Tropical Disturbances Research Articles

South Pacific Tropical Upper Tropospheric Trough (TUTT): variability and impact on the French Polynesian climate

The South Pacific Tropical Upper Tropospheric Trough (TUTT) is an elongated trough that appears in summer monthly averaged maps of the upper tropospheric flow over the ocean. We provide the first comprehensive description of the South Pacific TUTT and analyze its impact on the regional climate with 42 yr (1979-2020) of monthly data from ERA5, radiosonde, rain, keraunic data, and lightning flash rate from the Optical Transient Detector-Lightning Imaging Sensor. The data confirm the presence of the TUTT along a southeast-northwest axis from November to May. Located between 100 and 400 hPa, the TUTT is a cold-core with a relative vorticity minimum and a negative geopotential anomaly. Divergent, ascending flow with high relative humidity is found south and west of the TUTT axis, while convergent, descending flow with low relative humidity is observed north and east of the TUTT axis. The seasonal and long-term impacts of the TUTT on the local and regional climate is dependent on its location and its strength. The existence of the TUTT in the South Pacific is strongly dependent on the El Niño-Southern Oscillation (ENSO) phases, being stronger during a strong negative ENSO phase and disappearing from French Polynesia during a strong positive ENSO phase. Genesis of tropical disturbances east of 180°W is possible only if the TUTT is located east of 120°W. The environmental conditions associated with the TUTT, along with lightning and rainfall data from 3 sites in French Polynesia, show that lightning flash rates are higher during a negative ENSO phase than during a positive ENSO phase.

Contributions of the Large-Scale Environment to the Typhoon Genesis of Faxai (2019)

This study investigated the atmospheric and oceanic contributions to the genesis of Typhoon Faxai in 2019. Our statistical analysis using the tropical cyclone genesis score (TGS) attributed the tropical disturbance that developed into Faxai (Pre-Faxai) to easterly waves (EWs). The EW score evaluated by a grid version of the TGS (Grid-EW) averaged around the occurrence of Pre-Faxai was approximately twice as large as the climatological mean, and it was the second largest value in the past 38 years. The Pre-Faxai area with high Grid-EW scores could be traced back to the eastern North Pacific (ENP) around August 25, 2019. The lower-troposphere environment characterized by high Grid-EW scores was favorable for vortex formation because it provided a containment area for moisture entrained by the developing circulation or lofted by the deep convection therein. The Pre-Faxai area with high Grid-EW scores moved westward due to the background easterly flow over the ENP and then entered the western North Pacific (WNP). The Typhoon Intensity Forecast Scheme (TIFS) showed that the important environments for its genesis were ocean conditions and the vertical wind shear. The oceanic conditions contributed to the development of Pre-Faxai as it traveled over the WNP. The enhancement of vertical wind shear and subsequent suppression of the development of Pre-Faxai were caused by the lower-troposphere easterly winds associated with high EW scores. They were also caused by upper-troposphere westerly winds associated with an upper cold low northwest of Pre-Faxai. When the vertical shear decreased with weakening of the upper cold low, Pre-Faxai reached tropical storm intensity on September 4. Therefore, TGS and TIFS detected Pre-Faxai 10 days before the typhoon arose, which indicates that monitoring environmental factors such as EW and vertical wind shear are important for disaster prevention.

Analysis of Multi-Scale Hydrometeorological Triggering Flash Flood Event of the 13 July 2020 in North Luwu, South Sulawesi

During 12-13 July 2020, heavy rainfall had caused Masamba, Rongkong, and Rada rivers to overflow, causing flash floods in the North Luwu regency. This event resulted in many casualties; at least 38 people died and displaced thousands of people. This study presents an analysis of the multi-scale hydrometeorological settings that led to the development of these intense storms in the North Luwu fl ood. Boreal Summer Intraseasonal Oscillation (BSISO) Normalized PC2 has entered phases 4 and 5 on 7 - 13 July 2020, associated with cloud growth in the Indonesian Maritime Continent. Besides, the sub-seasonal tropical disturbance of Kelvin Waves is convectively active in the Sulawesi region on 7 - 13 July, increasing the intensity of rainfall at the location. Sea Surface Temperatures (SSTs) were in warmer conditions in the Bonne Bay region south of North Luwu, supplying more water vapor into the atmosphere. The easterly wind (Australian monsoon) enhanced the diurnal cycle of the water vapor mass movement from Bonne Bay to the highlands of North Luwu. From the observation of GPM satellite imagery, the accumulation of spatial rainfall on July 12 and 13 was concentrated in the eastern region of Sulawesi, which reached >150 mm/day and >50 mm/day in North Luwu.

Climatology of Frequency, Life Period, Energy and Speed for Tropical Disturbances and Cyclones over the Bay of Bengal

Tropical disturbances and cyclones are regularly formed at the Bay of Bengal basin. There are some common traits in them, albeit each one of them is unique. Discerning climatology for the basic features of any tropical cyclone is useful in numerous ways. This research has attempted to find a climatology for frequency, life period, energy, and speed for the tropical cyclones formed at the Bay of Bengal over a period of 31 years – from 1990 to 2020. The results elicit that there are marked changes in these aforementioned features. The total frequency, accumulated duration, and combined energy have escalated over the years. To be precise, these changes have taken effect more rigorously for the Post-monsoonal tropical cyclones. The overall translational speed has slightly diminished in recent years, except for the translational speed of cyclones formed during Pre-monsoon. These changes will have major ramifications on the lives and livelihoods of people, more so for those living in coastal areas. Hence, necessary actions are required to cut the probable losses and damages. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 10(1), 2021, P 23-31

The Impact of a Midlevel Dry Airflow Layer on Deep Convection in the Pre-Gabrielle (2013) Tropical Disturbance on 4–5 September

AbstractOn 4–5 September 2013, a relatively shallow layer of northerly dry airflow was observed just west of the core deep convection associated with the low-level center of the pre-Gabrielle (2013) tropical disturbance. Shortly thereafter, the core deep convection of the disturbance collapsed after having persisted for well over 24 h. The present study provides an in-depth analysis of the interaction between this dry airflow layer and the pre-Gabrielle disturbance core deep convection using a combination of observations, reanalysis fields, and idealized simulations. Based on the analysis, we conclude that the dry airflow layer played an important role in the collapse of the core deep convection in the pre-Gabrielle disturbance. Furthermore, we found that the presence of storm-relative flow was critical to the inhibitive effects of the dry airflow layer on deep convection. The mechanism by which the dry airflow layer inhibited deep convection was found to be enhanced dry air entrainment.

Ground Observation of Negative Sprites Over a Tropical Thunderstorm as the Embryo of Hurricane Harvey (2017)

AbstractWe report on the observation of six red sprites produced by negative cloud‐to‐ground (CG) lightning strokes in a tropical thunderstorm that later evolved into Hurricane Harvey (2017). Most of these sprites conformed to the morphological characteristics of typical negative sprites previously observed over continental thunderstorms. The sprite‐producing CG strokes were located in or near the deep convection region and their impulse charge moment changes all exceeded −500 C‐km. During the sprite generation stage, the CG lightning stroke rate declined and the tropical thunderstorm structure appeared stable. Our analysis suggests that tropical marine meteorological systems, such as tropical disturbances, depressions, and thunderstorms are more likely to be main production systems of negative sprites. The frequent occurrence of 18 gigantic jets produced by the same thunderstorm further indicates that the thundercloud charge structures of sprite‐producing oceanic thunderstorms are significantly different from that of continental thunderstorms.

Estimating zonal Ekman transport along coastal Senegal during passage of Hurricane Fred, 30\u201331 August 2015

We examine the role of zonal Ekman transport along the coast of Senegal on 30 August 2015 when the tropical disturbance associated with Tropical Cyclone Fred was located to the west of Senegal, causing considerable coastal damage in the southern Senegal–Gambia domain (south of Dakar, Senegal). Ten-meter winds from three Weather Research and Forecast model simulations were used to estimate zonal Ekman transport, when the maximum values were found on 30 August. These simulations are in agreement with limited coastal observations showing increasing southerly wind speeds during 30 August but overestimated relative to the three coastal stations. The strong meridional winds translate into increased zonal Ekman transport to the coast of Senegal on 30 August and are likely responsible for some coastal flooding. Ekman transport along the coast contributes significantly to the water-level variations during swell events. The use of a coupled ocean model will improve the estimates of Ekman transport along the Guinea-Senegalese coast. The observed damage suggests that artificial and natural barriers (mangroves) should be strengthened to protect coastal communities in Senegal.

Relative contributions of environmental factors on different time scales to tropical cyclogenesis over the eastern North Pacific

AbstractThe present study investigates relative contributions of large‐scale environmental factors on interannual, intraseasonal, and synoptic time scales to tropical cyclone (TC) genesis over the eastern North Pacific (ENP) during TC seasons of 1979–2013 from the perspective of TC genesis time and position. Conditional sorting displays that the synoptic component is more important in the contribution of lower‐level vorticity and mid‐level specific humidity to TC genesis compared to interannual and intraseasonal components. The convection contributes to TC genesis mainly through synoptic and intraseasonal components. Synoptic‐scale tropical disturbances mainly obtain barotropic eddy energy from climatological mean flows. TCs appear most frequently when vertical wind shear anomalies are between 0 and 3 m s−1, in which interannual and intraseasonal westerly wind anomalies make a positive contribution due to climatological easterly wind shear. When total SST exceeds 28°C, the interannual component of sea surface temperature (SST) is positive, and then it has a positive contribution to TC genesis. In addition, there are notable differences of relative contributions of different time scale components of large‐scale factors among the ENP, northern Atlantic Ocean, and western North Pacific.

Moisture Mode Theory’s Contribution to Advances in our Understanding of the Madden-Julian Oscillation and Other Tropical Disturbances

Moisture Mode Theory’s Contribution to Advances in our Understanding of the Madden-Julian Oscillation and Other Tropical Disturbances

Effects of Atlantic and Pacific Ocean Teleconnections on Corn Yield East of Puebla, Mexico: Case Studies

Ocean-atmospheric interactions have effects at different scales; forming microclimates, which can explain variations with climatic or natural anomalies, between meteorological processes. This research analyzes and identifies the relationship of the teleconnection hydrometeorological effects, which determine the distribution of precipitation in corn yield. The data were used from a semi-structured interview directed to corn producers, where seven years of case studies were identified for the eastern region of the state of Puebla, Mexico. The Graphics were made with “pentad scale distribution”. The results show the importance of geographical location for agricultural activities in relation to a valley with altitudinal gradient. In addition, the corn growth cycle is associated with tropical disturbances from east Puebla region as well as Hurricane activity. It was identified that the relationship of teleconnections and the distribution of rainfall are main factors that influence in the development good or bad of corn, showed in the yields, where the different phases of ENSO (EL NIÑO Southern Oscillation) have a differentiated impact on the availability of precipitation in this case studies of the present investigation.

Dissipation process of summer tropical easterly waves in Western North Pacific

Tropical easterly waves are common features in the trade wind zones and they are important sources of tropical cyclogenesis. Despite numerous studies have analyzed the genesis and maintenance of easterly waves in the Western North Pacific, few had examined their dissipation processes. Focusing on tropical easterly waves during May-September of 1979–2017, this study shows that most of the easterly waves (∼70 %) eventually dissipate when encountering the monsoon trough and associated westerlies, while 22 % were carried northward by the monsoonal southwesterly flows and became recurving disturbances. Less than 10 % of easterly waves propagate across the South China Sea against the prevailing monsoon westerlies and into the Indochina peninsula. The vorticity budget analysis illustrates that total vortex stretching in the lower troposphere is the key factor in propelling the small number of easterly waves westward, suggesting that stronger and more convectively active easterly waves tend to move further into the developed monsoon trough. This echoes the previous observation that tropical disturbances alone have a limited probability in developing into a typhoon, for those disturbances or easterly waves almost always need to interact with the monsoon trough or a monsoon gyre, as well as other intraseasonal features to sustain the organized convection and rotation.

Characteristics and mechanism of vertical coupling in the genesis of tropical cyclone Durian (2001)

The vertical coupling (VC) process and mechanism during the genesis of a tropical cyclone (TC) implied by the weak vertical shear of horizontal wind, one of the key factors impacting TC genesis, constitute important but unanswered fundamental scientific problems. This paper carried out a targeted investigation of this problem through numerical simulation and theoretical analyses. The main conclusions are as follows. Even if TC genesis occurs in a barotropic environment, a VC process still occurs between the trough (vortex) at the middle level and that at the lower level in the TC embryo area. VC mainly occurs at the tropical disturbance (TDS) stage. Only after the VC is accomplished can the tropical depression (TD) organize further by itself and develop into the tropical storm (TS) stage or the stronger tropical typhoon (TY) stage through the WISHE (wind-induced surface heat exchange) mechanism. In the VC process, vortical hot towers (VHTs) play vertical connecting roles and are the actual practitioners of the VC. Through the VHTs’ vertical connections, the middle- and lower-troposphere trough axes move towards each other and realize the VC. VHTs can produce intensive cyclonic vorticity in the lower troposphere, which is mainly contributed by the stretching term. The tilting term can produce a single dipole or double dipole of vorticity, but the positive and negative vorticity pairs offset each other roughly. While the stretching term ensures that the cyclonic rotations of the wind field in the middle and lower levels tend to be consistent, the tilting term acts to uniformly distribute the horizontal wind in the vertical direction, and both terms facilitate the VC of the wind field. With the latent heat of condensation, VHTs heat the upper and middle troposphere so that the 352 K equivalent potential temperature contour penetrates vertically into the 925–300 hPa layer, realizing the VC of the temperature field. While forming cloud towers, VHTs make the ambient air become moist and nearly saturated so that the 95% relative humidity contour penetrates vertically into the 925–400 hPa layer, realizing the VC of the humidity field. Due to the collective contributions of the VHTs, the embryo area develops into a warm, nearly saturated core with strong cyclonic vorticity. The barotropic instability mechanism may also occur during TC genesis over the Northwest Pacific and provide rich large-scale environmental vorticity for TC genesis. The axisymmetric distribution of VHTs is an important sign of TC genesis. When a TC is about to form, there may be accompanying phenomena between the axisymmetric process of VHTs and vortex Rossby waves.

The Seasonal Climatology of Tropical Disturbances and Its Associated Tracks over the Bay of Bengal During 1891-2020

The Bay of Bengal (BoB) is the import basin where tropical cyclone has been forming frequently compared to global perspective. The number and frequency of cyclone over the BoB varies year to year. The recent climatology of tropical cyclone and its track and landfall is very important to minimize casualties and damages. A statistical attempt has been made to know the seasonal number of cyclones over the BoB during 1891-2020 using BMD and IMD cyclone eAtlas data and the most devastating cyclones’ tracks of 1991, 1994, 2007, 2008, 2009 and 2020 are also drawn over the BoB. From the analysis it is found that the total number of cyclones formed over the BoB are 35, 123, 97 and 28 during pre-monsoon, monsoon, post-monsoon and winter seasons respectively whereas 58, 33, 103 and 26 for severe cyclonic storms respectively. The cyclone of all intensity over the BoB follows initially Westward/Northwestwards tracks, some of them re-curved and follows north/northeastwards tracks. Sometimes looping track is also found over the Bay of Bengal. It is also found that some cyclones have completed their journey in the BoB without landfall. This variation of cyclone makes the BoB basin more vulnerable and more challenging for forecasting of tropical cyclones.

The Effect of Non-Seasonal Climate Variations on Extreme Rainfall Events in Early Rainy Season Onset in Southest West Java Province

The region of Tasikmalaya, Garut, and Pangandaran (hereafter mentioned as Southeast West Java Province) experienced extreme precipitation that occurred on September 16, 2016, October 6, 2017, and November 5, 2018, which then followed by flood. The characteristics of these extreme rainfall events need to be communicated to the related disaster management agency and the local citizens as a part of understanding the risks and disaster mitigation. This paper aims to determine the relation between extreme rainfall and non-seasonal climate variations such as Madden Julian Oscillation (MJO), El Niño Southern Oscillation (ENSO), tropical storm, and local circulation that occur simultaneously. Atmosphere and ocean data, including daily rainfall, precipitable water, cloud satellite imagery, wind and sea surface temperature were used. Descriptive statistical analysis, atmospheric dynamics, and physical atmosphere were applied to characterize the event, spatially and temporally. The results showed that the MJO was a non-seasonal factor that always exists in these three early rainy season extreme rainfall events in the region. Other non-seasonal factors such as interaction between La Nina and tropical disturbance; La Nina and local circulation; and El Nino and local circulation also affected the extreme rainfall events. We conclude that the intra-seasonal climate variation of MJO and inter-annual climate anomaly of La Nina/ El Nino, tropical storm, and local circulation are among the weather generators for extreme rainfall during early rainy season (September to November) in the Southeast West Java Province.

Birth of a storm

Tropical Cyclones How do tropical cyclones form from smaller tropical disturbances? Only a fraction of tropical disturbances evolve into the highly energetic monsters called cyclones or hurricanes, so being able to predict which ones will make the transition could help improve forecasts. Ruppert et al. show that infrared radiation trapped by the deep clouds found in these precursor storms causes local heating that is key for their development into full-blown tropical cyclones. Incorporating these insights about the role of cloud-radiation feedback into numerical models could lead to better representations of tropical cyclone genesis and intensification. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.2013584117 (2020).

The critical role of cloud–infrared radiation feedback in tropical cyclone development

The tall clouds that comprise tropical storms, hurricanes, and typhoons-or more generally, tropical cyclones (TCs)-are highly effective at trapping the infrared radiation welling up from the surface. This cloud-infrared radiation feedback, referred to as the "cloud greenhouse effect," locally warms the lower-middle troposphere relative to a TC's surroundings through all stages of its life cycle. Here, we show that this effect is essential to promoting and accelerating TC development in the context of two archetypal storms-Super Typhoon Haiyan (2013) and Hurricane Maria (2017). Namely, this feedback strengthens the thermally direct transverse circulation of the developing storm, in turn both promoting saturation within its core and accelerating the spin-up of its surface tangential circulation through angular momentum convergence. This feedback therefore shortens the storm's gestation period prior to its rapid intensification into a strong hurricane or typhoon. Further research into this subject holds the potential for key progress in TC prediction, which remains a critical societal challenge.

Response of the Asian Summer Monsoon Precipitation to Global Warming in a High-Resolution Global Nonhydrostatic Model

AbstractThis study examined the responses of Asian monsoon precipitation to global warming on the regional scale, focusing on monsoon westerlies and monsoon trough. This is because the Asian monsoon precipitation is closely associated with tropical disturbances. To reproduce convective precipitation and tropical disturbances, this study used outputs of a high-resolution climate simulation. Two sets of approximately 30-yr simulations under present-day (control) and warmer climate conditions (global warming) were conducted by the 14-km Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with explicitly calculated convection. For understanding the spatial pattern of future precipitation changes, a further set of a 5-yr simulation [sea surface temperature (SST) + 4 K] was also conducted. Overall, the Asian summer monsoon was well simulated by the model. Precipitation increased as a result of global warming along the monsoon trough, which was zonally elongated across northern India, the Indochina Peninsula, and the western North Pacific Ocean. This increased precipitation was likely due to an increase in precipitable water. The spatial pattern of the increased precipitation was associated with enhanced cyclonic circulations over a large area along the monsoon trough, although it was difficult to determine whether the large-scale monsoon westerly was enhanced. This enhancement can be explained by future changes in tropical disturbance activity, including weak tropical cyclones. However, over part of South Asia, circulation changes may not contribute to the increased precipitation, suggesting regional characteristics. The regional increase in precipitation along the monsoon trough was mostly explained by the uniform increase in SST, whereas SST spatial patterns are important over some regions.

Vegetation and climate change during the Medieval Climate Anomaly and the Little Ice Age on the southern Cape coast of South Africa: Pollen evidence from Bo Langvlei

This paper presents continuous, high resolution fossil pollen and microcharcoal records from Bo Langvlei, a lake in the Wilderness Embayment on South Africa’s southern Cape coast. Spanning the past ~1300 years and encompassing the Medieval Climate Anomaly (MCA; c. AD 950–1250) and the Little Ice Age (LIA; c. AD 1300–1850), these records provide a rare southern African perspective on past temperature, moisture and vegetation change during these much debated periods of the recent geological past. Considered together with other records from the Wilderness Embayment, we conclude that conditions in the region during the MCA chronozone were – in the context of the last 1300 years – likely relatively dry (reduced levels of Afrotemperate forest pollen) and perhaps slightly cooler (increased percentages of Stoebe-type pollen) than present. The most significant phase of forest expansion, and more humid conditions, occurred during the transition between the MCA and the most prominent cooling phase of the LIA. The LIA is clearly identified at this locality as a period of cool, dry conditions between c. AD 1600 and 1850. The mechanisms driving the changes observed in the Bo Langvlei pollen record appear to be generally linked to changes in temperature, and changes in the influence of tropical circulation systems. During warmer periods, moisture availability was higher at Bo Langvlei, and rainfall was perhaps less seasonal. During colder periods, precipitation resulting from tropical disturbances was more restricted, resulting in drier conditions. While increased precipitation has been reported during the LIA from Verlorenvlei in the Western Cape as a result of an equatorward displacement of the westerly storm-track at this time, the opposing response at Bo Langvlei suggests that any increased influence of westerlies was insufficient to compensate for the concurrent reduction in tropical/local rainfall in the region.

Study of cyclogenesis of developing and non-developing tropical systems of NIO using NCUM forecasting system

A study has been carried out to understand the similarities and differences in a pair of developing and a non-developing tropical disturbances using NCMRWF Unified Model (NCUM) and the concepts of marsupial theory of tropical cyclogenesis. We focused on two systems, tropical cyclone (TC) Ockhi that attained Very Severe Cyclonic Storm (VSCS) strength, and another system that attained deep depression state, however, subsequently weakened. As per the first hypothesis of Marsupial Paradigm, genesis of both the systems is found to be associated with a westward moving parent disturbance. The pouch region associated with the parent disturbance is identified as a region of enhanced moisture. The vertical profiles of vorticity and convergence show that the pathway of genesis is bottom-up for both the systems. Further analyses have been carried out to understand why the depression failed to attain tropical cyclone strength. It has been evidenced that the vertical wind shear is too high (> 50 ms− 1) in the premises of the depression core during the post-genesis evolution. At the same time, the center of the depression happened to be in the close proximity of a ridge present over central Indian land mass. Additionally, significant evidence of dry air intrusion into the core of the depression is seen in the Total Precipitable Water (TPW) analysis and HYSPLIT back trajectory analysis. Furthermore, the depression immediately interacts with the colder sea water as it moves northwestward. Though the marsupial paradigm of tropical cyclogenesis is applicable in the North Indian Ocean as well, factors such as SST, wind shear, closeness to the land mass may play a critical role in further development of the system.

A Simulation Study of Kelvin Waves Interacting with Synoptic Events during December 2016 in the South China Sea and Maritime Continent

Abstract This study evaluates the model simulation of interaction between convectively coupled tropical disturbances in the South China Sea (SCS) and Maritime Continent (MC). The Model for Prediction Across Scales (MPAS) is used to simulate the major interaction events in December 2016 with a fixed 60-km horizontal resolution and a variable 60–15-km resolution. Compared with an observational analysis, the overall spatial and temporal evolution of simulated rainfall and circulation reveals the capability of MPAS for reproducing equatorial Kelvin waves (KWs), and the interactions with equatorial Rossby waves and off-equatorial mixed Rossby–gravity (MRG)/TD-type waves up to a 5–7-day lead in both fixed 60-km and variable 60–15-km resolutions. Two interaction events are further examined. One involves an MRG/TD wave, prevailing northeasterlies, and a Borneo vortex developed in SCS during 6–11 December. The other involves a KW converging with the easterly trade wind that led to an MRG/TD-type wave and the formation of Typhoon Nock-ten during 16–20 December. The MPAS 60–15-km resolution tends to produce stronger precipitation and more coherent vorticity structures in both interaction events. Increasing the resolution to 15 km contributes to better representation of finer spatial vorticity and rainfall structures.