On the quality of the ERA-Interim ozone reanalyses. Part I Comparisons with in situ measurements

Title
On the quality of the ERA-Interim ozone reanalyses. Part I Comparisons with in situ measurements
Report
Date Published
05/2010
Series/Collection
ERA Report Series
Document Number
2
Author
Rossana Dragani
Event Series/Collection
ERA Report
Abstract This is the first of two companion papers presenting an assessment of the quality of the ERA-Interim ozone reanalyses by comparisons with independent observations, during the period January 1989 to December 2008. Overall, ground-based ozone observations and satellite ozone products were used to validate both the three-dimensional ERA-Interim ozone analyses and the ERA-Interim total column ozone (TCO). This first part focusses on the assessment of the quality of the ERA-Interim ozone analyses against in-situ ozone data. The residuals between the ERA-Interim TCO and the ground-based Dobson TCO showed values within ±10% at high latitudes, and within ±5% elsewhere. The comparisons of the three-dimensional ERAInterim and ERA-40 ozone analyses with ozone sondes showed a dependence on the season, latitude, as well as on the period accounted for as a consequence of the varying ozone observing system actively used. In particular, the ERA-Interim ozone product benefitted from the assimilation of GOME ozone profiles (January 1996 - December 2002), particularly in the tropics. In the pre-GOME assimilation period, the residuals between the ozone sondes and their corresponding ERA-Interim ozone profiles were within ±10% in the tropics and at midlatitudes at most levels, and within ±20% at high latitudes. From January 1996 onwards, the level of agreement was within ±5% in the tropics and at high latitudes in summertime, and within ±10% at high latitudes in wintertime as well as at midlatitudes throughout the year. The comparisons also showed substantial improvements in ERA-Interim over the ERA-40 equivalent both at stratospheric and troposheric levels throughout the twelve year overlapping period. In particular, the RMS of the sondeanalysis differences were reduced up to 40% in the lower stratosphere, and between 20 and 50% in the troposphere when using ERA-Interim instead of ERA-40.
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