud/aerosol studies, additional continuum-like species

Horizontal Gradients

The ESA retrieval assumes a horizontally homogeneous atmosphere when retrieving each profile, e.g., that the atmospheric parameters retrieved from the 21km tangent point are the same as those encountered by the ray passing through the 18km tangent point as it crosses the 21km surface, even though these are displaced some 75km horizontally either side. One approach being explored by several groups is a 2-dimensional retrieval where a two dimensional field is retrieved in the vertical plane (e.g., a full orbit) rather than a single profile. However, this greatly increases the processing requirements. The approach adopted here will be a two-pass sequential estimator (Kalman smoother) where profiles are first retrieved assuming no gradient, then subsequently reretrieved using gradient information from the first pass.

Aims: improved accuracy, particularly in regions of sharp gradients, generation of larger datasets

Phase IV

MORSE v4

The last phase is intended only to allow for a period of support including code maintenance, bug-fixes, improvements to documentation and minor algorithm refinements based largely on user-feedback. This will culminate in a `final' release of MORSE v4.