A comparison of SSM/I-derived global marine surface-specific humidity datasets

Satellite-based microwave sensors have, since the 1980s, provided a means to retrieve near-surface marine specific humidity (q(a)), accurate estimation of which is necessary for climate and air-sea interaction applications. Seven satellite measurement-derived monthly mean humidity datasets are compared with one another and with a dataset constructed from in situ measurements. The means, spatial and temporal structures of the datasets are shown to be markedly different, with a range of yearly, global mean q(a) of approximate to 1gkg(-1). Comparison of the datasets derived using the same satellite measurements of brightness temperature reveals differences in q(a) that depend on the source of satellite data; the processing and quality control applied to the data; and the algorithm used to derive q(a) from the satellite measurements of brightness temperature. Regional differences between satellite-derived q(a) due to the choice of input data, quality control and retrieval algorithm can all exceed the accuracy requirements for surface flux calculation of approximate to 0.3gkg(-1) and in some cases can be several g kg(-1) in monthly means for some periods and regions.