Abstract :
The impact of humidity observations on forecast skill is explored by producing a series of global forecasts using
initial data derived from the ERA-40 reanalyses system, in which all humidity data have been removed during the data
assimilation. The new forecasts have been compared with the original ERA-40 analyses and forecasts made from them.
Both sets of forecasts show virtually identical prediction skill in the extratropics and the tropics. Differences between
the forecasts are small and undergo characteristic amplification rate. There are larger differences in temperature and
geopotential in the tropics but the differences are small-scale and unstructured and have no noticeable effect on the
skill of the wind forecasts. The results highlight the current very limited impact of the humidity observations, used to
produce the initial state, on the forecasts
