Summary: Tellus (2007), 59A, 336343 C 2007 The Author
Journal compilation C 2007 Blackwell Munksgaard
Printed in Singapore. All rights reserved
Improved simulation of water vapour and clear-sky
radiation using 24-hour forecasts from ERA40
By RICHARD P. ALLAN, ESSC, University of Reading, Reading, RG6 6AL, United Kingdom
(Manuscript received 22 August 2006; in final form 8 January 2007)
Monthly mean water vapour and clear-sky radiation extracted from the European Centre for Medium Range Weather
Forecasts 40-year reanalysis (ERA40) forecasts are assessed using satellite observations and additional reanalysis data.
There is a marked improvement in the interannual variability of column-integrated water vapour (CWV) over the oceans
when using the 24-hour forecasts compared with the standard 6-hour forecasts products. The spatial distribution of CWV
are well simulated by the 6-hour forecasts; using the 24-hour forecasts does not degrade this simulation substantially
and in many cases improves on the quality. There is also an improved simulation of clear-sky radiation from the 24-hour
forecasts compared with the 6-hour forecasts based on comparison with satellite observations and empirical estimates.
Further work is required to assess the quality of water vapour simulation by reanalyses over land regions. Over the
oceans, it is recommended that 24-hour forecasts of CWV and clear-sky radiation are used in preference to the standard
6-hour forecast products from ERA40.