Author: okelley

In the two weeks starting on December 25, 2019, several heavy storm systems impacted countries along the eastern Mediterranean. While January is typically one of the rainiest months of the year in this region, the rainfall totals are truly impressive and far above typical for this region and season.

NASA’s satellite-based near-realtime precipitation-estimation algorithm reports over 30 inches (760 mm) of accumulation during this period in isolated locations just off the coast of Syria and Cyprus, with accumulation in excess of 24 inches (610 mm) over a patch of northern Israel. News stories from cities in these regions report severe flooding, including in several cities in Israel, Latakia in Syria, and Chrysochous in Crete. Several low-pressure centers over the eastern Mediterranean have contributed to the sea-to-land flow of moist air that has fed the flooding.

NASA’s algorithm that combines precipitation estimates from a international fleet of satellites is called IMERG.  IMERG stands for Integrated Multi-satellitE Retrievals for GPM.  Visualization by O. Kelley.

By Friday morning, September 20, the rainfall from the remnant of Tropical Storm Imelda had increased to over 24 inches in some areas near the Gulf of Mexico coast between Beaumont and Houston, Texas. This rainfall was in excess of what had been forcasted a few days earlier and was due to Imelda’s forward motion ceasing for approximately 24 hours between Wednesday and Thursday afternoon. The image shows, with large “L” symbols, the location estimated by the National Hurricane Center for Imelda’s low-pressure center of rotation at various times over the past three days.

This near-realtime rain estimate comes from the NASA’s IMERG algorithm, which combines observations from a fleet of satellites, in near-realtime, to provide near-global estimates of precipitation every 30 minutes.

If one compares the IMERG satellite-based rain estimate to that from a National Weather Service ground radar, one sees that IMERG correctly identified the large region of heavy rainfall near Beaumont, but IMERG failed to resolve an extremely narrow band of heavy rainfall along Galveston Island. Such good detection of large rain features in realtime would be impossible if the IMERG algorithm merely reported the precipitation observed by the periodic overflights of various agencies’ satellites. Instead, what the IMERG algorithm does is “morph” high-quality satellite observations along the direction of the steering winds to deliver information about rain at times and places where such satellite overflights did not occur. Information morphing is particularly important over the majority of the world’s surface that lacks ground-radar coverage.

An “R” symbol on the image indicates a place where the rainfall from the remnant of Imelda caused a US Geological Survey river gauge to swell to “major flood” stage. “Major” flood generally means that nearby homes and roads were flooded. The river-gauge data shown here is intended merely to give a hint of what areas experienced flooding and is not intended to portray the complete extent of flooding. In addition, there were several preliminary reports of Imelda-spawn tornados on Wednesday and Thursday, September 18-19. Red circles on this image indicate the location of these tornado reports, as provided by NOAA’s Storm Prediction Center. Visualization by NASA Goddard.  Visualization and caption by O. Kelley.

Hurricane Dorian (2019) brought heavy rain to the Caribbean, along the US East Coast, and up to Canada.  NASA satellite-based precipitation estimates tracked the storm throughout its lifetime, as shown by the sequence of images below.

September 3, 2019: Hurricane Dorian over Grand Bahama and Abaco Islands

In the early hours of Tuesday, September 3, Hurricane Dorian had been stationary over the island of Grand Bahama for 18 hours, most of the time as a category 5 hurricane.  Storm-total rain accumulation over parts of Grand Bahama and Abaco islands have exceeded 24 inches according to NASA satellite-based estimates. On early Tuesday morning, Dorian’s central pressure has risen and its wind intensity had dropped to category 4 on the Saffir-Simpson scale.  In addition, Dorian had experienced an eyewall replacement cycle on September 2, so by Tuesday morning, the geographic extent of its tropical-storm-force winds had expanded.

These rain estimates come from the NASA IMERG algorithm, which combines observations from a fleet of satellites, in near-realtime, to provide global estimates of precipitation every 30 minutes. The storm-total rainfall at a particular location varies with the forward speed of the hurricane, with the size of the hurricane’s wind field, and with how vigorous the updrafts are inside of the hurricane’s eyewall.

The graphic also shows the distance that tropical-storm force (39 mph) winds extend from Hurricane Dorian’s low-pressure center, as reported by the National Hurricane Center.  The symbols H and TS represent a hurricane of various Saffir-Simpson categories or a tropical storm, respectively.  Visualization by Owen Kelley (NASA/GMU).

September 6, 2019: Hurricane Dorian Brings Tornadoes and Floods to the US East Coast

By Friday morning, September 6, Hurricane Dorian was located off the coast of North Carolina, having generated tornadoes the previous day as the northern rainband came ashore in North Carolina. NASA’s satellite-based realtime precipitation estimates suggest that, during the past day, most of the areas experiencing over 10 inches of rain accumulation remained offshore, while Dorian did drop heavy rain on South Carolina and North Carolina.

The preliminary reports of tornadoes were obtained from NOAA’s Storm Prediction Center, and are shown on the graphic as red circles. Since storm spotters are land based, these reports rarely capture any water spouts (tornado-like events over water) that might occur. As Hurricane Dorian interacted with the U.S. East Coast, the only tornado reports occurred from 4:50 AM to 5:00 PM EDT on September 5 in North and South Carolina.  Scientists think of a hurricane as a heat engine that converts the warmth of the sun-warmed ocean into the kinetic energy of the hurricane’s strong, horizontal wind. When these strong winds reach land, the increased friction of the land surface vs. the ocean surface can convert some of this kinetic energy into tornadoes within the hurricane.

The near-realtime rain estimates come from the NASA’s IMERG algorithm, which combines observations from a fleet of satellites, in near-realtime, to provide global estimates of precipitation every 30 minutes.  The storm-total rainfall at a particular location varies with the forward speed of the hurricane, with the size of the hurricane’s wind field, and with how vigorous the updrafts are inside the hurricane.  This graphic only shows precipitation that fell starting at 0000UTC on September 1, and therefore does not show the precipitation that fell in late August, prior to Hurricane Dorian’s approach to The Bahamas.

The graphic shows the distance that tropical-storm force (39 mph) winds extend from Hurricane Dorian’s low-pressure center, as estimated by the National Hurricane Center.  The Saffir-Simpson intensity category is the number following the “H” in the label on the image.  Visualization by Owen Kelley (NASA/GMU).

September 9, 2019: Dorian Reaches Canada

On Monday morning, September 9, Hurricane Dorian was a post-tropical storm after a mid-latitude weather front and cold seas had altered its tropical characteristics over the weekend. On Saturday and Sunday, Hurricane Dorian struck eastern Canada, causing wind damage and bringing heavy rainfall.  According to the Associated Press, a peak of 400,000 people were without power in Nova Scotia, Canada, because of Dorian.

This graphic shows precipitation that fell during the almost two-week period from August 27 to the early hours of September 9. The near-realtime rain estimates come from the NASA’s IMERG algorithm, which combines observations from a fleet of satellites, in near-realtime, to provide near-global estimates of precipitation every 30 minutes.

This year, NASA began running a improved version of the IMERG algorithm that does a better job estimating precipitation at high latitudes, specifically north of 60 degrees North latitude. The post-tropical remnant of Hurricane Dorian was approaching this cold region at the end of the period shown in this image. While the IMERG algorithm is still unable to estimate precipitation falling over ice-covered surfaces (such as Greenland), IMERG can now give a more complete picture of the water cycle in places such as Canada, which is, for the most part, free of snow cover at this time of year.

At one-day intervals, the image shows the distance that tropical-storm force (39 mph) winds extended from Hurricane Dorian’s low-pressure center, as estimated by the National Hurricane Center.  The Saffir-Simpson hurricane-intensity category is the number following the “H” in the label on the image. “TS” or “PT” indicate times when the storm was either at tropical storm strength or when the storm was categorized as post-tropical.

Red circles over North Carolina indicate preliminary reports of tornados from 4:50 AM to 5:00 PM EDT on September 5, provided by NOAA’s Storm Prediction Center.  Since storm spotters are land based, these reports rarely capture water spouts (tornado-like events over water) that may occur.  Lines of latitude and longitude are curved on this map projection, as would be seen by an observer in Earth orbit, so that both tropical and arctic regions can be shown on the same map with minimal distortion. By combining NASA precipitation estimates with other data sources, we can gain a greater understanding of major storms that effect our planet. Visualization by O. Kelley.

In early August 2019, a depression formed in the Bay of Bengal that moved over India contributing to heavy rainfall on India’s west coast. NASA’s satellite data analysis suggests that for August 5 though 11, two feet of rain fell in some places. This estimate is from the realtime multi-satellite algorithm called IMERG, which is run at NASA Goddard.

Larger image

The Global Precipitation Measurement (GPM) mission has released an improved version of a multi-satellite global precipitation-rate estimate. Using this algorithm, NASA has re-examined observations back to April 2000 to create a long-term archive of these high-quality rainfall and snowfall estimates. The algorithm is called IMERG, which stands for “The Integrated Multi-satelliE Retrievals for GPM.”

The IMERG algorithm stitches together the data an international constellation of satellite-borne sensors including infrared, passive microwave, and radar. The algorithm also uses estimate of tropospheric wind to morph precipitation observed at one time into data-sparse regions in earlier or later hours. The algorithm is the most sophisticated data-driven precipitation-estimation algorithm that NASA has ever developed, tested, and provided to the public. The current version of IMERG is designated Version 6.

The image above shows the earliest North Atlantic hurricane in the IMERG Version 6 2000-to-present archive, which is Hurricane Keith. Hurricane Keith formed in the Gulf of Mexico and reached category on the Saffir-Simpson scale before making landfall in Mexico. Using the QGIS application, this image was created from the Final IMERG Geographic Information System (GIS) daily product, a 24-hour summary of the 30-minute global 0.1 x 0.1 degree IMERG files. Both the original HDF5 files and these GIS TIFF translations together form the most popular data produce of the GPM mission, based on number of files that researchers download from NASA Precipitation Processing System (PPS).

To learn more about this data product, please read the Algorithm Theoretical Basis Document or visit the Global Precipitation Measurement (GPM) website. Since 2014, George Mason University’s Center for Earth Observing and Spatial Research (COESR0 employees working at NASA’s Precipitation Processing System (PPS) contributed to testing this algorithm and to creating the long-term archive of this algorithm’s output.

References:

NASA, 2019: The Integrated Multi-satelliE Retrievals for GPM (IMERG) Algorithm Theoretical Basis Document. 38 pp. Available online at https://pps.gsfc.nasa.gov/Documents/IMERG_ATBD_V06.pdf.

Kelley, O. 2019: The IMERG multi-satellite precipitation estimates reformatted as 2-byte GeoTIFF files for display in a Geographic Information System(GIS). Available online at https://pps.gsfc.nasa.gov/Documents/README.GIS.pdf.