Gravity Wave Propagation within Super Typhoon Jebi (credit: Meteorologist Jordan Rabinowitz)8/31/2018 
DISCUSSION: As of earlier today, Super Typhoon Jebi was making global headlines as it continued to impress both meteorological and non-meteorological communities alike as a powerful tropical cyclone spinning across part of the Western Pacific Ocean basin. As shown in the animated satellite imagery above (courtesy of the Himawari-8 satellite imager), there was very robust convection propagating both within and right around the edge of the eye wall and the central dense overcast region associated with Jebi. As a point of clarification, the central dense overcast within a tropical cyclone is the region wherein there is most often found to be the strongest thunderstorm activity wrapped around the immediate center and inner core regions of a given tropical cyclone. It goes without saying that Super Typhoon Jebi will remain to be a very legitimate and serious threat for the island nation of Japan in the coming days as it will most likely remain at least the equivalent of a major hurricane from the Tropical Atlantic basin.
As far as neat features go in association with Super Typhoon Jebi, one of the more interesting features currently being observed in association with Jebi is the consistent generation of gravity waves outward from the eye much earlier in the day on Friday morning (i.e., local time over in the Western Pacific Ocean basin). Gravity wave generation within the inner core region of intense tropical cyclones often occurs as a result of more rapid inner core pressure gradient changes and/or fluctuations. As a result of these more rapid inner core pressure changes occurring, the tropical cyclone dynamical response is sometimes visually observed in the form of gravity waves which are essentially vertical undulations in atmospheric flow. In a physical sense, gravity waves are effectively an atmospheric equilibrium-based response where the atmosphere is seeking an energetically-balanced state but is unable to do so within the inner-core of an intense tropical cyclone. Therefore, within the upper-most cloud deck associated with the inner core of more intense tropical cyclones there is often found to be at least a brief period during which gravity waves emanate from the eye wall and outward radially in all directions for at least 20 to 40 nautical miles or so (depending on the intensity of the given pressure gradient within a given storm near the time of peak intensity). It goes without saying that when gravity wave generation happens within the core of intense tropical cyclones for longer periods of time, this is especially when one should be concerned and attentive about future changes in intensity as well as forward track of a given tropical cyclone. To learn more about other neat stories pertaining to tropical cyclones from around the world, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz
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Why Hurricane Lane Is Considered a Rare Event (credit: Meteorologist Ash Bray / photo credit: NASA)8/31/2018  Hurricane Lane, once a Category 5, fell apart as it inched closer to the Hawaiian Islands from the south. Despite Lane’s beautiful collapse, this storm brought several feet of rain to some parts of the Big Island. Before Lane’s landfall, emergency officials were urging residents to gather supplies and prepare their property for the storm’s impact, producing scenes we normally associate with the Gulf region of the United States. But, why is it so unheard of for Hawaii to find itself in the path of a major hurricane? After all, Hawaii is an island state resting in the middle of the Pacific Ocean, where Pacific storms consistently reach extreme intensities farther to the West. However, Hawaii has only experienced landfalls from two hurricanes and two tropical storms since 1959. In 2014, it appeared that two hurricanes were on course to hit Hawaii back-to-back. Thankfully, one of the then-hurricanes, Iselle, was downgraded to a tropical storm before making landfall; Iselle’s twin, Julio, moved too far north, missing the islands.
It turns out Hawaii is in a “sweet spot” for a few reasons! Geographically speaking, the Pacific Ocean is exceptionally large (62.46 million-square-miles) while the Islands are quite small (10,931 square miles). Thus, the odds of a major hurricane striking such a small target is low, especially when compared to the odds of a hurricane striking along North America’s Gulf Coast or East Coast. In order for a hurricane to make direct landfall with Hawaii, the atmospheric conditions have to be perfect. Throughout May and October – peak hurricane season – a strong subtropical high-pressure feature sits to the north of Hawaii, acting as a barrier and driving storms westward. Cooler ocean waters to the north and east of the Hawaiian Islands also help to protect the region. Most years, the ocean temperatures are too cool for intense tropical systems to strike from the east. We can thank the regional ocean currents for this, as they bring cooler Alaskan waters down and along the eastern Pacific and West Coast regions. Ocean-surface temperatures tend to warm up the further south you go in the Northern Pacific Ocean, meaning a tropical system moving towards the Hawaiian Islands from the south would have a much higher probability of surviving. Any south or southwest wind aloft would help to steer a tropical system towards the Islands. During El Nino years, this situation changes slightly; El Nino allows warmer waters to creep further northward, getting into the tropical east-to-west track of tropical systems coming in from the eastern Pacific. This increase in ocean-surface temperatures also promotes tropical system formation closer to the Islands as well as across the eastern Pacific Ocean. El Nino also lessens the trade winds that occasionally decrease tropical activity, allowing tropical activity to the south to be drawn northward. Despite their rarity, when strong hurricanes strike Hawaii, they are often very damaging. In 1992, Hurricane Iniki – the most powerful Hawaiian hurricane (pre-Lane) – killed six and injured over 1,000 people, causing $1.8 billion in damages. Unlike the Gulf and Eastern Coast, large-scale evacuations are immensely challenging due to Hawaii’s remote, cramped geography. Unfortunately, this means that when a major tropical system is charted to strike Hawaii, many are left without shelters. According to a recent report, Hawaiian officials believe that when it comes to seeking shelter, most people would be left to fend for themselves. While most of the Island chain’s shelters are comprised of public schoolhouses, officials fear that these buildings – which are not always up to code due to high inspection costs – would not be able to withstand a major storm. Thankfully, Hurricane Lane dissipated greatly before striking the Hawaiian Islands. Lane was also bombarded with vertical wind shear and dry air which attacked Lane’s core, leading to the storm’s demise. Although Lane produced severe flooding and landslides on the Big Island, the storm surge and extreme winds were lessened, sparing the Islands from devastating wind damage. To learn more about other interesting meteorological topics, be sure to click here! © 2018 Meteorologist Ash Bray 
DISCUSSION: As another August gives way to September and the start of meteorological fall, this is about the time of the year where tropical cyclone activity begins to escalate rapidly. While the Atlantic hurricane season has been anything but active with only five official named storms so far, the Pacific hurricane season is the direct antithesis with a current total of 14 named storms; eight of these 14 storms attained hurricane classification and five of the eight were classified as major hurricanes. This includes the then-powerful Hurricane Lane which approached the state of Hawaii and was largely responsible for the extreme rainfall accumulations in several regions of the Big Island. Hurricanes Miriam and Norman are currently spinning about in the open waters of the Pacific, adding to the record values of accumulated cyclone energy.
The accumulated cyclone energy (ACE) index is a parameter that is used to quantify overall cyclonic activity. It’s calculation is intuitive – ACE equals the squared sum of the maximum single-minute sustained wind speed at six-hourly intervals (00, 06, 12, and 18 UTC) multiplied by 10^-4 (or 1/10,000). The squared velocity is an analog to the kinetic energy generated by the storm and accumulating that energy over the six-hourly intervals yields a value for ACE. Dimensional analysis reveals that the units can also be expressed as m2 / s2, which describes the energy per unit mass. In climatology, the average ACE in the eastern Pacific through the 31st of August is about 72 (assuming the unit is kt^2). However, the 2018 season has so far seen a value for ACE around 163 which is more than twice of climatology to the date (compared to the quieter Atlantic with current ACE of around 18). This indicates that tropical cyclone activity in the Pacific is characterized by hyperactivity. So what factor(s) are responsible for the frequent development of cyclones that we’ve witnessed this season in the Pacific? One of the key forcing mechanisms for the active Pacific season lies within the warming of the Pacific ocean related to the transition between a La-Niña pattern to more of a weak El-Niño pattern. A consequence with the onset of an El-Niño pattern is the deepening of the ocean thermocline in the Pacific, or depth at which ocean temperatures drop considerably, meaning that warmer waters are available at greater depth for storms to potentially use. Also at the global scale, the Madden-Julian Oscillation (MJO) is in a favorable phase for growth of tropical cyclones in the Pacific Ocean. Briefly, the MJO consists of eight phases with each individual phase focusing elevated levels of convection between the Indian and Pacific Oceans. Over the last month, the MJO signal was most prominent in phases six and seven (signal over the Western Pacific), although right now the signal is rather weak and focused over the Maritime Continent. Still, the presence of an MJO signal in the Western Pacific was enough to hint at enhanced convection over much of the Pacific basin. A synopsis at greater detail on the MJO signals can be found in this presentation. Another contributing factor is the lack of vertical wind shear. Without significant wind shear, thunderstorms surrounding the center of low pressure are free to grow unabated and vertically aligned while rotating, keeping the storm intact or perhaps intensifying. A host of scientific journal articles delve into the shear-intensity problem. An interesting paper that describes this effect at a high level can be accessed below: Paterson, L. A., B. N. Hanstrum, N. E. Davidson, and H. C. Weber, 2005: Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region. Mon. Wea. Rev., 133, 3644-3660. With three months still outstanding in the 2018 Pacific hurricane season, the high development frequency of tropical cyclones serves as a message to always remain vigilant for any activity that could form and threaten land. As an aside, it is always important to stay alert to upcoming tropical development, especially as the Atlantic basin approaches peak development during the months of September and October. To learn more about other important and relevant stories related to tropical cyclones, be sure to click here! © 2018 Meteorologist Brian Matilla  During the lifespan of a tropical cyclone, the system will go through what is known as an eyewall replacement cycle (ERC). They naturally occur when a tropical cyclone reaches major hurricane strength with winds of 115 mph or greater. When this happens, the rainbands outside of the main eyewall may strengthen and organize into a ring of thunderstorms. This is known as the outer eyewall. This newly formed eyewall moves slowly inward towards the inner eyewall and begins to take a toll on the tropical cyclones dynamics. It robs the inner eyewall of its moisture and angular momentum. In intense tropical cyclones, the strongest winds are located inside the main eyewall. As the outer eyewall takes control, it will “choke” out the inner eyewall and eventually replace it. This tends to weaken the tropical cyclone as it organizes with its newly formed eyewall. Once the ERC is complete, the storm will have a new smaller eyewall and may re-intensify depending on the environment it is propagating towards. However, sometimes an ERC can lead to a hurricane forming a large annular eyewall. This is when an eyewall is surrounded by a thick and organized ring of intense thunderstorm activity, but the storm lacks a few discrete convective features outside of the eyewall such as rainbands. This tends to give the cyclone a truck tire or donut shaped look as the eye expands, becomes larger and more asymmetric. Unlike typical intense tropical cyclones, this type of storm is not prone to the fluctuations in strength that comes with an eyewall replacement cycle. Annular tropical cyclones tend to hold their peak intensities for a longer period than do most intense tropical cyclones. Typhoon Soulik for example, a tropical cyclone in the western Pacific basin, just recently underwent this process before tracking through the southern Japanese islands. A tropical cyclone on average has an eyewall that is approximately 20-40 miles wide across. According to phys.org, Typhoon Souliks eye was between 40-50 nautical miles wide. Another example of an annular tropical cyclone was hurricane Isabel back in the 2002 Atlantic hurricane season. Hurricane Isabel displayed annular characteristics and had an eye that spanned about 40 nautical miles. The most recent update on Typhoon Soulik states that it is a category 3 typhoon with a centralized pressure of 950 mb and maximum sustained winds of 110 mph. The Japanese Meteorological Agency has Soulik continuing its northwest track before making a northeast turn towards the Korean Peninsula. Soulik is expected to maintain category 3 strength through Wednesday, but then start to weaken rapidly as it enters an unfavorable atmospheric environment. With that said, even when it makes landfall as a tropical storm, it will still be capable of producing flooding rains, damaging wind gusts and dangerous coastal conditions across the peninsula through Friday. Below is the projected path of Typhoon Soulik provided by the Japan Meteorological Agency. To learn more about other important and relevant stories related to tropical cyclones, be sure to click here! © 2018 Meteorologist Joseph Marino  
DISCUSSION: Over the course of any given tropical cyclone season in various oceanic basins spread around the world, it is well understood that tropical cyclone genesis and development conditions change on a fairly routine basis. During the course of this current 2018 Tropical Atlantic hurricane season there is certainly no exception to this seasonal tropical reality. Having said that, It is well understood that tropical cyclone genesis and development requires sea-surface temperatures to be at least or greater than 82°F (or 28°C). Thus, it takes fairly warm sea-surface temperatures to both support the formation and subsequent development of tropical cyclones both across the Tropical Atlantic basin and all other tropical cyclone basins around the world for that matter. Therefore, during situations wherein there are widespread sea-surface temperature regimes which are found to be well below-average, this creates concerns both within and beyond the global atmospheric science community for how active or inactive a given tropical cyclone season may end up being.
In helping to address this issue, there has been a tremendous amount of research done over the past several decades which has aimed to help uncover some of the mysteries and uncertainties behind this issue. More specifically, why some combination of atmospheric and oceanic conditions can facilitate the production of many intense tropical cyclones during some given period and during another season under similar conditions there is little to no tropical cyclone action. Among the many global collection of researchers which have worked to answer this question, one of the most premiere tropical cyclone and tropical meteorology researchers out there happens to be Dr. Kerry Emanuel from the Massachusetts Institute of Technology who is a world-renowned expert on this subject. Attached here are just a few examples of some of the comprehensive research work which has gone deep into working to better understand how tropical cyclone intensity forecasts have been and may be able to be further improved with time. Gilford, D. M., S. Solomon, and K. A. Emanuel, 2017: On the seasonal cycles of tropical cyclone potential intensity. J. Clim., 30, 6085-6096. Emanuel. K., and F. Zhang, 2016: On the predictability and error sources of tropical cyclone intensity forecasts. J. Atmos. Sci, 73, 3739-3747 Tang, B., and K. Emanuel, 2010: Midlevel ventilation's constraint on tropical cyclone intensity. J. Atmos. Sci., 67, 1817-1830. These and MANY other papers have gone to show that there is a tremendous amount of work that is currently still being done here in the 21st century to further expand our ability and accuracy in generating effective tropical cyclone forecasts (i.e., with respect to both tropical cyclone track, intensity, and more). Nonetheless, in looking at the graphic above (courtesy of Meteorologist Greg Postel from The Weather Channel), you can see how based on some of the expert and premiere work accomplished by Dr. Kerry Emanuel et al. over the years, there are several points to be made here. First off, there is no debate that there is still a favorable corridor across the Tropical Atlantic basin even as we head deeper into August 2018 as shown in the graphic above. This graphic effectively illustrates which sea-surface temperature zones would likely favor the support of a tropical cyclone of each respective categorical intensity if one were to form over the respective zones shaded in above according to the Saffir-Simpson hurricane intensity scale. Thus, even though the year has been relatively quiet up to this point due to less than favorable environmental conditions, it is critical to remember that it only ever takes one intense tropical cyclone to imprint on the memories of millions of people and their wallets and their government(s) for many years and decades to come. To learn more about other important and relevant stories related to tropical cyclones, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz |
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