Hurricane Henri Fights Wind Shear on Approach to New England (August 2021)

Tropical Storm Henri (20 Aug 2021)
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In the North Atlantic, the tropical system known as Henri reached hurricane status on Saturday, August 21, 2021.  At the time, it was approaching a landfall in New England. Between Friday and Sunday, Henri was observed three times by the Dual-frequency Precipitation Radar (DPR) on the core satellite of NASA’s Global Precipitation Measurement (GPM) mission.

The GPM satellite’s first two flyovers of Henri occurred just 10 hours apart and revealed a remarkably unchanging structure that was being impacted by wind shear.  A third overflight occurred a day later, when wind shear had abated and Henri had become more organized. The GPM mission’s observations, such as these, are processed in near real-time for use by weather agencies and researchers.

Above, the 3D image from the first overflight shows the volume of light precipitation associated with Henri at 1646 UTC (12:46 PM local time) on Friday (August 20), and the structure tells this chapter of Henri’s story. At this time, Henri’s center was approximately 750 miles south of the Connecticut coast.

Light precipitation is detected by a 20 dBZ radar reflectivity, with color-coded stripes indicating which portion of the precipitation reached an altitude of 8.5 km (green), 11 km (yellow), or 14.5 km (red) above the ocean surface. A letter “L” indicates the location of the system’s low-level center of rotation.

For several days, Henri had been battling strong wind shear.  Wind shear is when the horizontal wind near the Earth’s surface is blowing in a different direction or speed than the horizontal wind higher in the troposphere, the layer of the atmosphere that most storms are confined to. Henri’s low-level center of circulation was at its northern edge, with most of the precipitation occurring in a 100-kilometer-wide area to the south of the low-level center of circulation.  Wind shear can encourage the formation of strong convective cells in the inner core of a tropical system, but shear can also hamper those convective cell’s ability to strengthen that system.

Hot Towers and Heat Engines

In the first overflight of Henri on Friday morning, a “hot tower” convective cell was observed adjacent to the low-level circulation center, and this hot tower had strong enough updrafts to lift ice precipitation more than 16 kilometers (10 miles) above the ocean’s surface. Hot towers are convective cells with updrafts strong enough to lift precipitation up through the top of the troposphere.

A series of hot towers in the inner core of a tropical storm or hurricane indicates that a considerable amount of heat is being released inside the storm.  This heat could result in wind intensification if wind shear or other factors do not short circuit the process.

The second overflight of Henri (not shown) occurred late on Friday night, local time, which was early on Saturday morning in universal time (0232 UTC).  At that time, shear was still high and Henri still had a hot tower near its low-level center of rotation. By 11 AM Saturday morning (1500 UTC), shear had weakened and Henri had strengthened 5 knots to becoming a category-1 hurricane according to the National Hurricane Center.

Hurricanes are often said to contain “heat engines” because they convert some of the energy in the sun-warmed surface of the ocean into kinetic energy of the hurricane’s strong winds that rotate around its low-level center of rotation.  In this analogy, strong convective cells in the inner core of a hurricane are the pistons of its heat engine.

When a factor such as wind shear inhibits this energy conversion, one might say that all the pistons of the heat engine were firing but the engine wasn’t in gear.  In other words, vigorous convective cells were injecting heat into the atmosphere, but the hurricane could not convert that heat into a well defined eye circled by rapid winds.  This diagnosis of Henri is consistent with the two GPM satellite overflights during the 10-hour period on Friday, August 20.

Wind Shear Weaken,
Henri Strengthens into a Hurricane

Hurricane Henri (21 Aug 2021)
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As pictured above, the Dual-frequency Precipitation Radar flew over Henri for a third time late on Saturday night, local time (9:37 PM EDT), which was early Sunday morning in Universal Time (0137 UTC).  At this time, Henri’s center was approximately 230 miles south of the Connecticut coast. The precipitation of Henri’s inner core at this time was consistent with that of a weak hurricane. In this overflight, the DPR captured most but not all of the eye.  The precipitation along the southwest edge of the eye was outside of the field of view of the DPR.

On Sunday, Henri had an eye, an area at its center that was free from precipitation, and an eyewall, an arc of heavy precipitation around that eye.  Henri’s eyewall was relatively weak, with updrafts able to lift precipitation to less than 8 km altitude in most places.

In the Sunday morning overflight, there was, again, a hot tower near Henri’s center.  Given time and favorable circumstances, a series of hot towers in a hurricane’s eyewall can lead to intensification, according to NASA research.  For Hurricane Henri, further intensification was not forecast, nor did it occur, following this overflight.  Further intensification likely did not occur in part because Henri entered cold water off the coast of New England. Credits: Visualization and caption by O. Kelley.


Two Weeks of Heavy Rain and a Year of Drought Over Parts of China (July 2021)

China storms and drought (July 2021)
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During July 17 to 28, 2021, several storm systems brought heavy rain to parts of China and surrounding countries, while a nine-month-long drought persists in an adjacent part of China.  NASA’s multi-satellite precipitation algorithm has been monitoring this rainfall in near real-time, and the estimates are distributed to weather-forecasting agencies and disaster-monitoring organizations.  This algorithm is called IMERG, the Integrated Multi-satellitE Retrievals for GPM. GPM is the NASA / JAXA Global Precipitation Measurement mission, which launched its Core Observatory satellite in 2014.

Two Typhoons

IMERG precipitation estimates from July 17 to 28, 2021, are shown in the image above. Among the weather systems impacting the region were two landfalling typhoons – Cempaka and In-fa.

One can see that over a foot of rain has fallen during this 12 day period along most of the coast from Hong Kong west to Hanoi, Vietnam. This rain came mostly from Typhoon Cempaka during July 17 through 19.

Over a foot of rain has also fallen during this time near Shanghai, which is China’s most populous city.  That rain came mostly from Typhoon In-fa during July 25 to 28.  According to Buckingham (2021), parts of Zhoushan City, near Shanghai, received at least 30 inches of rain from In-fa between July 23 and 27. Over the Pacific Ocean near China, two rain gauges in the Global Historical Climatology Network (GHCN) reported 12-inch rain accumulations during the July 17 to 28 period at locations where NASA’s IMERG algorithm also reported similar accumulations.  These gauges are located on two small islands, Japan’s Nago and Kumejima islands. Point estimates of rainfall are represented in the image as circles that is colored using the same color scale used to display the IMERG data that covers the globe.  Nearby over the ocean, where no rain gauges exist, the IMERG algorithm reported significantly more rain accumulation along the path of Typhoon In-fa, over 30 inches of accumulation.

Two Additional Storm Systems

During the same time, a synoptic system brought over 30 inches of rain accumulation to the Chinese city of Zhengzhou and surrounding areas of Henan Province during July 19 to 22.  According to Childs (2021), over 31 inches fell during a three-day period.

Also separate from the two typhoons, a storm system brought accumulations in excess of 20 inches to the western coast of the Philippines, near Manila, during July 21 to 24.  These accumulations were detected by the IMERG algorithm and are consistent with rain gauge data in Manila.

While such accumulations are impressive, they are not unprecedented.  For example, Hurricane Harvey (2017) stalled over Texas and brought over 60 inches of rainfall to a small area over a period of four days (Blake and Zelinsky, 2018).  IMERG was also used to analyze the rainfall from Harvey

A Persistent Drought

Meanwhile, a drought in Fujian Province in coastal China has persisted since October 2020 with rainfall accumulation less than 25% of the typical accumulation for part of Fujian Province. Deviation from normal rainfall can be estimated using the long-term record of IMERG estimates that stretches back to the year 2000.

Wang (2021) and Ye (2021) report that China’s coast south of the Yangtze River, a region that includes Fujian Province, only received 20% to 50% of the normal amount of rainfall between October 2020 and February 2021. IMERG data shows that summer rains have relieved the drought in the provinces adjacent to Fujian, but not in Fujian itself.

The regional rainfall patterns seen during the past two weeks over China are significantly different than the Meiyu-Baiu pattern of rainfall that IMERG detected at this time last year. Credit: Visualization and caption by O. Kelley.


Rainfall that Matters: A Convective System over Arizona within the North American Monsoon (July 2021)

North American Monsoon region
Figure 1. Geographic extent of the North American Monsoon. Larger Image: click here

There is a monsoon that occurs in the southwestern U.S. each summer, and it brought heavy rain to the deserts of Arizona this week. This monsoon is less well known than India’s Summer Monsoon, but both monsoons have similar causes [1, 2, 3].

In western Mexico and the southern edge of the southwest U.S., most of the year’s rain typically falls in just three months: June, July, and August. The region is shown in light blue in the below climate map, which shows where summer rainfall predominates (Figure 1). This seasonal pattern is known as the North American Monsoon. The map was generated using the long-term record of NASA’s IMERG multi-satellite precipitation analysis. Climatologists and forecasters have used various dates to define the exact start and stop dates of the North American Monsoon [4].

This year’s monsoon season has officially begun, and the week of July 12-16 has been an active monsoon week for Southern Arizona [5].

On the morning of July 15, 2021, the National Weather Service issued a warning for severe thunderstorms over Arizona’s southern desert with rain rates of 1 inch per hour over ground that was already saturated by recent storms [6]. Under these circumstances, flash flooding is a concern. The location of this warning is indicated by the red circle on the climate map (31.72N latitude and 112.13W longitude in Figure 1).

The NASA / JAXA GPM Core Observatory satellite captured a 3D view of a convective storm system at this location at 8:37 a.m. local time on July 15. The data was collected by the satellite’s Dual-frequency Precipitation Radar (DPR). The 3D view is shown in Figure 2. This overflight of the storm system showed several convective cells embedded within it with precipitation rates at the Earth’s surface in excess of 1.4 inches per hour. The strongest of these cells contained updrafts that were sufficiently strong to lift ice precipitation overshooting through the top of the troposphere. The troposphere is the lowest layer of the atmosphere that normally contains the weather.

Arizona Storm (2021)
Figure 2. 3D observation of a convective system over Arizona contributing to the North American Monsoon. Larger image: click here.

Shown in red in Figure 2, the Dual-frequency Precipitation Radar detected a 20-dBZ radar-reflectivity signal at an altitude 14 kilometers above the Earth’s surface and over 14.8 km above sea level. In Figure 2, the yellow and green areas identify where 20-dBZ radar reflectivity was detected at 11 and 8.5 km altitude, respectively. Commercial airlines cruise at 9 to 13 kilometers. At lower altitudes, this convective cell contained radar reflectivity in excess of 50 dBZ, suggesting hail was present.

This GPM satellite overflight of the storm system occurred at 8:37 a.m., approximately half an hour before the National Weather Service issued its thunderstorm discussion for the area [6].

This portion of Arizona receives only 10 to 15 inches of precipitation in an entire year, on average, according to the long-term record from NASA’s IMERG multi-satellite precipitation algorithm. Local accumulation exceeded 2 inches in places due to the convective storm system that passed through this area on July 15, 2021.  This storm total was estimated both by the near real-time IMERG algorithm and also by the National Weather Service’s preliminary analysis of ground-radar data [].

It is typical of this area’s climate that heavy rainfall from convective systems such as this one contribute much of the area’s annual rainfall accumulation. According to the IMERG record, this portion of Arizona receives approximately 42% to 50% of its annual accumulation in June through August.  Visualization and caption by Owen Kelley (NASA/GMU).


(1) NASA, 2017: NASA Looks at the North American Monsoon. 1:26 minute video featuring NASA Scientist Dr. George Huffman,

(2) Branon, M., 2021 June 17: How the Weather Service cleared the air about Southwest monsoon season. Capital Weather Gang, Washington Post,….

(3) Adams, D. K., and A. C. Comrie, 1997: The North American Monsoon. Bulletin of the American Meteorological Society, 78, 2197-2213.

(4) June 15 is currently the National Weather Service’s official start date for the North American Monsoon.  Adams and Comrie (1997) propose June-August or July-September as possible date ranges for the monsoon.

(5) Arizona Daily Star, 2021 July 14: Tucson passes 2020 monsoon rainfall total; flash flood warning in effect,….

(6) NWS, 2021 July 15: Mesoscale Precipitation Discussion #0546 issued at 9:10AM EDT on Thursday, July 15, 2021. Weather Prediction Center (WPC), College Park, MD,….


Australia’s Heavy Rainfall: Desert and Coast (March 2021)

Australia Flooding (March 2021)

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During the week ending on March 23, 2021, two locations in Australia experienced unusually high rainfall totals. In the news, a persistent system brought flooding rains to Australia’s east coast from Brisbane to Sydney and points further south.

The preliminary estimate from NASA’s multi-satellite global precipitation analysis is that more than 24 inches fell just off the coast of Australia in 7 days with accumulations in coastal areas exceeding 16 inches. Near the Strzelecki Desert in central Australia, a storm system brought 8 inches of precipitation during the same 7-day period. Most of the rain fell during a 3-day period (0000 UTC on 20 March to 2359 UTC on 22 March).

These estimates come from the near-realtime version of NASA’s IMERG algorithm which combines microwave and infrared satellite observations with climatological datasets including rain-gauge
observations. In addition to near global coverage, another advantage of the IMERG algorithm is that it provides estimates covering approximately a 20-year period. This long-term record allows for comparisons of current precipitation events with long-term averages.

According to IMERG records, the past week’s rainfall accumulation along Australia’s eastern coast is more than 25% of the average annual accumulation there. The past week’s rainfall accumulation near the Strzelecki Desert is greater than the average annual accumulation there.

Data from the Australian Bureau of Meteorology confirms at least the main conclusions from these IMERG observations. Central Australia typically receives only 4 to 8 inches (100 to 200 millimeters) of precipitation a year, while a thin band of land along Australia’s eastern coast typically receives about ten times as much precipitation: 40 to 60 inches (1000 to 1500 millimeters) in a year.

In terms of 7-day storm totals, the Australian Bureau of Meteorology reports that there is a 1% chance each year of an event exceeding 498 or 177 millimeters (20 or 7 inches) of precipitation accumulation in Sydney or in the desert where New South Wales, Queensland, and South Australia meet. The desert rainstorm during the week ending on March 23, 2021, exceeded this threshold, while the flooding over Australia’s east coast did not.

Credits: IMERG data from NASA at Average annual accumulation from the Australia Bureau of Meteorology at One percent annual chance of 7-day rainfall accumulation from the Australian Bureau of Meteorology’s intensity-depth-frequency (IDF) data in the Design Rainfall Data System (2016) at Visualization and caption by O. Kelley.


Hurricane Sally in Context (Sept. 2020)

Hurricane Sally (2020) and Hurricane Harvey (2017)

One way to put a storm into context is to ­­­­­compare its precipitation to the average annual accumulation at that location. The IMERG multi-satellite precipitation algorithm estimates that Hurricane Sally brought over 20 inches of precipitation to the coast of Florida’s Panhandle during the 7 days ending September 18, 2020, at 0000 UTC. That accumulation was equivalent to 25% to 33% of that area’s average annual precipitation. This area is indicated in purple on the left half of the image.

The coast of Florida’s Panhandle is the wettest area along the entire US East Coast and Gulf Coast. It receives approximately 60 to 68 inches of precipitation in an average year, based on 19 years of estimates made by the IMERG algorithm.

When it comes to hurricane flooding along the Gulf Coast, Hurricane Sally does not top Hurricane Harvey which made landfall in late August, 2017. Like Sally, Harvey also dumped over 20 inches of rainfall, but over a larger area that included a major metropolitan area. Specifically, this area of southeastern Texas includes Houston, Texas, a city that typically averages approximately 55 to 60 inches of precipitation a year. In Houston, the 7-day accumulation from Hurricane Harvey was 50% to 75% of the city’s average annual accumulation. This area is indicated in red on the right half of the image. Houston area flooding contributed to Hurricane Harvey being one of the most costly hurricanes in US history.

Our ability to interpret near real-time estimates of precipitation over both land and ocean is assisted by the availability of a reference data set of 19 years of global precipitation estimates made by the same IMERG algorithm. Visualization and caption by O. Kelley.


A Year’s Worth of Rain in a Week for a City in Pakistan (Aug. 2020)

Pakistan Flood (August 2020)

In the last week of August 2020, Pakistan’s largest city, Karachi, received over 8 inches of rainfall according to NASA’s IMERG dataset, causing destructive flooding in the region. The amount of rain that fell that week is roughly equivalent to the amount that Karachi typically receives in an entire year, based on IMERG’s 19-year global climatology. In a typical year, most of Karachi’s rain will fall in July and August, but the rainfall during the week of August 23rd was unusually heavy.

The top panel of the three panels in this image shows the depth of the 7-day rainfall accumulation in inches (August 23 to 30, 2020). The light green color indicates at least 8 inches of rainfall in Karachi and locations further inland and to the east.

The middle panel divides the rain that fell during this seven day period by the average annual accumulation at each 0.1 x 0.1 degree grid box of the IMERG product.  Values in excess of 1 (red) indicate locations where a full year’s worth of rain fell in this one-week period. When such an extreme event occurs, it can be difficult for the built environment to function normally.

The bottom panel gives a sense of what is normal for the last week of August.  It shows what fraction of the annual total precipitation falls during the last week of August of a typical year.  Regions where at least 1/24 of a “year’s worth” of rain typically falls during the last week of August  normally experience part of their rainy season during the last week of August.  This is the case for the eastern portion of Pakistan and India.  Regions where less than 1/100 of a “year’s worth” of rain typically falls in the last week of August normally experience part of their dry season during the last week of August.  This is the case for western Pakistan and Afghanistan.

IMERG rainfall estimates are automatically generated in near real-time as part of NASA’s effort to monitor the Earth. The estimates are based on observations from an international constellation of satellite including the GPM satellite. Several months after an event occurs, the rainfall estimates are improved when NASA re-runs the IMERG algorithm using additional data sets that are not available in near real-time.


Hurricane Laura’s Hot Tower (Aug. 2020)

Hurricane Laura (2020)

Two days before landfall, Laura had been forecasted to experience a period of weakening before landfall but instead, it ended up intensifying and deepening until its inner core was interacting with land.  Consistent with NHC reports of continued intensifying is 3D evidence from the GPM satellite that Laura’s eyewall contained vigorous convection at 0300 UTC, 3 hours before landfall.

In this overflight, GPM’s Dual-frequency Precipitation Radar (DPR) saw a 15-km-tall hot tower in the eyewall. An nearby convective cell within the eyewall was shorter but contained an impressively strong 50-dBZ radar reflectivity signal in the mid-troposphere. Both observations are indicative of strong updrafts in the eyewall.  In the image, a indicates the low-pressure center of the hurricane, also know as the eye because it is occasionally cloud free; b indicates the hot tower in the eyewall convection, and c indicates the region of extremely heavy precipitation in the eyewall.

A hot tower means that the eyewall’s updrafts were strong enough to lift small ice hydrometeors up and out of the troposphere, the layer of the atmosphere that usually contains storm systems.  The 50 dBZ mid-troposphere signal means that updrafts were strong enough that ice particles where held aloft long enough to grow large, perhaps hail-sized, despite their fall speed increasing as they grew larger.


GPM Satellite Observes a Hot Tower in Squall Line off Florida’s Coast (April 2020)

GPM DPR observes Hot Tower over Ocean 20 April 2020

At 9:26PM on Sunday April 19, 2020, NASA’s GPM satellite observed an extremely vigorous convective storm cell embedded within a squall line traveling in advance of a tornado-producing weather system. This system spawned tornadoes earlier that evening along the US Gulf Coast and 12 hours later over Florida. The GPM satellite flew over the portion of this squall line that extended over the Atlantic Ocean, 350 miles (500 kilometers) east of Florida’s Coast.

The Dual-Frequency Precipitation Radar (DPR) observed a storm cell whose updrafts were strong enough to lift small ice particles into the stratosphere, 15.8 km above the ocean surface. Equally remarkable is that the updrafts were able to lift large ice particles, perhaps including hail, to a 14.5 km altitude. In round numbers, the precipitation radar detects small and large precipitating ice particles by means of a radar reflectivity of 20 or 40 dBZ, respectively.

As documented in NASA research, it is unusual, but not unheard of, for a precipitation cell over the ocean to reach such a high altitude. The vast majority of storm cells over the ocean fail to reach such high altitudes because the ocean’s surface temperature does not heat up as much as mid-latitude land surfaces can, denying storm cells an extremely unstable, warm, near-surface layer of air. Visualization and caption by O. Kelley.

For more information:

GPM website,

Kelley, O. A., J. Stout, M. Summers, and E. J. Zipser, 2010: Do
the tallest convective cells over the tropical ocean have slow
updrafts? Monthly Weather Review, 138, doi:10.1175/2009MWR3030.1.

Samenow, J., et al.: 20 April 2020, 10:11AM: Outbreak of tornadoes
and severe storms slams South for second straight
Sunday. Washington Post. Available online at

NOAA Storm Prediction Center, 20 April 2020: realtime preliminary
storm reports. Available online at


IMERG Reveals Two Weeks of Heavy Rain over the Eastern Mediterranean (Jan. 2020)

IMERG estimate of two weeks of heavy rainfall over the Mediterranean

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.


Estimating Tropical Cyclone Imelda’s Rain Total over Texas (Sept. 2019)

Tropical Storm Imelda over Texas (Sept 2019)

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.