Comparison of in-situ FISH measurements of water vapor in the UTLS with ECMWF (re)analysis data
Abstract. An evaluation of water vapor in the UTLS in the atmospheric ERA-Interim reanalysis data set is presented by using in-situ measurements from a large set of airborne measurement campaigns from 2001 to 2011 in the tropics, midlatitudes and polar regions. Water vapor measurements are derived from the Fast In-situ Stratospheric Hygrometer (FISH) and cover isentropic layers from 300–400 K (5–18 km). At the same time, the improvement of the ECMWF assimilation scheme representation of water vapor is addressed for time periods representing different cycles of the Integrated Forecast System (IFS). The ratio Δ(H2O) = H2OERA / H2OFISH is used as a simple measure for the difference between observations and the reanalyses. Overall, the reanalysis data reproduce around 87% of all FISH measurements within Δ(H2O) = 0.5–2, and 30% are within Δ(H2O) = 1.0 ± 0.1. Nevertheless, also strong over- and underestimations occur both in the troposphere and in the stratosphere. Δ(H2O) values indicate deviations of factors up to 10, with lower deviations in the stratosphere (Δ(H2O) = 0.5–4) than in the troposphere (Δ(H2O) = 0.5–10). In the tropical stratosphere the ratio is closer to 1 (Δ(H2O) = 0.5–2) than in the extratropical stratosphere where strong deviations occur (Δ(H2O) = 0.1–4). When considering operational analysis data, the agreement with FISH improves over the time, in particular when comparing water vapor fields for time periods before 2004 and after 2010. It appears that influences of tropical tropospheric and extratropical lower stratospheric processes on the water vapor distribution in the UTLS are particularly challenging, resulting in an overestimation of low and underestimation of high water vapor mixing ratios.