Stable isotope ratios: Hurricane Olivia

The oxygen and hydrogen isotopic compositions of rains from Hurricane Olivia (1994) in the eastern Pacific were measured. The rains were collected on 24 and 25 September during airplane flights conducted at an elevation of 3 km. Hurricane Olivia peaked in intensity to a category-4 storm between the two dates. Isotope ratios of rains from Hurricane Olivia were markedly lower (delta(18)O = -13.9parts per thousand to -28.8parts per thousand) than that of rain collected from a thunderstorm at an elevation of 2.3 km outside the influence of Olivia (delta(18)O = -3.8parts per thousand). A distinct decrease in isotope ratios from the first day to the next (delta(18)O = -18.4parts per thousand to -21.9parts per thousand) in Hurricane Olivia was attributed to decreased updraft velocities and outflow aloft. This shifted the isotopic water mass balance so that fewer hydrometeors were lifted and more ice descended to flight level. A decrease in the average deuterium excess from the first day to the next (d = 15.5 to 7.1parts per thousand) was attributed to an increase in the relative humidity of the water vapor `source' area. We hypothesize that the `source' region for the rain was in the boundary layer near the storm center and that because the hurricane was at peak intensity prior to the second day the relative humidity was higher.