# Presentation: 543692

## 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 

- 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

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

## 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

## 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

## Gamma distribution

- pp distrib. positively skewed, bounded on left by 0
- Shape (α) describes skewness
- Scale (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 = mean

**Notes:**
statistical estimators
pp > 0 only
shape < 1 has highest value occurring at x=0, shape > 20 approaches gaussian (normal) distrib  

## winter (JFM) precipitation

- scale

- shape

-        wet-day mean	          frequency

- 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

- * After Husak et al. (2007)

**Notes:**
overall, scale parameter echoes the winter gradient between low and high precipitation (from the central and northern plains to the southeast)
credit to Husak et al. 2007

## summer (JAS) precipitation

- Great Plains Low-Level Jet: scale-dominated
- North American monsoon
- Southeast: mean dominated by light/moderate

- scale

- shape

-          wet-day mean    	            frequency

## 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)

## 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

## summer

- 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

- prec. water

- precipitation

- MFC

- mean

- scale

## 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

## ENSO and winter precip.

## 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

- Shape of distribution is less sensitive to changes in climate

- Increased wet-day frequency during El Niño in southwest

## 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

## 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

## 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.