GPM IMERG Final Half Hourly (3IMERGHH V07)

[xvchang]

May I ask, for GPM IMERG Final Half Hourly (3IMERGHH V07), is its precipitation value the cumulative value for half an hour, the average value for half an hour, or the cumulative value for one hour and the average value for one hour？

[huffman]

Thanks for asking ... the IMERG value is provided from a single snapshot sometime during the half-hour period. Tests show that these passively sensed estimates have time correlations with time-averaged surface radar precipitation, so we take the snapshot as representative of the entire half hour. The units are intensive - stated as mm/hr. To get an estimated accumulation for the half hour, this rate needs to be applied for a half hour (so, rate * 0.5).

[xvchang]

Thank you very much for your response. I have a further question: If my threshold for heavy precipitation is 20mm/h, then on GPM would it be converted to 10mm?

[huffman]

If your threshold for each half hour map is 20 mm/hr, then yes, that means "at least 10 mm in each half hour period".

[xvchang]

Thank you for your answer. I wish you a happy life!

[xvchang]

I retrieved the 30-minute precipitation data for the China region in NetCDF (nc) format from the official website. Subsequently, I utilized Python code to extract the "precipitation" variable from the NetCDF data and convert it into GeoTIFF (tif) format. However, two issues have arisen:
1. While the data orientation appears correct when viewed in ArcMap, distortion occurs when the same data is opened in ENVI.
2. In an attempt to identify precipitation locations using GPM data through Python code, I used the previously generated GPM.tif file as input. Despite this, the resulting visualization exhibits a flipped orientation. Specifically, the precipitation area in the original image is concentrated in the lower-left and upper-right diagonal regions, whereas the generated visualization focuses on the upper-left and lower-right diagonal regions.

Upon further investigation, it was determined that the discrepancy arises due to a mismatch between the geographic reference information and the actual data content.
How can this be solved? I sincerely hope for your assistance. Thank you very much!

[GES DISC-EDL - ezamkoff]

This could be the dimension issue:

The variables are given in the (time, lon, lat) format. Typically, most datasets follow the (time, lat, lon) structure. After taking the transpose, it should work.

We have this FAQ:
https://disc.gsfc.nasa.gov/information/faqs?keywords=imerg%20orienation&title=How%20can%20I%20import%20and%20geolocate%20the%200.1-degree%20resolution%20IMERG%20native%20HDF5%20file%20in%20ENVI%3F