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.

Sources: http://journals.ametsoc.org/doi/pdf/10.1175/2007WAF2007031.1, http://rammb.cira.colostate.edu/resources/docs/annular_Knaff.pdf, http://earthobservatory.nasa.gov/IOTD/view.php?id=45780, http://en.wikipedia.org/wiki/Annular_tropical_cyclone, http://www.nhc.noaa.gov/archive/2014/ep09/ep092014.discus.013.shtml?, http://www.aoml.noaa.gov/hrd/tcfaq/D8.html

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.

Sources: http://www.cosmic-ray.org/reading/uhecr.html, http://www.boseinst.ernet.in/capss/doc/cosmic_primer.pdf, http://www.spaceflightnow.com/news/n0204/23quasars/, http://imagine.gsfc.nasa.gov/docs/features/news/22apr02.html, http://en.wikipedia.org/wiki/Oh-My-God_particlehttp://astronomy.swin.edu.au/cms/astro/cosmos/g/Greisen-Zatsepin-Kuzmin+Limit, http://arxiv.org/abs/0910.4168, http://science.gsfc.nasa.gov/662/boldt/BoldtSymp_Loewenstein.pdf

Thursday, February 26, 2015

Cosmic Rays 2

This is the second part of a two-part post about cosmic rays. For the first part, see here.

The first post dealt primarily with the composition of cosmic rays and the abundance of different types. The other major characteristic of cosmic rays is their energy.



The figure above illustrates the abundance of cosmic rays of different energies on a logarithmic scale. The x-axis shows the energy of cosmic rays in electron volts. An electron volt (eV) is a unit of energy, defined to be the energy required to move an electron through an electric potential of 1 volt. Protons, for example, the most common type of cosmic ray, have a rest energy of 9.38x108 eV, at the bottom end of the chart above. Even at rest, protons are considered to have energy given by the mass-energy relationship E = mc2. Protons (and other cosmic rays such as atomic nuclei) of larger velocity have even greater energies.

The y-axis of the chart indicates cosmic ray flux. Flux, generally speaking, refers to the quantity of something passing through a given surface. In this case, the flux refers to the number of cosmic rays of a certain energy passing through a given area of the Earth's atmosphere. For example, (as labeled on the graph) "1 m-2 s-1" indicates that one cosmic ray passes through each square meter of the Earth's atmosphere (viewed as a spherical surface) every second.

The blue line indicates the relationship between the energy of cosmic rays and the frequency with which they impact Earth (measured by flux). The fact that the curve is decreasing represents that higher energy cosmic rays are rarer. Uncertainty in the values causes the curve to widen for higher energies.

Finally, the graph is separated into three sections, indicating the typical origins of cosmic rays of different energies. The leftmost zone, the yellow zone, refers to solar cosmic rays, i.e. those originating in the Solar System.

Solar Cosmic Rays

Solar cosmic rays, also known as Solar Energetic Particles (SEP's), have energies up to a few gigaelectron volts (~1010 eV). The Sun emits these particles during coronal mass ejections (CME), or explosions of the Sun's atmosphere.



The above image shows a solar flare, which is similar but distinct from a CME in that most radiation released in electromagnetic (the explosion is bright) rather than cosmic rays. Solar flares do release cosmic rays, but to a lesser extent. Mass ejections, on the other hand, release huge quantities of SEP's, some of which are acclerated to over half the speed of light (these have the greatest energies). These events can produce particles that can interfere with the performance of satellites by penetrating their outer skin. The volume of high-energy radiation produced by the strongest events, such as the CME of August 1972, would be fatal to a human outside of the Earth's magnetic field.

Very rarely do solar (and other) cosmic rays reach the surface of the Earth, and, when they do, there are too few to cause radiation damage. However, by the chart above, there is an average of about 1000 such particles impacting every square meter of the surface of the outer atmosphere (outside the magnetic field) every second. Thus, there is potential for satellite damage, especially when this value increases during CME's.

Galactic Cosmic Rays

Galactic cosmic rays (GCR's) are cosmic rays originating from outside the Solar System, but within the Milky Way galaxy. Though they may have energies similar to those of solar cosmic rays, they also may be more energetic, falling into the range 1010 eV to 1015 eV, as illustrated by the blue zone of the figure.

The primary sources for GCR's are supernova remnants; the magnetic fields of these supernovae may accelerate ejected particles to over 99.9% of the speed of light. Through a process known as diffusive shock acceleration, the shock waves of the magnetic field of an exploding supernova emits impart massive amounts of energy to accelerate charged particles. However, other objects, such as so-called microquasars, also produce very high energy cosmic rays.



Microquasars are compact objects, such as white dwarfs, neutron stars, or black holes (see here) which have an ordinary companion star in a binary system. The gravitational pull of these objects sucks plasma off of their companion stars, adding them to an accretion disk as shown. Though a process not entirely understood (but known to involve the magnetic fields of the compact objects) matter from the accretion disk is shot out of the polar regions into relativistic jets, so named for the extremely high speeds of particles therein. Cosmic rays in these jets may be extremely energetic, on the order of 1-1000 TeV (1012-1015 eV). An example of a microquasar in the Milky Way is Cygnus X-3, named for being the third brightest X-ray source in the constellation Cygnus.

An X-ray image of Cygnus X-3


Though Cygnus X-3 is the third brightest in X-rays from Earth, it is more distant than Cygnus X-1 and Cygnus X-2, at a distance of 37,000 light-years (though still in the Milky Way). Further, it emits some of the highest-energy cosmic rays known to originate in the Milky Way galaxy, up to 1000 TeV. Note that the most energetic artificially accelerated particles produced on Earth by the Large Hadron Collider (LHC) have energies on the order of 10 TeV. Since the Earth receives about one particle with energy 1000 TeV per square meter per year, our planet is constantly bombarded by particles moving faster than anything manmade particle colliders have produced.

The final category of cosmic rays (corresponding to the purple area of the figure above) is extragalactic cosmic rays. They are discussed in the final post of this series, coming March 22.

Sources: http://www.euronuclear.org/info/encyclopedia/r/rest-energy.htm, http://solarphysics.livingreviews.org/open?pubNo=lrsp-2006-2&page=articlesu15.html, http://helios.gsfc.nasa.gov/sep.html, http://preppercentral.com/wp-content/uploads/2013/11/sun-big-solar-flare.jpg, http://hesperia.gsfc.nasa.gov/sftheory/spaceweather.htm, http://hyperphysics.phy-astr.gsu.edu/hbase/starlog/cygx3.html#c1, http://helios.gsfc.nasa.gov/gcr.html, http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/releng.html, http://www.andrewcollins.com/pics/cygx3_415k.jpg,

Monday, February 2, 2015

Cosmic Rays 1

The Earth constantly is bombarded with a large amount of radiation from the cosmos. Much of this radiation is electromagnetic, coming in the form of radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. All of these types, however, are composed of photons, massless particles which travel at the speed of light. Other subatomic particles also compose incoming radiation, falling into the umbrella of "cosmic rays".

We cannot take detailed images of the cosmos with cosmic rays like we can with electromagnetic radiation. This is because the particles composing this radiation are charged, and thus change direction and speed when influenced by the magnetic fields of the Sun and Earth. As a result, we cannot directly ascertain the direction from which these particles come. Nevertheless, cosmic rays help us understand the composition of objects in the cosmos. In addition, since the particles making up cosmic rays have mass, they often require huge amounts of acceleration to reach us. The fact that the particles reached Earth at all reveals clues about the nature of the objects from which they came.

Cosmic rays have two main characteristics: composition and energy. We explore each in turn.



Cosmic rays are generally composed of the nuclei of atoms, though other particles appear as well, such as electrons. The extreme energy and speed of these former atoms stripped them of their electrons, leaving positively charged nuclei and negatively charged electrons, each of which are influenced by magnetic fields, as described above. Of these types of particles, hydrogen and helium nuclei (protons and alpha particles), the lightest two elements, are by far the most abundant in cosmic rays. Further, the proportions of hydrogen and helium nuclei in cosmic rays, ~90% and ~9%, respectively, are very consistent with the proportion of atoms in the Universe which are hydrogen and helium.

The above diagram illustrates the relative abundance of the chemical elements (in increasing order) in the Universe, as estimated through indirect means such as spectroscopy, an observational method which uses facts about how different elements absorb and reflect light to discern the composition of celestial objects. Note the relative rarity of lithium, beryllium, and boron, despite the fact that these are very light elements (atomic numbers 3, 4, and 5). The reason for this lack is that these elements are not heavily produced during stellar fusion, occurring only as intermediates in nuclear reactions that either produce helium or heavier elements like carbon. However, the nuclei of these three elements compose 0.25% of cosmic rays, more than a million times greater than what we would expect from the Universe's composition!

It is believed that these elements are disproportionately represented in cosmic rays due to collisions between "original" cosmic rays containing protons and atoms in the interstellar medium, such as carbon or oxygen. Such interactions may produce energetic lithium, beryllium, or boron nuclei as a byproduct, and these particles subsequently reach Earth. The fact that the products of these collisions are energetic and thus more likely to reach Earth explains why these elements are disproportionally represented in cosmic rays. The nuclei of elements of higher atomic numbers also appear more frequently in cosmic rays than we would "expect" from the abundance graph above. This shows that cosmic rays tend to come from sources rich in heavy elements, such as supernovae.

Antiparticles also make infrequent appearances in cosmic rays. An estimated 0.01% of cosmic rays are composed of antimatter. In fact, cosmic rays led to the original discovery of antimatter. In 1928, Paul Dirac, using the mathematics of quantum theory, including Schrödinger's Wave Equation, predicted the existence of antimatter. Antimatter was at the time an undiscovered class of particles each of which would be an "opposite" counterpart to an ordinary particle (opposite in some properties, such as charge, but not in others, such as mass, which must always be positive or zero). One example is the antielectron, also known as the positron, which would have the same mass as the electron, but a positive, rather than negative, charge. This charge, through opposite, is of the same magnitude as the electron's. In 1932, Carl D. Anderson observed a cosmic ray composed of a positron, using a device known as a cloud chamber.

Antimatter, however, does not exist in abundance in the Universe. Matter and antimatter particles annihilate on contact, producing energy, so due to a slight matter-antimatter imbalance following the Big Bang (the origins of which are still unknown), the known Universe is dominated by matter. However, just as matter and antimatter particles can annihilate and form energy, energy may also spontaneously (during certain quantum interactions) be converted into a particle-antiparticle pair through a phenomenon known as pair production.



This diagram illustrates the formation of an electron (e-) and its antiparticle the positron (e+) from a gamma ray photon (γ). This instance of pair production often happens when a high-energy photon impacts the nucleus of an atom. The number of cosmic rays composed of antimatter which impact the Earth over a given period helps us to estimate the frequency of these reactions in space. Cosmic rays also often cause chain reactions in the Earth's atmosphere. In fact, this is how these rays are often detected.



The above image illustrates the cascade of particles that can occur when a cosmic ray (especially a powerful one) hits Earth's atmopshere. The interaction proceeds from top to bottom and begins when the cosmic ray impacts an atom (all atoms involved are denoted by circles). This produces several short-lived particles known as pions (with the symbols π+, π-, and π0 depending on the charge). The left branch indicates the decay of the neutral pion into gamma rays, each of which forms an electron-positron pair through pair production, (presumably through more interaction with atmospheric matter) releasing other gamma rays which do the same. The center shows the main decay mode of the pion into a muon (denoted μ) and a muon antineutrino (not shown) or the corresponding antiparticles (if the initial pion charge is reversed). Muons too decay after around 2.2 microseconds, leaving their characterisitc signiture of particles. Finally, the right branch indicates how incoming cosmic rays might impact and destablize atomuc nuclei. These nuclei then emit protons, neutrons, or tiny particles called neutrinos. Sometimes the emitted neutrons and protons hit other atoms, and the chain reaction continues.

The length of such a chain reaction depends on the energy of the original cosmic ray. Sufficiently energetic cosmic rays can cause huge chain reactions which reach the Earth's surface, allowing us to observe them. To see how energy affects the behavior of cosmic rays and can reveal more about their origins, see the next post.

Sources: http://helios.gsfc.nasa.gov/cosmic.html, http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html, http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmic.html#c2, http://imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html, http://abyss.uoregon.edu/~js/glossary/cosmic_rays.html, http://www.algebralab.org/practice/practice.aspx?file=Reading_RelativeAbundanceElementsUniverse.xml, http://jtgnew.sjrdesign.net/stars_fusion.html, http://www.whoi.edu/cms/files/ksims/2006/10/Cosmo_lect_2006_4SPP_14851.pdf, http://cosmos.phy.tufts.edu/~zirbel/ast21/sciam/AntiMatter.pdf, http://neutronm.bartol.udel.edu/catch/cr2b.gif,

Friday, January 9, 2015

Madden-Julian Oscillation

The Madden-Julian Oscillation (MJO) is a periodic fluctuation of convective activity in the tropics that impact the activity of the monsoon and the formation of tropical cyclones in various basins throughout the world.

Near the equator, weather is more chaotic and less structured than at midlatitudes. At midlatitudes, large cyclones and, sometimes, attached frontal boundaries, govern weather patterns. However, the formation of the vortices of these cyclones requires an effect called the Coriolis effect. The Coriolis effect, in essence, induces rotation in midlatitude regions because the rotation of the Earth carries points at different latitudes at different speeds. But at the equator and the area immediately around it, this effect is not significant enough to cause rotation under usual circumstances. Thus weather patterns in the tropics are rather caused by undulations of the Intertropical Convergence Zone (ITCZ), which seemed to produce shower and thunderstorm activity randomly.

However, some evidence contrary to the idea that the fluctuations were random has been known since antiquity. India's weather, for example, includes clearly demarcated dry and wet seasons in association with the monsoon, despite the fact that southern India's weather is predominantly affected by the activity of the ITCZ. In the 1970's, when more meteorological data became available, scientists Roland Madden and Paul Julian, after whom the oscillation is named, noted that there was a recurring cycle between increased precipitation and suppressed precipitation in the tropics, particularly notable in the Indian Ocean and the Pacific Ocean. More data emerged over the coming years, revealing that the anomalies in tropical precipitation tended to follow the equator, moving east around the globe over time.

The areas of increased and decreased thunderstorm activity associated with the MJO span several thousand miles; in areas of increased activity, the MJO is said to be in positive phase, and in areas of suppressed activity, in negative phase. Each "phase" propagates eastward around the globe at 4-8 meters per second, and therefore takes 30-60 days to travel around the Earth. This time interval, though not an exact value, is the period of the MJO, or in other words, the time necessary for the precipitation anomalies to return to roughly their initial conditions. At any given time, there are about 1-2 areas of increased convection and 1-2 areas of decreased convection that span the tropics.



The above water vapor imagery from the NOAA shows a dry air mass over much of the central Atlantic and Caribbean. The increased tendency for dry air masses to appear in an area of the tropics is thought to correlate with the negative phase of the MJO.

The MJO, as well as governing the onset of the monsoon in India and other locations, can also have an effect on tropical development, even in the Atlantic and Pacific basins. Dry air is often devastating to tropical cyclones, invading their circulations and weakening or even dissipating them. Though some parts of the typical Atlantic and Pacific hurricane seasons are more active then others, in any particular season, the tropical cyclone activity may modulate with the MJO, increasing during positive phase and decreasing in negative phase. In support of this theory, it is a well documented fact that the Atlantic basin tends to be quiet when the Eastern Pacific basin is active and vice versa. This is because these neighboring basins tend to be experiencing opposite phases of the MJO at any given time.

The study and modeling of the MJO is very important to meteorology becuase this oscillation brings order to the chaos of tropical weather. Greater understanding of the MJO will aid long-term and large-scale forecasts, and improve our ability to antipicate tropical cyclonogenesis.

Sources: http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/mjo.html, http://maddenjulianconversation.blogspot.com/2011/08/what-is-mjo-part-1.html, http://www.met.reading.ac.uk/~pete/mjo.html, http://en.wikipedia.org/wiki/Madden%E2%80%93Julian_oscillation, https://www2.ucar.edu/sites/default/files/news/2011/MJO_illus.jpg

Thursday, January 1, 2015

Derechos

Derechos are a meteorological phenomenon consisting of long lines of heavy thunderstorms persisting for several hours and traveling a large distance. Unlike typical pop-up thunderstorms which are usually localized, derechos bring all the threats of severe thunderstorms—heavy rainfall, hail, high winds, and possibly tornadoes—to large swaths of land. Thus derechos can be particularly damaging to life and property.

The word "derecho" is Spanish for "straight", and this name reflects the nature of the wind damage associated with the storms. For reasons discussed below, derechos cause "straight-line wind damage". For a thunderstorm complex to be a derecho, the complex must cause severe wind gusts (where "severe" refers to winds in excess of 50 kt or 58 mph) along a swath of land extending at least 250 miles in length.

The conditions that spawn derechos often involve the collision of moist tropical air and cool air from the polar regions, often along a cold frontal boundary. The warm, unstable nature of the tropical air causes it to rise near the border of the two air masses as the cold air slides beneath it. This process forms cumulonimbus clouds, very large clouds that can stretch from the lower atmosphere to over ten miles high. Such clouds are characteristic of thunderstorms, so storms tend to form along the frontal boundary, giving rise to a long ridge of clouds called a shelf cloud:

A shelf cloud, representing the leading edge of a line of thunderstorms


Another defining feature of derechos is the curvature the line of thunderstorms that develops as the system progresses. The curved radar signature left by these storms are called bow echoes, due to their likeness to archer's bows. This phenomenon is illustrated below by a composition of time-step radar images from a derecho on June 29, 2012.



Seven consecutive radar images are superimposed, showing the development of the bow echo. In addition, the image displays wind gusts observed at different locations during the progress of the derecho. Since several of these in fact were hurricane-strength gusts (exceeded 73 mph), and the storm traversed over 450 miles during its lifetime, this system does indeed meet the qualifications of a derecho.

As noted above, the hallmark of a derecho is straight-line wind damage. The sources for such winds is the downburst, a phenomenon in which cool air descends quickly to ground level and spreads in all directions, sometimes causing extremely high winds. Downbursts occur when a large air mass is rapidly cooled by the evaporation of water, the sublimation of ice directly into vapor, or the melting of ice crystals. Since all of these processes are endothermic, or require energy from the surrounding environment, they cause the air in the vicinity to cool. The mass of air, having been cooled, is now heavier than the surrounding air and accelerates toward ground level. When it reaches the ground, it is forced outward in all directions, causing winds spreading uniformly from the impact point, hence the straight-line winds.

The life cycle of a downburst


Within a downburst, which may span distances of several tens of miles, there are sometimes smaller-scale features known as microbursts, which are a few miles in length and contain especially intense winds. On a yet smaller scale are the burst swaths that sometimes occur in microbursts, areas of just thousands of square feet in which the wind speeds may rival those of a tornado. The straight-line winds of a derecho are also responsible for the bow echoes that appear on radar. Winds emanating from near the center of the squall line fan out and cause its edges to bend into the bow shape.

Meteorologists and climatologists, using data from the past 30 years, have analyzed how often and where derechos occur, predominantly within the United States.



The above map shows the frequency of derechos in different areas of the United States. Virtually no derechos have occurred in the west, and the most active area for derechos is the southeastern plains, which periodically have multiple derechos in a single year. Note also that these storms are most common in the area stretching from Minnesota to western Ohio during the warm season, and in an area from eastern Texas through the southern Gulf states during the colder months. It is likely that derechos occur regularly outside the United States, but very few thunderstorm events have been formally classified as such.

Sources: http://spanish.about.com/od/spanishvocabulary/a/derecho.htm, http://en.wikipedia.org/wiki/Derecho, http://www.spc.noaa.gov/misc/AbtDerechos/images/HampshireIL2008July10.gif, http://www.theweatherprediction.com/habyhints/288/, http://www.xweather.org/derecho, http://www.spc.noaa.gov/misc/AbtDerechos/bowechoprot.htm, http://www.crh.noaa.gov/iwx/?n=june_29_derecho, http://www.radiotimeline.com/wx-downburst.gif, http://www.spc.noaa.gov/misc/AbtDerechos/climatologypage.htm

Tuesday, December 9, 2014

2014 Season Summary

The 2014 Atlantic Hurricane Season had below-average activity, with a total of

9 cyclones attaining tropical depression status,
8 cyclones attaining tropical storm status,
6 cyclones attaining hurricane status, and
2 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

13 cyclones attaining tropical depression status,
12 cyclones attaining tropical storm status,
4 cyclones attaining hurricane status, and
1 cyclone attaining major hurricane status.

The season was in fact below-average, particularly in number of named storms (8) which was well below the 30 year average (12) and what I predicted. In addition, the season had the fewest named storms since the 1997 season. The number of hurricanes and major hurricanes were closer to average, and were higher than my predictions.

A weak El Nino event did develop by late spring of 2014, contributing to stronger upper-level winds across the Atlantic basin and inhibiting development. In addition, though sea surface temperatures were anomalously warm over the subtropical Atlantic basin, SST values remained near average closer to the equator; the Gulf of Mexico even trended below average during parts of the season, limiting development there (Tropical Storm Dolly was the only tropical storm-strength cyclone to exist there).

Adding to these factors was a near- to below-normal West African monsoon, which made tropical waves emerging into the east Atlantic less frequent and vigorous. Stable air over the Atlantic itself also inhibited cyclone formation. In contrast, the East Pacific basin had above-average activity, consistent with the presence of an El Nino. Most land areas in the Atlantic were spared significant damage this season, with only tropical storm landfalls in Mexico, Central America, and the eastern Caribbean, and a single landfalling hurricane, Hurricane Arthur, in the United States. A notable exception to this was Bermuda, which was hit directly by two hurricanes in a very short span (see below).

Some other notable facts and statistics concerning this season are:

  • Hurricane Gonzalo was the strongest storm of the season, a category 4 hurricane with 145 mph winds and a minimum pressure of 940 mb, making it the strongest Atlantic cyclone since Hurricane Igor of 2010
  • Hurricane Arthur made landfall in North Carolina on July 3 (EDT), making it the earliest in a season a hurricane has ever made landfall in the state
  • Hurricane Fay and Hurricane Gonzalo directly hit Bermuda on October 12 (Fay was a tropical storm at the time) and October 17, respectively, an unprecedented two direct hits in a six-day period for the island
  • Tropical Storm Hanna formed from the remnants of Tropical Storm Trudy of the East Pacific basin, which had made landfall in Mexico on October 18, dissipated, and crossed into the Gulf of Mexico


Overall, the 2014 Atlantic Hurricane Season was quiet, showing consistency with pre-season forecasts.

Sources: http://www.esrl.noaa.gov/psd/enso/mei/, http://www.ncdc.noaa.gov/teleconnections/enso/indicators/sea-temp-anom.php, http://www.noaanews.noaa.gov/stories2014/20141124_hurricaneseasonwrapup_2014.html

Wednesday, October 22, 2014

Tropical Storm Hanna (2014)

Storm Active: October 21-22, 27

On October 17, Tropical Storm Trudy formed in the eastern Pacific basin. The next day, it made landfall in Mexico and quickly dissipated over the mountainous terrain. On October 19, a low pressure system began to form over the Bay of Campeche from the remnants of Trudy. Producing scattered shower activity throughout the southwestern Gulf of Mexico, the system slowly meandered to the east-northeast and moved farther over water over the next few days. During the day of October 21, the low deepened significantly and gained definition, though the associated convection did not yet meet the criteria of a tropical cyclone. That night, however, a small but persistent area of thunderstorm activity developed near the center of circulation, and the system was designated Tropical Depression Nine.

Despite being over warm water, the system faced unfavorable atmospheric conditions, including shear out of the west and interaction with a frontal boundary to its northeast. This front caused heavy rain across the northern Yucatan through Cuba and the neighboring islands, but this moisture was not associated with Nine. On October 22, the system turned east-southeast towards the Mexican coast. Failing to strengthen, the system made landfall that evening as a weak tropical depression, and was downgraded to a remnant low just a few hours afterward.

On October 24, the system emerged over water on the eastern side of the Yucatan peninsula and began to drift generally east-southeast. Though atmospheric conditions were unfavorable, the low maintained its identity for the next few days, and concentrated thunderstorm activity reappeared during the day of October 26. By this time, the system had changed tack and was drifting westward toward the coast of Honduras. On the morning of October 27, despite the fact that a portion of the circulation was interacting with land, the low was producing gale force winds and had acquired an organized convective structure. It was therefore upgraded to Tropical Storm Hanna.

A few hours later, the center moved inland over northeastern Nicaragua. Hanna quickly lost definition and was downgraded to a tropical depression that evening. It then degenerated into a remnant low as it moved west-southwestward over the mountainous terrain of Central America. Despite this, heavy rains continued over portions of northern Nicaragua and southern Honduras, bringing 3-5 inches to many areas. On October 28, the low moved northwestward and emerged into the Gulf of Honduras, but it once again made landfall in Belize early on the 29th, eliminating any chance of redevelopment. Tropical moisture associated with Hanna eventually made it as far as the southwestern United States.



The above image shows Hanna on October 27, a few hours after regeneration into a tropical cyclone.



Despite mostly favorable conditions, Hanna did not significantly intensify during its lifetime due to land interaction.

Sources: http://upload.wikimedia.org/wikipedia/commons/0/01/Hanna_Oct_27_2014_1600Z.jpg, http://upload.wikimedia.org/wikipedia/commons/6/63/Hanna_2014_track.png

Sunday, October 12, 2014

Hurricane Gonzalo (2014)

Storm Active: October 12-19

During the first week of October, a tropical wave entered the eastern Atlantic and tracked generally westward. It did not show signs of organization until October 10, when shower activity began to increase in concentration. Despite a large mass of dry air to its north, the disturbance developed rapidly. On October 12, curved bands became evident around a well-defined center of circulation. Since aircraft data indicated that gale-force winds were occurring in the vicinity of the center, the system was designated Tropical Storm Gonzalo early that afternoon.

Situated over an environment of warm water, unstable air, and low wind shear, only dry air slightly slowed development. Convective bands wrapped around a primitive eye feature that evening and steady strengthening began. Meanwhile, a trough to the north of the system steered it westward toward the Leeward Islands. During the morning of October 13, Gonzalo's center passed among these islands, bringing tropical storm conditions to much of the region as it continued to intensify and deepen. Later that day, the system turned to the northwest and gained enough organization to be upgraded to a hurricane as it passed near the Virgin Islands. Though the center passed to the east, the large area of deep convection associated with Gonzalo stretched as far as Puerto Rico.

As the hurricane exited the Caribbean overnight, an eye began to consistently appear on satellite imagery. Pressures continued to decline, and the system underwent rapid intensification through the morning of October 14. Though the convection remained somewhat lopsided (with most of the deep convection south of the eye), Gonzalo became the second major hurricane of the 2014 season later that day. Meanwhile, the cyclone continued to round the edge of a ridge to its north, and its motion gradually turned poleward. The eye contracted during the morning of October 15, indicating that an eyewall replacement cycle had begun and stabilizing Gonzalo's intensity as a low-end Category 4 hurricane. Gonzalo was the first category 4 hurricane to form in the Atlantic since 2011's Ophelia.

As is usual in such cycles, the cyclone's eye clouded over late that morning as an outer eyewall formed, and internal dynamics caused Gonzalo to weaken slightly over the following 12 hours. Overnight, the system completed its northward turn and its eyewall replacement, with a large, symmetrical eye forming by the morning of October 16. Meanwhile, the banding structure and outflow had also improved, and Gonzalo restrengthened into a category 4. Later that day, the system reached its peak intensity of 145 mph winds and a minimum pressure of 940 mb before the inner core was once again disrupted that evening, leading to gradual weakening. Caught in a south-southwesterly flow, the cyclone also began to accelerate to the north-northeast toward Bermuda that evening.

By the morning of October 17, conditions were deteriorating in Bermuda, as outer bands began to sweep across the island. During the afternoon, the Gonzalo's eye reappeared, and weakening temporarily ceased, with the cyclone at category 3 hurricane strength. Around 8:30 pm EDT that evening, the center of the hurricane passed directly over Bermuda bringing significant storm surge to the coastline as well as sustained winds to hurricane force. During that evening, upper-level winds increased somewhat, putting Gonzalo on a steady weakening trend as it accelerated away from Bermuda.

The cyclone moved north of the Gulf stream during the morning of October 18, and convection began to disappear from the southern half of the circulation. As a result, the system weakened to a category 1 hurricane. Despite plummeting ocean temperatures however, Gonzalo maintained a well-defined eyewall through that evening. Overnight, the system sped past offshore of Newfoundland, causing gusty winds with its broadening windfield. By the morning of October 19, Gonzalo was racing northeast across the northern Atlantic at forward speed of over 50 mph. The cyclone finally became extratropical above 50°N that afternoon. The system subsequently passed near the United Kingdom on October 21 before being absorbed near the Arctic Circle.



Gonzalo experienced several fluctuations in intensity as a major hurricane due to internal dynamics. Even in the above image, a concentric set of eyeballs seems to be forming.



Remarkably, both Tropical Storm Fay and Hurricane Gonzalo passed very near or directly over Bermuda over the course within a period of less than one week!

Sources: http://upload.wikimedia.org/wikipedia/commons/7/7a/Gonzalo_Oct_16_2014_1745Z.jpg, http://upload.wikimedia.org/wikipedia/commons/1/18/Gonzalo_2014_track.png

Friday, October 10, 2014

Hurricane Fay (2014)

Storm Active: October 10-13

Around October 8, persistent shower and thunderstorm activity appeared in conjunction with a low pressure system located northeast of the Windward Islands. As the low moved west-northwestward over the following day, environmental conditions improved. On October 9, the trough associated with the circulation center became visibly curved with the development of a semicircular rain band about the low's north and east sides. Meanwhile, surface pressures continued to fall, and the circulation became much better defined by the morning of October 10. At this time, the system was organized enough to be designated Subtropical Depression Seven.

Through the evening, an area of deeper convection within the rain band developed northwest of the center. Since aircraft reconnaissance data indicated that higher winds were occurring in this area, the intensity of the system increased significantly, and Seven was upgraded to Subtropical Storm Fay. By the morning of October 11, Fay had come to the western edge of a subtropical ridge and had assumed a northward motion toward Bermuda. In addition, the area of deep convection moved close enough to the center and became symmetrical enough that Fay transitioned to a tropical storm that same morning.

Later that day, despite moderate shear aloft, the convective canopy covered Fay's center for the first time. The central pressure continued to drop meanwhile, and Fay intensified to near-hurricane strength. By this time, conditions were rapidly deteriorating in Bermuda. Early in the morning on October 12, the center of the cyclone passed almost directly over Bermuda, bringing winds gusting to hurricane strength, 3-5 inches of rain, and large sea swells. The system continued to curve to the east and accelerate as it passed the island that day. During the afternoon, Fay briefly developed a small eye, and became more symmetrical as shear temporarily lessened. As a result, the cyclone was upgraded to a minimal hurricane and reached its peak intensity of 75 mph winds and a pressure of 986 mb.

During the evening and overnight, however, wind shear increased substantially, quickly weakening the system back below hurricane strength and displacing its convection to the northeast of the center. On October 13, a frontal boundary moving off of the United States was steering Fay nearly due east, and the interactions between the two systems contributed to the tropical storm's dissipation later that day. The remnant vortex of Fay became embedded in the same front by the afternoon.



The above image shows Fay passing near Bermuda on October 12.



Fay's track includes square points, indicating a time at which the cyclone was subtropical.

Sources: http://upload.wikimedia.org/wikipedia/commons/a/a9/Fay_Oct_12_2014_1455Z.jpg, http://upload.wikimedia.org/wikipedia/commons/8/83/Fay_2014_track.png