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The frequency distribution of daily precipitation over the U.S.

The frequency distribution of daily precipitation over the U.S. Emily J. Becker 1 , E. Hugo Berbery 1 , and R. Wayne Higgins 2 1: Department of Atmospheric and Oceanic Science, Univ. of MD 2: Climate Prediction Center, NOAA/NWS/NCEP. Outline.

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The frequency distribution of daily precipitation over the U.S.

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  1. The frequency distribution of daily precipitation over the U.S. Emily J. Becker1, E. Hugo Berbery1, and R. Wayne Higgins2 1: Department of Atmospheric and Oceanic Science, Univ. of MD 2: Climate Prediction Center, NOAA/NWS/NCEP

  2. Outline Objective: Examine the seasonal characteristics of the frequency distribution of precipitation and its dependence on the lower frequency modulation of ENSO during winter • Introduction, Data and Method • Seasonal precipitation frequency distribution • Seasonal frequency distribution of precipitation-related variables • ENSO modulation of precipitation and precipitation-related variables • Summary & conclusions

  3. Daily precipitation • Changes in the frequency and intensity of daily precipitation within a season can occur independently from changes in seasonal mean • More extreme events can lead to high streamflow, flooding (Groisman et al. 2001) • Often see greater increase in extremes than in mean total

  4. Data: NARR • North American Regional Reanalysis • 1979-present (1979-2005 for this study) • 32-km horizontal resolution, 45 vertical layers • Developed with 2003 version of the Eta model; Eta model coupled with Noah land surface model • Precipitation is assimilated • Extremely high agreement between NARR precip. and observations

  5. Gamma distribution • pp distrib. positively skewed, bounded on left by 0 • Shape (α) describes skewness • Scale (β) stretches or shrinks along x-axis α β = mean

  6. winter (JFM) precipitation • scale-dominated areas (large scale, small shape): more precipitation received from extremes • shape-dominated areas (large shape, small scale): more precipitation from more-frequent lighter events * • in winter, most areas with high wet-day mean are scale-dominated • northern Plains and Great Lakes are shape-dominated • Atlantic and Gulf of Mexico coasts are scale-dominated wet-day mean frequency scale shape * After Husak et al. (2007)

  7. summer (JAS) precipitation Great Plains Low-Level Jet: scale-dominated North American monsoon Southeast: mean dominated by light/moderate wet-day mean frequency scale shape

  8. precipitation-related factors • Precipitable water (total atmospheric water vapor contained in a vertical column, mm) • Vertically-integrated moisture flux convergence (MFC): earlier work (Silva and Berbery, 2006) suggests model forecasts have better representation of MFC distribution than of precipitation • Also convective available potential energy (CAPE)

  9. winter prec. water precipitation MFC • Mean prec. water generally similar to mean precipitation, but scale parameters (representing contribution of extremes) not strongly linked • MFC and precip. scale parameters have strong resemblance

  10. summer prec. water precipitation MFC • Prec. water much larger in summer; distribution is close to Gaussian (shape > 20) over most of country • GPLLJ and NAM evident in both prec. water and MFC mean • MFC reflects both mean and extreme precipitation mean scale

  11. ENSO modulation • 5 strongest El Niño winters and 5 strongest La Niña winters in 1979-2005 record using CPC’s Oceanic Niño Index El Niño 200 hPa zonal wind La Niña 200 hPa zonal wind

  12. ENSO and winter precip.

  13. ENSO and winter precip. Areas with major changes in mean precip: southwest, Great Plains (Lower Missouri River basin), Ohio River basin, southeast Most of these areas show changes in extremes with same spatial extent and greater magnitude Increased wet-day frequency during El Niño in southwest Shape of distribution is less sensitive to changes in climate

  14. ENSO: prec. water and MFC prec. water precipitation MFC Mean pr. water: little change with ENSO MFC changes are similar to precipitation; same areas, lesser magnitude

  15. summary & conclusions • During winter, mean precipitation over most of the U.S. is dominated by contribution of heavy and extreme daily events • Throughout the year, areas of the country with more variable and extreme rainfall have strong daily MFC • Precipitation frequency distribution not strongly linked to prec. water distribution • Most areas with large change in mean precipitation between ENSO phases have greater change in extremes • ENSO-related change in MFC distribution similar to precipitation

  16. references • Becker, E.J., E.H. Berbery, and R.W. Higgins, 2008: The frequency distribution of daily precipitation over the United States. J. Climate, submitted • Higgins, R.W., V. Silva, J. Larson, and W. Shi, 2007. Relationships between climate variability and fluctuations in daily precipitation over the United States. J. Climate, 15, 3561-3579. • Husak, G.J., J. Michaelsen, and C. Funk, 2007. Use of the gamma distribution to represent monthly rainfall in Africa for drought monitoring applications. Int. J. Climatol., 27, 935-944.

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