Precursory signals in analysis ensemble spread

Time evolution of the ensemble spread of an experimental ensemble atmospheric reanalysis ALERA is examined in relation to various meteorological phenomena. The analysis ensemble spread increases about two days prior to westerly bursts in the eastern Indian Ocean. Precursory signals are also found in the monsoon onset. The analysis ensemble spread is large at the leading edge of the Somali jet and it grows as the jet extends eastward. Over Vietnam analysis ensemble spread is maximized several weeks before the maximum of the monsoon westerlies. In the stratosphere the analysis ensemble spread takes the maximum value a few days prior to a sudden warming. Our findings indicate that the ensemble analysis contains additional information on atmospheric uncertainty of scientific interest, which may also have practical value.