North Pacific Basin Research Articles

The impact of freshening over the Antarctic Ocean on Pacific Decadal Oscillation

<p>The profound influence of the Antarctic Ocean freshening on the Pacific Decadal Oscillation (PDO) is investigated in this study by utilizing a series of fully coupled ocean-atmosphere 400-year-modeling experiments. The simulated results derived from the Fast Ocean-Atmosphere Model (FOAM) can reasonably identify the spatial pattern and time period (10–20 years and 20–50 years) of the observed PDO with slightly weak amplitudes. In the sensitivity experiment (Southern Ocean Water Hosing), 1.0 Sv (Sverdrup, 1Sv = 1.0 × 106 m<sup>3</sup>/s) freshwater flux is uniformly imposed over the Antarctic Ocean for 400 years. As a response to this Antarctic Ocean freshening, the Tropical Pacific Ocean displays a normal “La Niña pattern”, while the low-frequency variability within the North Pacific Ocean is much weakened. Preserving the PDO’s spatial pattern, the multidecadal (20–50 years) magnitude becomes weak and shifts toward higher frequency. In contrast, the decadal magnitude of the PDO (10–20 years) is slightly reinforced and also shifts towards higher frequency. Dynamical analysis indicates that the shortening of the PDO multidecadal variability is mainly caused by the acceleration of the first-baroclinic-mode Rossby waves. The spreading of the fresh anomalies and associated increasing stratification in the North Pacific Ocean result in the shortening of the long Rossby wave propagation to cross the subtropical North Pacific basin. A heat budget analysis further shows that the upper-ocean thermodynamic variability in relationship to the stratification oscillation in the North Pacific Ocean is mainly associated with the anomalous behaviors of the meridional advection, heat flux and ocean mixing.</p>

Increased tropical cyclone intensification time in the western North Pacific over the past 56 years

It has been projected that the influence of anthropogenic climate change on tropical cyclone (TC) intensity could be detected by the end of the century although significant increasing trends in TC intensity metrics have been found based on the currently available historic records. The human influences on TC intensity have been debated for about two decades because of a lack of quantitative assessment of the contributions of large-scale environmental factors and track shifting. As an extension of a previous study, we show that the observed rise in the percentage of intense TCs in the western North Pacific basin over the past 56 years resulted from the combined influence of the track shifting and temporary changes in environmental factors. The influence of environmental factors was primarily owing to the decrease of environmental vertical wind shear and the warming of sea surface temperature (SST). While a small part of the observed rise in the percentage of intense TCs resulted from SST warming, the track shifting increased the TC intensification time by 18.2% (11.3 h) over the past 56 years, accounting for more than one-third of the observed percentage increase in intense TCs. Since track shifting is also projected in the global warming experiments, this study suggests that anthropogenic climate change may intensify TCs by shifting TC prevailing tracks.

A tropical cyclone intensity prediction model using conditional generative adversarial network

This study proposes a model for predicting tropical cyclone (TC) intensity evolution by utilizing the conditional Wasserstein generative adversarial network with gradient penalty (CWGAN-GP) for TC-related risk assessment. The TC intensity evolution is treated as a random variable, which is the output of a nonlinear system conditioned on a series of input random variables that characterize both the TC state (e.g., translation speed and potential intensity) and environment (e.g., vertical wind shear and ocean mixed layer depth). To represent the high-dimensional non-Gaussian probabilistic characteristics, the system is modeled by the CWGAN-GP and calibrated concerning the dataset of 1010 historical TCs from the western North Pacific basin. The over-ocean and overland branches of the model are calibrated separately and then merged. The suitability of the established model is validated by comparing the predicted TC intensity to the observations and the results of existing models such as those based on linear regression. Numerical results indicate that the proposed model successfully replicates the probabilistic properties such as the non-Gaussian marginal distribution of the intensity change and the input-output joint distribution and moments between the TC intensity and predictors, which the linear regression model cannot provide. Further application examples are also carried out by using the model to simulate the evolution of historical TC events and the extreme TC intensities in Southern China, demonstrating its potential role in the TC hazard assessment.

Can Tibetan Plateau Snow Depth Influence the Interannual Association between Tropical Indian Ocean Sea Surface Temperatures and Rapidly Intensifying Typhoons?

Abstract This study finds that the observed decrease in winter–spring Tibetan Plateau snow depth since 2000 has played an important role in weakening the correlation between rapidly intensifying tropical cyclones over the western North Pacific and tropical Indian Ocean sea surface temperatures (SSTs). Tibetan Plateau snow depth modulates convective activity through changes in the South Asian high. Increased Tibetan Plateau snow depth promotes basinwide tropical Indian Ocean cooling in spring through a Gill-type response. In the following seasons, downward latent heat flux anomalies associated with a weaker monsoon circulation contributes to warm SST anomalies over the western tropical Indian Ocean, thus favoring a positive phase of the Indian Ocean dipole. This evolution of tropical Indian Ocean SSTs associated with anomalously high Tibetan Plateau snow depth potentially weakens the relationship between rapidly intensifying tropical cyclone frequency and spring tropical Indian Ocean SSTs. When the effect of Tibetan Plateau snow depth is removed via partial correlation analysis, we find a significant relationship between spring tropical Indian Ocean SSTs and rapidly intensifying tropical cyclones as well as corresponding large-scale environmental factors. The results of this study enhance understanding of changes in tropical cyclone intensity and have implications for seasonal forecasting of tropical cyclone intensity over the western North Pacific basin. This study also emphasizes the importance of Tibetan Plateau thermal forcing in atmosphere–ocean coupling.

Analysis of Landfalling Rapidly Weakening Tropical Cyclones in the Philippines

Rapid weakening (RW) of tropical cyclones (TCs) is defined as the 90th percentile of all 24-h over-water weakening rates in the Western North Pacific (WNP) basin, corresponding to a decrease of at least 20 kt in the JMA dataset and 25 kt in the JTWC dataset. RW tends to occur along the 20 – 30° N latitude of the WNP, which makes the probability of RW TC landfall in the Philippines low. Over the study period from 1951–2020, a total of 468 and 563 WNP RW TCs were recorded, where only 17 and 19 of those made landfall in the country based on the JMA and JTWC datasets, respectively. Analysis of potential wind threats of landfalling RW TCs shows significantly lower hazards than non-RW TCs, except for those that make landfall on northern Luzon. RW occurs more frequently outside of the southwest monsoon or Habagat season. Simulations of two recently landfalling RW TCs – Typhoon (TY) Maysak (2015) and TY Yutu (2018) – using the Weather Research and Forecasting (WRF) model show the decrease in the equivalent potential temperature (𝜽e), a measure of the amount of heat and moisture in the atmosphere, in the TC inner core region can be used to diagnose RW. Constantly decreasing 𝜽e values below 400 𝑲 caused by cooler underlying sea surface temperature and/or dry air intrusion lead to TC RW. RW can also occur in low-shear environments. Environmental conditions that result in RW are typically observed from October–April of the following year, which explains the higher occurrence frequency of RW in the inactive TC season of the WNP. While the impacts of RW TCs are lower, over-forecasting a TC in one event can lead to a complacent populace for the next, as well as damage the reputation of forecasters, hence the importance of understanding RW.

Genetic analyses reveal a non-panmictic genetic structure in the sablefishAnoplopoma fimbriain the northern Pacific

AbstractThe sablefish Anoplopoma fimbria is distributed on the continental shelf of the North Pacific, has a high commercial value for both fisheries and aquaculture, and represents a shared resource between countries in the North Pacific basin. In the present study, we extend the geographic range surveyed in previous studies and reexamine the population’s genetic structure by integrating phylogeographic patterns of mitochondrial DNA and microsatellite DNA markers. Our results contradict the proposal that sablefish constitute a single population throughout their distributional range. We observed a signal of ancient divergence in mtDNA that differentiates the North Pacific West Coast from the rest of the sample sites, and microsatellite markers reveal a contemporary isolation of Mexican sablefish. Our findings show genetic differences between localities that should be explored in more detail to fully understand the interconnectedness that appears to exist between populations.

A Self-Organizing Maps Analysis of Wintertime North Pacific Jet Stream Variability

Abstract Previous research regarding the intraseasonal variability of the wintertime Pacific jet has employed empirical orthogonal function (EOF)/principal component (PC) analysis to characterize two leading modes of variability: a zonal extension or retraction and a ∼20° meridional shift of the jet exit region. These leading modes are intimately tied to the large-scale structure, sensible weather phenomena, and forecast skill in and around the vast North Pacific basin. However, variability within the wintertime Pacific jet and the relative importance of tropical and extratropical processes in driving such variability, is poorly understood. Here, a self-organizing maps (SOM) analysis is applied to 73 Northern Hemisphere cold seasons of 250-hPa zonal winds from the NCEP–NCAR reanalysis data to identify 12 characteristic physical jet states, some of which resemble the leading EOF Pacific jet patterns and combinations of them. Examination of teleconnection patterns such as El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) provide insight into the varying nature of the 12 SOM nodes at inter- and intraseasonal time scales. These relationships suggest that the hitherto more common EOF/PC analysis of jet variability obscures important subtleties of jet structure, revealed by the SOM analysis, which bear on the underlying physical processes associated with Pacific jet variability as well as the nature of its downstream impacts.

Evaluation of Western North Pacific Typhoon Track Forecasts in Global and Regional Models during the 2021 Typhoon Season

The track forecasts of tropical cyclones (TC) in the western North Pacific (WNP) basin during 2021 typhoon season with five global models and four regional models are evaluated here. The results show that the average direct position errors (DPEs) of the global and regional models are approximately 80, 150, 200, 300, and 400 km at 24 h, 48 h, 72 h, 96 h, and 120 h lead-times, respectively. The European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (ECMWF-IFS) achieved the best track forecast performance at each lead among the five global models. Among the four regional models, The China Meteorological Administration Tropical Regional Atmosphere Model for the South China Sea (CMA-TRAMS) attained the smallest DPEs within a 72 h lead, while The Hurricane Weather Research and Forecasting (HWRF) achieved the best track forecast performance at 96 h and 120 h leads. Most of the models produced an obvious westward systematic bias on track forecast from a 24 h to a 120 h lead. Further correlation and cluster analyses indicate that initial TC intensity and size and environmental steering flow can be regarded as good predictors for TC DPEs. TCs with a stronger initial intensity, a bigger initial size, and a larger environmental steering flow in general attain a smaller DPE, and the improvements may go up to 36% at short lead-time.

Meridional Survey of the Central Pacific Reveals Iodide Accumulation in Equatorial Surface Waters and Benthic Sources in the Abyssal Plain

AbstractThe distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152°W from the shelf of Alaska to Papeete, Tahiti. The transect included oxygenated waters near the shelf of Alaska, the full water column in the central basin in the North Pacific Basin, the upper water column spanning across seasonally mixed regimes in the north, oligotrophic regimes in the central gyre, and the equatorial upwelling. Iodide concentrations are highest in the permanently stratified tropical mixed layers, which reflect accumulation due to light‐dependent biological processes, and decline rapidly below the euphotic zone. Vertical mixing coefficients (Kz), derived from complementary 7Be data, enabled iodide oxidation rates to be estimated at two stations. Iodide half‐lives of 3–4 years show the importance of seasonal mixing processes in explaining north‐south differences in the transect, and also contribute to the decrease in iodide concentrations with depth below the mixed layer. These estimated half‐lives are consistent with a recent global iodine model. No evidence was found for significant inputs of iodine from the Alaskan continental margin, but there is a significant enrichment of iodide in bottom waters overlying deep sea sediments from the interior of the basin.

Interannual Variations of the β Drift of Tropical Cyclones over the Western North Pacific

Abstract Tropical cyclone (TC) movement consists essentially of the large-scale environmental steering and the β drift, and the latter is also affected by large-scale environmental flows. However, few previous studies have been conducted to examine the interannual variations of the β drift of TCs in the western North Pacific (WNP) basin and the corresponding relationship with the large-scale environmental flow. Using the observed TC tracks and reanalysis dataset during 1965–2019, it is found that the β drift plays a more important role in the TC motion than environmental steering at the interannual time scale. The westward (northward) component of the β drift in the environment with anticyclonic relative vorticity is slower (faster) than that with cyclonic relative vorticity, while an equatorward (poleward) relative vorticity gradient decreases (increases) the westward component of the β drift, in agreement with previous numerical simulations. It is also found that the southwestward migration of the WNP subtropical high can decrease (increase) the westward (northward) component of the β drift. This study suggests that environmental influence on the β drift should be considered when understanding interannual variations of TC movement and tracks, which are important to TC landfall, emergency management, and hazard mitigation. Significance Statement The β drift is an important component of TC movement and is affected by large-scale environmental flows. However, few previous studies have been conducted to examine the interannual variations of the β drift of TCs in the western North Pacific basin and the corresponding relationship with the large-scale environmental flow. This study finds that the β drift plays a more important role in the TC motion than environmental steering at the interannual time scale, and the interannual variations of the β drift are also controlled by large-scale environmental flows. It is suggested that environmental influence on the β drift should be considered when understanding interannual variations of TC movement and tracks, which are important to TC landfall, emergency management, and hazard mitigation.

Dynamics and Impacts of the North Pacific Eddy-Driven Jet Response to Sudden Stratospheric Warmings

Abstract In this study, observations and simulations are used to investigate the mechanisms behind the different surface responses over the North Pacific and North Atlantic basins in response to sudden stratospheric warmings associated with a polar-night jet oscillation event (PJO SSWs). In reanalysis and a free-running preindustrial simulation, on average, a negative North Atlantic Oscillation (NAO) response is seen, corresponding to an equatorward shift of the eddy-driven jet. This is considered as the canonical tropospheric response to PJO SSWs. In contrast, the response over the North Pacific is muted. This basin-asymmetric response is shaped by the North Pacific air–sea interactions spun up by the tropospheric precursor to PJO SSWs, which prevent the Pacific eddy-driven jet from responding to the downward influence from the stratosphere. To isolate the downward influence from the sudden warming itself from any preconditioning of the troposphere that may have occurred prior to the warming, a nudging technique is used by which a reference PJO SSW is artificially imposed in a 195-member ensemble spun off from a control simulation. The nudged ensembles show a more basin-symmetric negative Northern Annular Mode (NAM) response, in which the eddy-driven jet shifts equatorward in both the Pacific and Atlantic sectors. Monitoring the atmospheric and oceanic conditions in the North Pacific before and at the onset of PJO SSWs may be useful for forecasting whether a basin-asymmetric negative NAO or basin-symmetric negative NAM response is more likely to emerge. This can be further used to improve subseasonal-to-seasonal predictions of weather and climate. Significance Statement Stratospheric sudden warming events (SSWs) occur when the eastward winds usually found above the Arctic in the winter spontaneously and rapidly reverse. Following their occurrence, the Northern Hemisphere surface westerlies move southward, sometimes over both the North Atlantic and North Pacific and other times over the North Atlantic only. We therefore wanted to understand this uncertainty in the North Pacific surface westerlies response. We find that the North Pacific surface westerlies response to SSWs can be muted by air–sea interactions over the North Pacific. Our results highlight the importance of monitoring the atmospheric and oceanic conditions in the North Pacific before the occurrence of SSWs to forecast whether the Pacific westerlies are likely to respond to SSWs.

NOAA's operational satellite ocean heat content products

ABSTRACT NOAA generates a suite of Ocean Heat Content (OHC) products for the North Atlantic and the North and South Pacific basins on a daily basis. Knowledge of the upper Ocean Heat Content is a critical aid in assessing the ocean's impact on Tropical Cyclones (TCs) and weather events. In-situ and satellite measurements of Sea Surface Temperature (SST) and Sea Surface Height Anomaly (SSHA) are used to generate the OHC product suite. The evaluated suite includes the 20°C (D20) and 26°C (D26) isotherm depths, the mixed layer depth (MLD) and mapping errors. Two cases are presented within the context of situational awareness showing the underling oceanic variability during the lifecycles of TC's Irma and Dorian.

Distinct Responses of North Pacific and North Atlantic Summertime Subtropical Anticyclones to Global Warming

Abstract The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change.

Factors affecting climate variability of basin‐wide western North Pacific tropical cyclone intensity

AbstractChanges in the duration of tropical cyclones (TCs) and the embedded large‐scale environment along with TC tracks have a substantial impact on basin‐wide TC intensity. However, their relative importance in changes of basin‐wide TC intensity remains unclear. This study focuses on their contributions to change in TC intensity over the western North Pacific (WNP) basin using a TC intensity model. First, simulations using a TC intensity model with inputs of TC tracks and the embedded environment adequately capture the climate variability of the observed basin‐wide WNP TC intensity. It suggests that the climate variability of basin‐wide TC intensity largely depends on the synergistic changes in large‐scale environment and TC tracks. Sensitivity tests indicated that changes in TC tracks generally appear to be the most important factor affecting basin‐wide TC intensity on the interannual and interdecadal timescales. Changes in large‐scale environment play a limited role especially for weak‐to‐moderate TCs (i.e., tropical storms and Cat.1–3 TCs), while comparable impacts of change in TC tracks and large‐scale environment are found for intense TCs (i.e., Cat.4–5 TCs). Given that TC tracks remain unchanged, sensitivity simulations further show that TC duration plays a more crucial role in controlling basin‐wide TC intensity than the environmental fields. Moreover, weak‐to‐moderate TCs appear to be more sensitive to duration than intense TCs, in consistent with the dramatic changes in basin‐wide TC intensity for westward TCs that form in the South China Sea and the Philippine Sea with short TC duration. Results of this study would be helpful for climate prediction of basin‐wide TC intensity over the WNP basin.

Statistical Framework for Western North Pacific Tropical Cyclone Landfall Risk through Modulation of the Western Pacific Subtropical High and ENSO

Abstract Seasonal predictions of tropical cyclone (TC) landfalls are challenging because seasonal landfall count not only depends on the number and spatial distribution of TC genesis, but also whether those TCs are steered toward land or not. Past studies have separately examined genesis and landfall as a function of large-scale ocean and atmospheric environmental conditions. Here, we introduce a practical statistical framework for estimating the seasonal count of TC landfalls as the product of a Poisson model for seasonal TC genesis and a logistic model for landfall probability. We compute spatial variations in TC landfall and genesis by decomposing TC activity in the western North Pacific (WNP) basin into 10° × 10° bins, then identify coherent regions where El Niño–Southern Oscillation (ENSO) and the western extent of the Pacific subtropical high (WPSH) have significant influences on seasonal landfall count. Our framework shows that ENSO and the WPSH are weakly related to basinwide landfalls but strongly related to regional genesis and landfall probability. ENSO modulates the zonal distribution of TC genesis, consistent with past work, whereas the WPSH modulates the meridional distribution of landfall probability due to variations in steering flow associated with the Pacific subtropical high. These spatial patterns result in four coherent subregions of the WNP basin that define seasonal landfall variations: landfall count increases in the southwestern WNP during a positive WPSH and La Niña, the south-central WNP during a positive WPSH and El Niño, the eastern WNP during a negative WPSH and El Niño, and the northern WNP during a negative WPSH and La Niña.

Simulations on multiple tropical cyclones event associated with monsoon trough over the western North Pacific

AbstractIn this study, the formation of an idealized multiple tropical cyclones (MTCs) event within a monsoon trough (MT) region is investigated by using the weather forecasting model (WRF_ARW). Sensitivity experiments are conducted by specifying different initial conditions. It is revealed that both dynamical and thermodynamical conditions associated with the MT are important in triggering MTCs event over the western North Pacific basin. In the composite active years, which have a strong MT with higher ambient moisture, an MTCs event is produced. In contrast, no MTCs event occurs in the inactive years, which confirms the previous observational study. The possible pathway for the formation of MTCs event is proposed. In the active years, under favourable moist environments, the first TC is generated faster through the greater barotropic kinetic energy conversion. Once the first tropical cyclone (TC) is generated, the energy dispersion induced low‐level Rossby wave train acts as a precursor to the second TC. Furthermore, the upper‐level asymmetric outflow jet acts as a dynamical forcing to induce vertical motion, which builds up a favourable environmental condition for a second TC development. This work provides some insights into the formation of MTCs event.

Quantile regression analysis of time-space variation characteristics of tropical cyclones in the west North Pacific basin under global warming

ABSTRACT The enormous economic losses and casualties were caused by tropical cyclones in the southeast coastal areas of China every year. In order to understand the time-space variation characteristics of tropical cyclones (including intensity, minimum central atmosphere pressure, duration, and generation position) in the global and the western North Pacific basin, the monthly and interannual variation characteristics of tropical cyclones from 1949 to 2018 were analyzed by the quantile regression method and least square regression method. The results show that the global climate temperature and the annual maximum wind speed of tropical cyclones have a tendency to increase under different quantiles. While the maximum wind speed of tropical cyclones generated in the WNP basin basically decreases with the increase of years in El Niño, La Niña, and normal years under different quantiles. It is obvious that the interannual variation of the maximum wind speed of tropical cyclones is affected by the ENSO events. Similarly, minimum central atmospheric pressure, duration, and generation position of TCs from 1949 to 2018 are also analyzed in the present study. The results of this study can provide an effective reference for data analysis and trend prediction of tropical cyclones in the western North Pacific basin.

Quantifying Heavy Precipitation throughout the Entire Tropical Cyclone Life Cycle

Abstract Tropical cyclones (TCs) and their associated precipitation can have devastating impacts on the areas affected, with outcomes ranging from mudslides to inland flash flooding. Previous studies have used a fixed radius around the TC to isolate storm-related precipitation. One previous study instead used a dynamic radius of 8 m s−1 winds, but the wind field of the TC can deteriorate or shift quickly after landfall or the onset of extratropical transition (ET). This study uses a dynamical radius derived from the 500-hPa geopotential height in and around the TC to define TC- and post-tropical cyclone (PTC)-related heavy precipitation, allowing for the analysis of precipitation with tropical origins after the official demise of the original TC. Climatologies are constructed, indicating a maximum in TC- and PTC-related heavy precipitation in the west North Pacific and a secondary maximum in the east North Pacific. PTC-related heavy precipitation accounts for as much as 40% of the annual heavy precipitation in the northwest portion of the west North Pacific basin and 3.13% of heavy precipitation globally. We observe that the major hurricane stage contributes on average 2.6% of the global TC- and PTC-related precipitation, while the less intense but more common tropical storm stages of the TC life cycle contribute 85.7% of this observed precipitation. This analysis framework can be further extended to assess model biases and climate projections of TC and PTC precipitation.

An Analysis of Tropical Cyclone Vortex and Convective Characteristics in Relation to Storm Intensity Using a Novel Airborne Doppler Radar Database

Abstract This analysis introduces a novel airborne Doppler radar database, referred to as the Tropical Cyclone Radar Archive of Doppler Analyses with Re-centering (TC-RADAR). TC-RADAR comprises over 900 analyses from 273 flights into TCs in the North Atlantic, eastern North Pacific, and central North Pacific basins between 1997 and 2020. This database contains abundant sampling across a wide range of TC intensities, which facilitated a comprehensive observational analysis on how the three-dimensional, kinematic TC inner-core structure is related to TC intensity. To examine the storm-relative TC structure, we implemented a novel TC center-finding algorithm. Here, we show that TCs below hurricane intensity tend to have monopolar radial profiles of vorticity and a wide range of vortex tilt magnitudes. As TC intensity increases, vorticity becomes maximized within an annulus inward of the peak wind, the vortex decays more slowly with height, and the vortex tends to be more aligned in the vertical. The TC secondary circulation is also strongly linked to TC intensity, as more intense storms have shallower and stronger lower-tropospheric inflow as well as larger azimuthally averaged ascent. The distribution of vertical velocity is found to vary with TC intensity, height, and radial domain. These results—and the capabilities of TC-RADAR—motivate multiple avenues for future work, which are discussed. Significance Statement Acquiring observations of the inner core of tropical cyclones (TCs) is a challenge due to the hazardous conditions inherent to the storm. A proven method of sampling the TC core region is the use of airborne radar. This study presents a novel database comprising over 900 airborne radar analyses collected in storms between 1997 and 2020, which is freely available to the research community. Here we demonstrate the utility of the database by examining how the three-dimensional structure of the TC core region changes depending upon the intensity of the storm. By identifying how the baseline TC vortex structure varies with TC intensity, this work provides the foundation for multiple future research avenues and model evaluation efforts.

An Analog Comparison between Rapidly and Slowly Intensifying Tropical Cyclones

Abstract To better understand the conditions that favor tropical cyclone (TC) rapid intensification (RI), this study assesses environmental and storm-scale characteristics that differentiate TCs that undergo RI from TCs that undergo slow intensification (SI). This comparison is performed between analog TC pairs that have similar initial intensity, vertical wind shear, and maximum potential intensity. Differences in the characteristics of RI and SI TCs in the North Atlantic and western North Pacific basins are evaluated by compositing and comparing data from the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA5) and the Gridded Satellite (GridSat) dataset. In the period leading up to the start of RI, RI TCs tend to have a stronger and deeper vortex that is more vertically aligned than SI TCs. Additionally, surface latent heat fluxes are significantly larger in RI TCs prior to the intensity change period, compared to SI TCs. The largest surface latent heat flux differences are initially located to the left of shear; subsequently, upshear and right-of-shear differences amplify, resulting in a more symmetric distribution of surface latent heat fluxes in RI TCs. Increasing azimuthal symmetry of surface latent heat fluxes in RI TCs, together with an increasing azimuthal symmetry of horizontal moisture flux convergence, promote the upshear migration of convection in RI TCs. These differences, and their evolution before and during the intensity change period, are hypothesized to support the persistence and invigoration of upshear convection and, thus, a more symmetric latent heating pattern that favors RI.