A fast H2O total column density product from GOME – Validation with in-situ aircraft measurements

[摘要] Atmospheric water vapour is the most important greenhouse gas which is responsible forabout 2/3 of the natural greenhouse effect, therefore changes in atmospheric water vapour in achanging climate (the water vapour feedback) is subject to intense debate.H₂O is also involved in many important reaction cycles of atmospheric chemistry, e.g. in the productionof the OH radical. Thus, long time series of global H₂O data are highly required. Since 1995the Global Ozone Monitoring Experiment (GOME) continuously observes atmospheric tracegases. In particular it has been demonstrated that GOME as a nadir lookingUV/vis-instrument is sensitive to many tropospheric trace gases. Here we present a new, fastH₂O algorithm for the retrieval of vertical column densities from GOME measurements. Incontrast to existing H₂O retrieval algorithms it does not depend on additional information likee.g. the climatic zone, aerosol content or ground albedo. It includes an internal cloud-,aerosol-, and albedo correction which is based on simultaneous observations of the oxygendimer O₄. From sensitivity studies using atmospheric radiative modelling we conclude thatour H₂O retrieval overestimates the true atmospheric H₂Overtical column density (VCD) by about 4% for clear sky observations in the tropics and sub-tropics, while it can lead to anunderestimation of up to -18% in polar regions. For measurements over (partly) cloudcovered ground pixels, however, the true atmospheric H₂O VCD might be in generalsystematically underestimated. We compared the GOME H₂O VCDs to ECMWF model dataover one whole GOME orbit (extending from the Arctic to the Antarctic) including alsototally cloud covered measurements. The correlation of the GOME observations and themodel data yield the following results: a slope of 0.96 (r² = 0.86) and an average bias of5%. Even for measurements with large cloud fractions between 50% and 100% an averageunderestimation of only -18% was found. This high accuracy of our GOME H₂Odata is also confirmed by the excellent agreement with in-situ aircraft measurements during the MINOScampaign in Greece in summer 2001 (slope of 0.97 (r² = 0.86), and an average bias of only0.2%). Our H₂O algorithm can be directly adapted to the nadir observations of SCIAMACHY(SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY) which waslaunched on ENVISAT in March 2002. Near real time H₂O column data from GOME andSCIAMACHY might be of great value for meteorological weather forecast.

[学科分类] 大气科学