A MODEL FUNCTION OF THE GLOBAL BOMB TRITIUM DISTRIBUTION IN PRECIPITATION 1960-1986

We have developed, using the World Meteorological Organization/International Atomic Energy Agency (WMO/IAEA) tritium data set, a global model function for predicting the annual mean concentration of decay-corrected bomb tritium (TU81N) in precipitation over the time period 1960-1986. The model consists of two reference time histories or factors, calculated from a factor analysis of the zonally averaged global data set, and global maps of the two spatial coefficients associated with the factors. By combining the reference curves with the appropriate coefficient values taken from these maps, an estimate of the tritium time history for particular locations can be produced. The predicted error for the integrated tritium concentration from the annual model is approximately 3-10% and, in a comparison with previous work (Weiss and Roether, 1980), provides a statistically better fit of the tritium data at most stations. We also discuss the seasonal cycle of tritium in precipitation and present estimates for the amplitude and phase for tritium using a simple seasonal model. At most of the WMO/IAEA monitoring stations, our four-parameter seasonal model accounts for greater than 80% of the variance in the monthly observations.