Meteorological features of observed trends in U.S. heavy precipitation events

Trends in the type of meteorological events behind the documented increases in U.S. heavy precipitation during the latter part of the 20th Century have not been identified. This study is investigating whether there have there been secular changes in the frequency, intensity, and other characteristics of the meteorological phenomena producing heavy precipitation. A set of 930 long-term stations, distributed throughout the U.S., was used to identify heavy precipitation events over the period of 1895-2006. Heavy precipitation events were defined as 1-day precipitation totals exceeding the threshold for a 1 in 5 year recurrence. The meteorological cause of each event was identified as one of the following: extra-tropical cyclone, frontal, tropical cyclone, mesoscale convective system, air mass convection, or unknown. Several datasets were employed to determine the cause including reanalysis data, surface fields of temperature and precipitation, and cyclone tracks. The identification was made by the judgment of the authors. The result of this effort is a catalog of heavy precipitation event causes. This talk will describe the climatological characteristics of the causes, including trends.

An additional analysis investigated possible relationships between the frequency of heavy events and circulation anomalies on seasonal time scales as they relate to moisture advection. This line of inquiry used a simple metric of the horizontal flux of atmospheric moisture: the product of the component of the horizontal wind into (or out of) the continental US and the absolute atmospheric moisture content at the pressure level of 850 hPa. A number of statistically significant correlations were identified. Moisture flux across the north Pacific transect is positively (negatively) correlated with heavy precipitation events in the northwest (southwest). Interestingly, moisture flux across the south Pacific transect is positively correlated with heavy precipitation events in both the west and northwest regions, but there is a negative correlation with events in the southeast region. The moisture flux in the west Gulf is positively correlated with heavy precipitation events in the east north central, west north central, and south regions and negatively correlated with events in the northeast and southeast regions. The moisture flux in the east Gulf is positively correlated with heavy precipitation events in the central and south regions and negatively correlated with events in the west region. Moisture flux across the Atlantic transect is positively correlated with heavy precipitation events in the northeast and southeast regions. The results of these analyses are physically reasonable, and suggest that trends in coastal moisture fluxes may be a useful tool for analyses of inland heavy precipitation.