The U.S. Global Precipitation Measurement mission (GPM) science team is developing a long-term dataset based on intercalibrated estimates from the international constellation of precipitation-relevant satellites and other data. The Integrated Multi-satellitE Retrievals for GPM (IMERG) merged precipitation product (IMERG) is computed at the half hour, 0.1 x 0.1 resolution globally in three "Runs"Early, Late, and Final (4 hours, 14 hours, and 3.5 months after observation time, respectively). GPM is well into computing the new Version 06, which will be the first time IMERG covers the last two decades and routinely provides morphed estimates in polar regions where the surface is snow- and ice-free.A few salient features of the IMERG algorithm will be summarized, then representative examples of IMERG products will be shown. This starts with basic results, such as animations of maps, then extends to preliminary analyses of dataset characteristics. For example, the diurnal cycle demonstrates improvements over V05

Bolvin, David T.

Braithwaite, Dan

Hsu, Kuolin

Huffman, George J.

Joyce, Robert

Kidd, Christopher

Nelkin, Eric

Sorooshian, Soroosh

Tan, Jackson

Xie, Pingping

NASA Technical Reports Server

Heading for 20 years of Quasi-Global Precipitation
with the new Version 06 IMERG
George J. Huffman(1), David T. Bolvin(1,2), Eric Nelkin(1,2), Jackson Tan(1,3), 
and Dan Braithwaite(4), Kuolin Hsu(4), Robert Joyce(5,6),
Christopher Kidd(1,7), Soroosh Sorooshian(4), Pingping Xie(6)
(1) NASA/GSFC Earth Sciences Division – Atmospheres
(2) Science Systems and Applications, Inc.
(3) Univ. Space Res. Assoc.
(4) Univ. of California Irvine
(5) Innovim
(6) NOAA/NWS Climate Prediction Center
(7) Univ. of Maryland / ESSIC
george.j.huffman@nasa.gov
https://ntrs.nasa.gov/search.jsp?R=20190025770 2019-08-31T11:56:52+00:00Z
We want 3-hourly observations, globally
• sampling the diurnal cycle
• morphed microwave loses skill outside ±90 
min
The current GPM constellation includes:
• 5 polar-orbit passive microwave imagers
•   3 SSMIS, AMSR-2, GMI
• 6 polar-orbit passive microwave sounders
•   3 MHS, 2 ATMS, SAPHIR*
• input precip estimates 
• GPROF (LEO PMW)
• PERSIANN-CCS (GEO IR)
• CORRA (combined PMW-Ku radar)
• GPCP SG (monthly satellite-gauge)
The constellation is evolving
• launch manifests are assured for sounders, 
sparse for imagers
1. Introduction – The Constellation
IMERG is a unified U.S. algorithm based on
• Kalman Filter CMORPH – NOAA/CPC
• PERSIANN CCS – U.C. Irvine
• TMPA – GSFC
• PPS (GSFC) processing environment
IMERG is a single integrated code system for near-real 
and post-real time
• multiple runs for different user requirements for 
latency and accuracy
• “Early” – 4 hr (flash flooding)
• “Late” – 14 hr (crop forecasting)
• “Final” – 3 months (research)
• time intervals are half-hourly and monthly (Final 
only)
• 0.1º global CED grid
• morphed precip, 60º N-S in V05, 90º N-S in V06
• IR covers 60º N-S
2. IMERG – Quick Description (1/2)
Half-hourly data file (Early, Late, Final)
1 [multi-sat.] precipitationCal
2 [multi-sat.] precipitationUncal
3 [multi-sat. precip] randomError
4 [PMW] HQprecipitation
5 [PMW] HQprecipSource [identifier]
6 [PMW] HQobservationTime
7 IRprecipitation
8 IRkalmanFilterWeight
9 [phase] probabilityLiquidPrecipitation
10 precipitationQualityIndex
Monthly data file (Final)
1 [sat.-gauge] precipitation
2 [sat.-gauge precip] randomError
3 GaugeRelativeWeighting
4 probabilityLiquidPrecipitation [phase]
5 precipitationQualityIndex
IMERG is adjusted to GPCP V2.3 monthly climatology 
zonally to achieve a bias profile that we consider 
reasonable
• GPM Version 04, 05, 06 core products have similar 
zonal profiles (by design)
• these profiles are systematically low in the 
extratropical oceans compared to
• GPCP V2.3 monthly Satellite-Gauge product
• Behrangi Multi-satellite CloudSat, TRMM, 
Aqua (MCTA) product
• over land GPCP adjustment provides a first cut at 
the adjustment to gauges that the final calibration in 
IMERG enforces
• similar bias concerns apply during TRMM
2. IMERG – Quick Description (2/2)
Half-hourly data file (Early, Late, Final)
1 [multi-sat.] precipitationCal
2 [multi-sat.] precipitationUncal
3 [multi-sat. precip] randomError
4 [PMW] HQprecipitation
5 [PMW] HQprecipSource [identifier]
6 [PMW] HQobservationTime
7 IRprecipitation
8 IRkalmanFilterWeight
9 [phase] probabilityLiquidPrecipitation
10 precipitationQualityIndex
Monthly data file (Final)
1 [sat.-gauge] precipitation
2 [sat.-gauge precip] randomError
3 GaugeRelativeWeighting
4 probabilityLiquidPrecipitation [phase]
5 precipitationQualityIndex
PMW
sensor
2. IMERG – Examples of Data Fields
PMW
time into 
half hour
PMW
precip
IR precip
IR weight
cal precip
probability of 
liquid phase
Quality 
Index
2 July 2015 
0030 UTC
Half-hourly QI (revised)
• approx. Kalman Filter correlation
• based on 
• times to 2 nearest PMWs (only 1 for 
Early) for morphed data
• IR at/near time (when used)
• where r is correlation, and  the i’s are for 
forward propagation, backward 
propagation, and IR
• or, an approximate correlation when a 
PMW is used for that half hour
• revised to 0.1º grid (0.25º in V05)
• thin strips due to inter-swath gaps
• blocks due to regional variations
• snow/ice masking will drop out microwave 
values
2. IMERG – Quality Index (1/2)
D.Bolvin (SSAI; GSFC)
The goal is a simple “stoplight” index
• ranges of QI will be assigned
• good 0.6-1
• use with caution 0.4-0.6
• questionable 0-0.4
• is this a useful parameter?
Monthly QI (unchanged)
• Equivalent Gauge (Huffman et al. 1997) in gauges / 2.5ºx2.5º
• where r is precip rate, e is random error, and H and S are source-specific error constants
• invert random error equation
• largely tames the non-linearity in random error due to rain amount
• some residual issues at high values
• doesn’t account for bias
• the  stoplight ranges are
• good > 4
• use with caution 2-4
• questionable < 2
• note that this ranking points out uncertainty
in the values in light-precip areas that
nearly or totally lack gauges (some
deserts, oceanic subtropical highs)
2. IMERG – Quality Index (2/2)
Month Qual. Index  Dec 2016 0 4 8 12  16 20+
D.Bolvin (SSAI; GSFC)
Morphing vector source switched to MERRA-2/GEOS FP
Morphed precip extended from 60º N-S (V05 and earlier) to 90º N-S, but 
• masked out for icy/snowy surfaces
Half-hourly Quality Index modified
• t=0 values estimated (set to 1 in V05)
• shifted to 0.1º grid ( 0.25º in V05)
Full intercalibration to CORRA
• V05 took shortcuts
Modifications for TRMM era
• compute calibrations for older satellites against TRMM
• compute TRMM-era microwave calibrations in the band 33ºN-S and 
• blend with adjusted monthly climatological GPM-era microwave calibrations over 25º-90º N and S
Revisions to internals raises the maximum precip rate from 50 to 200 mm/hr and no longer discrete
• files bigger due to less compressibility
• allows really tiny numbers
2. IMERG – V06 Upgrades
Following the CMORPH approach
• for a given time offset from a microwave overpass
• compute the (smoothed) average correlation 
between
• morphed microwave overpasses and microwave 
overpasses at that time offset, and
• IR precip estimates and microwave overpasses 
at that time offset and IR at 1 and 2 half hours 
after that time offset
• for conical-scan (imager) and cross-track-scan 
(sounder) instruments separately
• the microwave correlations drop off from t=0, 
dropping below the IR correlation within a few hours 
(2 hours in the Western Equatorial Pacific)
3. Some Details – Key Points in Morphing (1/3)
Western 
Equatorial 
Pacific Ocean
Aug.-Oct. 2017
J. Tan (USRA; GSFC)
Following the CMORPH approach
• for a given time offset from a microwave overpass
• compute the (smoothed) average correlation between
• morphed microwave overpasses and microwave 
overpasses at that time offset, and
• IR precip estimates and microwave overpasses at 
that time offset and IR at 1 and 2 half hours after 
that time offset
• for conical-scan (imager) and cross-track-scan 
(sounder) instruments separately
• the microwave correlations drop off from there, 
dropping below the IR correlation within a few hours (2 
hours in the Western Equatorial Pacific)
• at t=0 (no offset), imagers are better over oceans, 
sounders are better or competitive over land
3. Some Details – Key Points in Morphing (2/3)
L2 correlation at t=0  Aug.-Oct. 2017
Imager
Sounder
J. Tan (USRA; GSFC)
Tested vectors computed on a 5ºx5º template 
every 2.5º, interpolated to 0.1ºx0.1º based on
• MERRA2 TQV (vertically integrated vapor)
• MERRA2 PRECTOT (precip)
• CPC 4-km merged IR Tb (as in V05 IMERG)
• NULL (no motion)
On a zonal-average basis, compute the Heidke
Skill Score for
• merged GPROF precip (HQ) propagated for 
30 min. 
• compared to HQ precip observed in the 
following 30 min.
• TQV is consistently at/near the top
• further research is expected for V07
3. Some Details – Key Points in Morphing (3/3)
J. Tan (USRA; GSFC)
−90 −60 −30 0 30 60 90
latitude
0.500
0.525
0.550
0.575
0.600
0.625
0.650
0.675
0.700
H
S
S
None IR (V05) PRECTOT TQV
Calibration sequence is
• CORRA calibrated to GPCP over ocean 
outside 30ºN-S
• GPM constellation calibrated to CORRA
Adjustments working roughly as intended
• CORRA is low at higher latitudes
• adjustments in Southern Ocean are 
large and need analysis
• IMERG subsetted to coincidence 
with CORRA is much closer to 
CORRA
4. Early Results – Calibration 
D. Bolvin (SSAI; GSFC)
Latitude
CORRA
CORRA
Average June-July-
August for 2014 to 2018
• data re-sorted to give 
the same LST over 
the whole globe
• surface cycles 
between Blue Marble 
and Night Lights
• adding the TRMM 
era will smooth out 
the results
4. Early Results – JJA Diurnal Cycle (GPM Era) 
Reminiscent of TMPA, but
• more detailed, broader spatial coverage
• no interpolations between the 3-hourly times
• less IR-based precip used (which tends to have a 
phase lag)
Reminiscent of IMERG V05, but 
• better data coverage at higher latitudes
• less “flashing” due to inter-satellite differences 
and morphing
• and still have artifacts along ice edges
J. Tan (USRA; GSFC)
Average June-July-August for 2014 to 2018 (5 
summers) for 6 states
Compared to Multi-Radar Multi-Sensor 
(MRMS), IMERG Final shows:
• lower averages
• lower amplitude cycle in Colorado
• higher amplitude cycle in Iowa
• very similar curve shapes, peak times
This version of MRMS only starts in 2014, so 
an extended comparison would have to use 
different data 
4. Early Results – JJA Diurnal Cycle in Central U.S. (GPM Era) 
J. Tan (USRA; GSFC)
IMERG-Final – solid
MRMS – dashed
Harvey loitered over southeast Texas for a 
week
• Multi-Radar Multi-Sensor (MRMS) 
considered the best estimate
• some questions about the details of the 
gauge calibration of the radar estimate
• over land
• Uncal (just the intercalibrated satellite 
estimates) under(over)-estimated in Area 
1(2)
• should be similar in NRT Late Run
• Cal (with gauge adjustment) pulls both 
areas down
• microwave-adjusted PERSIANN-CCS IR 
has the focus too far southwest
4. Early Results – Hurricane Harvey, 25-31 August 2017, IMERG and MRMS (1/2)
J. Tan (USRA; GSFC)
IMERG largely driven by microwave 
overpasses (dots)
• except duplicate times
• not just time interpolation
• systems move into / out of the box 
between overpasses
• satellites show coherent differences from 
MRMS
• microwave only “sees” the solid 
hydrometeors (scattering channels), 
since over land
• IR looks at Tb within “clustered” data
• both are calibrated to statistics of 
time/space cubes of data
• Cal is basically ( Uncal x factor )
• short-interval differences show some 
cancellation over the whole event
• but several-hour differences can be 
dramatic
4. Early Results – Hurricane Harvey, 25-31 August 2017, IMERG and MRMS (2/2)
J. Tan (USRA; GSFC)
V06 Final Run starts June 2000, currently 
processed into 2010, plus the GPM era
V06(V05) is higher(lower) than TMPA
TMPA and TRMM-era IMERG have a 
strong semi-annual signal
• GPM-era IMERG is dominated by the 
annual cycle
Short-term “trend”
• similar for all GPM-era datasets
• flatter for IMERG than for TMPA in 
TRMM era
Behavior in 2009-10 is under review
4. Early Results – Ocean (50ºN-S) Timeseries
D. Bolvin (SSAI; GSFC)
Early March 2019: began Version 06 IMERG Retrospective Processing
• the GPM era was launched first, Final Run first
• the TRMM era Final Run reprocessing is underway
• complete data should finish in May
• 4 km merged global IR data files continue to be delayed for January 1998-January 2000
• the run builds up the requisite 3 months of calibration data starting from February 2000
• the first month of data is for June 2000
• the initial 29 months of data will be incorporated when feasible
• Early and Late Run Retrospective Processing uses Final intermediate files, so they come after Final
• Final is always ~3.5 months behind, so the Early and Late retrospective processing have to wait on 
Final Initial Processing to fill in the last 3 months before May 2019
• Early and Late Run Initial Processing started ~1 May
5. Schedule and Final Remarks (1/3)
underway
done
und rway
coming
Development Work for V07
• multi-satellite issues
• improve error estimation
• develop additional data sets based on observation-model combinations
• work toward a cloud development component in the morphing system
• general precipitation algorithmic issues
• introduce alternative/additional satellites at high latitudes (TOVS, AIRS, AVHRR, etc.)
• evaluate ancillary data sources and algorithm for Prob. of Liq. Precip. Phase
• work toward using PMW retrievals over snow/ice
• work toward improved wind-loss correction to gauge data
Version 07 release should be in about 2 years (2021-22?)
5. Schedule and Final Remarks (2/3)
IMERG is being 
upgraded to V06 in 
Spring 2019
• the product structure 
remains the same
• Early, Late, Final
• 0.1ºx0.1º half-
hourly (and 
monthly in Final)
• new source for 
morphing vectors
• higher-latitude 
coverage
• extension back to 
2000 (and eventually 
1998)
• improved Quality 
Index
5. Schedule and Final Remarks (3/3)
J. Tan (USRA; GSFC)
See https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=4285


Heading for 20 Years of Quasi-Global Precipitation with the New Version 06 IMERG

Heading for 20 Years of Quasi-Global Precipitation with the New Version 06 IMERG

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