This Directory Interchange Format (DIF) describes a collection of fields for the GPM Level 3 IMERG  *Final* Half Hourly 0.1 x 0.1 degree V03 (GPM_3IMERGHH) at the NASA Goddard Earth Sciences (GES) Data and Information Services Center (DISC).

The Integrated Multi-satelliE Retrievals for GPM (*IMERG*) is the unified U.S. algorithm that provides  the  Day-1  multi-satellite  precipitation  product  for  the  U.S.  GPM  team.     The precipitation  estimates  from  the  various  precipitation-relevant  satellite  passive  microwave (PMW) sensors comprising the GPM constellation are computed using the 2014 version of the Goddard Profiling Algorithm (GPROF2014), then gridded, intercalibrated to the GPM Combined Instrument product, and combined into half-hourly 0.1°x0.1° fields.  These are provided to both the Climate Prediction Center (CPC) Morphing-Kalman Filter  (CMORPH-KF) Lagrangian time interpolation scheme and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks – Cloud Classification System (PERSIANN-CCS) re-calibration scheme.  In parallel, CPC assembles the zenith-angle-corrected, intercalibrated “even-odd” geo- IR fields and forward them to PPS for use in the CMORPH-KF Lagrangian time interpolation scheme and the PERSIANN-CCS computation routines.   The PERSIANN-CCS estimates are computed (supported by an asynchronous re-calibration cycle) and sent to the CMORPH-KF Lagrangian time interpolation scheme.   The CMORPH-KF Lagrangian time interpolation (supported by an asynchronous KF weights updating cycle) uses the PMW and IR estimates to create half-hourly estimates.  The IMERG system is run twice in near-real time

•    “Early” multi-satellite product ~4 hr after observation time and
•    “Late” multi-satellite product ~12 hr after observation time,

and once after the monthly gauge analysis is received

•     “Final” satellite-gauge product ~2 months after the observation month.

The baseline is for the (near-)real-time Early and Late half-hour estimates to be calibrated with climatological coefficients that vary by month and location, while in the Final post-real-time run the multi-satellite half-hour estimates are adjusted so that they sum to a monthly satellite-gauge combination.   In all cases the output contains multiple fields that provide information on the input data, selected intermediate fields, and estimation quality.

Release document:
http://pmm.nasa.gov/sites/default/files/document_files/IMERG_FinalRun_Day1_release_notes.pdf

Other key documents:

Technical document and acronyms:
http://pmm.nasa.gov/sites/default/files/document_files/IMERG_doc.pdf

Algorithm Theoretical Basis Document: 
http://pmm.nasa.gov/sites/default/files/document_files/IMERG_ATBD_V4.4.pdf

In brief, the input precipitation estimates computed from the various satellite passive microwave sensors are intercalibrated to the GPM Combined Instrument product (because it is presumed to be the best snapshot GPM estimate), then “morphed” and combined with microwave precipitation-calibrated geo-IR fields, and adjusted with monthly surface precipitation gauge analysis data (where available) to provide half-hourly and monthly precipitation estimates on a 0.1° lat./long. grid over the domain 60°N-S. Precipitation phase is diagnosed using analyses of surface temperature, humidity, and pressure. The current period of record is mid-March 2014 to the present (delayed by about 2 months).The Integrated Multi-Satellite Retrievals for GPM (IMERG) algorithm is designed to leverage the international constellation of precipitation-relevant satellites to create a long record of uniformly time/space gridded precipitation estimates for the globe. The algorithm is focused on creating the best estimate at each time step, meaning that it is not a Climate Data Record, although the ideal is as homogenous a record as possible.