1. 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

2. Outline 1.Introduction, Data and Method 2.Seasonal precipitation frequency distribution 3.Seasonal frequency distribution of precipitation- related variables 4.ENSO modulation of precipitation and precipitation- related variables 5.Summary & conclusions Objective: Examine the seasonal characteristics of the frequency distribution of precipitation and its dependence on the lower frequency modulation of ENSO during winter

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

4. Data: NARR North American Regional ReanalysisNorth American Regional Reanalysis 1979-present (1979-2005 for this study)1979-present (1979-2005 for this study) 32-km horizontal resolution, 45 vertical layers32-km horizontal resolution, 45 vertical layers Developed with 2003 version of the Eta model; Eta model coupled with Noah land surface modelDeveloped with 2003 version of the Eta model; Eta model coupled with Noah land surface model Precipitation is assimilatedPrecipitation is assimilated Extremely high agreement between NARR precip. and observationsExtremely high agreement between NARR precip. and observations

5. Gamma distribution pp distrib. positively skewed, bounded on left by 0pp distrib. positively skewed, bounded on left by 0 Shape (α) describes skewnessShape (α) describes skewness Scale (β) stretches or shrinks along x-axisScale (β) stretches or shrinks along x-axis BlueRed mn7.84.8 α0.580.64 β13.47.5 α β = mean α β = mean

6. winter (JFM) precipitation scaleshape wet-day mean frequency wet-day mean frequency scale-dominated areas (large scale, small shape): more precipitation received from extremesscale-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 *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-dominatedin winter, most areas with high wet-day mean are scale-dominated northern Plains and Great Lakes are shape-dominatednorthern Plains and Great Lakes are shape-dominated Atlantic and Gulf of Mexico coasts are scale-dominatedAtlantic and Gulf of Mexico coasts are scale-dominated * After Husak et al. (2007)

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

8. precipitation-related factors Precipitable water (total atmospheric water vapor contained in a vertical column, mm)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 precipitationVertically-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)Also convective available potential energy (CAPE)

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

10. summer Prec. water much larger in summer; distribution is close to Gaussian (shape > 20) over most of countryPrec. 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 meanGPLLJ and NAM evident in both prec. water and MFC mean MFC reflects both mean and extreme precipitationMFC reflects both mean and extreme precipitation prec. water precipitationMFC 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 Index5 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. 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 Shape of distribution is less sensitive to changes in climate Increased wet-day frequency during El Niño in southwest

14. ENSO: prec. water and MFC prec. water precipitationMFC 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 eventsDuring 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 MFCThroughout 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 distributionPrecipitation frequency distribution not strongly linked to prec. water distribution Most areas with large change in mean precipitation between ENSO phases have greater change in extremesMost areas with large change in mean precipitation between ENSO phases have greater change in extremes ENSO-related change in MFC distribution similar to precipitationENSO-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, submittedBecker, 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.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.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.