Wide Band Power and Harmonic Amplitude of Precipitation Alex Ruane John Roads Scripps Institution of Oceanography / UCSD Ramat Gan, Israel: July, 2006 This work supported by NOAA and NASA

Introduction and Background Datasets and Methodology General Wide Band Behavior Discussion Overview

Currently, atmospheric parameterizations in global reanalyses tend to focus on longer-term means at the expense of extreme events –Too many small storms, not enough floods or droughts –AGCMs are not producing proper statistical distributions of sub- seasonal events Goal is to simulate proper evolution, exchanges, and statistics of water and energy cycles throughout the day over diverse regions at multiple spatial and temporal scales –What are the frequency characteristics of reanalysis output? … of satellite products? –Are the limitations in parameterization or dynamics? –How are biases passed throughout the water and energy cycles? Motivation for Diurnal Examination

GSM-based Global Reanalyses –T62L28, 3-hourly output, initialized 4x daily –Covering – –Weekly variation in SST Data Sets Satellite Precipitation Products NCEP / DOE Reanalysis-2ECPC SFM Reanalysis CMORPH ±60 N/S, 3-hourly values covering PERSIANN ±50 N/S, 6-hourly values covering TRMM 3B42 ±50 N/S, 3-hourly values covering

Harmonic Reconstruction Analysis –Fit diurnal and semidiurnal harmonics to 3.5 years of precipitation at 3-hr resolution No filling required Methodology Fourier-based Wide Bands –Filled in missing points with a cubic spline interpolation –Performed Fast Fourier Transform on 3-hr time series –Averaged frequency bands into several wide bands Low-Frequency: Periods longer than 30 days Synoptic: Periods between 2 days and 30 days High-Frequency: Periods shorter than 2 days Summertime Precipitation Example

Downward SW Radiation Flux from SFM Low- Freq Synoptic High- Freq Harmonic

Prior Expectations General theory for precipitation would predict: –Low-frequency power in ITCZ and at high-latitudes –Synoptic power over mid-latitude storm tracks –High-frequency power in ITCZ and over land Especially arid regions –Strong diurnal and semidiurnal harmonics over: Coastlines Mountains ITCZ Diurnal harmonic often described as explaining high percentage of variance over land areas

TRMM Behavior Low- Freq Synoptic High- Freq Harmonic

CMORPH Behavior Low- Freq Synoptic High- Freq Harmonic

PERSIANN Behavior Low- Freq Synoptic High- Freq Harmonic

SFM Behavior Low- Freq Synoptic High- Freq Harmonic

RII Behavior Low- Freq Synoptic High- Freq Harmonic

Zonal Mean Land/Sea Variance Explained RII SFM CMORPH PERSIANN TRMM

General Frequency Characteristics –Low-frequency power is very weak for precipitation Likely due to unchanging baseline value of non-precipitating events –High-frequency variability dominates most locations Likely due to short-lived convective events –Land has more power in high-frequency than ocean Particularly over arid regions –Oceans have more synoptic power than land Particularly over sinking portion of Hadley Cell General Harmonic Characteristics –Extremely low variance described Likely due to sporadic nature of precipitation –Most variance explained over mountainous regions and regions where there were no strong events Discussion (1)

Both approaches are enlightening –Fourier approach Full frequency characteristics Relies on interpolation of missing values Strongly affected by temporal averaging –Harmonic approach Can be used on irregularly observed data Performs very well when applied to strongly averaged series May be restricted to physically understood variations –Differences Fourier approach brings out much more of the expected high-frequency variation Characteristic precipitation behaviors may be more readily identified using Fourier approach Discussion (2)

Large biases evident in frequency characteristics –Among satellite products TRMM appears to over-emphasize high-frequency variations CMORPH appears to behave the most like our expectations, but is overwhelmed by particularly strong events PERSIANN has the whitest spectrum –Among GSM reanalyses SFM has spuriously large low-frequency and synoptic power over tropical oceans RII has a spectrum that looks much more like we expected Discussion (3)

Apply these approaches to each component of the water and energy cycle Run long-term experiments to test the sensitivity of these components’ frequency characteristics to: – Land-surface schemes –Cloud schemes –Convective schemes Future Work