Data Frequently Asked Questions

Data

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There are several sources for downloading and viewing data which allow you to subset the data to only include specific parameters and/or geographic locations. These include the GES DISCGiovanni and STORM. In Giovanni you can obtain data for a specific country, U.S. state, or watershed by using the "Show Shapes" option in the "Select Region" pane.

There are several sources for downloading and viewing data which allow you to subset the data to only include specific parameters and/or geographic locations. These include the GES DISCGiovanni and STORM. In Giovanni you can obtain data for a specific country, U.S. state, or watershed by using the "Show Shapes" option in the "Select Region" pane.

The TRMM satellilte has been decommissioned and stopped collecting data in April 2015. The transition from the Tropical Rainfall Measuring Mission (TRMM) data products to the Global Precipitation Measurement (GPM) mission products has completed as of August 2019. The GPM IMERG dataset now includes TRMM-era data from June 2000 to the present, and other TRMM-era data has been reprocessed with GPM-era algorithms and is now available on the GPM FTP servers. TMPA data production ended as of December 31st, 2019 and the TRMMOpen FTP server has been shut down. Historical TMPA data is still available to download from the NASA GES DISC at: https://disc.gsfc.nasa.gov/datasets?keywords=TMPA&page=1

Click here for more details on the transition from TMPA to IMERG. 

GPM data products can be divided into two groups (near real-time and production) depending on how soon they are created after the satellite collects the observations. For applications such as weather, flood, and crop forecasting that need precipitation estimates as soon as possible, near real-time data products are most appropriate.  GPM near real-time (GMI & DPR) products are generally available within a few hours of observation.  For all other applications, production data products are generally the best data sets to use because additional or improved inputs are used to increase accuracy.  These other inputs are only made available several days, or in some cases, several months, after the satellite observations are taken, and the production data sets are computed after all data have arrived, making possible a more careful analysis.

For the GPM IMERG dataset, IMERG Early and Late Runs are the near real-time products, while IMERG Final Run is the research / production product. Click here to learn more about the differences between IMERG Early, Late and Final. 

The resolution of Level 0, 1, and 2 data is determined by the footprint size and observation interval of the sensors involved.  Level 3 products are given a grid spacing that is driven by the typical footprint size of the input data sets.

For our popular multi-satellite GPM IMERG data products, the spatial resolution is 0.1° x 0.1° (or roughly 10km x 10km) with a 30 minute temporal resolution.

Visit the directory of GPM & TRMM data products for details on the resolution of each specific products.

Browse our directory of GPM & TRMM data products to locate your desired algorithm, then click on the links in the algorithm description under "Documentation". All documentation is also available at the Precipitation Processing System website

IMERG

IMERG provides a data field that estimates the probability that the retrieved precipitation amount is “liquid”, which is defined to include “mixed” (liquid and solid) precipitation.  In retrospect the field name should have been “ice”, but “liquid” had already been set.  The rational is that mixed precipitation is very rare and transient, so it should be lumped with either “liquid” or “ice”.  Furthermore, the primary effects of “ice” are to 1) prevent the falling precipitation from immediately entering the hydrological system (until it melts), and 2) to create (potentially) dangerous travel conditions.  “Mixed” typically ends up not creating either of these effects, so lumping it with “liquid” seems appropriate. 

Even given this basic definition, there are numerous forms of precipitation, and it might not be obvious how they end up being classified in IMERG.  The key fact is that the phase is computed diagnostically at present, based on work by Guosheng Liu (Florida State University) and students.  The Liu scheme uses data from a numerical model or model analysis to compute a “specification”, without reference to the satellite data, including whether or not IMERG estimates that precipitation is occurring, or even possible to estimate.  Thus, probabilityLiquidPrecipitation (pLP) is a globally complete field whenever the relevant model data exist.  An additional factor is that there is a conceptual difference between how the half-hourly phase is computed and how phase is defined in this probability framework for the monthly data.  We will handle the half-hourly first, for which the Liu specification equation is directly relevant.

Liu determined that the primary factor for phase is the surface wet bulb temperature (Tw), a combination of temperature and humidity, with small contributions from the low-altitude Tw lapse rate and the surface pressure, and with systematic differences between ocean and land areas.  In practice, the fitted probability as a function of Tw is converted to separate look-up tables for ocean and land.

Typical results for different forms of precipitation are:

  • Rain:  Ordinary falling liquid typically happens for Tw>0°C, so pLP is high.
  • Freezing Rain:  Liquid that freezes upon contact with the Earth's surface typically falls in Tw<0°C, so pLP is low.
  • Snow, ice pellets, snow pellets:  These frozen hydrometeors occur around or below Tw<0°C, so pLP varies from around 50% to very low.
  • Sleet:  Frozen droplets (U.S. definition) typically fall in Tw<0°C, so pLP is usually below 50%.
  • Mixed snow and rain; falling slush:  The mixed category is likely to occur around the pLP=50% mark.  If one uses 50% as a liquid/solid threshold, that implies that mixed cases will end up in both categories, depending on the details.
  • Hail:  Hail typically occurs when the surface air temperature is well above freezing (i.e., on summer afternoons).  Thus, pLP is very high.  But, hail is even rarer than mixed and unlikely to be correctly specified in this scheme, and anyway, in such conditions it rapidly melts and so is properly lumped into "liquid".
  • Dew and frost:  These phenomena are not forms of precipitation.  They are liquid or solid water that condenses directly at the Earth's surface.  For this reason, any amount of surface accumulation due to dew or frost is not included in the IMERG precip estimate.

As the time interval for the data values lengthens, it becomes increasingly likely that both liquid and solid might have fallen, at which point the meaning of pLP should change to “what fraction of the estimated precipitation amount fell as liquid or mixed?”  This is the definition of pLP for both the monthly IMERG Final Run pLP and the set of GIS IMERG files (TIFF+WorldFile) providing estimated accumulations longer than three hours.

Q1: How closely should the monthly satellite-gauge combined precipitation datasets follow the gauge analysis?

A1: The combined precipitation research team at Goddard has major responsibility for the Global Precipitation Climatology Project monthly Satellite-Gauge combined product, the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) 3B43 monthly product (previously), and the IMERG Final Run monthly product.  In each case the multi-satellite data within the product are averaged to the monthly scale and combined with the Global Precipitation Climatology Centre's (GPCC) monthly surface precipitation gauge analysis (see https://www.dwd.de/EN/ourservices/gpcc/gpcc.html).  In each case the multi-satellite data are adjusted to the large-area mean of the gauge analysis, where available (mostly over land), and then combined with the gauge analysis using a simple inverse estimated-random-error variance weighting.  In all three data sets the gauge analysis has an important or dominant role in determining the final combined value for grid boxes in areas with "good" gauge coverage.  Regions with poor gauge coverage, such as central Africa have a higher weight on the satellite input.  The oceans are mostly devoid of gauges and therefore mostly lack such gauge input.  Isolated island stations are deleted from the GPCC gauge analysis before use because they are usually not representative of the surrounding ocean, and frequently not even the island as a whole.

Q2: How closely related are the short-interval multi-satellite precipitation datasets to the monthly satellite-gauge combined precipitation datasets?

A2: The short-interval GPCP is the One-Degree Daily (1DD), the short-interval TMPA is 3B42 (which is 3-hourly), and the short-interval IMERG is the half-hourly.  In each case the short-interval data are adjusted with a simple, spatially varying ratio to force the multi-satellite estimates to approximately average up to the corresponding monthly product, although with controls on the ratios to prevent unphysical results.  Thus, monthly-average values for the short-interval data should be close to the mean values for the monthly datasets, which the developers consider more reliable than the short-interval datasets.  In fact, compared to datasets that lack the adjustment to the monthly satellite-gauge estimates, the 1DD, 3B42, and IMERG Final half-hourly datasets tend to score better at timescales longer than a few days.  This is presumably because the random error begins to cancel out as more samples are averaged together, while the bias error remains. 

As an example, see:

Bolvin, D.T., R.F. Adler, G.J. Huffman, E.J. Nelkin, J.P. Poutiainen, 2009:  Comparison of GPCP Monthly and Daily Precipitation Estimates with High-Latitude Gauge Observations.  J. Appl. Meteor. Climatol., 48(9), 1843–1857.

In the previous TMPA data set, each data value provided a precipitation rate based on one (or perhaps two) satellite snapshots during the TMPA’s 3-hour analysis period. IMERG values are based on a single microwave snapshot during its half-hour analysis period, or a morphed/Kalman filter interpolation if no microwave values are available.  The values are expressed in the intensive units mm/hr; it is usually best to assume that this rate applies for the entire half-hour period.  If you wish to regrid to a finer time and/or space grid, note that many interpolation schemes have the property of suppressing maxima in precipitation and expanding rain events into neighboring zero-amount periods, so it is critical that you examine this issue and state what scheme was used in documentation and papers. 

The main difference between the IMERG Early and Late Run is that Early only has forward propagation (which basically amounts to extrapolation forward in time), while the Late has both forward and backward propagation (allowing interpolation).  As well, the additional 10 hours of latency allows lagging data transmissions to make it into the Late run, even if they were not available for the Early (see below). 

There are two possible factors which contribute to differences in the IMERG Late Run and Final Run datasets:

  1. The Late Run uses a climatological adjustment that incorporates gauge data. In Versions 05 and 06 no adjustment has been applied.  In Version 04 this was a climatological adjustment to the Final run, which includes gauge data at the monthly scale. For Version 03  the TRMM V7 climatological adjustment of the TMPA-RT to the production TMPA was used (which includes gauge at the monthly scale) because this at-launch algorithm didn't yet have any Late and Final data from which to build the climatological adjustment.  The Final run uses a month-to-month adjustment to the monthly Final Run product, which combines the multi-satellite data for the month with GPCC gauge.  Its influence in each half hour is a ratio multiplier that's fixed for the month, but spatially varying.
  2. The Late Run is computed about 14 hours after observation time, so sometimes a microwave overpass is not delivered in time for the Late Run, but subsequently comes in and can be used in the Final.  This would affect both the half hour in which the overpass occurs, and (potentially) morphed values in nearby half hours.

The satellite sensor difference could be examined by comparing the satellite sensor data field in the Late and Final Run datasets for each half hour.  Since the gauge adjustment is a constant multiplier, a time series should show a constant ratio between the Late and the Final Runs for the entire month (except for cases where the satellite sensor is changing, just as for the ocean).

We always advise people to use the Final Run for research.  The vast majority of grid boxes have fairly similar Late and Final values, so it makes sense to stick to metrics that are more resistant to occasional data disturbances.  Extreme values are more sensitive to these details; medians, means, and root-mean square difference are less sensitive. 

The following table provides a quick reference for the IMERG variables that can be visualized using Giovanni.

Product

Variable and Description

GPM_3IMERGHHE

30-min averaged data

Merged microwave-only precipitation estimate [Final]

Precipitation estimates from combining microwave data from the GMI, TMI, and other partner instruments.

Random error for gauge-calibrated multi-satellite precipitation [Final, Early, Late]

This is an estimate of the non-systematic component of the error. The exact variable name depends on the product, but all begin with "Random error..."

Microwave satellite observation time [Final]

Observation time of the microwave precipitation estimates given as minutes from the beginning of the current half-hour.

Microwave satellite source identifier [Final]

This is an integer between 0 and 24 that corresponds to the instrument from which the microwave precipitation estimate was taken. See *3IMERGHH data fields* in the Integrated Multi-satellitE Retrievals for GPM (IMERG) Technical Documentation 

Weighting of IR-only precipitation relative to the morphed merged microwave-only precipitation estimate [Final]

This is the weighting of the infrared data in the final merged estimate, given in percent. Zero means either no IR weighting or no precipitation.

IR-only precipitation estimate [Final]

This is the microwave-calibrated infrared precipitation estimate.

Multi-satellite precipitation estimate with (climatological) gauge calibration [Final, Early, Late]

This is the precipitation estimate that is calibrated with monthly gauge data for the Final Run. This variable is recommended for most users. Note: The gauge calibration used in Early and Late would be climatological, but in V06 no calibration is applied in either Early or Late.

Multi-satellite precipitation estimate [Final]

This is the precipitation estimate that has not been calibrated with gauge data.

Accumulation-weighted probability of liquid precipitation phase [Final]

This is the probability of liquid precipitation. The probabilities are calculated globally regardless of whether precipitation is actually present

   

GPM_3IMERGM

1 month averaged data

Weighting of observed gauge precipitation relative to the multi-satellite precipitation estimate

This is the percent weighting of the surface gauge data.

Merged satellite-gauge precipitation estimate

This is the precipitation estimate that has been calibrated with gauge data. This variable is recommended for most users.

Accumulation-weighted probability of liquid precipitation phase

This is the probability of liquid precipitation. The probabilities are calculated globally regardless of whether precipitation is actually present.

Random error for merged satellite-gauge precipitation

This is an estimate of the non-systematic component of the error.

1. Most of the low-orbit microwave satellites downlink their data once or twice an orbit (which is about 90 minutes long) to the operating agency. [Except GMI comes down in 5-min. granules.] The agencies compute the Level 1B files, package them, and ship the data around. PPS gets all the NOAA and EUMETSAT data through NOAA, DMSP from NRL Monterey, and the Japanese AMSR2 through NASA, SAPHIR (now intermittently and generally not in near-real time) through ISRO, and GPM DPR and GMI within GPM. The global merged geo-IR data are assembled by NOAA/NWS/CPC, and the Precipitation Processing System (PPS) accesses GEOS FP forecasts through the Goddard NCCS and Autosnow from NOAA. By about 3 hours after observation time the geo-IR and about 85% of the microwave (depending on how systems feel that day) have arrived at PPS. PPS converts the microwave data to precip estimates and computes the IMERG Early Run, which only uses forward propagation morphing (plus IR in the Kalman filter) by about 4 hours after observation time.  [One subtle point is that the IR half-hour data fields come in pairs, so both halves of the hour are processed together, meaning the second half hour almost always has shorter latency than the first. Another is that IMERG uses forecasts to get the analysis of vertically integrated vapor, which is what IMERG V06 uses to estimate system motion for the morphing. So, hitches in delivery of the latest GEOS FP are not a big deal – IMERG just uses the most recent forecast sequence, whatever that is.]

2. To compute the IMERG Late Run, which entails both forward and backward propagation morphing (plus IR), processing has to wait long enough that the following microwave overpass has a chance to occur and then follow the delivery chain described above. So, PPS waits 11 hours to capture a reasonably complete set of "next overpass" data, and then start processing. The nominal latency stated in the documentation is 14 hours, but it's more like 12 in recent data.

3. The IMERG Final Run uses MERRA2 for the vertically integrated vapor, GPCC monthly Monitoring Analysis for gauge, and revised precipitation retrievals that depend on ERA-5 (and ERA-I up through mid-2019).  The pacing items are the GPCC and ERA-5, usually giving a latency of about 3.5 months.

Compared to previous versions, Version 06 IMERG introduces additional coverage at the high latitudes for the precipitation fields in all Runs -- Early, Late, and Final.  IMERG continues to use a merged geosynchronous infrared brightness temperature analysis to provide IR-based precipitation estimates.  The requisite analysis (provided by NOAA/NWS/Climate Prediction Center) covers the latitude band 60°N-S, so a "full" IMERG analysis is possible there.  At higher latitudes (in both hemispheres) IR-based estimates cannot be included, so the coverage in the complete precipitation fields is "partial" -- limited to gridboxes for which there is no snow/ice on the surface.  Some of the other data fields, specifically the merged microwave estimates and the precipitation phase, are provided for the entire globe.