The Role of Atmospheric Water Vapor in the Observed Upward Trend in Extreme Precipitation

The United States has experienced a large increase in the frequency and intensity of extreme precipitation events over the past 50 years. At the same time, both the globe and the U.S. have warmed substantially, most likely due to increases in greenhouse gas concentrations. This warming has been accompanied by global increases in atmospheric water vapor. In theory this should increase the capacity of the atmosphere to produce intense precipitation. The objective of this study is to determine whether atmospheric water vapor increases are contributing to the upward trend in extreme precipitation. The specific question is whether the water vapor conditions in the temporal and spatial vicinity of historical extreme events have been changing. While water vapor is increasing globally, this does not necessarily mean that it is increasing during the small subset of times and places of the occurrence of extreme precipitation.

Analysis of over 3,000 Cooperative Observer stations for the period of 1949-2013 produced a dataset of heavy precipitation events exceeding thresholds for a variety of recurrence intervals and durations. An initial analysis focused on daily events exceeding a 5-year average recurrence interval for the period of 1970-2013 (when higher quality radiosonde data are available). This analysis focused on the northeastern United States, which has experienced the largest increase in extreme precipitation. For each extreme event, the water vapor environment was characterized by the precipitable water measured on that day from the nearest Integrated Global Radiosonde Archive (IGRA) observation. Initial results indicate an increase in water vapor associated with extreme precipitation events. Recent events have occurred in a moister environment than events in the earlier part of the period of analysis. Work continues, and results for multiple regions and multi-day events will also be presented.