Since 2014, NASA has run the IMERG multi-satellite algorithm to estimate how much precipitation falls over the globe every 30 minutes on a grid with 0.1-degree resolution in latitude and longitude. Each grid box is approximately a square seven miles to a side at the Equator.
Every few years, the team of scientists that develop the IMERG algorithm deliver a new version of the software. They give the algorithm code to the NASA Goddard Precipitation Processing System (PPS) to run. Analysts and computer programmers with George Mason University’s CEOSR assist with running the algorithm and detecting anomalies in its output. When a new version of IMERG is delivered, NASA goes back over 23 years of satellite observations to give updated precipitation estimates that cover every 30 minutes of that entire period.
In 2024, NASA released Version 7 of the IMERG algorithm. An example accumulation from this version of the algorithm is shown above, covering five and a half days of observations of Hurricane Debby. Debby impacted the United States East Coast in August 2024. For part of that time, Debby was stalled off the coast of South Carolina, which means that the hurricane brought on shore a great deal of moisture which fell as large rainfall accumulations.
CEOSR team members used ancillary datasets to help evaluate the quality of the IMERG precipitation accumulations of Hurricane Debbie. The red triangles are locations of preliminary tornado reports associated with Hurricane Debbie according to the NOAA Storm Prediction Center. The violet-colored bow ties are locations where the USGS measured major
flooding at a steam gauge, as reported by the NOAA Water Prediction System. The orange and red squares are locations where rain gauges measured accumulation in excess of 14 or 17 inches, respectively. These gauge data were published in a discussion by the NOAA Weather Prediction Center.
The image’s filled contours in the image show precipitation estimates from the near real-time run of Version 7 IMERG. These near real-time estimates are known as the Late IMERG run. Weather forecasting agencies, disaster management organizations, and researchers can download these estimates approximately 16 hours after the satellite observations were collected. The path taken by the low-pressure center of Hurricane Debby is show with the white dashed line as reported in real-time by the National Hurricane Center.
As can be seen in the image, the maximum accumulation reported by the rain gauges occurred mostly over South Carolina within 50 miles of the coast. Many of the tornadoes and flooding stream gauges were also located this close to the sea. In contrast, the near real-time accumulations from IMERG were largest further inland or off the coast.
Because IMERG relies heavily on satellite estimates, it does not always capture the fine-scale structure of precipitation from severe storms with sharp precipitation gradients. Over much of the world’s oceans and even parts of continents, ground-based precipitation estimates are sparse, so there is interest in improving the IMERG algorithm based on comparisons with ground data where plentiful, such as over much of the United States.
CEOSR researcher Owen Kelley examined Version 7 IMERG accumulations from other hurricane impacting the United States coast in prior years to see if the new Version 7 algorithm always has errors matching the spatial pattern seen here with Hurricane Debby (2024). He determined that the new algorithm does not habitually make this mistake. NASA and CEOSR researchers will continue to evaluate the new version of IMERG with the aim of helping NASA to release an improved version in approximately 2 years.