Onset Of Rapid Intensification Research Articles

Diversity of Tropical Cyclones Rapid Intensification

AbstractThe study investigates the rapid intensification (RI) of tropical cyclones (TCs) in the Northwestern Pacific. We found that rapid changes in the maximum wind speed (Vmax) and the minimum central pressure (Pmin) are not always concurrent. RI cases can be categorized into three types: (a) RIv, only Vmax strengthens rapidly; (b) RIp, only Pmin decreases rapidly; (c) RIpv, rapid changes in Vmax and Pmin occur concurrently. At the onset of RI, RIv‐type TCs exhibit the weakest intensity and the smallest size, with deep convection concentrated in the inner‐core region; RIp‐type TCs are characterized by the strongest cyclone intensity and the largest outer‐core size, with strong convection covering the inner‐ and outer‐core regions; RIpv‐type TCs show moderate intensity, size, and convection distribution. For RIpv, significant strengthening of wind profile is concentrated in the inner‐core region, while for RIp it is more prominent in the outer‐core.

Revisiting Environmental Wind and Moisture Calculations in the Context of Tropical Cyclone Intensification

Abstract Deep-layer vertical wind shear and midtropospheric relative humidity (RH) are explored in and around environments of all intensifying North Atlantic tropical cyclones (TCs) between 1980 and 2021 using reanalysis data. Shear and RH are averaged within the standard environmental annulus of 200–800 km, along with a 100–600-km annulus, and a 0–250-km radius to represent the inner core and TC itself. Distributions of shear and RH at onset along with a time series of evolution from 48 h prior to and after onset of three different intensification rates, slight [5–10 kt (24 h)−1; 1 kt ≈ 0.51 m s−1], moderate [15–25 kt (24 h)−1], and rapid [≥30 kt (24 h)−1], are analyzed. RH is also investigated within different shear environments and in shear-relative quadrants around the storm. While low shear and high RH are found to be most favorable for rapid intensification (RI), there is still a significant probability that RI will occur within less favorable environments. RI cases decrease in 850–200-hPa shear in the 24 h leading up to RI, whereas slight intensification cases increase, which is evident in both the standard shear and a shallower layer at 48 h prior to onset. The inner-core RH for RI increases prior to onset whereas it decreases in the environments. RH analysis by shear-relative quadrants demonstrates the importance of moistening in the upshear-right quadrant before onset of RI. Results indicate the potential value of multiple annuli and shear-relative analysis for moisture and a shallower, 925–400-hPa layer for shear in RI forecasting. Significance Statement The purpose of this study was to investigate the wind and moisture around different areas of intensifying North Atlantic tropical cyclones between 1980 and 2021 using reanalysis data. Average wind and moisture evolve differently around the onset of different intensification rates as well as in different near and far regions from the storm center. These results provide additional indicators that forecasters may consider when examining how the environment around the storm and the situation in the inner core may influence its future intensity.

Onset of Tropical Cyclone Rapid Intensification: Evaluating the Response to Length Scales of Sea Surface Temperature Anomalies

Abstract Predicting the rapid intensification (>15.0 m s−1 increase in 10 m wind speed over 24 h or less) of tropical cyclones (TC) remains a challenge in the broader context of numerical weather prediction largely due to their multiscale dynamics. Ocean observations show that the size and magnitude of sea surface temperature (SST) anomalies associated with cold wakes and ocean eddies play important roles in TC dynamics. In this study, a combination of spectral and structure function analyses is utilized to generate realistic realizations of multiscale anomalies characteristic of the SST conditions in which Hurricane Irma (2017) underwent rapid intensification (RI). We investigate the impact of the length scale of these SST anomalies and the role of translation speed on the variance in RI onset timing. Length-scale-induced convective asymmetries, in addition to the mean magnitude of SST anomalies beneath the storm eye, are shown to modulate the variance in RI onset timing. The size of the associated SST length scales relative to the storm size is critical to the magnitude of variance in RI onset timing, as smaller length scales are shown to lack the spatial extent required to induce preferential convective asymmetries. Storm translation speed is also shown to influence the variance in RI onset timing for larger-length-scale ensembles by altering the exposure time of the eye to these SST anomalies. We find that an interplay between SST-induced convective asymmetries, the magnitude of SST anomalies underneath the eye/eyewall, and storm translation speed play crucial roles in modulating the variance in RI onset timing. Significance Statement The characteristics of sea surface temperature (SST) anomalies in the tropical cyclone near-environment are inherently multiscale in nature as a result of interactions between various dynamical processes in the ocean. Assuming a uniform SST beneath storms in numerical simulations limits the predictability of how air–sea interaction affects the physics of rapid intensification (RI). In this study, the influence of realistic multiscale SST anomalies on RI onset timing is investigated. Our results suggest that the length scale of SST anomalies (in addition to its magnitude) modulate the distribution of convection, creating asymmetries around the RMW that can influence the predictability of RI onset. This effect is further modulated by storm translation speed, with the most prominent impact seen in slow-moving storms.

On the Evolution of Tropopause Layer Cooling Over Tropical Cyclone

AbstractThis study investigates the evolution of tropopause layer cooling (TLC) in an idealized tropical cyclone (TC) as well as the physical mechanism for its formation throughout its lifetime. TLC development is closely related to the intensity change of the TC. The TLC within the inner core of a TC coincides with the occurrence of the TC warm core, whereas the enhancement of the TLC precedes the strengthening of the warm core and the intensification of the TC. This phenomenon suggests that the TLC could be a valuable signal for predicting TC intensity and intensity change. After the onset of rapid intensification, the TLC continues to enhance until it is replaced by the warm core in the inner core region, and it expands to the outer core area and forms a cold ring above the TC outflow layer. The budget analyses of the azimuthal mean potential temperature indicate that advection plays a leading role during all stages of the TLC evolution. Prior to and during the rapid intensification stages of a TC, advection (due to eddy motion) contributes to a cooling of the upper layer above the TC center. This upper‐level cooling is assumed to be related to the uplift caused by overshooting and convection within the inner core. However, the advection process caused by the azimuthal vertical motion dominated the broad cooling across the whole TLC area.

Modulation of Asymmetric Inner‐Core Convection on Midlevel Ventilation Leading up to the Rapid Intensification of Typhoon Lekima (2019)

AbstractThis study investigates how the asymmetric inner‐core convection can modulate midlevel ventilation preceding rapid intensification (RI) of sheared tropical cyclones (TC) based on two numerical experiments of Typhoon Lekima (2019) with warm (CTL) and relatively cool (SSTA0) sea surface temperatures (SSTs). The midlevel ventilation is dominated by an inflow layer between heights of 5 and 10 km in the upshear‐left quadrant, while the convective development in the upshear‐left quadrant acts to induce asymmetric midlevel outflow. These outflows, together with the reduced tilt, lessen the strength of midlevel inflow in a quadrant‐mean sense. Due to a slower boundary‐layer recovery of low‐entropy air over cooler SST, convective development in the upshear‐left quadrant in SSTA0 is suppressed, causing stronger midlevel inflows than that in CTL. A backward trajectory analysis initialized near the RI onset of CTL shows that the TC in CTL is less susceptible to the intrusion of midlevel dry air from left‐of‐shear and upshear‐left quadrant due to the weaker upshear‐left midlevel inflow. Moreover, the intruding air parcels in SSTA0 tend to originate farther away from TC center and lower the eyewall equivalent potential temperature more effectively. A column‐integrated moist static energy (MSE) budget reveals a dramatic increase in MSE in inner‐core region associated with the positive changes of horizontal MSE advection radially outward of enhanced convective rainbands preceding RI onset in CTL. In contrast, the MSE increases more slowly in SSTA0 due to the weaker upshear convection, inducing a less favorable thermodynamic conditions that is potentially responsible for a delayed RI onset.

A Rapid Intensification Warning Index for Tropical Cyclones Based on the Analog Method

AbstractPrediction of the rapid intensification (RI) of tropical cyclones (TCs) remains challenging. In this paper, by using information from the early period following TC formation, the rapid intensification warning index (RIWI) is developed based on the analog method. A 10‐year cross‐validation and data from Hurricane Ida (2021) are used to verify its potential application. Results show that the RIWI can efficiently discriminate between RI and non‐RI storms and has a significant positive correlation with the lifetime maximum intensity (LMI) of the TCs. By using this index, an early warning can be issued ∼30 hr before the onset of RI, which is much earlier than the predictions made using the probabilistic Statistical Hurricane Intensity Prediction System RI index. In addition, by using the RIWI as a predictor, the prediction of LMI provides an early estimate of TC severity.

Characteristics of the upper-level outflow and its impact on the rapid intensification of Typhoon Roke (2011)

In this study, we investigate the structural characteristics of the upper-level outflow and its impact on the rapid intensification (RI) of Typhoon Roke (2011), which experienced an evident outflow transformation from equatorward to poleward during its RI period. The simulations by the Weather Research and Forecasting Model suggest that the upper-level outflow extends from 100 hPa to 150 hPa, with an upper-level warm core at around 150 hPa. The upper-level outflow is enhanced ahead of the typhoon intensification, which is closely related to the outflow-environment interaction. Further analyses indicate that at the early stage of Roke (2011) before the RI, the strong equatorward outflow and the updraft south of the typhoon center are enhanced, favoring the onset of RI. During the RI period, the strong divergent flow near the entrance of the southwesterly jet in front of the upper-level trough, induces the poleward outflow. The eddy flux convergence of angular momentum inward propagated to the typhoon center from a 1000-km radius further enhances the poleward outflow and leads to the development of the vertical motion north of the typhoon center. Then Roke (2011) intensifies rapidly. Simultaneously, the shallow weak positive potential vorticity (PV) anomaly south of the southwesterly jet increases the inner-core PV, favoring the sustained intensification of Roke (2011). After Roke (2011) reaches its peak intensity, its intensity decreases due to the increase of vertical wind shear and the approaching of the southwesterly jet. It is indicated that the interaction between the upper-level outflow and the upper-tropospheric trough has significant influence on the RI of TC.

Variability of Environmental Conditions for Tropical Cyclone Rapid Intensification in the Western North Pacific

Abstract How environmental conditions vary among rapidly intensifying tropical cyclones (TCs) and which factors can help offset negative factors for intensification were examined using a dataset of geostationary satellites and environmental diagnostics. The dataset contains TCs in the western North Pacific from 1995 to 2020. A cluster analysis was performed to classify different morphologies of TC cloud patterns at the onset of rapid intensification (RI). Six clusters were identified, and each cluster had a distinct set of environmental conditions. Three clusters (clusters 1, 3, and 5) had some conditions unfavorable for RI. Cluster 1 TCs were exposed to moderate vertical (850–200 hPa) shear (∼6 m s−1). Relatively high sea surface temperature, a moist environment, and movement toward environments with weak vertical shear, high equivalent potential temperature, and high ocean heat content are potential factors that resist the effects of vertical shear. Cluster 3 TCs were characterized by a large 30-kt wind radius and moderate vertical shear (1 kt ≈ 0.51 m s−1). Large storm size and a moist environment caused by large-scale, strong, low-level convergence are possible factors for vortex resiliency against shear. Cluster 5 TCs were located in a very dry environment. Weak vertical shear and small storm size are factors that may offset the negative effects of dry air and ocean cooling. The results suggest that in the case of RI with negative conditions for intensification, other factors can offset the negative impacts of those conditions and that suitable combinations of environmental conditions and TC structural features are important for RI.

How Frequently Does Rapid Intensification Occur after Rapid Contraction of the Radius of Maximum Wind in Tropical Cyclones over the North Atlantic and Eastern North Pacific?

Abstract The phenomenon that rapid contraction (RC) of the radius of maximum wind (RMW) could precede rapid intensification (RI) in tropical cyclones (TCs) has been found in several previous studies, but it is still unclear how frequently and to what extent RC precedes RI in rapidly intensifying and contracting TCs in observations. In this study, the statistical relationship between RMW RC and TC RI is examined based on the extended best track dataset for the North Atlantic and eastern North Pacific during 1999–2019. Results show that for more than ∼65% of available TCs, the time of the peak contraction rate precedes the time of the peak intensification rate, on average, by ∼10–15 h. With the quantitatively defined RC and RI, results show that ∼50% TCs with RC experience RI, and TCs with larger intensity and smaller RMW and embedded in more favorable environmental conditions tend to experience RI more readily following an RC. Among those TCs with RC and RI, more than ∼65% involve the onset of RC preceding the onset of RI, on average, by ∼15–25 h. The preceding time tends to be longer with lower TC intensity and larger RMW and shows weak correlations with environmental conditions. The qualitative results are insensitive to the time interval for the calculation of intensification/contraction rates and the definition of RI. The results from this study can improve our understanding of TC structure and intensity changes.

Identifying the rapid intensification of tropical cyclones using the Himawari‐8 satellite and their impacts in the Philippines

AbstractTropical cyclones (TCs) that undergo rapid intensification (RI), defined as the upper 95th percentile increase of TC maximum winds in a 24‐hr period, are likely to pose a bigger threat to affected countries in the western North Pacific (WNP). In the Philippines, of the 522 TCs that made landfall from 1951 to 2020, 146 TCs (28%) underwent RI. The majority (82%) of these RI TCs made landfall with at least typhoon intensity, in contrast, only 12% of landfalling non‐RI TCs were typhoons. As the region in the WNP basin where most TC RI occurs (bounded by 123°–140°E and 10°–20°N) is located just to the east of the Philippines, TC RI is typically followed by landfall in the Philippines, many of which do so at their peak intensity. Consequently, results suggest that TCs undergoing RI have a larger impact on the country. Comparison of socioeconomic data between landfalling RI and non‐RI TCs showed that the former tend to have larger impacts in terms of population affected and number of deaths. Hence, determining TC RI at its onset can aid in the early warning of impending TC intensification. In this study, we explored the use of infrared brightness temperature (Bt) satellite data in the identification of TC RI onset. The 10.4 μm window band of the Himawari‐8 satellite was used to measure the amount of active convection that typically occurs before or during the onset of TC RI. RI onset is determined if 100% of pixels within a 50 km radius from the TC centre have Bt ≤225 K. Assessment of satellite Bt data that fit this set of criteria followed by 25 and 35 kt RI in the next 24 hr have positive Pierce skill scores of 0.47 and 0.48, respectively, false alarm ratio of 0.13–0.20, and probability of false detection of 0.02–0.03.

Dynamics and Predictability of Tropical Cyclone Rapid Intensification in Ensemble Simulations of Hurricane Patricia (2015).

Hurricane Patricia (2015) over the eastern Pacific was a record‐breaking tropical cyclone (TC) under a very favorable environment during its rapid intensification (RI) period, which makes it an optimal real case for studying RI dynamics and predictability. In this study, we performed ensemble Kalman filter analyses at Patricia's early development stage using both traditional observations and the Office of Naval Research Tropical Cyclone Intensity (TCI) field campaign data. It is shown that assimilating the inner‐core TCI observations produces a stronger initial vortex and significantly improves the prediction of RI. Analysis of observation sensitivity experiments shows that the deep‐layer dropsonde observations have high impact on both the primary and secondary circulations for the entire troposphere while the radar observations have the most impact on the primary circulations near aircraft flight level. A wide range of intensification scenarios are obtained through two sets of ensemble forecasts initialized with and without assimilating the TCI data prior to the RI onset. Verification of the ensemble forecasts against the TCI observations during the RI period shows that forecast errors toward later stages can originate from two different error sources at early stages of the vortex structure: One is a timing error from a delayed vortex development such that the TC evolution is the same but shifted in time; the other is due to a totally different storm such that there is no moment in time the simulated storm can obtain a correct TC structure.

The Implication of Outflow Structure for the Rapid Intensification of Tropical Cyclones under Vertical Wind Shear

Abstract The implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best track, reanalysis, and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s−1) or strong (>11 m s−1) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (<4.5 m s−1) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.

Hitting time of rapid intensification onset in hurricane-like vortices

Predicting tropical cyclone (TC), rapid intensification (RI) is an important yet challenging task in current weather forecast due to our incomplete understanding of TC nonlinear processes. This study examines the variability of RI onset, including the probability of RI occurrence and the timing of RI onset, using a low-order stochastic model for TC development. Defining RI onset as the first hitting time for a given subset in the TC-scale state space, we quantify the probability of the occurrence of RI onset and the distribution of the timing of RI onset for a range of initial conditions and model parameters. Based on asymptotic analysis for stochastic differential equations, our results show that RI onset occurs later, along with a larger variance of RI onset timing, for weaker vortex initial condition and stronger noise amplitude. In the small noise limit, RI onset probability approaches one and the RI onset timing has less uncertainty (i.e., a smaller variance), consistent with observation of TC development under idealized environment. Our theoretical results are also verified against Monte Carlo simulations and compared with explicit results for a general one-dimensional system, thus providing new insights into the variability of RI onset and helping better quantify the uncertainties of RI variability for practical applications.

On occurrence of rapid intensification and rainfall changes in tropical cyclones over the North Indian Ocean

AbstractThe present study examines the climatological characteristics and possible triggers of rapid intensification (RI) of tropical cyclones (TCs) during 1990–2019 over the North Indian Ocean (NIO). RI is defined as an increase in maximum sustained surface wind speed of 30 knots (15.4 m⋅s−1) or more in a 24 hr duration. In the NIO basin, the threshold of 24 hr intensity change represents the 93rd percentile using a 3‐min sustained wind, while, it is 95th percentile in the Atlantic basin, where 1‐min sustained wind is used. A total of 46 TCs (~38% of the total TCs) have exhibited RI at an average rate of 1–2 TCs year−1 over the NIO. A significant increase of RI‐TCs is seen from the year 2000 onwards over the region. The maximum RI‐TCs occurrence is found in the post‐monsoon season. The majority of TCs (~48%) undergo the RI phase within a 12–24 hr time during the depression stage. About ~35% (26%) of the TCs are retained in the RI phase for the duration of at least 24 hr (36 hr). Most of the RI‐TCs move northwestward (38%) and westward (31%) direction 6 hr before the RI onset with slow/normal translation speeds. During the RI phase, ~72% of TCs travels a distance of ~150–450 km. The TC inner‐core region receives heavy rainfall, and a ~ 3 mm⋅hr−1 increment is noticed 12 hr before the RI to the RI onset. Most of the RI‐TCs made landfall over eastern states of India showing the vulnerability of the regions. Composite analysis demonstrated that higher precipitable water (~55 mm), surface flux (500 W⋅m−2), cyclone heat potential (50–60 kJ⋅cm−2) in the moderate shear (6–8 m⋅s−1) environment favours the RI process. This study highlights the significance of RI‐TCs and triggering conditions for the RI onset over the NIO region.

Intensity and microphysical properties surrounding the rapid intensification in landfalling Super Typhoons over China during the summer and autumn seasons

AbstractThis study investigates the seasonal intensity variabilities of typhoons (TYs) over the western North Pacific that made landfall in the mainland China during the summer (June–August) and autumn (September–November) seasons for the period of 1979–2017. Using the latest climate reanalysis dataset (ERA5), cloud microphysical properties surrounding the rapid intensification (RI) period within the Super TYs (the maximum wind speed exceeding 51 ms−1) are explored to verify whether there exists microphysical precursor in TY intensity change like RI. The major results are: (a) The total number of landfalling TYs in summer is 188, almost two times of the number in autumn, and 25.5% of them can reach the Super TYs category, while 39.5% of the landfalling TYs in autumn became Super TYs with a shorter lifespan. (b) At least half of the summer and autumn cases have undergone the RI before they reached their maximum intensity, and more autumn cases experienced RI as they approached the coastal areas. (c) Snow and rain contents within the vortex exhibit consistent increasing trends during the RI period in both seasons. It is found that ice and snow contents show relatively distinct increase prior to the intensity change during the RI period in autumn cases, while no obvious signal exists in summer. The short‐term microphysical change and the released latent heat may be insufficient in supporting the onset of RI in Super TYs, and the differences of large‐scale environmental conditions between summer and autumn are mostly responsible for the distinct evolutions of intensity and microphysical characteristics in such strong systems.

Boundary Layer Recovery and Precipitation Symmetrization Preceding Rapid Intensification of Tropical Cyclones under Shear

AbstractThis study investigates the precipitation symmetrization preceding rapid intensification (RI) of tropical cyclones (TCs) experiencing vertical wind shear by analyzing numerical simulations of Typhoon Mujigae (2015) with warm (CTL) and relatively cool (S1) sea surface temperatures (SSTs). A novel finding is that precipitation symmetrization is maintained by the continuous development of deep convection along the inward flank of a convective precipitation shield (CPS), especially in the downwind part. Beneath the CPS, downdrafts flush the boundary layer with low-entropy parcels. These low-entropy parcels do not necessarily weaken the TCs; instead, they are “recycled” in the TC circulation, gradually recovered by positive enthalpy fluxes, and develop into convection during their propagation toward a downshear convergence zone. Along-trajectory vertical momentum budget analyses reveal the predominant role of buoyancy acceleration in the convective development in both experiments. The boundary layer recovery is more efficient for warmer SST, and the stronger buoyancy acceleration accounts for the higher probability of these parcels developing into deep convection in the downwind part of the CPS, which helps maintain the precipitation symmetrization in CTL. In contrast, less efficient boundary layer recovery and less upshear deep convection hinder the precipitation symmetrization in S1. These findings highlight the key role of boundary layer recovery in regulating the precipitation symmetrization and upshear deep convection, which further accounts for an earlier RI onset timing of the CTL TC. The inward-rebuilding pathway also illuminates why deep convection is preferentially located inside the radius of maximum wind of sheared TCs undergoing RI.

Effect of Scale-Aware Planetary Boundary Layer Schemes on Tropical Cyclone Intensification and Structural Changes in the Gray Zone

AbstractHorizontal grid spacings of numerical weather prediction models are rapidly approaching O (1 km) and have become comparable with the dominant length scales of flows in the boundary layer; within such “gray-zone”, conventional planetary boundary layer (PBL) parameterization schemes start to violate basic design assumptions. Scale-aware PBL schemes have been developed recently to address the gray-zone issue. By performing WRF simulations of Hurricane Earl (2010) at sub-kilometer grid spacings, this study investigates the effect of the scale-aware Shin-Hong (SH) scheme on the tropical cyclone (TC) intensification and structural changes in comparison to the non-scale-aware YSU scheme it is built upon. Results indicate that SH tends to produce a stronger TC with a more compact inner core than YSU. At early stages, the scale-aware coefficients in SH gradually decrease as the diagnosed boundary layer height exceeds the horizontal grid spacing. This scale-aware effect is most prominent for the nonlocal subgrid-scale vertical turbulent fluxes, in the non-precipitation regions radially outside of the convective rainband, and from the early stage through the middle of rapid intensification (RI) phase. Both the scale awareness and different parameterization of the nonlocal turbulent heat flux in SH reduce the parameterized vertical turbulent mixing, which further induces stronger radial inflows and helps retain more water vapor in the boundary layer. The resulting stronger moisture convergence and diabatic heating near the TC center account for the faster inner-core contraction before RI onset and the higher intensification rate during the RI period. Potential issues of applying these two PBL schemes in TC simulations and suggestions for improvements are discussed.

Application of Unsupervised Learning Techniques to Identify Atlantic Tropical Cyclone Rapid Intensification Environments

AbstractTropical cyclone (TC) track forecasts have improved in recent decades while intensity forecasts, particularly predictions of rapid intensification (RI), continue to show low skill. Many statistical methods have shown promise in predicting RI using environmental fields, although these methods rely heavily upon supervised learning techniques such as classification. Advances in unsupervised learning techniques, particularly those that integrate nonlinearity into the class separation problem, can improve discrimination ability for difficult tasks such as RI prediction. This study quantifies separability between RI and non-RI environments for 2004–16 Atlantic Ocean TCs using an unsupervised learning method that blends principal component analysis with k-means cluster analysis. Input fields consisted of TC-centered 1° Global Forecast System analysis (GFSA) grids (170 different variables and isobaric levels) for 3605 TC samples and five domain sizes. Results are directly compared with separability offered by operational RI forecast predictors for eight RI definitions. The unsupervised learning procedure produced improved separability over operational predictors for all eight RI definitions, five of which showed statistically significant improvement. Composites from these best-separating GFSA fields highlighted the importance of mid- and upper-level relative humidity in identifying the onset of short-term RI, whereas long-term, higher-magnitude RI was generally associated with weaker absolute vorticity. Other useful predictors included optimal thermodynamic RI ingredients along the mean trajectory of the TC. The results suggest that the orientation of a more favorable thermodynamic environment relative to the TC and midlevel vorticity magnitudes could be useful predictors for RI.

Understanding the characteristics of rapid intensity changes of Tropical Cyclones over North Indian Ocean

North Indian Ocean (NIO), which comprises of Bay of Bengal (BoB) and Arabian Sea (AS) basins, is one of the highly potential regions for Tropical Cyclones (TCs) in the world. Significant improvements have been achieved in the prediction of the movement of TCs, since the last decade. However, the prediction of sudden intensity changes becomes a challenging task for the research and operational meteorologists. Hence, the present study focuses on finding the climatological characteristics of such intensity changes over NIO regions. Rapid Intensification (RI) is defined as the 24-h maximum sustained surface wind speed rate equal to 30 knots (15.4 ms−1). The results suggest that the TCs formed over the NIO basin are both seasonal and basin sensitive. Since 2000, a significant trend is observed in RI TCs over the basin. At least one among three cyclones getting intensified is of RI category. More number of RI cases have been identified in the BoB basin than the AS. The post-monsoon season holds more RI and rapid decay cases, with 63% and 90% contribution. Most of the TCs are attaining RI onset in their initial stage. Further, India is receiving more landfalling RI TCs, followed by Bangladesh and Oman. The east coast of India, Tamil Nadu, and Andhra Pradesh are the most vulnerable states to these RI TCs. The cyclogenesis locations associated with RI cases hold higher moisture, and sea surface temperature as compared to the Non-RI cases.

Nonlinear Forecast Error Growth of Rapidly Intensifying Hurricane Harvey (2017) Examined through Convection-Permitting Ensemble Assimilation of GOES-16 All-Sky Radiances

AbstractThe dynamics and predictability of the rapid intensification (RI) of Hurricane Harvey (2017) were examined using convection-permitting initialization, analysis, and prediction from a cycling ensemble Kalman filter (EnKF) that assimilated all-sky infrared radiances from the Advanced Baseline Imager on GOES-16. The EnKF analyses were able to evolve the various scales of the radiance fields associated with Harvey close to those observed, including those associated with scattered individual convective cells before the onset of rapid intensification (RI) and the organized vortex-scale convective system during and after RI. This was true for more than 3 days of a continuous assimilation cycling. Deterministic forecasts initialized from the EnKF analyses captured the rapidly deepening intensity of Harvey more than 24 h prior to its onset. To explore the predictability of Harvey’s intensity during RI, ensemble probabilistic forecasts and sensitivity analyses were conducted. It was found that significant ensemble spread growth was induced by initial perturbations individually in either the wind or moisture fields. The nonlinear interactions between wind and moisture perturbations further limited the predictability of the intensification process of Harvey by increasing the uncertainty in the simulated wind and moisture distributions and modifying the convective activity and its feedback on vortex flow. This study highlights both the importance of better initializing the dynamic and moisture state variables simultaneously and the potential contribution of satellite all-sky radiance assimilation on constraining them and their associated convective activity that impacts RI of tropical cyclones.