Assessing the diurnal cycle of precipitation in

the CSU MMF (T42)

Michael S. Pritchard

Richard C. J. Somerville

John O. Roads

Scripps Institution of Oceanography

Questions How does the mean daily cycle of JJA

precipitation in the MMF compare to CAM, and to satellite retrievals from TRMM?

Does the MMF admit the sorts of propagating organized convective systems that in nature cause a nocturnal maximum in summer rainfall over the central U.S.?

CMMAP team meeting, Ft. Collins, July 2008

Methods CSU MMF & CAM simulations

Single-summer with hourly output T42 resolution MMF: 32 (x-z) CRM points per host grid-cell

TRMM “3B42” merged rainfall retrieval 3 hourly climatology, 2000-2006 Merged precipitation product using all 3 sensors

CMMAP team meeting, Ft. Collins, July 2008

Preliminary results 1. Harmonic analysis 2. Broadness of the diurnal maximum 3. Animation: Are there propagating

systems connected to nocturnal maxima over the central U.S.?

CMMAP team meeting, Ft. Collins, July 2008

Harmonic analysis At each grid point

Compute “mean summer day” at each grid point Least-squares fit to a 24-hour sinusoid

Plot maps of phase, amplitude Only where the fit is reasonable

Harmonic analysis% variance, 24-hour harmonic

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0% 100%

Satellite (TRMM 3B42) Standard model

Multi-scale model

CMMAP team meeting, Ft. Collins, July 2008

Harmonic analysisPhase, 24-hour harmonic

1am 5am 10am 3pm 8pm 12am

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TRMM 3B42 Standard model

Multi-scale model

CMMAP team meeting, Ft. Collins, July 2008

Broadness of diurnal maximum

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CMMAP team meeting, Ft. Collins, July 2008

CMMAP team meeting, Ft. Collins, July 2008

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Broadness of diurnal maximum

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6 years of satellite

Standard model

Multi-scale model

20 years of rain gauge

CMMAP team meeting, Ft. Collins, July 2008

Nocturnal maxima in theCentral U.S.?SatelliteObservations

Multiscalemodel

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CMMAP team meeting, Ft. Collins, July 2008

CMMAP team meeting, Ft. Collins, July 2008

Preliminary Conclusions Superparameterization improves several aspects

of diurnal precipitation at T42 resolution: Phase of the 24-hr harmonic delayed over continents Improved phase variability linked to topography Broadness of diurnal peak shows improved regionally

But… Current MMF does not capture the central U.S.

propagating precipitation mechanism

CMMAP team meeting, Ft. Collins, July 2008

Acknowledgements

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