Daehyun Kim

This study introduces a newly defined turbulence patch that includes continuous turbulence reports of in situ aircraft data, separating at least 5‐min from others. This enables the individual turbulence patch to be characterized more easily by their possible sources than by an isolated data point. Using 5 years of satellite observations, we classified new turbulence patches into convectively induced turbulence (CIT) and non‐CIT. We also classified patches into long‐ and short‐duration events. Patch duration shows clear latitudinal differences, especially for CIT. CIT duration is linked to the size of underlying convective systems and strength of background winds. Signal analysis shows that CIT in low latitude exhibits a high skewness and kurtosis, indicating highly variable turbulence with occasional extreme fluctuations. Meanwhile, long‐duration non‐CIT patches occur frequently under enhanced Clear‐Air‐Turbulence (CAT) diagnostics, indicating an association between CAT‐prone conditions and encounter duration. These findings support turbulence risk assessment and nowcasting.

The influence of the spatial organization of tropical convection on the production of extreme instantaneous localized tropical rain rates remains poorly constrained, both in observations and in models. Using retrievals from the Ku‐band radar onboard the GPM satellite, we identify tropical precipitation features (PFs) and quantify their morphological characteristics. We find that PFs that produce extreme instantaneous rain rates are on average larger and have a higher convective area fraction than the average PF. Further, PFs whose convection is more spatially concentrated generate more intense maximum precipitation rates. We compare these observational results to the output of four DYAMOND models. We find that, while there is some model diversity in how sensitive maximum rain rate is to PF morphology, the models all capture the general relationship between PF morphology and extreme precipitation rates.

Global warming has intensified extreme precipitation, a trend projected to continue as warming persists. However, how extreme precipitation characteristics respond to global warming mitigation strategies remains poorly understood. This study examines East Asian summer extreme precipitation responses under a net-zero anthropogenic carbon dioxide (CO ₂ ) emission scenario. Despite substantial cooling over East Asia following atmospheric CO ₂ concentration decline, the intensity of extreme precipitation events continues to increase for about 30 years and does not subsequently weaken. This counterintuitive result arises mainly from intensified upward motion during extreme precipitation events, driven by enhanced baroclinicity and strengthening of the upper-tropospheric jet, which together overcompensate the cooling effect. These are essentially due to the hysteresis in the Atlantic (i.e., Atlantic Meridional Overturning Circulation slowdown), Southern (i.e., sustained warming), and Pacific (i.e., El Niño–like warming) Oceans. Our findings highlight that achieving net-zero anthropogenic CO ₂ emissions is insufficient to mitigate intensifying extreme rainfall events in the densely populated East Asian countries.

We use a newly developed tropical cyclone (TC) seed tracking data set in ERA5 to investigate how the tangential wind structure of a seed differs between those that develop into TCs (“developing seeds”) and those that do not (“non‐developing seeds”). Focusing on the concept of “structural compactness” , a vortex structural parameter that characterizes the planetary Rossby wave drag on vortex intensity, we find that most developing seeds maintain relatively compact low‐level dynamical structure throughout their lifetimes, whereas non‐developing seeds exhibit a broader range of structures, with many being significantly incompact. This structural difference is reflected in the genesis probability of seeds: a seed with a lower has a lower probability of developing into TC. Furthermore, for incompact seeds , the maximum time they persist in ERA5 tracks agrees well with a theoretical prediction that stronger planetary Rossby wave drag weakens such seeds more rapidly, limiting their persistence. This suggests that planetary Rossby wave drag will rapidly filter out incompact seeds within a few days after seed formation, and only seeds that are sufficiently compact would be expected to survive long enough to potentially undergo TC genesis. These findings highlight the importance of considering the seed's internal dynamical structure, rather than focusing solely on large‐scale environmental conditions, when evaluating TC genesis potential. By incorporating seed compactness alongside traditional environmental parameters, we can gain a more comprehensive understanding of the seed‐to‐cyclone transition in different climates.

Tropical cyclone (TC) rapid intensification (RI) is driven by complex interactions of TCs with multiple environmental factors. Using observations and two HighResMIP models (CNRM‐CM6‐1‐HR and HadGEM3‐GC31‐HM) that better simulate TCRI, we examine how six environmental factors jointly influence TCRI. Observations show that while TCRI occurrence probability increases with the number of supportive conditions, it is strongly suppressed when even a few unsupportive ones are present. For the same number of supportive conditions, the number of unsupportive conditions largely controls the TCRI occurrence. In the models, the sensitivity of TCRI occurrence frequency to the number of supportive and unsupportive conditions is found to be weaker than the observed, caused by unrealistic representation of the effect of vertical wind shear and low free‐troposphere humidity on TCRI. Our results emphasize that unsupportive environmental conditions must be explicitly considered alongside supportive ones when diagnosing and predicting RI.

Global storm resolving models (GSRMs) provide an excellent testbed for understanding the role of small-scale processes in tropical cyclogenesis. In this study, we assess tropical cyclone (TC) genesis and frequency in the Geophysical Fluid Dynamics Laboratory’s eXperimental System for High‐resolution prediction on Earth‐to‐Local Domains version 2021 (X-SHiELD v2021). Compared to observations, X-SHiELD produces significantly fewer TCs, albeit with realistic representations of both the number of pre-TC disturbances (TC seeds) and the mean state relative to ERA5. We find that the TC seed-to-TC transition rate is underestimated in X-SHiELD because the spin-up of TC seed vortex is inefficient. The inefficient TC seed spin-up in X-SHiELD is associated with a weaker-than-observed low-level inflow, which is coupled to mesoscale ascent near the center that is too weak and too top-heavy. The biases in vertical velocity are associated with a dry bias at low levels, which is caused by the overly active shallow convection scheme. Our results underscore the importance of subgrid scale processes, particularly low level mixing through shallow convection, in the representation of TC genesis process even in kilometer scale models.

As global warming may influence convective organization, it is important to observationally constrain the effects of convective organization on the production of extreme instantaneous tropical rain rates. It is also important to evaluate the ability of global storm-resolving models in capturing this coupling. We quantify the spatial morphology of tropical precipitation features that contain extreme tropical rain rates. We compare these morphological measures to the morphology of extreme precipitation features as simulated by a global storm-resolving model. We find that in both observations and the model simulation, precipitation features with extreme rain rates are on average larger and have higher convective area fraction than the average precipitation feature. Further, among precipitation features with a similar amount of total convection, those with spatially concentrated convective cores generated on average more intense maximum precipitation rates. Our evaluated model underestimates the influence of the spatial concentration of convection on maximum rain rate.

Not-yet-known not-yet-known not-yet-known unknown Although stratiform heating plays a crucial role in tropical convective systems, it remains unclear (1) how stratiform heating changes with surface warming and (2) how those changes affect convectively coupled equatorial waves. This study analyzes changes in the stratiform heating structure and convectively coupled Kelvin waves (CCKWs) associated with surface warming using a set of aquaplanet simulations (-4K, CTL, +4K). Results show that the melting level rises with warming, causing ice particles falling from the stratiform clouds to melt at lower pressure levels (higher altitudes). The upward shift of melting-induced cooling results in a decrease in temperature and vertical motion variability associated with stratiform clouds in the lower free-troposphere and upper boundary layer. These changes lower the degree to which stratiform (i.e., the second baroclinic mode) and deep convective clouds (i.e., the first baroclinic mode) are coupled within CCKWs, causing CCKWs to weaken and accelerate with warming.

Understanding the relation between large‐scale (100 km) tropical atmospheric motions and small‐scale convective circulations remains a challenge, despite such multiscale interactions playing a crucial role in the dynamics of large‐scale circulations. In this study, a 40‐day simulation made with a global storm‐resolving model at 4 km horizontal resolution is used to simultaneously characterize large‐ and small‐scale convective circulations and examine their relationships. The large‐scale motions are characterized by the area‐averaged vertical mass flux profile over tropical domains of similar size to the grid scale of contemporary climate models; small‐scale motions are computed as deviations of vertical mass flux from the large‐scale mean. We find that the simulated large‐scale circulations tend to evolve in a systematic way that bears qualitative resemblance to the canonical evolution of tropical convective systems, with large‐scale regimes progressing, on average, from weak but widespread subsidence, to shallow ascent/congestus clouds, to deep convection, to top‐heavy stratiform anvils, and finally returning to a suppressed state. Utilizing the moisture‐space framework, we compare the composite structures of the small‐scale circulations in different large‐scale regimes. We find that the structure of the large‐scale vertical motions strongly constrains the shape of the embedded small‐scale circulations. We further identify tropical anvil clouds as mediators of deep convection's up‐scale influence on changes in the large‐scale circulation.

This study evaluates the subseasonal‐to‐seasonal (S2S) prediction skill of the Madden‐Julian Oscillation (MJO) and its impacts on the east Pacific (EP) tropical cyclogenesis in the Unified Forecast System (UFS) during boreal summer (May–October). Utilizing four experimental versions, Prototypes 5–8, the study finds that although the UFS generally captures the MJO's propagation characteristics near the initialization time, it encounters difficulty in accurately predicting the propagation speed and decay rate of the MJO beyond 15 days. Specifically, the phase transition rate of the MJO in the UFS is slower than observed, although this behavior is improved in Prototype 8. The UFS overestimates intraseasonal east Pacific genesis potential index anomalies as a function of the MJO phase. Analysis of the vertically integrated moist static energy (MSE) budget reveals that all four UFS prototypes underestimate the damping effect of vertical MSE advection and the amplifying effect of longwave radiative heating, indicating weaknesses in tropical convective parameterization and cloud radiative feedbacks, although these biases are somewhat improved in Prototype 8. These deficiencies result in less efficient vertical MSE export, weaker damping of MJO convection, slow MJO propagation, and delayed MJO's impacts on the EP. Thus, improving the UFS's ability to simulate MJO propagation and maintenance processes is crucial for better predicting the MJO's effects on EP TC genesis and enhancing S2S forecast capability for MJO‐modulated TC activity.