Reconnaissance Aircraft Research Articles

From the Editors of the Special Issue on Multiplatform and Multifunctional RF Systems

The idea for this Special Issue on Multiplatform and Multifunctional RF Systems [(MPRFS) and (MFRFS)] was conceived in 2020 on the strength of a very successful workshop on Multiplatform RF Systems organized by the European Defence Agency (EDA). The editors were further motivated by the realization that modern platforms such as fighter jets, drones, reconnaissance aircraft, frigates, destroyers, and submarines have limited real estate for the inclusion of even more sensors—giving rise to the field of MFRFS. MFRFS represents a research and technology endeavor to realize multiple RF functions such as radar, electronic attack (EA), electronic support (ES), and communications (Comms) in a platform– sensor suite through the sharing of hardware architectures. There is no standard definition for MFRFS, but one interpretation is the use of active electronically scanned array (AESA) antennas for radar, EA, ES, and Comms so that the overall number of antennas distributed over a given platform may be reduced. There are, of course, other valid interpretations not predicated on the use of ASEAs, but rather aim to consolidate multiple functions into one suite of common hardware. The concept of MPRFS is based on the controlled utilization of RF systems that are available within an interconnected and spatially dislocated multiplatform constellation. Individual platforms may be strategically positioned and managed to improve constellation performance in complex operational environments. MPRFS and MFRFS are areas of active research and this is set to intensify in the coming years. We, as guest editors, are pleased to publish this first special issue dedicated to the field of MPRFS and MFRFS and, as the field matures, we are confident that future special issues will follow. This special issue comprises four interesting contributions. Each is briefly summarized.

A decade (2011–2020) of tropical cyclone reconnaissance flights over the South China Sea

A decade has passed since the Hong Kong Observatory (HKO) and the Government Flying Service (GFS) jointly commenced operational tropical cyclone (TC) reconnaissance over the South China Sea in 2011, currently the only such programme in the region and a first since the USA terminated aircraft reconnaissance in the western North Pacific in 1987. This paper reviews the HKO–GFS flights between 2011 and 2020, including 24 storm-penetrating flights down to 2000ft and 28 dropsonde missions resulting in 190 profiling measurements, which have supported operational forecasting and warning of TCs as well as contributed to scientific research and regional collaboration.

Multi‐GNSS Airborne Radio Occultation Observations as a Complement to Dropsondes in Atmospheric River Reconnaissance

AbstractVariations in the water vapor that atmospheric rivers (ARs) carry toward North America within Pacific storms strongly modulates the spatiotemporal distribution of west‐coast precipitation. The “AR Recon” program was established to improve forecasts of landfalling Pacific‐coast ARs and their associated precipitation. Dropsondes are deployed from weather reconnaissance aircraft and pressure sensors have been added to drifting ocean buoys to fill a major gap in standard weather observations, while research is being conducted on the potential for airborne Global Navigation Satellite System (GNSS) radio occultation (ARO) to also contribute to forecast improvement. ARO further expands the spatial coverage of the data collected during AR Recon flights. This study provides the first description of these data, which provide water vapor and temperature information typically as far as 300 km to the side of the aircraft. The first refractivity profiles from European Galileo satellites are provided and their accuracy is evaluated using the dropsondes. It is shown that spatial variations in the refractivity anomaly (difference from the climatological background) are modulated by AR features, including the low‐level jet and tropopause fold, illustrating the potential for RO measurements to represent key AR characteristics. It is demonstrated that assimilation of ARO refractivity profiles can influence the moisture used as initial conditions in a high‐resolution model. While the dropsonde measurements provide precise, in situ wind, temperature and water vapor vertical profiles beneath the aircraft, and the buoys provide surface pressure, ARO provides complementary thermodynamic information aloft in broad areas not otherwise sampled at no additional expendable cost.

US WITHDRAWAL FROM THE OPEN SKIES TREATY: CONSEQUENCES FOR INTERNATIONAL SECURITY AND STABILITY

The arms control regime consists of several documents, and the United States of America and the Russian Federation as the State-Parties ensured the implementation and effectiveness of them mostly. However, the growing conflict between these States due to Russia's numerous violations of international law and obligations under international agreements has also harmed arms control. The withdrawal of both states from strategically important documents, which were tools for maintaining transparency and mutual control, is a result of the decline in trust and the lack of a constructive dialogue between Washington and Moscow. The Open Skies Treaty is one of these documents. The article examines the United States' reasons for the withdrawal from the Treaty and reveals the consequences of such a decision by the administration of President Trump. It is noted that the agreement, the idea of ​​which belonged to the United States, was signed immediately after the end of the Cold War and was to help build trust and openness in the new environment. It is noted that the Treaty ensures international stability through the ability of each party to openly collect information on the armed forces, activities, and operations of another party through coordinated flights of reconnaissance aircraft over the territory of the member states. However, problems are gradually accumulating due to Russia's actions to restrict flight zones in violation of the provisions of the Open Sky Treaty, which has become especially acute since 2014. That made the United States think about the meaning of participating in an agreement where one of the parties violates its provisions. The authors outline the consequences of the U.S. decision to leave the agreement; in particular, the focus is on the reactions of the U.S. European partners and Ukraine. They also examined applications and countermeasures by Russia that has also announced its withdrawal from the agreement. Reflecting on the prospects for international security and arms control, the preconditions for the potential return of the United States and Russia to the Open Sky Treaty are outlined.

Extraction and mapping of component information of ancient buildings in Huizhou based on UAV technology

The component information collection of ancient buildings is an important form of heritage protection in Huizhou. The application of UAV-based mapping technology has the advantages of small size, flexibility, and high precision. It has been widely used in the surveying and mapping projects of ancient building. Drones are tiny unmanned aerial vehicles that can be remotely controlled from a distance, i.e., drones. In appearance, they resemble helicopters or reconnaissance aircraft, and their versatility makes them a valuable asset. In this paper, the UAV technology was applied to photograph and map the components of ancient buildings in Huizhou. It is found that the UAV shooting, dynamic video, mapping and measurement can achieve the image acquisition, and the physical data such as image information of the building components can be captured by the UAV. Besides, the radio adaption method was used in the UAV shooting to ensure the faster sub-packet transfer rate and lower error packet rate, which can greatly improve the shooting quality of component information extraction for the ancient building in Huizhou. The numerical finding for the proposed method enhances the comparison of data rate under better communication quality and comparison of data rate under poor communication quality.

Indicators of accuracy for determining the coordinates of radio emission sources in the short wave direction finding network

Direction finding networks have found application in radio monitoring, radio intelligence and passive radar systems. The operation of the direction-finding network in the short-wave range has a number of distinctive features, namely, long range of direction finders (up to several thousand km) due to ionospheric propagation of radio waves and high sensitivity of narrow-band signal receivers. In addition, the distance between direction finders can be hundreds or thousands of kilometers. Therefore the calculations should be carried out due to the location of the direction finders and radio sources on a spherical surface. In this work, analytical relationships are obtained for calculating the accuracy indicators of the estimation of coordinate information (latitude and longitude) at the output of the direction finding network in a rather general form in relation to the features of the short-wave range. The problem is solved in a geographic coordinate system for an arbitrary number of direction finders (two at least) and with their arbitrary location on the surface of Earth. To carry out a comparative analysis and assess the quality of coordinate information for various options for placing direction finders, it is proposed to display accuracy indicators using working zones (for example, round). The use of working areas allows a visual assessment on the map overall spatial pattern for accuracy indicators direction-finding network. The results of the calculation of working areas direction-finding network shortwave when placing it on the territory of Ukraine in the case of the smallest real errors direction-finding, and a mutual separation distance finders maximum permissible selected. The calculation results reflect the limiting possibilities for the accuracy of determining the coordinates of radio emission sources for such a direction finding network with a minimum number of direction finders (3 or 4). The given method of calculating working zones allows for the implementation of the best accuracy indicators to choose a specific option for placing direction finders on the territory of the country, taking into account the influence of all factors (approach of positions, availability of access roads, conditions for accommodating service personnel, etc.). As an example, the work considers 3 options for the location of direction finders with the maximum separation on the territory of Ukraine. The developed technique can also be used for other passive radar systems with direction finding coordinates, when it is necessary to take into account the spherical form of the Earth. Such a system can include two or more aerial reconnaissance aircraft with direction finders on board, as well as one aircraft or unmanned vehicle that determines coordinates by the method of multiple direction finding on the flight route.

한국공군의 첨단 항공기 도입에 따른 안전관리방안 연구

The purpose of this study is to ensure safety by proactively identifying hazards that could be derived from changes in mission form and environment as the advanced aircraft such as F-35A stealth fighter, KC-330 Multi-role transport and tanker, RQ-4B high altitude unmanned reconnaissance aircraft, etc are introduced that the Republic of Korea Air Force(ROKAF) has never been operated so far. To this end, the safety management methods based on proactive and predictive approaches used in advanced countries(US Air Force, UK Royal Air Force, Royal Australian Air Force) operating aircraft types same or similar things being newly powered by the ROKAF were reviewed. In addition, the direction for improvement of the safety management methods operating in the ROKAF and the measures necessary for establishment of the new safety management techniques to be applied were suggested.

Potential low bias in high-wind drag coefficient inferred from dropsonde data in hurricanes

AbstractUnderstanding momentum exchange at the air-sea interface is important for accurate hurricane predictions and understanding fundamental storm dynamics. One method for estimating air-sea momentum transfer in high winds is the flux-profile method, which infers surface momentum fluxes and the corresponding drag coefficient from mean velocity profiles obtained from either dropsondes or meteorological towers, under the assumption that the boundary-layer wind profile at low altitudes exhibits a logarithmic profile with height. In this study, we use dropsonde data from reconnaissance aircraft, as well as “virtual sondes” from a turbulence-resolving simulation of an intense tropical cyclone, to critically analyze the diagnosis of drag coefficient CD at hurricane-force wind speeds. In particular, the “roll-off” of the drag coefficient, where CD decreases at 10-m wind speeds ¿ 35 m s−1, is called into question based on uncertainty due to relatively low sample size and a lack of robustness of the flux-profile at high winds. In addition, multiple factors appear to favor an underestimate of CD at hurricane-force winds relative to their true values, including uncertainty in the height of recorded dropsonde data, violation of Monin-Obukhov similarity theory near the eyewall, and the short vertical extent of the logarithmic layer. Due to these and other related sources of uncertainty, it is likely that a quantitative limit has been reached in inferring the specific values of u* and CD using the flux-profile method, while at the same time the potential for underestimation may cast doubt on the CD–U10 relationship inferred from this method at high winds.

Re-examining Tropical Cyclone Fullness Using Aircraft Reconnaissance Data

Re-examining Tropical Cyclone Fullness Using Aircraft Reconnaissance Data

日本初のアイウォール貫通型航空機観測により捉えられた2017年台風第21号（ラン）の二重暖気核構造

Upper-tropospheric aircraft reconnaissance was carried out for Typhoon Lan (2017) using a civil jet with a newly developed dropsonde system. This was the first time a Japanese research group observed the inner core of an intense typhoon using dropsondes. This paper describes the warm-core structure in the eye and the associated thermodynamic and kinematic features of the eyewall. During two days of reconnaissance, the typhoon preserved its peak intensity in an environment with strengthening vertical shear. The dropsondes captured a double warm-core structure with a higher perturbation temperature in the middle and upper troposphere, which persisted between the two missions. The two warm cores showed a difference in equivalent potential temperature (θe) of more than 10 K, suggesting different air origins. Saturation-point analysis suggests that the air observed in the upper warm core was entrained from the eyewall. The eyewall updraft in the left-of-shear semicircle had a two-layer structure with a higher θe and lower absolute angular momentum on the inner side of the updraft core. Analyses of the saturation point and parcel method suggest that the warmer air with a θe exceeding 370 K on the inner side of the updrafts originated from the eye boundary layer and was eventually transported into the upper warm core. These results led us to hypothesize that the vertical transport of high-θe air from the eye boundary layer contributed to the continuous eye warming in the upper troposphere against the negative effect of strengthening environmental wind shear on storm intensity. This study demonstrates the significance of eyewallpenetrating upper-tropospheric reconnaissance for monitoring the warm-core structure in the present situation, where accurate measurements of both humidity and temperature for calculating θe can only be made with dropsonde-type expendables.

The manifestation of coordination failures in service triads

PurposeThis paper aims to investigate governance in service triads, specifically studying significant steering and connecting coordination failures, to reveal typically hidden characteristics and consequences.Design/methodology/approachThis study focuses on coordination functions and activities between a buyer (a government department), a customer (a military service) and two service providers. Rich data on these normally confidential service ties are drawn from an official report into the causes of a fatal accident involving a UK reconnaissance aircraft and specifically from the evidence presented regarding the earlier development of its complex safety case. The authors also analysed a range of additional secondary data sources.FindingsThe authors examine the sources, drivers and manifestation of coordination failures. The authors uncover a series of coordination failures driven from the bridge position, revealing that while bounded rationality and opportunism influenced steering coordination failures, connecting coordination failures were associated with knowledge asymmetry, dyadic inertia and unethical practices.Practical implicationsOrganisations and governments delivering complex projects and knowledge-intensive professional services should guard against outsourcing the “coordination” activity to a third party, thereby relinquishing the bridge position. Handing over the bridge position to an integrator would leave the client vulnerable to coordination dysfunctions such as bounded rationality, opportunism, knowledge asymmetry, dyadic inertia and unethical practices.Originality/valueThe study links the previously separate research streams of service triads and inter-organizational coordination. While extant research pays attention to mainly positive control functions, this study focuses on all three actors in two (failed) service triads – and highlights the impact of coordination activities and failures.

ФРАНЦУЗЬКІ АВІАТОРИ ТА АВІАЦІЙНІ ЧАСТИНИ НА СХІДНОМУ ФРОНТІ ПЕРШОЇ СВІТОВОЇ ВІЙНИ

One of the forms of military assistance that France provided to allied Russia during the First World War was the deployment of aviation specialists. During 1916-1917, two air contingents were sent to Russia. The first of them numbered only 30 people and was not the consolidated unit – the aviators who were part of it were distributed among the various Russian aviation units. This approach proved unsuccessful: a few weeks after the arrival of the French pilots, they had to move from one section of the front to another, more important one. This negative experience was taken into account when forming the second contingent. In 1917, France sent two full-fledged air squadrons to Russia, as well as the necessary units to provide a total of almost 270 people. The second air contingent operated on the South-Western Front (and the adjacent section of the Romanian Front). The choice of the South-Western Front was conditioned by the fact that in 1917 the largest operations of the Russian army were planned here.The French aviation contingent was armed with quite modern fighter planes and reconnaissance aircraft. The personnel was largely formed at the expense of volunteers, which contributed to the high morale and activity of the aviation contingent. This is evidenced by the results of a series of air battles conducted by French pilots. At the same time, the fighting of the French squadrons was affected by the general situation at the front. They suffered significant losses in equipment during the retreat in July 1917. However, despite the collapse of the Russian army in the autumn of 1917, French aviators continued their hostilities until the armistice between the Bolshevik government of Russia and Germany. Their further stay on the territory of Ukraine, which lasted until February 1918, was characterized by the desire to maintain neutrality and ensure repatriation.

Field measurements of Tropical Storm Aere (1619) via airborne GPS‐dropsondes over the South China Sea

AbstractIn the past two decades, Global Positioning System (GPS)‐dropsonde has been developed as an effective tool to explore the internal structures of tropical cyclones (TCs). To facilitate the short‐term forecasting of the TCs' intensity and track over the Hong Kong Flight Information Region (HKFIR), Hong Kong Observatory (HKO) launched in 2016 reconnaissance campaigns by using airborne GPS‐dropsondes. On October 7, 2016, when Tropical Storm Aere (1619) got close to Hong Kong, 10 dropsondes were released from a reconnaissance aircraft at 10 km altitude to sample the TC at different storm‐relative positions. This offers a valuable opportunity to examine the functional performance of the dropsonde system. In the study, the validity of dropsonde measurements is first examined, mainly by comparing the results with those collected contemporaneously by radiosonde balloons from a nearby station. Observational results of the structural characteristics are then discussed. Evidence was observed for the invasion of the storm's inner structure by background atmosphere, which was unfavourable for the intensification of Aere in the following time. Meanwhile, a height‐resolving model of the TC's pressure field is proposed. It is also observed that the parachutes of two dropsondes failed to deploy during the descending period. The results presented here are expected to provide useful information for a better understanding of the in‐situ measurements from dropsondes deployed over the HKFIR, and for advancing the knowledge on TC's inner structure.

Adjoint-Derived Impact of Assimilated Observations on Tropical Cyclone Intensity Forecasts of Hurricane Joaquin (2015) and Hurricane Matthew (2016)

AbstractThe adjoint-derived observation impact method is used as a diagnostic to derive the impact of assimilated observations on a metric representing the forecast intensity of a tropical cyclone (TC). Storm-centered composites of observation impact and the model background state are computed across 6-hourly analysis/forecast cycles to compute the composite observation impact throughout the life cycle of Hurricane Joaquin (2015) to evaluate the impact of in situ wind and temperature observations in the upper and lower troposphere, as well as the impact of brightness temperature and precipitable water observations, on intensity forecasts with forecast lengths from 12 to 48 h. The compositing across analysis/forecast cycles allows for the exploration of consistent relationships between the synoptic-scale state of the initial conditions and the impact of observations that are interpreted as flow-dependent interactions between model background bias and correction by assimilated observations on the TC intensity forecast. The track of Hurricane Matthew (2016), with an extended period of time near the coasts of Florida, Georgia, and the Carolinas, allows for a comparison of the impact of aircraft reconnaissance observations with the impact of nearby overland rawinsonde observations available within the same radius of the TC.

A Consensus Approach for Estimating Tropical Cyclone Intensity from Meteorological Satellites: SATCON

ABSTRACTA consensus-based algorithm for estimating the current intensity of global tropical cyclones (TCs) from meteorological satellites is described. The method objectively combines intensity estimates from infrared and microwave-based techniques to produce a consensus TC intensity estimate, which is more skillful than the individual members. The method, called Satellite Consensus (SATCON), can be run in near–real time and employs information sharing between member algorithms and a weighting strategy that relies on the situational precision of each member. An evaluation of the consensus algorithm’s performance in comparison with its individual members and other available operational estimates of TC intensity is presented. It is shown that SATCON can provide valuable objective intensity estimates for poststorm assessments, especially in the absence of other data such as provided by reconnaissance aircraft. It can also serve as a near-real-time estimator of TC intensity for forecasters, with the ability to quickly reconcile differences in objective intensity methods and thus decrease the uncertainty and amount of time spent on the intensity analysis. Near-real-time SATCON estimates are being provided to global operational TC forecast centers.

Evidence of unidentified «aeroplanes» in Viciebsk guberniya in 1914–1915: a look through the prism of the era

The article is devoted to the Belarusian anti-aeroplane campaign of 1914–1915, i. e. to counteract the speculation that rapidly spread across the provinces about the appearance of enemy aeroplanes and airships behind the front line. Documents and materials from the National Historical Archive of Belarus, State Archive of Russian Federation and State Archive of Pskov Region funds concerning the verification of reports about aeroplanes in the Viciebsk guberniya are reviewed and analyzed. It is noted that the mass appearance of rumors about airplanes took place in the Viciebsk guberniya in several stages: at first, according to the descriptions of eyewitnesses, they were only a shapeless flying mass, and later information about them became more specific and even reports about their descent were checked. All this led to a serious problem – under the influence of panic, cases of superstitious soldiers firing at their own planes became more frequent, which could directly affect the course of military operations, in particular, the supply of advanced weapons and the latest intelligence. It is also important to consider the impact of the issue on the rear, which resulted primarily in the population fanning rumors directed against rich landowners, and the growth of social tension as a result. It is shown that the local anti-aeroplane campaign had a Baltic and Pskov «scurf», naturally complementing the actions of local authorities and the population. It is concluded that most of the reports from the Viciebsk guberniya and its borders, cited in the documents, do not describe observations of real combat or reconnaissance aircraft of the enemy, but only give an incorrect interpretation by eyewitnesses of any natural or astronomical phenomena as well as your own aeroplanes that occurred due to phobic images of new equipment in the eyes of the local population, reinforced by military hysteria and militaristic psychosis.

UAV strike threat protection system using neural network analysis units

Unmanned aerial vehicles (UAVs), which have proved their ability to conduct aerial reconnaissance much more effectively than manned aircraft and perform other combat support tasks by striking at the enemy, became one of the means of new weapons in military conflicts of the late 20th and early 21st centuries. Today it is impossible to imagine an army without an UAV, because they point to the target, bring artillery, adjust fire, transmit intelligence directly to the headquarters, and most importantly, save the lives of fighters. This paper presents an analysis of the use of UAVs for photo and video shooting in the interests of intelligence, which as a result gives large arrays of images in real time. The system of pattern recognition on the ground, recognition of speech information, quick mapping of the area before Bonwit operations, face recognition in the interests of identification, recognition of radio signals under conditions of adversary's use of active interference - this is not a complete list of tasks by which they can find application of the properties of neural networks and neural network analysis. Unmanned reconnaissance aircraft together with manned and space reconnaissance form the reconnaissance triad. Military experts from developed countries of the world believe that in a modern combat situation, reconnaissance UAVs can more effectively and efficiently, in comparison with reconnaissance pilots, solve airborne reconnaissance missions. This reduces the time to bring the received intelligence information to the appropriate governing body. The success of using an UAV depends not only on the quality of the devices themselves, but also on the training of pilots, the model of combat use used, compliance with operating conditions, routine maintenance of the resource, that is, the availability of service support and a repair base, as well as the necessary conditions for storage and mobility UAV crew when moving along the front. The analysis allows us to conclude that the role of reconnaissance unmanned aircraft in the aerial reconnaissance system for operational support in reconnaissance is growing.

QUADCOPTER FLIGHT CONTROLLER DESIGN ON THE OTHTT SYSTEM PROTOTYPE (OVER THE HORIZON TARGET TRACKING)

The development of drone technology at this time is very rapid with a variety of functions and uses, including use in the military sphere. In this study, researchers made a prototype of a drone that can identify the target while determining the latitude and longitude of the target. This research was carried out because the Navy had defense equipment, a ship that had the ability to shoot missiles. This missile possession is important because in addition to being a deterence effect, modern naval battles are dominated by long-distance warfare or Over The Horizon. In missile fire, accurate intelligence is needed to determine the position of the target. So far what has been done is sending submarines or reconnaissance aircraft. This is very inefficient because in addition to requiring large resources, the risk of losing personnel and material is also very large. To realize this, researchers create a system that can overcome existing limitations. The focus of research conducted by researchers at this time is to create a flight controller on the drone. This system uses the Arduino Mega 2560 PRO Embed as a flight controller that is connected to other parts such as batteries, brushless motors, ESC (Electronic Speed Control) and propeller so that it can move the drone according to operating needs. At the time of testing, vertical flight test, horizontal flight test, pitch and roll control test and altitude control test were carried out. Meanwhile, to get stability when flying, then you need the right PID settings on the flight controller. Based on the test results obtained Kp = 7.5, Ki = 0.045 and Kd = 24, so that the drone can fly stably. From the test results it can also be concluded that the drone has been able to carry out its basic functions to fly, approach the target and return to the ground station after carrying out the mission.
 Keywords: Drone, Control, Stability.

DESIGN AND DEVELOPMENT OF TARGET DESIGN WITH BASIC IMAGE PROCESSING ON OTHTT SYSTEM PROTOTYPE (OVER THE HORIZON TARGET TRACKING)

Based on Law No. 34 of 2004, the Indonesian Navy has the main task of maintaining state sovereignty in territorial waters. Various attempts have been made to strengthen the Navy so that it can carry out these basic tasks, one of which is to add defense equipment, namely ships that have the ability to shoot missiles. This missile possession is important because in addition to being a deterence effect, modern naval battles are dominated by long-distance warfare or Over The Horizon. In missile fire, accurate intelligence is needed to determine the position of the target. So far what has been done is sending submarines or reconnaissance aircraft. This is very inefficient because in addition to requiring large resources, the risk of losing personnel and material is also very large. Based on this, the researcher makes a prototype of a vehicle that can identify and determine the position of a target. The vehicle in the form of a UAV is equipped with GPS, altimeter, compass and camera. This equipment will provide data in the form of UAV latitude and longitude position, UAV height, UAV heading and UAV camera angle. From the four data above, the ground station on themissile carrier gets the target's latitude and longitude data based on image processing. While the method used is color thresholding which can distinguish a target based on its color. After several trials, the UAV has been able to carry out the target positioning up to a height of 15.4 meters. At these heights the target latitude can be determined with a system accuracy of 3.3 meters. To be used as a reference for missile firing, it is necessary to increase the capability of the equipment specifications to reach the desired distance.
 Keywords: Target, Latitude, Longitude, Image Processing.

The Rapid Intensification and Eyewall Replacement Cycles of Hurricane Irma (2017)

Abstract The initiation of a rapid intensification (RI) event for a tropical cyclone (TC) at major hurricane intensity is a rare event in the North Atlantic basin. This study examined the environmental and vortex-scale processes related to such an RI event observed in Hurricane Irma (2017) using a combination of flight-level and airborne radar aircraft reconnaissance observations, microwave satellite observations, and model environmental analyses. The onset of RI was linked to an increase in sea surface temperatures and ocean heat content toward levels more commonly associated with North Atlantic RI episodes. Remarkably, Irma’s RI event comprised two rapidly evolving eyewall replacement cycle (ERC) episodes that each completed in less than 12 h. The two ERC events displayed different secondary eyewall formation (SEF) mechanisms and vortex evolutions. During the first SEF event, a secondary maximum in ascent and tangential wind was observed at the leading edge of a mesoscale descending inflow jet. During the ensuing ERC event, the primary eyewall weakened and ultimately collapsed, resulting in a brief period of weakening. The second SEF event displayed characteristics consistent with unbalanced boundary layer dynamics. Additionally, it is plausible that both SEF events were affected by the stagnation and axisymmeterization of outward-propagating vortex Rossby waves. During the second ERC event, the TC continued to rapidly intensify, which is a stark contrast to the ERC paradigm described in the literature. The differing ERC evolutions appear linked to the vortex response to changing environmental conditions. The results presented here underscore the utility of frequent aircraft reconnaissance observations for an improved understanding of TC dynamics.