Currently, NCEP/NCAR Reanalysis (Kistler, et al., 2001, and Kalnay et. al, 1996) is still the most comprehensive and coherent global analysis including the era before satellite measurements were used. Significant drying trends in the upper air humidity, however, were detected starting from 1949. More than 3% relative humidity per decades is found at 500 hPa in the mid latitudes. Even stronger trends are found in the higher latitudes and altitudes. The trends impact many Reanalysis parameters, such as cloud fractions and radiative forcing, which are diagnosed from the Reanalysis global data assimilation system. The finding has profound implications on the global energy balance, e.g. a 10% change of longwave cloud radiative forcing over three decades.

In search of the validity of the trends, a comparison of the Reanalysis with an AMIP simulation is conducted. The analysis is done based on 30 sample stations where the rawinsondes were known to comply with standards. The result shows the trends are more sever before 1957 International Geophysical Year. Further investigations on the response time-constants of hygrometers show a wide range of variance, as well as tremendous delay in the dryer and thinner environment; from 2 seconds at the surface to 5 minutes at 300 hPa for the hair- type hygrometer.

This result suggests strongly that the early Reanalysis are biased high due to the slower response of the legacy hygrometers. As the hygrometer within a rawinsonde ascends, its slow response registers the high humidity of the lower moister levels at the higher dryer levels. As the technology of hygrometer progressed, some of the problems were alleviated, but generated an artificial trend as seen by Reanalysis. Further time series associated with the changes of instrument/reporting algorithms, as well as details of the regional analyses, will be presented. The implication for global climate variations, and suggestions for future Reanalysis will also be discussed.

References:

Kalnay, E. and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project, Bull. Amer. Meteor Soc., 77, 437-471

Kistler, R, and Coauthors, 2001: The NCEP-NCAR 50-Year Reanalysis: Monthly Means CD-ROM and Documentation, Bull. Amer. Meteor Soc., 82, 247-267