The Frequency Distribution of Daily Precipitation over the United States

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This study examines the seasonal frequency distribution of daily precipitation and related variables over the United States using the North American Regional Reanalysis. Regions where the seasonal mean precipitation is dominated by heavy and extreme daily events or by more consistent lighter events are identified. The distributions are related to the variability of the vertically integrated moisture flux convergence (MFC) and precipitable water. The modulation of daily precipitation by ENSO and the Madden-Julian Oscillation (MJO) during winter is investigated.

Assuming that the frequency of daily precipitation can be approximated by a gamma distribution, the scale and shape parameters are useful proxies to estimate the observed frequency distribution of precipitation.  During winter, most areas of the country with high mean precipitation have a strong contribution from extreme events, particularly along the West and Gulf Coasts.  During summer, the wettest areas of the country are Florida, where the mean precipitation is dominated by more-frequent light and moderate rainfall days, and the central Plains, dominated by variable rains and extreme events.  Throughout the year, the MFC mean and scale parameter patterns strongly resemble those of precipitation, and areas with more heavy and extreme precipitation have stronger MFC daily values.  These results suggest that the frequency distribution of MFC can be used as a proxy for the frequency distribution of modeled forecast precipitation. 

Changes in the winter total precipitation between the phases of ENSO are largely attributable to changes in the heavy and extreme events.  Areas showing increased mean precipitation during the warm phase show an even greater increase in extremes.  Similar to precipitation, strong values of MFC are more sensitive to ENSO phase than is the mean MFC.  While the ENSO variability of the frequency distribution of MFC shows a strong relationship to that of precipitation, the variability of precipitable water does not.  MJO modulation of winter daily precipitation over the central U.S. occurs primarily during MJO Phases 5 and 6, when MJO-related enhanced convection is located in the western Pacific.  During these phases, the winter storm track is enhanced, and positive MFC anomalies are present in the central U.S.