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NASA's Hurricane and Severe Storm Sentinel (HS3) airborne mission sent an unmanned Global Hawk aircraft this morning to study newborn Tropical Depression 14 in the central Atlantic Ocean that seems primed for further development. The Global Hawk left NASA's Wallops Flight Facility on Wallops Island, Va., this morning for a planned 26-hour flight to investigate the depression.

NASA's latest hurricane science field campaign began on Sept. 7 when the Global Hawk flew over Hurricane Leslie in the Atlantic Ocean. HS3 marks the first time NASA is flying Global Hawks from the U.S. East Coast.

According to Chris Naftel, project manager of NASA's Global Hawk program at NASA's Dryden Flight Research Center, Edwards Air Base, Calif., the Global Hawk aircraft took off at 7:06 a.m. EDT and headed for Tropical Depression 14, which at the time of take-off, was still a developing low pressure area called System 91L.

At 1500 UTC (11 a.m. EDT), Tropical Depression 14 was located near 16.3 North latitude and 43.1 West longitude, about 1,210 miles (1,950 km) east of the Lesser Antilles. The depression had maximum sustained winds near 35 mph. It was moving to the west near 10 mph (17 kmh) and had a minimum central pressure of 1006 millibars.

The National Hurricane Center expects Tropical Depression 14 to strengthen into a tropical storm over the next 48 hours, and turn to the northwest.

On Sept. 10, the Tropical Rainfall Measuring Mission (TRMM) satellite passed over Tropical Depression 14, when it was known as low pressure System 91L and data from TRMM's Microwave Imager (TMI) and Precipitation Radar (PR) were used to create a rainfall analysis. The data was overlaid on a combination infrared and visible image from TRMM's Visible and InfraRed Scanner (VIRS) and showed that System 91L was getting organized and that convective storms reaching heights of about 13km (~8.1 miles) were dropping heavy rain to the northwest and northeast of the center of the circulation.

The HS3 mission targets the processes that underlie hurricane formation and intensity change. The data collected will help scientists decipher the relative roles of the large-scale environment and internal storm processes that shape these systems.

HS3 is supported by several NASA centers including Wallops; Goddard; Dryden; Ames Research Center, Moffett Field, Calif.; Marshall Space Flight Center, Huntsville, Ala.; and the Jet Propulsion Laboratory, Pasadena, Calif. HS3 also has collaborations with partners from government agencies and academia.

HS3 is an Earth Venture mission funded by NASA's Science Mission Directorate in Washington. Earth Venture missions are managed by NASA's Earth System Science Pathfinder Program at the agency's Langley Research Center in Hampton, Va. The HS3 mission is managed by the Earth Science Project Office at NASA's Ames Research Center.

Researchers from the Radiation Belt Storm Probes' Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) team at the University of Iowa have released a new recording of an intriguing and well-known phenomenon known as "chorus," made on Sept. 5, 2012.

The Waves tri-axial search coil magnetometer and receiver of EMFISIS captured several notable peak radio wave events in the magnetosphere that surrounds Earth. The radio waves, which are at frequencies that are audible to the human ear, are emitted by the energetic particles in Earth's magnetosphere.

"People have known about chorus for decades," says EMFISIS principal investigator Craig Kletzing, of the University of Iowa. "Radio receivers are used to pick it up, and it sounds a lot like birds chirping. It was often more easily picked up in the mornings, which along with the chirping sound is why it's sometimes referred to as 'dawn chorus.'"

This recording was made by many members of the EMFISIS team, including Terry Averkamp, Dan Crawford, Larry Granroth, George Hospodarsky, Bill Kurth, Jerry Needell and Chris Piker.

You can listed to the audio of the phenomenon known as "chorus" radio waves within Earth's magnetosphere that are audible to the human ear, as recorded on Sept. 5, 2012, by RBSP's Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS). Five six-second "events" are captured in this sample, and they are played end-to-end, one right after the other, without gaps:

http://www.nasa.gov/wav/687014main_emfisis_chorus.wav

While the RBSP teams at NASA's Kennedy Space Center and Cape Canaveral Air Force Station celebrated a job well done following the 4:05 a.m. EDT launch of the Radiation Belt Storm Probes on Thursday, Aug. 30, another group of RBSP engineers and scientists celebrated at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. But for many of those at APL's Mission Operations Center (MOC) that morning, their job on the RBSP mission was about to get very busy.

Immediately after launch, RBSP entered a 60-day commissioning phase of operations, where all of the spacecrafts' systems and instruments are activated, monitored, and made ready for the two-year primary science mission.

After the deployment of both spacecraft from the Centaur stage of the Atlas V rocket some 90 minutes after launch, the RBSP team at the MOC went immediately to work. Their job was to establish contact with the twin probes, and make sure the spacecraft deployed their solar panels and were receiving power from them.

With those power and communications systems checked out, the RBSP spacecraft and teams had little time to celebrate — there was much to do on RBSP's first day in orbit. The twin Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) booms (two on each spacecraft, located at the edges of two solar panels) were the first instruments to be powered up and deployed. This was done so that the magnetic signatures of the other instruments could be observed as they were powered up. In addition to providing science data for the EMFISIS team, magnetometers on the booms are used by the mission operations team (along with sun sensors) to help determine the attitude of the spacecraft, which in this case is the angle at which they are pointed at the sun.

Additionally, the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument was turned on — though only with low voltage, just enough to power up the Engineering Radiation Monitor (ERM), which keeps track of the amount of radiation entering RBSP.

The first Saturday of the mission (Sept. 1) saw the first full powering-up of one of the many instruments on the spacecraft. At about 3 a.m. EDT, the Relativistic Electron Proton Telescope (REPT) instrument of the Energetic Particle, Composition, and Thermal Plasma Suite (ECT) aboard spacecraft A was turned on, and useable data began to immediately stream back to the REPT team. REPT-B was powered up 12 hours later.

Saturday's achievements didn't stop there: The Relativistic Proton Spectrometer (RPS) on spacecraft B was turned on, while its sibling on spacecraft A was powered up on Sunday, Sept. 2.

During the first two weeks of orbit, the spacecraft completed a series of small changes in velocity and also adjusted the angle at which they face the sun, known as "precession." These were done to optimize the orbit and operation of the spacecraft.

"Things are going very smoothly with the spacecraft," says Ray Harvey, RBSP mission operations manager. "We've also begun to send out preliminary test data for the space weather broadcast from the spacecraft, in the same format as the final broadcast will be, so the partner institutions can verify they are receiving it."

On Wednesday, Sept. 5, the Instrument Data Processing Unit (IDPU) for Electric Field and Waves Suite (EFW) was powered up to prepare for the upcoming deployment of EFW's four booms (per spacecraft), and on Thursday, Sept. 6, the eight Magnetic Electron Ion Spectrometers (MagEIS, another of ECT's three instruments) were powered up; each spacecraft has four MagEIS instruments that measure widely different energy ranges.

The next major instrument activity is the EFW boom deployment, which begins on Sept. 13, when both RBSP spacecraft will be spun up to seven RPM from their normal five RPM. This will prepare them for the change in momentum following the initial deployment of the EFW spin-plane booms. The doors containing the booms will open, and then on Friday, Sept. 14, the first four meters of the booms will be deployed. Over the following days, more of each boom will be deployed every day, until the four booms (each is 50 meters long) are fully out. In roughly the middle of this process, the RBSP MOC team will also send a command to open the door to the aperture on the RBSPICE instrument that will allow it to begin full science operations.

The final RBSP instrument to be powered up will be an ECT instrument: the Helium Oxygen Proton Electron (HOPE) instrument, which will be powered up sometime in mid to late October, after the spacecraft have deployed all their booms and completed their commissioning-phase maneuvers.

RBSP is part of NASA's Living With a Star Program to explore aspects of the connected sun-Earth system that directly affect life and society. LWS is managed by the agency's Goddard Space Flight Center in Greenbelt, Md. APL built the RBSP spacecraft and manages the mission for NASA.

Gardeners and landscapers may want to rethink their fall tree plantings. Warming temperatures have already made the U.S. Department of Agriculture's new cold-weather planting guidelines obsolete, according to Dr. Nir Krakauer, assistant professor of civil engineering in The City College of New York's Grove School of Engineering.

Professor Krakauer developed a new method to map cold-weather zones in the United States that takes rapidly rising temperatures into account. Analyzing recent weather data, he overhauled the Department of Agriculture's latest plant zone map released in January.

The new USDA Plant Hardiness Zone Map, which predicts which trees and perennials can survive the winter in a given region, was a long time coming. Temperature boundaries shown in the latest version have shifted northward since the last one appeared in 1990. But the true zones have moved even further, according to Professor Krakauer's calculations.

"Over one-third of the country has already shifted half-zones compared to the current release, and over one-fifth has shifted full zones," Professor Krakauer wrote this summer in the journal "Advances in Meteorology."

This means that fig trees, once challenged by frosty temperatures above North Carolina, are already weathering New York City winters thanks to changing temperatures and the insulating effect of the metropolis. Camellias, once happiest south of Ohio, may now be able to shrug off Detroit winters.

The USDA divides the country into zones based on their annual minimum temperatures — frigid dips that determine which plants perish overnight or live to flower another day. (Each zone has a minimum temperature range of 10 degrees Fahrenheit; half zones have a 5-degree range.)

Professor Krakauer found a weakness in how the agency came up with the zones, however. The USDA averaged annual minimum temperatures over a 30-year span, from 1976 to 2005, but winters have warmed significantly over that period. Zones now average about 2 degrees Fahrenheit warmer than the USDA's 30-year average.

"What is happening is that the winter is warming faster than the summer. Since [my] hardiness temperatures are based on minimum temperatures each year, they are changing faster than the average temperatures," Professor Krakauer said. He found that these lowest yearly temperatures warmed roughly two and a half times faster than the average temperatures.

His analysis also showed that the country is changing unevenly; more warming is occurring over the eastern interior and less in the Southwest.

Professor Krakauer's technique will allow gardeners and farmers to reassess what will survive the next year's winter more frequently than the USDA can produce a new map. "The idea is that you could use this method to keep updating the zone map year by year instead of waiting for the official map — just keep adding new data and recalculate."

He noted that similar analyses could distinguish long-lasting climate trends — in wind or rainfall, for example — from year-to-year weather variations to distinguish between what some are calling the recent "weird weather" and the natural variations in global weather.

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Journal Reference:

1. Nir Y. Krakauer. Estimating Climate Trends: Application to United States Plant Hardiness Zones. Advances in Meteorology, 2012; 2012: 1 DOI: 10.1155/2012/404876

 If you think this summer was hot, it's nothing compared to the summer of 1913, when the hottest temperature ever recorded was a searing 134 degrees Fahrenheit in Death Valley, Calif. But while that reading was made 99 years ago, it is only being recognized today by the World Meteorological Organization (WMO) as the most extreme temperature ever recorded.

That's because an international team of meteorologists recently finished an in-depth investigation of what had been the world-record temperature extreme of 58 degrees Celsius (136.4 F), recorded on Sept. 13, 1922, in El Azizia, Libya. The group found that there were enough questions surrounding the measurement and how it was made that it was probably inaccurate, overturning the record 90 years to the day it was recorded.

"We found systematic errors in the 1922 reading," said Randy Cerveny, an ASU President's Professor in the School of Geographical Sciences and Urban Planning. "This change to the record books required significant sleuthing and a lot of forensic records work," added Cerveny, who also is the Rapporteur of Climate and Weather Extremes for the WMO, the person responsible for keeping worldwide weather records.

Officially, the "new" world record temperature extreme is 56.7 C (134 F), recorded on July 10, 1913, at Greenland Ranch in Death Valley, Calif.

"In the heart of every meteorologist and climatologist beats the soul of a detective," said Cerveny. In this case the weather detectives had to work around an unfolding revolution in Libya.

Cerveny said the El Azizia temperature had long been thought as dubious. It was recorded in 1922 at what then was an Italian army base.

The international meteorological team — which included experts from Libya, Italy, Spain, Egypt, France, Morocco, Argentina, the United States and the United Kingdom — identified five major concerns with the El Azizia temperature record. They included the use of antiquated instrumentation, a likely inexperienced observer, an observation site which was not representative of the desert surroundings, poor matching of the extreme to other nearby locations and poor matching to subsequent temperatures recorded at the site.

The WMO evaluation committee concluded the most compelling scenario for the 1922 event was that a new and inexperienced observer, not trained in the use of an unsuitable replacement instrument that could be easily misread, improperly recorded the observation. The reading was consequently in error by about 7 degrees Celsius (12.6 F).

The detective work Cerveny describes included finding and examining the original log sheet, which he said was very useful. In reconstructing the events, Cerveny describes a person new to making temperature measurements being asked to make the measurements with a "Six-Bellini thermometer," which even by 1922 standards was an obsolete piece of technology. By reviewing the logs, it became apparent that the person who recorded the temperature was transposing what he read from the thermometer, consistently scoring the readings in the wrong column of the log.

"One of the problems with a Six-Bellini thermometer is that the indicator — the pointer — to the temperature scale could conceivably be read at the top of the pointer or the bottom of the pointer," Cerveny explained. "If an inexperienced observer used the top of the pointer rather than the bottom, he would have been as much as 7 C in error. "

Other telling forensic information included the general location of where the measurement was made — El Azizia is roughly 35 miles southwest of Tripoli, which is on the Mediterranean coast — and the fact that the record temperature pretty much stood out among all of the other recorded values near the El Azizia location.

"When we compared his observations to surrounding areas and to other measurements made before and after the 1922 reading, they simply didn't match up," Cerveny said.

Investigation during a revolution

The investigation was launched in 2010 and soon after the revolution in Libya started to form. The Libyan official on the team (Khalid El Fadli, director of the climate section of the Libyan National Meteorological Center) fell out of contact with the rest of the team for about eight months and the investigation went into a suspended state. Then El Fadli sent word that he was safe (although he and his family left Tripoli for a while to avoid being accidently shot in the turmoil) and he could resume his role in the investigation. But another three weeks passed before El Fadli was heard from again.

"Khalid El Fadli did this at great risk to himself," Cerveny said. "He was an official of the previous regime, so when the revolution began to turn, his safety was a key concern."

Fortunately, after the revolution, El Fadli could resume his duties as a lead meteorologist with the new government and the investigation started up again.

Beyond establishing bragging rights, Cerveny said the world record highest temperature does have some important uses.

"This is the highest recorded temperature of where people live, so this type of data can help cities that exist in such environments to design buildings that are best suited for these extremes," he said. "Knowing the maximum temperatures certain materials must endure leads to better products and designs. That's why many auto manufactures have test tracks in the hot Mohave desert.

Cerveny added that there also are important basic science implications in this finding.

"This investigation demonstrates that, because of continued improvements in meteorology and climatology, researchers can now reanalyze past weather records in much more detail and with greater precision than ever before," Cerveny explained. "The end result is an even better set of data for analysis of important global and regional questions involving climate change."

A full list of weather and climate extremes is available at the WMO Archive of Weather and Climate Extremes (http://wmo.asu.edu/). This includes the world's highest and lowest temperatures, rainfall, heaviest hailstone, longest dry period, maximum gust of wind, as well as hemispheric weather and climate extremes.

Severe UK winters, like the 'big freeze' of 2009/10, can now be better forecast months in advance using the Met Office's latest model, new research suggests.

A new study, published today, Friday 14 September, in IOP Publishing's journal Environmental Research Letters, compares the latest seasonal forecast system to the one previously used and shows that it can better warn the UK of extreme winter weather conditions.

Dubbed the 'high-top' system, it is different from the previous system as it takes into account phenomenon known as sudden stratospheric warmings (SSWs), which have previously been shown to be responsible for cold surface conditions.

'SSWs occur when the usual westerly winds in the stratosphere – between 10 and 50km altitude – break down. This causes a reversal in the westerly winds in the stratosphere, generating a signal that can often burrow down to the Earth's surface over the course of a few weeks,' said lead author of the study David Fereday.

'This reduces the occurrence of surface westerly winds that bring mild air to northern Europe in winter from the North Atlantic. Instead, northern Europe experiences cold and blocked conditions that can cause extreme low temperatures, as happened in winter 2009/10.'

The Met Office's current long-range forecasting system, GloSea4, is able to simulate weather conditions in higher parts of the atmosphere. This was not a feature available in the forecast system used for the 2009/10 long-range outlook.

GloSea4 uses a computer model which simulates winds, humidity and temperatures on an approximately 150km-spaced grid of points at a range of vertical heights from the surface to beyond the stratosphere which is why it is able to represent SSWs more realistically.

In the study, the researchers compared the forecasts made during the 2009/10 winter with the low-top model, to retrospective forecasts with the high-top model. The forecasts started on dates in October and November and predicted conditions from December to February.

The high-top model predicted conditions that were more closely matched to the observed severe conditions in 2009-10, especially in the late winter..

The high top version of the GloSea4 forecasting system has been in operation since late 2010 and provided useful guidance to weather forecasters in the following two winters (2010/11 and 2011/12).

Co-author of the study, Jeff Knight, said: 'By October 2010, the high top version of the GloSea4 system was indicating an increased chance of a cold start to winter. That year December was the second-coldest in 350-years of records. It also highlighted the possibility that conditions in late winter were likely to be less harsh, which was indeed the case.

'In 2011, GloSea4 predicted that a mild, westerly winter was likely. This turned out to be the case — only the first two weeks of February 2012 were cold. The inclusion of the high top model is one of a series of planned improvements to long range forecasts.'

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Journal Reference:

1. D R Fereday, A Maidens, A Arribas, A A Scaife, J R Knight. Seasonal forecasts of northern hemisphere winter 2009/10. Environmental Research Letters, 2012; 7 (3): 034031 DOI: 10.1088/1748-9326/7/3/034031

NewsPsychology (Sep. 17, 2012) — Over the next few weeks, an ER-2 high altitude research aircraft operating out of NASA’s Wallops Flight Facility in Wallops Island, Va., will take part in the development of two future satellite instruments. The aircraft will fly test models of these instruments at altitudes greater than 60,000 feet to gather information researchers can use to develop ways to handle data future spaceborne versions will collect.

NASA Wallops will be the temporary home of one of NASA’s ER-2 research aircraft. The ER-2 from NASA’s Dryden Aircraft Operations Facility in Palmdale, Calif., will carry two instruments, the Cloud-Aerosol Transport System (CATS) and the Multiple Altimeter Beam Experimental Lidar (MABEL). CATS and MABEL are test beds for instruments to be carried by future satellite missions, and because they are both high-altitude laser instruments they will share space on the ER-2 in part as a way to lower costs for both teams. The ER-2’s deployment began on Sept. 7 and will end no later than Sept. 27.

CATS is a high spectral resolution lidar that uses a laser to gather data about clouds and aerosols. Aerosols are tiny particles in the atmosphere such as dust, smoke or pollution. Similar instruments on existing satellites, such as CALIPSO, can detect aerosol plumes, but cannot determine what they are made of.

“You have to make some assumptions,” said atmospheric scientist Matt McGill at NASA’s Goddard Space Flight Center in Greenbelt, Md. CATS can better detect aerosol particle properties, allowing researchers to better determine what kind of aerosols the plumes are made of and improve studies of aerosol transport and cloud motion. CATS was designed as a test instrument for the future Aerosol-Cloud Ecosystems (ACE) satellite mission, which is still in its planning stages, and a version of CATS will be installed on the International Space Station in mid-2013.

MABEL is a laser altimeter built to simulate the primary instrument on ICESat-2, scheduled for launch in 2016. ICESat-2 will study land and sea ice and vegetation. In April, a NASA ER-2 carrying MABEL flew surveys of land and sea ice out of Keflavik, Iceland, which yielded large amounts of data that researchers are using to develop algorithms for ICESat-2.

This time around, MABEL will measure vegetation along the U.S. East Coast, which will provide data useful for developing methods for determining the amount and thickness of vegetation coverage. This involves measuring both the tops of tree canopies and ground level at the same time, which Kelly Brunt, a cryospheric scientist at NASA Goddard, said is a challenging task. The ICESat-2 team’s need to measure deciduous forest canopies is in part of why these flights will operate out of Wallops. “We can’t get the type of vegetation canopy we need flying out of Dryden,” Brunt said. The ER-2 will be surveying forests and grasslands from Maine to the Florida Everglades.

In addition to CATS and MABEL, the ER-2 will carry a Cloud Physics Lidar (CPL) instrument that will be used to detect clouds and aerosols that could hinder MABEL’s performance. “We need to know what’s between MABEL and the surface,” said McGill.

These flights will coincide with NASA’s Hurricane and Severe Storms Sentinel, or HS3, campaign. HS3 is an airborne mission where a NASA Global Hawk unpiloted aircraft will overfly hurricanes and severe storms to measure properties such as wind, temperature, precipitation, humidity and aerosol profiles. One of the instruments it carries is a CPL identical in design to the one on board the ER-2. The Global Hawk is capable of flying at altitudes greater than 60,000 feet for more than 28 hours at a time and will be operated by pilots back on the ground.

The flights are sponsored by the Earth Science Division of NASA’s Science Mission Directorate in Washington.

For more information about MABEL, visit: http://www.nasa.gov/topics/earth/features/mabel-maiden.html

For more information about CPL, visit: http://cpl.gsfc.nasa.gov

For more information about NASA’s HS3 Mission, visit: http://www.nasa.gov/HS3

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Story Source:

The above story is reprinted from materials provided by NASA.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

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The second Metop satellite was launched today (Sept. 17) from the Baikonur cosmodrome, in Kazakhstan, atop a Russian Soyuz launcher.

Metop-B will ensure the continuity of the weather and atmospheric monitoring service provided by its predecessor Metop-A, which has been circling the globe from pole to pole, 14 times a day, since 2006 and has now exceeded its design lifetime.

The Soyuz-Fregat vehicle lifted off at 16:28 GMT on Monday, Sept. 17. The Fregat upper stage manoeuvred to release the satellite into a polar orbit at an altitude of 810 km some 69 minutes later, over the Kerguelen Archipelago, in the Indian Ocean.

Metop-B, developed for EUMETSAT's polar satellite system, is now under the control of ESA's Operations Centre in Darmstadt, Germany.

In the coming days, its systems will be tested before it is handed over to EUMETSAT, also based in Darmstadt, for six months of commissioning of its payload before entering routine service with Metop-A.

For ESA's Director General Jean-Jacques Dordain: "The launch of this second Metop satellite has taken place about two and a half months after that of MSG-3; this is a perfect illustration of the vitality of Europe's weather satellite programmes developed in cooperation between ESA and EUMETSAT. The fact that the next generation satellites in line are already being prepared by ESA shows the strong commitment of Member States of both organisations to continue and improve collecting data that are supporting not only weather-forecasting but also monitoring and understanding of climate change. Such services are demonstrating daily the economical and societal value of investing in space infrastructure."

Volker Liebig, ESA's Director of Earth Observation Programmes, commented: "Metop-B will become operational while Metop-A is still active and performing well. This will ensure the continuity of the service without any risk of interruption in the data feed. Meanwhile, we are working with EUMETSAT to prepare the future with the second generation of European polar satellites."

Unlike the Meteosat satellites, which are watching about half of our planet from a fixed vantage point almost 36 000 km above the Gulf of Guinea, Metops work at lower altitude and fly over the whole globe to provide additional data on the atmosphere.

Beyond weather monitoring, the Metop and Meteosat satellites are part of ESA's effort on climate watch, which includes the experimental Earth Explorer satellites, to probe Earth and its atmosphere.

Three Earth Explorers have been launched since 2009 — the GOCE gravity mapper, the SMOS water satellite and the CryoSat ice satellite — and more are in preparation.

In 2013, ESA will start launching Sentinel satellites to monitor our environment and climate under the Global Monitoring for Environment & Security (GMES) initiative with the European Commission.

Extreme precipitation in the tropics comes in many forms: thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones like the recent Hurricane Isaac.

Global warming is expected to intensify extreme precipitation, but the rate at which it does so in the tropics has remained unclear. Now an MIT study has given an estimate based on model simulations and observations: With every 1 degree Celsius rise in temperature, the study finds, tropical regions will see 10 percent heavier rainfall extremes, with possible impacts for flooding in populous regions.

"The study includes some populous countries that are vulnerable to climate change," says Paul O'Gorman, the Victor P. Starr Career Development Assistant Professor of Atmospheric Science at MIT, "and impacts of changes in rainfall could be important there."

O'Gorman found that, compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change. "It seems rainfall extremes in tropical regions are more sensitive to global warming," O'Gorman says. "We have yet to understand the mechanism for this higher sensitivity."

Results from the study are published online this week in the journal Nature Geoscience.

A warm rain will fall

Global warming's effect on rainfall in general is relatively well-understood: As carbon dioxide and other greenhouse gases enter the atmosphere, they increase the temperature, which in turn leads to increases in the amount of water vapor in the atmosphere. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.

Scientists have been developing models and simulations of Earth's climate that can be used to help understand the impact of global warming on extreme rainfall around the world. For the most part, O'Gorman says, existing models do a decent job of simulating rainfall outside the tropics — for instance, in mid-latitude regions such as the United States and Europe. In those regions, the models agree on the rate at which heavy rains intensify with global warming.

However, when it comes to precipitation in the tropics, these models, O'Gorman says, are not in agreement with one another. The reason may come down to resolution: Climate models simulate weather systems by dividing the globe into a grid, with each square on the grid representing a wide swath of ocean or land. Large weather systems that span multiple squares, such as those that occur in the United States and Europe in winter, are relatively easy to simulate. In contrast, smaller, more isolated storms that occur in the tropics may be trickier to track.

An intensity of extremes

To better understand global warming's effect on tropical precipitation, O'Gorman studied satellite observations of extreme rainfall between the latitudes of 30 degrees north and 30 degrees south — just above and below the Equator. The observations spanned the last 20 years, the extent of the satellite record. He then compared the observations to results from 18 different climate models over a similar 20-year period.

"That's not long enough to get a trend in extreme rainfall, but there are variations from year to year," O'Gorman says. "Some years are warmer than others, and it's known to rain more overall in those years."

This year-to-year variability is mostly due to El Niño — a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Niño causes localized warming and changes in rainfall patterns and occurs independent of global warming.

Looking through the climate models, which can simulate the effects of both El Niño and global warming, O'Gorman found a pattern. Models that showed a strong response in rainfall to El Niño also responded strongly to global warming, and vice versa. The results, he says, suggest a link between the response of tropical extreme rainfall to year-to-year temperature changes and longer-term climate change.

O'Gorman then looked at satellite observations to see what rainfall actually occurred as a result of El Niño in the past 20 years, and found that the observations were consistent with the models in that the most extreme rainfall events occurred in warmer periods. Using the observations to constrain the model results, he determined that with every 1 degree Celsius rise under global warming, the most extreme tropical rainfall would become 10 percent more intense — a more sensitive response than is expected for nontropical parts of the world.

"Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming," O'Gorman says. "But they also provide an estimate of what that sensitivity is, which should be of practical value for planning."

The results of the study are in line with scientists' current understanding of how global warming affects rainfall, says Richard Allan, an associate professor of climate science at the University of Reading in England. A warming climate, he says, adds more water vapor to the atmosphere, fueling more intense storm systems.

"However, it is important to note that computer projections indicate that although the rainfall increases in the wettest regions — or similarly, the wet season — the drier parts of the tropics … will become drier still," Allan says. "So policymakers may have to plan for more damaging flooding, but also less reliable rains from year to year."

The frozen cap of the Arctic Ocean appears to have reached its annual summertime minimum extent and broken a new record low on Sept. 16, the National Snow and Ice Data Center (NSIDC) has reported. Analysis of satellite data by NASA and the NASA-supported NSIDC at the University of Colorado in Boulder showed that the sea ice extent shrunk to 1.32 million square miles (3.41 million square kilometers).

The new record minimum measures almost 300,000 square miles less than the previous lowest extent in the satellite record, set in mid-September 2007, of 1.61 million square miles (4.17 million square kilometers). For comparison, the state of Texas measures around 268,600 square miles.

NSIDC cautioned that, although Sept. 16 seems to be the annual minimum, there's still time for winds to change and compact the ice floes, potentially reducing the sea ice extent further. NASA and NSIDC will release a complete analysis of the 2012 melt season next month, once all data for September are available.

Arctic sea ice cover naturally grows during the dark Arctic winters and retreats when the sun re-appears in the spring. But the sea ice minimum summertime extent, which is normally reached in September, has been decreasing over the last three decades as Arctic ocean and air temperatures have increased. This year's minimum extent is approximately half the size of the average extent from 1979 to 2000. This year's minimum extent also marks the first time Arctic sea ice has dipped below 4 million square kilometers.

"Climate models have predicted a retreat of the Arctic sea ice; but the actual retreat has proven to be much more rapid than the predictions," said Claire Parkinson, a climate scientist at NASA Goddard Space Flight Center, Greenbelt, Md. "There continues to be considerable inter-annual variability in the sea ice cover, but the long-term retreat is quite apparent."

The thickness of the ice cover is also in decline.

"The core of the ice cap is the perennial ice, which normally survived the summer because it was so thick," said Joey Comiso, senior scientist with NASA Goddard. "But because it's been thinning year after year, it has now become vulnerable to melt."

The disappearing older ice gets replaced in winter with thinner seasonal ice that usually melts completely in the summer.

This year, a powerful cyclone formed off the coast of Alaska and moved on Aug. 5 to the center of the Arctic Ocean, where it churned the weakened ice cover for several days. The storm cut off a large section of sea ice north of the Chukchi Sea and pushed it south to warmer waters that made it melt entirely. It also broke vast extensions of ice into smaller pieces more likely to melt.

"The storm definitely seems to have played a role in this year's unusually large retreat of the ice," Parkinson said. "But that exact same storm, had it occurred decades ago when the ice was thicker and more extensive, likely wouldn't have had as prominent an impact, because the ice wasn't as vulnerable then as it is now."

NASA scientists derive 2012 sea ice concentration data from microwave instruments aboard Defense Meteorological Satellite Program satellites. The wind data in the visualization is from the National Centers for Environmental Prediction.