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Abstract Severe convective storms and tornadoes rank among nature’s most hazardous phenomena, inflicting significant property damage and casualties. Near-surface weather conditions are closely governed by large-scale synoptic patterns. It is crucial to delve into the involved multiscale associations to understand tornado potential in response to climate change. Using clustering analysis, this study unveils that leading synoptic patterns driving tornadic storms and associated spatial trends are distinguishable across geographic regions in the U.S. Synoptic patterns with intense forcing featured by intense upper-level eddy kinetic energy and a dense distribution of Z500 fields dominate the increasing trend in tornado frequency in the southeast U.S., generating more tornadoes per event. Conversely, the decreasing trend noted in certain regions of the central Great Plains is associated with weak upper-level synoptic forcing. These findings offer an explanation of observational changes in tornado occurrences, suggesting that the physical mechanisms driving those changes differ across regions. 

Tropical cyclones are expected to intensify more rapidly with warming, but relatively little work has examined whether they could expand more rapidly with warming, too. Recent theory predicts that peak expansion rate should increase with sea surface temperature (SST), and physical arguments suggest this dependence should be specifically on the relative SST, i.e. the SST difference from the tropical mean. We test this hypothesis with historical observational data, in which SST variations are primarily variations in relative SST. Both average and peak expansion rates are found to systematically increase with relative SST globally across the Northern Hemisphere (27.2 and 37.5 km/d/K) and within each individual basin. Results are robust across both reanalysis and Best Track observational datasets. Uniform-SST aquaplanet simulations show a much weaker dependence of maximum expansion rate on absolute SST, suggesting that the dominant dependence is on relative SST. Hence, mean global warming is not expected to strongly change storm size dynamics, but patterns of sea surface warming may play an important role in determining how storm size, and hence coastal risk, may change in the future. This work can also help improve forecasting of the wind field and its hazards and impacts at landfall. 

Abstract Tropical cyclones (TCs) are often generated from preexisting “seed” vortices. Seeds with higher persistence might have a higher chance to undergo TC genesis. What controls seed persistence remains unclear. This study proposes that planetary Rossby wave drag is a key factor that affects seed persistence. Using recently developed theory for the response of a vortex to the planetary vorticity gradient, a new parameter given by the ratio of the maximum wind speed (V_max) to the Rhines speed at the radius of maximum wind (R_max), here termed “vortex structural compactness” (C_υ), is introduced to characterize the vortex weakening by planetary Rossby wave drag. The relationship between vortex compactness and weakening rate is tested via barotropicβ-plane experiments. The vortex’s initialC_υis varied by systematically varying their initialV_maxandR_maxin idealized wind profile models. Experiments are also conducted with real-world seed vortices from reanalysis data, which possess natural compactness variability. The weakening rate depends strongly on the vortex’s initialC_υacross both idealized and real-world experiments, and the initial axis-asymmetry introduces minor differences. Experiments doubling the size of seed vortices cause them to weaken more rapidly, in line with other experiment sets. The dependence of the weakening rate on initial compactness can be predicted from a simple theory, which is more robust for more compact vortices. Our results suggest that a seed’s structure strongly modulates how long it can persist in the presence of a planetary vorticity gradient. Connections to real seeds on Earth are discussed. Significance StatementThis study explores the evolution of tropical cyclone (TC) seeds, which are preexisting weakly rotating rainstorms, in a simple setting that isolates the dynamical effects of the rotating sphere. It is not clear why some seeds can persist for a longer duration and might have a higher chance to eventually undergo genesis. We proposed that a factor called “planetary Rossby wave drag” plays a crucial role in this process. To investigate this, we introduce a new parameter called “compactness” to describe how the size and intensity of a seed vortex determines how quickly it will weaken due to this drag. We conducted experiments with numerical simulations and real-world TC seeds to test our ideas. Our findings show that the initial compactness of seeds strongly influences how quickly they weaken. We have developed a formula to predict how quickly these seeds weaken based on their compactness, which is especially accurate for more compact seeds. This research helps us understand how planetary Rossby wave drag affects the persistence of a TC seed and, ultimately, how it might impact the frequency of TCs. 

Abstract Despite its high tropical cyclone (TC) density, the Eastern North Pacific (ENP) basin has received relatively little research attention on landfall variability. This study investigates the climatological seasonal cycle and interannual variability of TC landfalls in the ENP. We find that the basin is characterized by a bimodal distribution of landfalls, with peaks in June and September–October. Using a composite analysis of high and low landfall years, we show that this distribution is primarily driven by landfall probability rather than genesis. The absence of landfalls during July is due to enhanced easterlies from the Caribbean Low‐Level Jet entering the ENP through gaps in the Americas Cordillera. High landfall years feature enhanced easterly wind reversals from a northward‐shifted Intertropical Convergence Zone. These additional steering winds drive hurricanes ashore in the vulnerable region of southwest Mexico. This study provides valuable insights for improving TC landfall forecasts and preparedness in the region. 

Abstract Genesis potential indices (GPIs) are widely used to understand the climatology of tropical cyclones (TCs). However, the sign of projected future changes depends on how they incorporate environmental moisture. Recent theory combines potential intensity and midtropospheric moisture into a single quantity called the ventilated potential intensity, which removes this ambiguity. This work proposes a new GPI (GPI_υ) that is proportional to the product of the ventilated potential intensity and the absolute vorticity raised to a power. This power is estimated to be approximately 5 by fitting observed tropical cyclone best track and ECMWF Reanalysis v5 (ERA5) data. Fitting the model with separate exponents yields nearly identical values, indicating that their product likely constitutes a single joint parameter. Likewise, results are nearly identical for a Poisson model as for the power law. GPI_υperforms comparably well to existing indices in reproducing the climatological distribution of tropical cyclone genesis and its covariability with El Niño–Southern Oscillation, while only requiring a single fitting exponent. When applied to phase 6 of the Coupled Model Intercomparison Project (CMIP6) projections, GPI_υpredicts that environments globally will become gradually more favorable for TC genesis with warming, consistent with prior work based on the normalized entropy deficit, though significant changes emerge only at higher latitudes under relatively strong warming. The GPI_υhelps resolve the debate over the treatment of the moisture term and its implication for changes in TC genesis favorability with warming, and its clearer physical interpretation may offer a step forward toward a theory for genesis across climate states. Significance StatementTropical cyclones cause significant human impacts globally, yet we currently do not understand what controls the number of storms that form each year. Tropical cyclone formation depends on fine-scale processes that our climate models cannot capture. Thus, it is common to use parameters from the background environment to represent regions favorable for cyclone formation. However, there are a variety of formulations because the link between environment and cyclone formation is complicated. This work proposes a new method that unifies a few common formulations, which helps resolve a divergence in current explanations of how tropical cyclone formation may change under climate change. 

Abstract Minimum central pressure (P_min) is an integrated measure of the tropical cyclone wind field and is known to be a useful indicator of storm damage potential. A simple model that predictsP_minfrom routinely estimated quantities, including storm size, would be of great value. Here, we present a simple linear empirical model for predictingP_minfrom maximum wind speed, a radius of 34-kt (1 kt ≈ 0.51 m s⁻¹) winds (R_34kt), storm center latitude, and the environmental pressure. An empirical model for the pressure deficit is first developed that takes as predictors specific combinations of these quantities that are derived directly from theory based on gradient wind balance and a modified Rankine-type wind profile known to capture storm structure inside ofR_34kt. Model coefficients are estimated using data from the southwestern North Atlantic and eastern North Pacific from 2004 to 2022 using aircraft-based estimates ofP_min, extended best track data, and estimates of environmental pressure from Global Forecast System (GFS) analyses. The model has a near-zero conditional bias even for lowP_min, explaining 94.2% of the variance. Performance is superior to a variety of other model formulations, including a standard wind–pressure model that does not account for storm size or latitude (89.2% variance explained). Model performance is also strong when applied to high-latitude data and data near coastlines. Finally, the model is shown to perform comparably well in an operation-like setting based solely on routinely estimated variables, including the pressure of the outermost closed isobar. Case study applications to five impactful historical storms are discussed. Overall, the model offers a simple, fast, physically based prediction forP_minfor practical use in operations and research. Significance StatementSea level pressure is lowest at the center of a hurricane and is routinely estimated in operational forecasting along with the maximum wind speed. While the latter is currently used to define hurricane intensity, the minimum pressure is also a viable measure of storm intensity that is known to better represent damage risk. A simple empirical model that predicts the minimum pressure from maximum wind speed and size, and based on the physics of the hurricane wind field, does not currently exist. This work develops such a model by using wind field physics to determine the important parameters and then uses a simple statistical model to make the final prediction. This model is quick and easy to use in weather forecasting and risk assessment applications. 

Abstract A tropical cyclone (TC) can generally be divided into three regions: inner core with vigorous convection, intermediate region with intermittent convection, and far outer region with less convective activity. The different physics in these three regions suggest correspondingly different wind structure models. In this study, we combine the inner‐core wind model from Tao et al. (2023,https://doi.org/10.1029/2023gl104583, T23), the outer wind model from Emanuel (2004,https://texmex.mit.edu/pub/emanuel/PAPERS/Energetics_Structure.pdf, E04), and a transition model of a modified Rankine vortex to create a new fast and analytical model for the complete radial structure of the TC wind field. The T23 model captures inner‐core wind variation with small errors, while the E04 model reproduces the broad outer wind structure at large radii well. The new wind model combines the strengths from both T23 and E04 models without the need for statistical fitting, showing great potential in reproducing the full range of simulated and observed TC winds. 

Abstract The effect of warming on severe convective storm potential is commonly explained in terms of changes in vertically integrated (“bulk”) environmental parameters, such as CAPE and 0–6-km shear. However, such events are known to depend on the details of the vertical structure of the thermodynamic and kinematic environment that can change independently of these bulk parameters. This work examines how warming may affect the complete vertical structure of these environments for fixed ranges of values of high CAPE and bulk shear, using data over the central Great Plains from two high-performing climate models (CNRM and MPI). To first order, projected changes in the vertical sounding structure are consistent between the two models: the environment warms approximately uniformly with height at constant relative humidity, and the shear profile remains relatively constant. The boundary layer becomes slightly drier (−2% to 6% relative humidity) while the free troposphere becomes slightly moister (+1% to 3%), with a slight increase in moist static energy deficit aloft with stronger magnitude in CNRM. CNRM indicates enhanced low-level shear and storm-relative helicity associated with stronger hodograph curvature in the lowest 2 km, whereas MPI shows near-zero change. Both models strongly underestimate shear below 1 km compared to ERA5, indicating large uncertainty in projecting subtle changes in the low-level flow structure in climate models. The evaluation of the net effect of these modest thermodynamic and kinematic changes on severe convective storm outcomes cannot be ascertained here but could be explored in simulation experiments. Significance StatementSevere thunderstorms and tornadoes cause substantial damage and loss of life each year, which raise concerns about how they may change as the world warms. We typically use a small number of common atmospheric parameters to understand how these localized events may change with climate change. However, climate change may alter the weather patterns that produce these events in ways not captured by these parameters. This work examines how climate change may alter the complete vertical structure of temperature, moisture, and wind and discusses the potential implications of these changes for future severe thunderstorms and tornadoes. 

Abstract This study investigates how entrainment’s diluting effect on cumulonimbus updraft buoyancy is affected by the temperature of the troposphere, which is expected to increase by the end of the century. A parcel model framework is constructed that allows for independent variations in the temperature (T), the entrainment rateε, the free-tropospheric relative humidity (RH), and the convective available potential energy (CAPE). Using this framework, dilution of buoyancy is evaluated withTand RH independently varied and with CAPE either held constant or increased with temperature. When CAPE is held constant, buoyancy decreases asTincreases, with parcels in warmer environments realizing substantially smaller fractions of their CAPE as kinetic energy (KE). This occurs because the increased moisture difference between an updraft and its surroundings at warmer temperatures drives greater updraft dilution. Similar results are found in midlatitude and tropical conditions when CAPE is increased with temperature. With the expected 6%–7% increase in CAPE per kelvin of warming, KE only increases at 2%–4% K⁻¹in narrow updrafts but tracks more closely with CAPE at 4%–6% in wider updrafts. Interestingly, the rate of increase in the KE withTbecomes larger than that of CAPE when the later quantity increases at more than 10% K⁻¹. These findings emphasize the importance of considering entrainment in studies of moist convection’s response to climate change, as the entrainment-driven dilution of buoyancy may partially counteract the influence of increases in CAPE on updraft intensity. Significance StatementCumulonimbus clouds mix air with their surrounding environment through a process called entrainment, which controls how efficiently environmental energy is converted into upward speed in thunderstorm updrafts. Our research shows that warmer temperatures will exacerbate the moisture difference between cumulonimbus updrafts and their surroundings, leading to greater mixing and less efficient conversion of environmental energy into updraft speeds. This effect should be considered in future research that investigates how climate change will affect cumulonimbus clouds. 

Abstract Tropical cyclones are known to expand to an equilibrium size on thefplane, but the expansion process is not understood. In this study, an analytical model for tropical cyclone outer-size expansion on thefplane is proposed. Conceptually, the storm expands because the imbalance between latent heating and radiative cooling drives a lateral inflow that imports absolute vorticity. Volume-integrated latent heating increases more slowly with size than radiative cooling, and hence, the storm expands toward an equilibrium size. The predicted expansion rate is given by the ratio of the difference in size from its equilibrium valuer_t,eqto an environmentally determined time scaleτ_rtof 10–15 days. The model is fully predictive if given a constantr_t,eq, which can also be estimated environmentally. The model successfully captures the first-order size evolution across a range of numerical simulation experiments in which the potential intensity andfare varied. The model predictions of the dependencies of lateral inflow velocity and expansion rate on latent heating rate are also compared well with numerical simulations. This model provides a useful foundation for understanding storm size dynamics in nature.