Sunday, August 30, 2015

Hurricane Fred (2015)

Storm Active: August 30-

On August 27, the National Hurricane Center began to monitor a disturbance that was still well inland over the west African coast for tropical development. By the time it entered the Atlantic early on August 29, the system already had a well-defined circulation and convection increased immediately. Though initially very far south, the disturbance was able to gain latitude after moving over water due to a weakness in the subtropical ridge north of the Cape Verde Islands. Early on August 30, the circulation became well-defined, indicating the formation of Tropical Depression Six. Surrounded by warm water and low wind shear, the depression intensified shortly after its classification, becoming Tropical Storm Fred just a few hours later. Fred was named at longitude 18.9°W, only the fourth known Atlantic system to be named east of 19°W.

The system continued to strengthen quickly during the day, exhibiting healthy outflow and a dense area of central convection. Meanwhile, its outer bands began to affect the Cape Verde Islands, bringing wind and rain as well as a storm surge to the islands. Overnight, Fred began to develop an eye, and was upgraded to a hurricane. The system began to pass through the Cape Verde Islands as a category 1 hurricane during the morning of August 31, one of the strongest known cyclones ever to affect this area. While near the islands, Fred reached its peak intensity of 85 mph winds and a central pressure of 986 mb. Immediately after passing the northern islands that afternoon, however, the system encountered less favorable conditions including higher shear and lower atmospheric instability. Thunderstorm activity diminished rapidly overnight, and Fred weakened to a tropical storm.

As of 8:00 am EDT August 31, 2015, Hurricane Fred had sustained winds of 65 mph, a minimum central pressure of 997 mb, and was moving toward the west-northwest at 12 mph.

Tuesday, August 25, 2015

Tropical Storm Erika (2015)

Storm Active: August 24-29

Around August 20, a tropical wave moved off of the African coastline. It passed just south of the Cape Verde islands shortly afterward, and moved swiftly into the central Atlantic. A low pressure center soon developed along the wave, and it begin to organize in earnest by August 23. The next day, the system acquired enough definition to be considered a tropical cyclone. Since an Atlantic buoy measured gale force winds near the center, it was classified Tropical Storm Erika.

Erika initially struggled with moderate wind shear and dry air aloft. On August 25, the center became exposed as convection retreated into the southeast quadrant. However, thunderstorm activity made a comeback overnight as the system approached the Leeward Islands from the east. Conditions deteriorated in the islands that night, for while Erika still possessed no visible banding features and was poorly organized, it packed a large area of heavy rainfall and thunderstorm activity. The center reformed to the south during the morning of August 27 as Erika entered the Caribbean.

Continuing to struggle with strong shear, the system remained badly organized that day, and though it passed fairly close to Puerto Rico from the south, most of the strongest winds remained in the southeastern quadrant over open water. By the morning of August 28, Erika had turned west-northwestward toward the island of Hispaniola. Later that day, the cyclone moved ashore on the south coast near the border of the Dominican Republic and Haiti, bringing scattered areas of heavy rain and tropical storm force winds. Land interaction weakened and disorganized the system further, making the center tremendously difficult to locate overnight. Though the circulation emerged over water, the system could not recover and Erika dissipated during the morning of August 29 near Cuba. The remnants of Erika continued toward the northwest, bringing rain to Cuba and eventually Florida. Moisture associated with Erika also contributed to thunderstorm activity over the Carolinas during the final days of August.

Erika failed to achieve significant organization due to constant vertical wind shear. The above image shows Erika over the Lesser Antilles.

Erika dissipated on August 29; the last several points of its track indicate the positions of its remnants over the following day.

Tuesday, August 18, 2015

Hurricane Danny (2015)

Storm Active: August 18-24

At the beginning of the third week of August, a the most vigorous tropical wave of the season thus far moved off of the African coastline. The system developed a surface low pressure center almost immediately. The low moved westward, tracking well south of the Cape Verde Islands early on August 16. Within two days, the low had acquired a closed circulation, and was classified Tropical Depression Four. Low shear conditions dominated the central Atlantic at the depression's low latitude (near 10°N) and the system strengthened to Tropical Storm Danny within hours.

The system experienced modest strengthening through the morning of August 19, but the center became exposed during the day as Saharan dry air infiltrated the northern half of the circulation. Meanwhile, a weakness in the ridge north of Danny allowed it to slow its forward motion and turn toward the west-northwest. Overnight, the cyclone's convective structure recovered, and a small area of concentrated thunderstorm activity appeared near Danny's center by the morning of August 20. Later that morning, an eye formed at the center of the system and Danny rapidly intensified into a category 1 hurricane, the first of the season. After its reformation, the structure of the system was very compact: tropical storm force winds extended only about 50 miles from the center, and hurricane force winds only 10 miles.

Surrounded by a low-shear environment and recovered from its encounter with dry air, Danny continued to strengthen over the next day. A period of rapid deepening brought the hurricane to its peak intensity as a category 3 major hurricane during the afternoon of August 21, with sustained winds of 115 mph and a central pressure of 974 mb. After attaining this peak intensity, however, wind shear increased and the system's cloud pattern decayed. Danny had become a minimal hurricane by the afternoon of August 22 and weakened to a tropical storm overnight. Meanwhile, the ridge north of the cyclone reformed, turning its motion back toward the west and increasing its forward speed. On August 23, Danny's center became exposed, and it became a minimal tropical storm that night. The system finally reached the Caribbean the morning of August 24, passing near Guadeloupe now as a tropical depression. By late that morning, Danny had lost its closed circulation and degenerated into a trough of low pressure in the far eastern Caribbean. Its remnants continued to spread areas of heavy rain westward over Puerto Rico on August 25 before fading away.

The above image shows Danny on August 21 when it briefly attained major hurricane intensity.

Danny dissipated on August 24, just as it entered the Caribbean Sea.

Monday, July 13, 2015

Tropical Storm Claudette (2015)

Storm Active: July 13-14

Late on July 11, a low pressure center moved off of the coast of North Carolina and entered the Atlantic Ocean. Thunderstorms became more concentrated near the low when its circulation encountered warm water early on the 12th, but the system was not yet tropical. Relatively low wind shear allowed the system to organize as it tracked northeast over the next day. By the afternoon of July 13, it had lost its frontal characteristics and was named Tropical Storm Claudette.

As Claudette accelerated away from land, conditions became unfavorable almost immediately as wind shear significantly increased. By the evening, thunderstorm activity was confined to the northeastern quadrant. Over the next day, the tropical storm steadily weakened from its peak intensity of 50 mph. The circulation was devoid of significant convection by the evening of July 14 and the cyclone became post-tropical.

Shortly after becoming a tropical cyclone, Tropical Storm Claudette began to experience strong shear that exposed the center.

Claudette formed from a frontal low that originated over North Carolina.

Tuesday, June 16, 2015

Tropical Storm Bill (2015)

Storm Active: June 15-20

On June 12, disorganized thunderstorms began to appear in the northwestern Caribbean and the neighboring areas of Belize and the Yucatan Peninsula. The trough of low pressure associated with the activity moved northwest over the Yucatan Peninsula the following day and deepened, producing a well-defined arc of convection extending into the Gulf of Mexico. By June 14, the disturbance was generating gale-force winds but had not yet acquired a well-defined circulation. Vigorous storm activity continued as the system moved into the Gulf. Atmospheric conditions improved in the vicinity of the disturbance on June 15 and it became more concentrated. Since tropical storm force winds were already occurring on the eastern side of the circulation, the system was designated Tropical Storm Bill late on June 15.

Through the morning of June 16, an area of high pressure over the southeastern United States steered Bill quickly towards the coast of Texas. The storm experienced slight strengthening, reaching its peak intensity of 60 mph winds and a central pressure of 997 mb before making landfall in the central Texas coast just before noon (local time). Bill weakened over land but maintained a well-defined circulation that was very prominent on radar imagery. The system turned northward that night and continued to bring flooding rains to Texas and Oklahoma as it moved inland and weakened to a tropical depression on June 17.

Tropical Depression Bill weakened only gradually over the next few days, maintaining its identity remarkably well even hundreds of miles inland. It turned slowly toward the east, moving through Oklahoma and Arkansas on June 18 and into Kentucky and Missouri the following day. 3 to 6 inches of rain were recorded along the first states in Bill's path and totals from 2 to 4 inches continued over a wide swath around the center over the east-central United States. On June 20, Bill finally became post-tropical and began to produce severe thunderstorm activity across the mid-Atlantic region as it interacted with a frontal boundary.

The above image shows Tropical Storm Bill just after landfall in Texas.

Despite spending less than two days over water as a tropical cyclone, Bill persisted far inland. This was in part due to the wet soil along its path (there had been a good deal of rain before Bill) that provided moisture to fuel the cyclone's circulation. This phenomenon is also known as the "brown ocean effect."

Thursday, May 14, 2015

Professor Quibb's Picks – 2015

My personal prediction for the 2015 North Atlantic Hurricane Season (written May 13, 2015) is as follows:

9 cyclones attaining tropical depression status*,
7 cyclones attaining tropical storm status*,
3 cyclones attaining hurricane status, and
1 cyclone attaining major hurricane status.

*Note: Tropical Storm Ana formed on May 8, almost a month before official start of the hurricane season and before I published my predictions.

The above prediction anticipates a significantly below-average hurricane season, particularly relative to the past few decades. It calls for just over half the typical number of tropical storms, and less than half of the usual hurricanes and major hurricanes. If this prediction were to come true, 2015 would be the least active season since 1994.

Several factors are stacked against tropical cyclone formation this season. First, a weak El Nino event that developed in 2014 has persisted so far this year. In addition, predictions indicate that it will strengthen over the coming months. Already, the anomalously high sea surface temperatures in the Pacific associated with an El Nino have caused a stronger jet stream and stronger wind shear across the United States and the neighboring Atlantic.

The relative scarcity of Atlantic hurricanes over the 2013 and 2014 seasons also suggests that we finally may be entering the "cool phase" of the Atlantic Multi-Decadal Oscillation (AMO), an apparent cycle of above-normal and below-normal hurricane activity whose period is measured in decades. We do not completely understand this cycle and cannot predict it, but the elevated activity for the 15 years ending in 2012 and the subsequent lull could indicate a reversal. Currently, cool water temperatures abound throughout the tropical Atlantic, supporting the hypothesis that we are entering a "cool" period. Regardless, the low Atlantic sea surface temperatures will help to suppress cyclone formation.

Finally, sea-level pressures over areas of the north central Atlantic have been higher than normal over the past few months. Though I wrote elsewhere that a stronger high-pressure system over the central Atlantic tends to steer Atlantic hurricanes towards land and that the strong high appears with the La Nina, and not the El Nino, the truth is more complicated. Both its position and strength influence hurricane activity in different ways and the latter may not correlate with the ENSO cycle. This year, there is both an El Nino and a higher intensity Bermuda high, and both of this factors tend to reduce hurricane activity.

My estimated risks for different parts of the Atlantic basin are as follows (with 1 indicating very low risk, 5 very high, and 3 average):

U.S. East Coat: 1
A combination of a strong jet stream and cool ocean waters will result in a very low risk of landfall for the U.S. East Coast. Ironically, Tropical Storm Ana made landfall in South Carolina before the season even officially began. However, this was due to a blocking pattern that should occur only infrequently, particularly if the El Nino strengthens.

U.S. Gulf Coast/Northern Mexico: 3
The anomalies in water temperatures are less pronounced in the Gulf of Mexico, so this region is at greater risk for landfalling cyclones. The Bay in Campeche in particular may be a birthplace of one or more short-lived tropical storms this season.

Yucatan Peninsula and Central America: 3
The high sea-level pressures this season will enhance trade winds, causing the potential for westward-tracking, fast-moving systems through the central Caribbean and into the Central American states. Despite the expected quiet season, this region will have about average risk.

Caribbean Islands: 2
The Caribbean can expect some activity, but El Nino-related dry air may lead to the demise of some tropical systems before they can undergo much strengthening. This region probably has little to fear from Cape Verde-type strong hurricanes this year.

Overall, the 2015 season is expected to be quiet, and possibly historically quiet. The precise strength of the developing El Nino is the main uncertainty in predicting the level of activity. Regardless, even quiet years can have devastating storms, so be sure to always practice hurricane preparedness!


Tuesday, May 12, 2015

Hurricane Names List – 2015

For the North Atlantic Basin, the list for naming tropical cyclones in 2015 is as follows:


This list is the same as the list for the 2009 season because no names were retired that year.

Friday, May 8, 2015

Tropical Storm Ana (2015)

Storm Active: May 8-11

During the first few days of May, a stationary front stalled over the northwest Caribbean sea, extending through the Bahamas. By May 5, an upper-level trough had formed over the central Bahamas, generating a widespread area of showers and thunderstorms extending into eastern Florida. A weak surface low formed in association with the trough the next day and moved close to the Floridian coast. Due to a blocking pattern situated over the mid-Atlantic states, the low moved little over the next few days, drifting only slowly northward. Meanwhile, the low deepened over the warm Gulf stream waters southeast of the Carolinas. By May 8, the system was exhibiting gale force winds in a broad region encompassing its center as it meandered a few hundred miles off of the South Carolinian coastline. Late that night, the low acquired enough convective organization to initiate advisories, but the large radius of maximum winds led to its classification as Subtropical Storm Ana.

Despite relatively favorable sea surface temperatures, dry air impeded development of convection near the center of Ana over the next day. Through most of the day May 8, almost all thunderstorm activity was confined to the southern and eastern portions of the system's circulation and the center remained isolated. This also contributed to Ana remaining a subtropical cyclone that day as it moved little. During the night, significant cloud cover finally developed near the center and the broader structure disappeared, indicating that Ana had transitioned to a tropical storm early on May 9. By this time, it had acquired a slow north-northwestward motion towards the U.S. coastline. During the day, convective banding structure improved, but Ana simultaneously began to move over the cooler water nearest to the coastline. As a result, cloud tops warmed, though the system maintained its approximate peak intensity of 60 mph winds and a minimum pressure of about 998 mb. Meanwhile, bands of rain began to move into parts of the Carolinas. Overnight, Ana weakened as it interacted with the aforementioned cooler water and land, but made landfall just south of the border between the Carolinas at 6:00 am EDT, May 10, still as a tropical storm.

The cyclone quickly weakened to a tropical depression over land and began to move toward the north and north-northeast as the ridge over the mid-Atlantic shifted eastward and diminished. By May 11, Ana was well-inland over North Carolina but still maintained its identity as a weak tropical depression. Late that night, Ana degenerated into a remnant low over eastern Virginia. The low was absorbed soon after. Ana became the second-earliest cyclone to make a landfall in the United States when it did so on May 10, behind only a cyclone of February 1952.

The above image shows Ana approaching the United States coastline on May 9, just after transitioning to a tropical storm.

Ana stalled southeast of the Carolinas due to a blocking ridge situated to its north.

Wednesday, April 15, 2015

Annular Tropical Cyclones

An annular tropical cyclone is a tropical cyclone (various types of which are hurricane, typhoon, tropical storm, or simply cyclone, depending on location and intensity) with certain distinguishing structural characteristics. These characteristics not only affect the appearance of the tropical cyclone but also its evolution and interaction with the surrounding environment.

The word "annular", meaning "ring-shaped", describes the shape of this type of system. The image below demonstrates the visual differences between ordinary and annular tropical cyclones.

The top image shows a "normal" tropical cyclone, Hurricane Igor of 2010. The bottom image is Hurricane Isabel of 2003, during an annular stage. Though the cyclones were of comparable intensities, they differ greatly in structure. A typical powerful tropical cyclone will have a relatively small eye and spiral rain bands emanating far from the center of circulation. Annular tropical cyclones, on the other hand, have large, symmetric eye features, and are almost perfectly circular. In addition, they tend to be smaller and more compact than other cyclones (the images above are not to the same scale).

Annular tropical cyclones were not recognized as a distinct category until 2002, at which time the concept was introduced in a paper (see here). This idea was created in an effort to explain a class of cyclones which not had a different appearance but also had a notorious tendency to defy prediction, especially intensity forecasting.

In an effort to objectively define whether a given tropical cyclone is annular, researchers developed an algorithm to measure the characteristics of annular tropical cyclones. Using satellite data to measure cloud heights, eye radii, and the like, the algorithm takes a satellite image of a cyclone and outputs a numerical value: the annular hurricane index. If this value is less than zero, the cyclone is non-annular, but if the value is greater than zero, it is annular.

Since the algorithmic method derives a numerical value from a still satellite image, it follows that the index may change with time, and thus that a single tropical cyclone may at one time be annular and at other times not. This reflects observational intuition: weak cyclones by their very nature do not have the symmetry and eye structures which factor into the annular hurricane index. Being annular is a phase of a tropical cyclone's evolution, not an inherent property of a cyclone, and cyclones can further be annular at different intensities and for different durations.

The above image appears in the paper introducing the annular hurricane index (see here for the source). It shows several stages of evolution of a single hurricane, once again Hurricane Isabel of 2003. The different colors on the infrared images indicate various cloud top temperatures. The cooler a cloud top, the higher its altitude, and (roughly) the stronger the storm at that location. In the first image (from September 11), the strengthening category 4 Isabel still possesses visible banding features to the south and east and is notably asymmetric, resulting in a negative annular hurricane index. By the second image on September 12, Isabel was a category 5 hurricane, had lost all bands, and was close to circular in structure. With an index of 1.58, the cyclone met the criteria of an annular cyclone on this date. The third image, taken September 14, shows the hurricane, still a category 5, with quite circular cloud coverage and still little banding. However, the distribution of the coldest cloud tops is asymmetric, resulting in a slightly negative index for this date. By September 18, the hurricane had weakened to a category 2, and some dry air had invaded the system. The spiral structure contributed to the index being far below zero for this date.

Annular tropical cyclones also intensify and weaken quite differently from typical cyclones. While on intensifying trends, typical hurricanes and typhoons will often fluctuate in intensity once they reach major hurricane wind speeds (111 mph and up). The cause of this phenomenon is called the eyewall replacement cycle.

An eyewall replacement cycle is a process during which the eyewall (the ring of strong thunderstorms surrounding the eye) of a cyclone contracts, dissipates, and is replaced by another. The image above, from the Hurricane Research Division of the NOAA, illustrates such a cycle in the evolution of Hurricane Wilma, the most intense Atlantic hurricane ever recorded. The top row shows a series of visible satellite pictures, the second row infrared images, and the bottom vertical cross-sections through the center of the hurricane, showing approximate cloud heights.

The first column illustrates Wilma's compact eye at the time of its record peak intensity on October 19. In the second column, taken October 20, a new ring of clouds, the secondary eyewall, has completely surrounded the first. When this occurs, the first eyewall loses access to moisture and weakens, causing the eye to visibly cloud over just as it does in the second column. With a less-defined center of circulation, the maximum winds decrease and the pressure rises (in this case, Wilma weakened from a category 5 to a category 4). By the third image (from October 21) the first eyewall has dissipated completely, and the secondary wall has become the new primary one, causing the intensity of Wilma to level out (at least temporarily).

However, annular cyclones generally do not experience eyewall replacement cycles, instead maintaining their characteristic large, circular eyes for days at a time. This property exemplifies a more general trend: annular cyclones respond less to changes in their environment than regular tropical cyclones. In particular, having reached their peak intensity, they, in the absence of very cold water or land interaction, tend to weaken only very slowly.

The above graph compares annular hurricanes (6 cases) and other Atlantic hurricanes (56 cases) that did not encounter especially hostile conditions (again, land and very cold water). The intensity trends of the cyclones were averaged and normalized, so that a v-value of 1 corresponds to the peak intensity of a cyclone. Notably, annular hurricanes strengthen slightly more slowly and weaken significantly more slowly than their ordinary counterparts. Examples include 2014's Hurricane Iselle in the Pacific, which maintained hurricane intensity over marginal sea surface temperatures much longer than expected by forecasters. Iselle ultimately made landfall in Hawaii as a tropical storm, a very rare occurrence.

Annular cyclones form a very distinct class of cyclones that exhibit abnormal behavior. Though we do not yet fully understand how and why these cyclones form and act as they do, we continue to make advances in their identification and prediction, including the development of the annular hurricane index. Further research will help us comprehend these cyclones and more adequately prepare for them.


Sunday, March 22, 2015

Cosmic Rays 3

This is the final post in a series on cosmic rays. For the first, see here.
We began by showing another graph of the cosmic ray flux at various energies.

It so happens that this graph very nearly follows an inverse cube relationship (which appears straight on log-log graphs). That is, cosmic rays with double the energy are about eight times rarer in the cosmos. The above diagram also shows the small deviations from the inverse cube relationship that occur at high densities, known as the "knee" and the "ankle".

The first major deviation from the inverse cube law, the so-called "knee", occurs around 1015 eV. Theoretically, this is near the maximum energy that a charged particle could have and still be contained in the Milky Way by the galaxy's galactic halo. The Earth receives a slightly larger flux of these energies than expected because particles slowed to this energy remain in the Milky Way while higher energy particles do not. The transition to extragalactic particle origins forms the "ankle" of the graph.

Extragalactic Cosmic Rays

Cosmic rays with energies more than about 1015 electron volts seldom, if ever, originate in our galaxy. Such cosmic rays could originate from particularly powerful supernovae, or, accelerated by the same mechanism of magnetic shock waves, from two colliding galaxies. However, it is likely that most cosmic rays originate in active galactic nuclei, such as quasars.

The above is an artist's rendering of a quasar. As the name suggests, quasars are very similar to microquasars, except much larger (similar features include the accretion disk and relativisitic jets, both illustrated above). The compact object involved, rather than being a neutron star or stellar black hole, is a supermassive black hole, the kind found at the center of most galaxies (including the Milky Way). Such black holes often exceed one million solar masses.

The characteristic property of active galactic nuclei is that they consume matter at an extraordinarily rapid rate. As a result, the energy and radiation released far outshine the rest of the galaxy. In fact, many quasars are hundreds or thousands of times more luminous than our entire Milky way, corresponding to a luminosity sometimes exceeding a trillion suns. However, over time, the supermassive black hole consumes all nearby matter, and the galaxy becomes dormant, becoming in many respects like our own. For this reason, quasars tend to be young galaxies, and thus the ones we see tend to be quite distant (that is, we see them as they were a long time ago). Even the nearest quasars are still billions of light-years away.

Cosmic rays from these distant quasars may have energies up to about 5x1019 eV (50 million trillion eV). However, this value is the theoretical upper-limit for the energy of cosmic rays from distant sources, also known as the Greisen-Zatsepin-Kuzmin Limit (GZK limit). Computed independently in the 1960's by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin, the GZK limit comes about from the interactions between cosmic ray particles and the cosmic background radiation (see also here), the leftover radiation from shortly after the Big Bang which permeates the Universe. In theory, if cosmic rays with higher energy travel a sufficient distance, interactions with the cosmic background radiation will reduce them to the GZK limit.

However, this is not the end of the story. It has been confirmed that cosmic rays with even higher energies have hit the Earth, though such events are rare: cosmic rays with energy exceeding the GZK limit hit a given square kilometer of the atmosphere only about once per decade. The most commonly accepted explanation for these particles is that they have not yet traveled far enough for the cosmic background radiation to slow them down, and therefore come from sources within about 200 million light-years.

Recently, scientists have proposed that quasar remnants (galactic nuclei that were formerly active) could eject such particles, though being relatively quiet in electromagnetic emissions. Unlike quasars, so-called "retired quasars" are very common in our region of the cosmos, some within 100 million light-years. Very massive, rotating supermassive black holes are the most probable candidates for the origin of these cosmic rays.

The elliptical galaxy M60 (above) is one plausible source for cosmic rays with energies exceeding the GZK limit. The galaxy is only about 55 million light-years away, and harbors a supermassive black hole of 4.5 billion solar masses, among the largest known.

Likely the current record-holder for the most energetic cosmic ray ever observed was aptly named "Oh-My-God" particle. This particle (most likely a proton) was observed in Utah on October 15, 1991. It had an energy of approixmately 3x1020 eV, roughly six times the GZK limit. Since this energy is about 50 J, it is comparable to the kinetic energy of a pitched baseball, all in a single particle! Upon Earth impact, this particle was traveling at roughly 99.99999999999999999999951% of the speed of light. However, the Oh-My-God particle, and other similar cosmic rays, are no threat to Earth or its indigineous life, due to their infrequency and their interactions with the atmosphere before reaching Earth's surface.

Nevertheless, studying the composition and energy of cosmic rays is very important to astrophysics. By observing cosmic rays in addition to collecting images using electromagneitc radiation, we obtain a more complete picture of our Universe.