Add new IMERG processed data

[hannahker]

Looks like we now have earlier IMERG data available:

Dear Science User,
PPS has processed the Version 07B IMERG Early Run and Late Run datasets from January 1998 to May 2000. These join the Final Run in now covering the start of the TRMM era. Note that the NOAA Climate Data Record (CDR) of Gridded Satellite Data from ISCCP B1 (GridSat-B1) Infrared Channel Brightness Temperature dataset is used for the Version 07B datasets for the period until 2000-02-07 19:59:59 UTC. Thereafter, the NOAA Climate Prediction Center (CPC) Merged 4 km IR dataset is used for the Version 07B datasets starting from 2000-02-07 20:00:00 UTC. An additional note is that the IR calibrations used in IMERG gradually shift from GridSat to CPC over the period February through May 2000, although the actual IR estimates switch at the date/time 2000-02-07 20:00 UTC.
These extensions to the datasets are available now. The Version 07B IMERG Early Run dataset encompasses from January 1998 (the beginning of TRMM data) up until around 4 hours after the present observation time. The Version 07B IMERG Late Run dataset covers from January 1998 (the beginning of TRMM data) up until around 14 hours after the present observation time. The previously extended Version 07B IMERG Final Run dataset covers from January 1998 (the beginning of TRMM data) up until around 4 months after the present observation time.