GRWG Web Meeting 2011-08-09

GSICS Web Meeting on Form of GSICS Correction - Diurnal Biases, Offset Terms and Other Dependencies

Agenda

1. Tim Hewison (EUMETSAT) – Handling Dependencies in the GSICS Corrections
2. Fangfang Yu (NOAA) – How to deal with Diurnal Cycles and Midnight Calibration Biases in GSICS Corrections
3. Tim Hewison (EUMETSAT) – Fixing Offset/Slope Terms in GSICS Correction - How and Why
4. Tim Hewison (EUMETSAT) – Exploring algorithms for Meteosat-HIRS Inter-Calibration
5. Tim Hewison (EUMETSAT) – Strategy for inter-calibration of satellite series - Choice of Spectral Correction

Attendees

Chair: Tim Hewison

CMA: Xiuqing Hu, Xu Na

EUMETSAT: Tim Hewison, Sebastien Wagner, Rob Roebeling, Bertrand Theodore

JMA: Arata Okuyama, Yoshihiko Tahara, Hiroaki Tsuchiyam, Yuki Kosaka

KMA: Dohyeong Kim

NOAA: Fangfang Yu

Summary

Some of the GEO-LEO hyperspectral IR GSICS Corrections currently in demonstration mode and under development show significant variations during the diurnal cycle. This is mostly related to the midnight blackbody calibration problem. The current ATBD could be modified to produce separate corrections for the periods of the diurnal cycle affected by this problem. However, this will take time. Meanwhile, the Users' Guides for these products should clearly specify the range of conditions over which they are valid - in particular, the periods of the diurnal cycle. These periods can be derived from the analysis Fangfang presented. It is also recognised that a thorough uncertainty analysis of these products requires considerable time to produce. An alternative would be to perform an analysis of time series of the GSICS Corrections to provide a Type A evaluation of the random part of the uncertainty. GPRCs are encouraged to conduct such an analysis for both the 'normal' part of the diurnal cycle, and the part affected by the midnight calibration problem and report these in their Users Guide for these products. Further analysis should also be conducted to analyse the contribution of the following factors to the apparent diurnal cycles: incidence angle (to resolve any scan-dependence) and surface type.

The approach proposed by Tim Hewison to allow producers of GSICS Corrections to fix their coefficients (e.g. OFFSET=0 when it is not significant) was generally supported. Tim will prepare a modified ATBD to explain the process and distribute this, together with examples of the original and modified GSICS Corrections to the GRWG for review.

The representatives of the GRWG agreed a desire to develop a systematic approach within to the problem of developing "Spectral Corrections" to account for spectral response function differences between the monitored and reference instruments of inter-calibration products. Tim Hewison posed a series of questions to help develop a consistent approach to this issue. Although no final answers were agreed, several of these questions resulted in common responses from the meeting attendees. In particular, it was felt that multiple channels of the reference instrument should be allowed as inputs, although they should remain static throughout a given product, where possible. Similarly, the spectral correction should have the same form for each pair of satellite instruments (although some terms may become zero due to availability). Most importantly, it was felt that we should not allow any subjectivity in the selection of the form/channel combinations to ensure the inter-calibration products remain impartial to the producers' influence. This highlights the need to develop a systematic approach to the generation of these spectral conversion functions and the products derived from them.