|Title||4D-Var assimilation of MERIS total column water vapour retrievals over land|
|Year of Publication||2009|
|Secondary Title||Technical Memorandum|
Experiments with the active assimilation of total column water vapour retrievals from Envisat MERIS observations have been performed at ECMWF focusing for the summer 2006 AMMA ﬁeld campaign period. A mechanism for data quality control, observation error deﬁnition and variational bias correction have been developed so that the data can be safely treated within 4D-Var like other observations that are currently assimilated in the operational system. While data density is limited due to its restriction to daylight and cloud-free conditions, a systematic impact on mean moisture analysis was found with distinct regional and seasonal features. The impact can last 1-2 days into the forecast but has little impact on forecast accuracy both in terms of moisture and dynamics. This is mainly explained by the weak dynamic activity in the areas of largest data impact. Evaluation with radiosonde observations revealed a strong dependence on radiosonde type. Compared to Vaisala RS92 observations, the addition of MERIS total column water vapour observations produced neutral to positive impact while contradictory results were obtained when all radiosonde types were used in generating the statistics. This highlights the issue of radiosonde moisture biases and the importance of sonde bias correction in NWP.