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

Outline

1.1. Introduction, Data and MethodIntroduction, Data and Method2.2. Seasonal precipitation frequency distributionSeasonal precipitation frequency distribution3.3. Seasonal frequency distribution of precipitation-related Seasonal frequency distribution of precipitation-related

variablesvariables4.4. ENSO modulation of precipitation and precipitation-ENSO modulation of precipitation and precipitation-

related variablesrelated variables5.5. Summary & conclusionsSummary & conclusions

Objective: Examine the seasonal characteristics Objective: Examine the seasonal characteristics of the frequency distribution of precipitation and of the frequency distribution of precipitation and its dependence on the lower frequency its dependence on the lower frequency modulation of ENSO during winter modulation of ENSO during winter

Daily precipitation

• Changes in the frequency and intensity of daily Changes in the frequency and intensity of daily precipitation within a season can occur precipitation within a season can occur independently from changes in seasonal meanindependently from changes in seasonal mean

• More extreme events More extreme events can lead to high can lead to high streamflow, flooding streamflow, flooding (Groisman et al. 2001)(Groisman et al. 2001)

• Often see greater Often see greater increase in extremes increase in extremes than in mean totalthan in mean total

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 Developed with 2003 version of the Eta

model; Eta model coupled with Noah land model; Eta model coupled with Noah land surface modelsurface model

• Precipitation is assimilatedPrecipitation is assimilated• Extremely high agreement Extremely high agreement

between NARR precip. between NARR precip. and observationsand observations

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

Blue Red

mn 7.8 4.8

α 0.58 0.64

β 13.4 7.5

α β = meanα β = mean

winter (JFM) precipitation

scalescale shapeshape

wet-day meanwet-day mean frequency frequency

• scale-dominatedscale-dominated areas (large scale, small shape): areas (large scale, small shape): more precipitation received from extremesmore precipitation received from extremes

• shape-dominatedshape-dominated areas (large shape, small scale): areas (large shape, small scale): more precipitation from more-frequent lighter events * more precipitation from more-frequent lighter events *

• in winter, most in winter, most areas with high areas with high wet-day mean are wet-day mean are scale-dominatedscale-dominated

• northern Plains and northern Plains and Great Lakes are Great Lakes are shape-dominatedshape-dominated

• Atlantic and Gulf of Atlantic and Gulf of Mexico coasts are Mexico coasts are scale-dominatedscale-dominated

* After Husak et al. (2007) * After Husak et al. (2007)

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

scalescale shapeshape

wet-day mean wet-day mean frequency frequency

precipitation-related factors

• Precipitable waterPrecipitable water (total (total atmospheric water vapor atmospheric water vapor contained in a vertical column, contained in a vertical column, mm)mm)

• Vertically-integrated moisture flux Vertically-integrated moisture flux convergence (MFC)convergence (MFC): earlier work (Silva : earlier work (Silva and Berbery, 2006) suggests model and Berbery, 2006) suggests model forecasts have better representation of forecasts have better representation of MFC distribution than of precipitation MFC distribution than of precipitation

• Also Also convective available potential energy convective available potential energy (CAPE)(CAPE)

winterprec. waterprec. water precipitationprecipitation MFCMFC

• Mean Mean prec. waterprec. water generally similar to mean precipitation, but scale parameters (representing contribution of extremes) generally similar to mean precipitation, but scale parameters (representing contribution of extremes) not strongly linkednot strongly linked

• MFCMFC and precip. scale parameters have strong resemblance and precip. scale parameters have strong resemblance

summer

• Prec. water much larger in summer; distribution is close to Prec. water much larger in summer; distribution is close to Gaussian (shape > 20) over most of countryGaussian (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. waterprec. water precipitationprecipitation MFCMFC

meanmean

scalescale

ENSO modulation

• 5 strongest El Niño winters and 5 strongest 5 strongest El Niño winters and 5 strongest La Niña winters in 1979-2005 record using La Niña winters in 1979-2005 record using CPC’s Oceanic Niño Index CPC’s Oceanic Niño Index

El Niño 200 hPa zonal windEl Niño 200 hPa zonal wind La Niña 200 hPa zonal windLa Niña 200 hPa zonal wind

ENSO and winter precip.

ENSO and winter precip.Areas with major Areas with major changes in changes in mean mean precipprecip: southwest, : southwest, Great Plains (Lower Great Plains (Lower Missouri River Missouri River basin), Ohio River basin), Ohio River basin, southeast basin, southeast

Most of these areas Most of these areas show changes in show changes in extremesextremes with same with same spatial extent and spatial extent and greater magnitude greater magnitude

Shape Shape of distribution is less of distribution is less sensitive to changes in climatesensitive to changes in climate

Increased Increased wet-day frequencywet-day frequency during El Niño in southwestduring El Niño in southwest

ENSO: prec. water and MFC

prec. waterprec. water precipitationprecipitation MFCMFC

Mean Mean pr. waterpr. water: little : little change with ENSOchange with ENSO

MFCMFC changes are similar to precipitation; changes are similar to precipitation; same areas, lesser magnitudesame areas, lesser magnitude

summary & conclusions

• During winter, mean precipitation over most of the During winter, mean precipitation over most of the U.S. is dominated by contribution of heavy and U.S. is dominated by contribution of heavy and extreme daily eventsextreme daily events

• Throughout the year, areas of the country with more Throughout the year, areas of the country with more variable and extreme rainfall have strong daily MFCvariable and extreme rainfall have strong daily MFC

• Precipitation frequency distribution not strongly Precipitation frequency distribution not strongly linked to prec. water distributionlinked to prec. water distribution

• Most areas with large change in mean precipitation Most areas with large change in mean precipitation between ENSO phases have greater change in between ENSO phases have greater change in extremesextremes

• ENSO-related change in MFC distribution similar to ENSO-related change in MFC distribution similar to precipitationprecipitation

references

• Becker, E.J., E.H. Berbery, and R.W. Higgins, 2008: The Becker, E.J., E.H. Berbery, and R.W. Higgins, 2008: The frequency distribution of daily precipitation over the frequency distribution of daily precipitation over the United States. United States. J. ClimateJ. Climate, submitted, submitted

• Higgins, R.W., V. Silva, J. Larson, and W. Shi, 2007. Higgins, R.W., V. Silva, J. Larson, and W. Shi, 2007. Relationships between climate variability and Relationships between climate variability and fluctuations in daily precipitation over the United States. fluctuations in daily precipitation over the United States. J. ClimateJ. Climate, , 1515, 3561-3579., 3561-3579.

• Husak, G.J., J. Michaelsen, and C. Funk, 2007. Use of the Husak, G.J., J. Michaelsen, and C. Funk, 2007. Use of the gamma distribution to represent monthly rainfall in gamma distribution to represent monthly rainfall in Africa for drought monitoring applications. Africa for drought monitoring applications. Int. J. Int. J. Climatol.Climatol., , 2727, 935-944., 935-944.