Abstract Radar scans of winter storms frequently show enhancements in equivalent radar reflectivity factor (Z e ) with in the -18°C to -12°C cloud layer, often referred to as the “Dendritic Growth Layer” (DGL). However, the microphysical structures responsible for these radar signatures remain poorly understood due to limited in-cloud in situ validation. This study leverages coordinated airborne radar and in situ observations collected during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. We analyzed 581 vertical profiles of Ku-band Z e gradient (dZ Ku /dz), each averaged over 10 km (∼1 minute) segments and grouped into five clusters using a k-means clustering algorithm. Two clusters exhibited local maxima in the magnitude of dZ Ku /dz ≥ 10 dBZ e km −1 and corresponding increases in Ku-Ka dual-frequency ratio (DFR) ≥ 1.5 dB with in the DGL. Coincident in situ observations from one of these clusters revealed larger particle sizes and the presence of aggregates across the DGL. However, habit analyses from three independent imaging probes showed that pristine dendrites were rare across all clusters, comprising only a small fraction of observed particles. Instead, complex polycrystals, particularly side planes and polycrystalline plates, dominated the habit population in the DGL, with large aggregates prevalent only in the cluster exhibiting enhanced radar signatures. Thermodynamic observations showed that ice supersaturation largely remained below the criterion for dendritic growth. We hypothesize that the aggregation of predominately non-dendritic polycrystals produced the observed radar enhancements in the DGL, with only a minor contribution from dendrites.
Abstract Tropical cyclones (TCs) moving into the midlatitudes often undergo notable structural changes as they transition into extratropical cyclones (ETCs). This transition, known as the extratropical transition (ET), results from interactions between the TC and the midlatitude baroclinic environment. To elucidate the dynamical processes underlying ET, this study proposes a cyclone-following local energetics framework and applies it to TC Hagibis (2019) in the western North Pacific. The energy cycle intensifies during the ET phase, with its spatial structure becoming markedly asymmetric relative to that in the TC phase. In both the TC and ET phases, eddy available potential energy increases as its generation and energy conversion from background flow dominate over the baroclinic energy conversion. During ET, enhanced diabatic heating, warm temperature anomalies, and vertical motion appear ahead of the cyclone due to its interaction with the midlatitude jet. This leads to an asymmetric distribution of enhanced energy sources and energy conversions for eddy available potential energy. While eddy kinetic energy also increases in the TC phase as baroclinic energy conversion exceeds lateral boundary flux, it decreases and redistributes asymmetrically during ET due to enhanced divergence of eddy geopotential flux along the boundary of the western North Pacific subtropical high and the upper-level divergent TC outflow associated with jet–TC interaction. These results show that local energetics can effectively capture asymmetric thermal and dynamical processes during ET, offering a physical interpretation of TC-to-ETC transition processes.
Abstract Three-dimensional (3D) structure of the residual circulation in the upper troposphere and lower stratosphere during solstice seasons was investigated using a 3D residual flow applicable to both stationary and transient waves. This circulation corresponds to the shallow branch of the Brewer-Dobson circulation in the lower stratosphere and seamlessly connects with the tropospheric circulation. The 3D residual vertical flow was compared with the precipitation distributions. The residual meridional flow was analyzed separately to isolate contributions from stationary waves, transient waves, and ageostrophic disturbances. The relation between residual meridional flow attributed to ageostrophic disturbances and that induced by gravity wave forcing was investigated. For residual flow analysis, MERRA-2 data was used, while ERA5 data was employed to examine gravity-wave driven meridional flow. Key findings at the 200 hPa and 100 hPa surfaces are as follows: regions with strong upward residual vertical flow at both 200 hPa and 100 hPa closely correspond to high precipitation regions, while downwelling regions align with relatively weak precipitation. At 200 hPa, residual meridional flow induced by stationary and transient waves is predominantly poleward, whereas that attributed to ageostrophic disturbances is primarily equatorward. Although transient waves driving poleward residual flows are primarily baroclinic waves, poleward flow driven by transient waves was also identified under weak baroclinic conditions. At 100 hPa, contributions from stationary and transient waves, and ageostrophic disturbances to poleward residual flow were comparable. For residual meridional flow due to ageostrophic disturbances at both levels, its distribution corresponds well to that of gravity-wave driven meridional flow in the mid- and high latitudes.
Abstract Most atmospheric models treat radiative transfer as a 1D process, producing differences in surface flux termed 3D radiative effects. Modeled shortwave 3D radiative effects on domain average surface flux are positive with the sun overhead (i.e., 1D surface flux is artificially dim) and negative when the sun is near the horizon. Using a comprehensive sample of 3D radiative effects from shallow cumulus, deep convection, and stratocumulus LES cloud scenes, we decompose the 1D to 3D change in surface flux into changes due to the amount of intercepted direct radiation and scattered light produced (cloud cover), the fate of scattered light (diffuse transmissivity), and their interaction. The decomposition reveals that cloud cover, rather than transmissivity or their interaction, is the primary driver for how 3D cloud radiative effects change as the sun lowers. Next we develop a simple, quantitative model to predict 3D radiative effects based on solar zenith angle and three measures of the cloud field: vertically-projected cloud cover, domain mean aspect ratio (defined height-to-width), and 1D diffuse transmissivity. We find that across all clouds scenes and the broader parameter space explored, 3D radiative effects always change from positive to negative values as the sun lowers. The sign change occurs because transmissivity enhancement remains roughly constant with solar zenith angle while 3D cloud cover expands super-linearly, causing diminishing positive effects to eventually be outpaced by growing negative effects. Higher cloud aspect ratios accelerate this transition; higher initial coverage delays it due to cloud overlap. The model improves process-level understanding, revealing the importance of accurately representing how clouds interact with the direct beam.
Abstract This study is motivated by the observation of a unique intraseasonal power in the upper-tropospheric equatorial meridional winds over the Western Hemisphere during boreal winter. The presence of intraseasonal power at both westward and eastward wavenumbers is unusual and intriguing, as the dominant tropical modes of intraseasonal variability typically exhibit little amplitude in equatorial meridional winds and are instead characterized by strong zonal wind perturbations. Diagnostic analyses provide observational evidence of intraseasonal disturbances displaying spatial structures and dynamical characteristics consistent with mixed Rossby–gravity waves (MRGWs). Additionally, these MRGWs are confined to the upper troposphere. A systematic relationship between the location and amplitude of the intraseasonal MRGWs and the upper-tropospheric westerlies suggests that background circulation is fundamental to their existence. This hypothesis is investigated through a set of diagnostic analyses guided by the dispersion relation of a linear shallow-water model incorporating a homogeneous background flow. The results explain not only the emergence of intraseasonal MRGWs but also the overall distribution of equatorial meridional wind power throughout the troposphere. The strength and direction of the background flow govern the observed spatial and temporal characteristics of MRGWs via Doppler shifting of intrinsic MRGWs. Consequently, the spectral power distribution of equatorial meridional wind perturbations across pressure levels represents a composite of MRGWs that have been Doppler shifted by a range of background flow regimes. Significance Statement This study investigates the origin of previously unexplored intraseasonal perturbations in upper-tropospheric equatorial meridional winds over the central–eastern Pacific during boreal winter. Multiple diagnostic analyses demonstrate that these disturbances exhibit the structural and dynamical characteristics of canonical mixed Rossby–gravity waves (MRGWs). Further examination reveals they are intrinsic MRGWs whose dispersion relation is Doppler shifted by the background circulation. The spectral characteristics of equatorial meridional winds throughout the troposphere are found to be largely explained by this Doppler effect. The ability of linear theory to account for a substantial fraction of a dominant mode of tropical variability highlights its potential to improve prediction of tropical weather and subseasonal variability.
Abstract Turbulent fluctuations of supersaturation, driven by water vapor and temperature variability, significantly influence cloud droplet activation and droplet size distributions (DSDs). Accurate modeling of this variability is challenging because of the complex interactions among turbulent mixing and the phase change itself, affecting variabilities and covariabilities of scalar fields. In Chandrakar et al. (2022, 2023), we introduced a new subgrid-scale model to capture supersaturation variability in such complex conditions. In this study, the performance of this model, coupled with droplet growth via a set of Lagrangian stochastic differential equations in large-eddy simulations (LES), is evaluated against the reference direct numerical simulations (DNS) of the Pi Convection Cloud Chamber. Lagrangian statistics of water vapor, temperature, supersaturation, and droplet growth from LES with the new subgrid model capture both the magnitudes and structures in DNS at two different sidewall forcings. Most importantly, the LES coupled with the subgrid-scale model reproduces DSDs from the reference DNS for both forcings. DNS results also show notable differences in the Lagrangian statistics of water vapor, temperature, supersaturation, droplet size, and their cross-correlations under clean versus polluted cloud conditions. A relatively broader DSD in the clean condition and damped supersaturation fluctuations in the polluted condition occur. A theoretical scaling of the second-order structure function of droplet size is presented, which is consistent with simulation results. Overall, this study provides a tested framework for improved modeling of subgrid-scale interactions between the supersaturation field and cloud particles in future atmospheric cloud simulations.
Abstract Colder supercell outflow—generally linked to lower boundary layer relative humidity—is often detrimental to tornadogenesis and maintenance. However, the rear flank is anything but homogeneous; surface temperatures within a single rear-flank downdraft may vary by as much as 10–20 K over just a few kilometers. In this study, we analyze how local variations in supercell outflow and the near-inflow environment might influence tornadogenesis and evolution. This is accomplished with a 25-member ensemble of high-resolution idealized simulations, with each member made unique by the addition of a small region of cooler air prior to tornadogenesis. In each simulation, the resulting “blob” of cooler near-surface air advects toward the developing vortex and, in some cases, meaningfully alters the resulting vortex-scale evolution. Regardless of the initial blob location, all of the ensemble members featured a vortex with weaker peak intensity than the blob-less control run. Blobs inserted in the near inflow exhibited the least impact on tornadogenesis and evolution; the blob encountered the rear-flank gust front and was advected away from the low-level updraft. Vortices were meaningfully weaker in simulations with a blob inserted in the forward flank. Although vortex-bound Lagrangian vorticity diagnostics were similar between this run and the control run, the blob resulted in a suboptimal horizontal separation of the developing vortex from the low-level updraft core. These findings highlight the sensitivity of vortex development to local cooling, perhaps reminiscent of rear-flank internal surges or cold pools from cell mergers in the real atmosphere. Significance Statement Some relationships between the larger-scale background environment and storm-scale characteristics exist that aid in real-time tornado prediction. One involves the link between low-level relative humidity and storm outflow temperature. Motivated by the often-heterogeneous nature of storm outflow, we examine the sensitivity of tornado development and evolution to controlled regions of cooling as the tornado forms. Our results show that tornado evolution can be quite sensitive to cool pockets of air, particularly those located upstream of the developing tornado. These findings highlight how sensitive simulated tornadoes can be to small changes in their surroundings and the reasons for this behavior.
Hail melting and shedding of surface water govern how hailstones evolve below the melting layer and modulate precipitation, downdrafts, and cold pools. We present controlled wind tunnel experiments on pure ice spheres (10-24 mm) at 228C and varying humidity, combining high-speed video with digital holography to track melting, onset of shedding, shed water mass, and drop size spectra. Melting rates collapse onto a single normalized curve, enabling a humidity-independent parameterization of meltwater fraction versus relative time (ratio of instantaneous and total melting time). We provide Reynolds number relations as functions of ice core mass and a Sherwood number fit, implying slightly enhanced ventilation relative to smooth spheres. The retainable surface water mass develops nonlinear with ice core mass and peaks at a hailstone size of approximately 15 mm. The shed water mass per event follows power laws in Reynolds number and ice core mass, and the bimodal drop spectrum spans cloud drizzle to raindrop sizes. Implementing these parameterizations into a box model reproduces time series of terminal velocity, maximum dimension, volume, and meltwater fraction. Thus, they are applicable in cloud models to determine the critical meltwater mass before shedding, the amount of water released per shedding event, and the resulting shed drop size distribution. Embedding them in a one-dimensional (1D) kinematic microphysics model shows that the recycling of liquid water by shedding and its impact on drop and hail spectra strongly depends on the incoming hail size distribution.
Abstract This paper investigates the dynamics governing multibanded cloud and precipitation structures in extratropical cyclones through a case study from the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. On 1 February 2020, a low pressure system emerged off the North Carolina coast at 1200 UTC, deepening by 7 hPa in 6 h as it accelerated northeast over the Atlantic. High-resolution Geostationary Operational Environmental Satellite (GOES) visible imagery revealed multiple bands of high-reflectance cloud to the north/northeast of the center, along with clusters of convective cells closer to the core. Wavelet analysis identified a dominant multiband wavelength of 30 km and a secondary peak at 15–20 km. Airborne radar measurements from IMPACTS flights showed deep convection near the center and narrow, elevated reflectivity bands linked to the multiband features farther out. Numerical simulations reproduced the multibands, enabling exploration of their dynamical origin. Intrinsic phase speed and polarization relation calculations revealed that, contrary to several previous studies, the dominant multibands were not gravity waves. Instead, the bands were identified as manifestations of inflection-point instability due to the presence of counterrotating secondary circulations and the vertical wind shear profile with low Richardson numbers near the upper-level outflow. Gravity waves were present in the low- to midlevels generated by convection, but they did not account for the strong perturbations in the mid–upper levels. These new scientific insights into the governing dynamics of multibanded structures in extratropical cyclones highlight the role of shear-driven hydrodynamic instabilities. Significance Statement Extratropical cyclones along the U.S. East Coast can produce intense bands of snow and other types of precipitation that cause travel chaos and potential loss of life. In this work, a storm of this type was studied using NASA aircraft and satellite data as well as computer simulations. The results show that the storm bands were not caused by atmospheric waves, as scientists often thought, but by sharp changes in the wind speed with height called shear. This shear created unstable layers that rolled and mixed the air, forming organized cloud and precipitation patterns. Understanding these fundamental physical processes can help forecasters better predict when and where heavy snowbands will form, improving warnings and securing public safety.
Abstract By diagnosing 30 years of atmospheric reanalysis, this study presents the three-dimensional distribution of wave energy fluxes (WEFs) globally within monthly climatological fields. The results, combined with a theoretical analysis, reveal two key dynamical features for midlatitude waves in westerly jets. First, in the lower troposphere, wave-activity (WA) flux is directed upward, while the WEF is directed downward, originating from adiabatic (winter) and diabatic (summer) sources at the middle troposphere. The ratio between the vertical components of WEF and WA flux provides a measure confirming negative WA in the bulk lower troposphere. Second, the present study introduces a rotational flux in the zonal-vertical plane to an existing WEF diagnosis, which acts only in the presence of the vertically trapped modal structure of waves. This rotational flux provides upwind sinking and downwind rising of vertical WEFs, offering a way to identify the counter-shear nature of zonal WEFs. The upper layer of this system exhibits westward WEFs that are outcropped on the polar side of the westerly jets. The rotational flux fades in tropical regions, determining cross-equatorial upper-tropospheric WEFs in the eastern tropical Pacific in relation to the zonal variation of the Hadley upwelling during austral summer.
Abstract Accurate prediction of nocturnal convection remains a forecasting challenge, particularly in stable nighttime environments where atmospheric bores can influence convective development. Previous studies suggest that low-level jets (LLJs) and nocturnal stable boundary layers provide favorable environmental conditions for atmospheric bore generation, yet their relative roles remain unclear. This study performs a series of idealized numerical simulations to isolate and quantify the impacts of LLJ strength and thickness, as well as the strength and depth of the low-level stable layer, on the formation and maintenance of cool-pool-driven bores. The results show that bore intensity increases with LLJ strength, whereas a thicker LLJ, associated with weaker vertical wind shear, weakens wave trapping and shortens bore longevity. In the absence of an LLJ, a cold pool can still produce a bore in environments characterized by a moderately deep stable layer, corresponding to a partially blocked regime, with stronger low-level stability supporting longer-lived bore propagation. In contrast, extremely shallow or excessively deep isolated stable layer cannot support bore propagation without LLJ support. Overall, both the LLJ and low-level stable layer play essential and complementary roles in promoting bore generation and sustaining bore evolution under various environmental conditions. Significance Statement More accurate prediction of nocturnal convection is needed in some regions due to heavy rainfall and flooding, and improving the representation of bores is a required part of this needed improvement. This study investigates how cold-pool-driven bores respond to variations in low-level jet (LLJ) strength and depth, as well as to different configurations of the low-level stable layer. The results show that a strong LLJ enables a cold pool to drive more intense bores, whereas a sufficient strong stable layer can support bore propagation even in the absence of an LLJ. These findings provide new insights into the physical mechanisms governing bore formation and evolution across diverse environmental conditions.
The radius of maximum wind (RMW) of a tropical cyclone (TC) plays a critical role in shaping the wind field and associated hazards. Observations report RMW values as small as 5 km, yet the existence and magnitude of a physical lower bound remain unclear. Using an axisymmetric boundary layer diagnostic model, we examine how small the RMW can become before the inner core becomes unstable and undergoes rapid expansion. A nondimensional parameter defined as the horizontal mixing length normalized by the RMW, lh = lh/rm, emerges as a key control in the model. We identify a critical threshold of lh = 0:2, above which horizontal turbulent diffusion controls the boundary layer outflow structure and induces an abrupt outward shift of the boundary layer ascent center. This ascent shift acts as a robust indicator of structural instability and imposes a strong constraint on the minimum achievable RMW. For a typical horizontal mixing length of lh = 1000 m, the implied lower bound of the RMW is approximately 5 km. The ascent shift resembles eyewall replacement cycles and arises from the horizontal-turbulent-diffusion-induced modification of the supergradient wind distribution. These results highlight a potentially underappreciated pathway for inner-core reorganization in compact TCs and suggest that horizontal turbulent diffusion contributes to the self-regulation of the TC inner-core structure.
Abstract Mixed-layer similarity is a cornerstone framework for interpreting and modeling turbulence in studies of the convective boundary layer (CBL), providing a basis to collapse turbulence statistics across a wide range of conditions. However, most previous studies of the CBL have focused on dry air, leaving the influence of moisture on turbulence scaling laws largely unexplored. Using a database of 24 large-eddy simulations of the moist (but cloud free) CBL encompassing two values of global stability − z i / L , three values of evaporative fraction E f , and four values of humidity entrainment flux ratio φ wq , we examine the influence of humidity on the scaling of turbulence variances and flux profiles in the mixed-layer of the CBL. While mixed-layer scaling generally performs well for small evaporative fractions ( E f = 0.2), neither mixed-layer nor entrainment-based scaling is sufficient to collapse turbulence statistics at higher evaporative fractions ( E f = 0.5 and 0.8). Rather, flux and variance profiles become highly sensitive to the value of φ wq once the surface latent heat flux is comparable to the sensible heat flux. Moreover, we observe significant differences in the entrainment flux ratio for heat, entrainment zone depth, and partitioning of the buoyancy production of turbulent kinetic energy into contributions from heat and moisture. These results highlight the role of water vapor as an active scalar in the moist CBL, whose dissimilarity with temperature leads to the breakdown of classic mixed-layer scaling. Significance Statement The individual role of moisture within the framework of mixed-layer similarity theory has not been systematically characterized. In this study, we explore mixed-layer similarity scaling across 24 idealized large-eddy simulations of the moist convective boundary layer (CBL) that can be described by four dimensionless variables: height, stability, evaporative fraction, and entrainment humidity flux ratio. By decomposing the buoyancy production of turbulence kinetic energy budget term into separate contributions by heat and humidity, we find a strong dependence on the thermodynamic boundary conditions of each case. Our results indicate that traditional mixed-layer similarity theory is increasingly unable to account for the role of humidity as an active scalar on turbulence statistics within the moist CBL with increasing evaporative fraction.
Twenty-year satellite observations have shown pronounced offshore propagation of the rainfall diurnal cycle at the northern coast of New Guinea. Propagation speed has a wide range from 5 to 15 m s-1 even when mean background winds are weak. This study investigates the mechanisms driving this variability in propagation speed, including why the slower-propagation rainfall events can travel more than 600 km, using the Maritime Continent Austral Summer Climatology v1.0, which provides 10-yr high-resolution model simulations. Days with pronounced propagation are analyzed, and the top 30% and bottom 30% of propagation speeds are classified as faster and slower cases, respectively. The faster-propagation group exhibits more widespread scattered rainfall patterns, primarily driven by inertia-gravity waves generated by land-sea thermal contrast. In this group, clearer skies allow greater daytime absorption of shortwave radiation, which amplifies the land-sea temperature contrasts and thereby generates stronger inertia-gravity waves that drive faster offshore rainfall propagation. Conversely, the slower group displays more concentrated rainfall, closely associated with cold pool dynamics farther offshore. Stronger low-level wind shear plays a key role in this group by favoring convection initiation along the leading edges of cold pools. The balance between low-level wind shear and cold pool intensity promotes more organized convection, whose offshore movement is primarily controlled by the cold pool propagation speed, resulting in slower but long-distance rainfall propagation. A key finding is that both inertia-gravity waves and cold pools can sustain offshore rainfall propagation over distances exceeding 600 km in New Guinea, in contrast to traditional studies that emphasize gravity waves as the primary driver of far-offshore propagation.
The dominant turbulence length scale A in the atmospheric boundary layer characterizes the size of the most energetic turbulent eddies. It plays a key role in scale-adaptive planetary boundary layer (PBL) schemes for numerical weather prediction models operating at kilometer-scale resolutions. However, the vertical profile of A for the entire boundary layer has remained elusive due to deficiencies in its quantitative definition. Specifically, conventional definitions of A rely on the magnitude of the Reynolds-averaged flux T, hence fail when T is close to zero, leading to discontinuities and fluctuations in the A profile. To overcome this deficiency, this study proposes an alternative definition of A that does not depend on the value of T. Instead, it determines A as the grid resolution that maximizes the intergrid variance of the subgrid-scale flux. The idea behind the proposed definition is illustrated with a wavenumber analysis. A comparative evaluation among A profiles extracted from large-eddy simulations (LESs) of the convective boundary layer (CBL) with the new and the conventional definitions is conducted. While achieving close agreement with the conventional definitions for the most part, the new definition provides physically meaningful and vertically continuous estimates of A for the entire CBL. Applying the new definition, self-similar profiles of the normalized A for the vertical sensible heat and momentum fluxes are obtained for use in scale-adaptive PBL schemes.
Abstract We propose a simple model of Rossby–Kelvin instability based on a flow with zero potential vorticity gradients except for a single discontinuity in potential vorticity at the jet latitude. The basic state has constant angular momentum equatorward of the jet latitude and uniform relative vorticity on its poleward side. With no potential vorticity gradients equatorward of the jet, Rossby waves are trapped, and the rotational circulation in the tropics is entirely determined by the vorticity perturbation at the vorticity front. The simple model produces unstable modes in good agreement with previous studies of Rossby–Kelvin instability and with the modes found to drive superrotation in some idealized simulations. The simplified framework facilitates an interpretation of the instability in terms of the interaction between the divergent Kelvin wave circulation and the rotational Rossby wave circulation. It is shown that the growth of the modes is associated with the spatial correlation between equatorial divergence and anticyclonic subtropical vorticity and requires an eastward tilt with latitude of the zonal wind perturbation. Significance Statement In fluid mechanics, instabilities arise when small perturbations to a background flow known as eddies grow spontaneously by exchanging energy or other related quantities with that background flow. In the atmosphere, these instabilities are important not only for producing weather variability but also for determining the mean climate through their associated eddy transports of heat and momentum. Baroclinic instability, for instance, has long been known as an essential ingredient for the extratropical circulation. In this paper, we are concerned with a different form of fluid instability known as Rossby–Kelvin instability that is more relevant for the tropical circulation. Because of the parameter regime for which it happens, this instability is more complicated and poorly understood than baroclinic instability. We propose a simplified model of Rossby–Kelvin instability that facilitates the calculation of the unstable eddies and helps rationalize the physics of this instability.
Faceted ice crystals with hollowing are common in atmospheric cold clouds, yet there exist few analytical theories of their growth from the vapor. Previously published analytical theories do not allow the source of surface steps, and step formation mechanism, to vary with location on the crystal surface. We develop and extend two analytical models for the vapor growth of single crystalline ice that include growth during hollowing by allowing the formation of steps from various mechanisms near facet corners or at the center of crystal facets. A hybrid capacitance-based theory for step-mediated growth near crystal corners is also developed, and it approximately reproduces faceted growth during hollowing. Comparisons of these theories with crystals grown in a free-fall chamber suggest that growth at temperatures above-10 degrees C is consistent with steps nucleating near the corners of the most rapidly growing faces, but dislocations emerging at the facet centers are required to explain the slow-growing faces. Nearly isometric crystals can be explained by dislocations emerging from the face middles. In contrast, crystals grown on a substrate at temperatures near-50 degrees C are best explained by steps nucleated near the facet corners. In all cases, crystal growth is very sensitive to hollowing once the rim width becomes relatively narrow. This result indicates that quantification of crystal growth requires careful rim-width measurements of very hollowed faces.
This study investigates mesoscale potential vorticity (PV) variability in the upper troposphere and lower stratosphere (UTLS) in tropical cyclones (TCs) using 9-km resolution, six-hourly ECMWF operational forecast analyses, in order to characterize its behavior during TC life cycles. TCs exhibit a diurnal cycle in deep convection with a maximum overnight. Since mesoscale variability of UTLS PV is caused by deep convection, it should also exhibit a diurnal cycle. Spatial standard deviations, σ PV , and means are calculated in a 6°×6° domain following a storm track. The distinctive maximum in σ PV at the tropopause is caused by the upward increase in mean static stability. A key time was chosen to be the first 6 pm after reaching C1. All storms reached C2 the night after the key time. We required at least a six day track, and compared 3 days before and after reaching C2. This method was first applied to TC Yasa. We then analyzed a total of TCs which met these selection criteria in the South Pacific during 2016–2024 (13 C1–C2 and 10 C3–C5). A strong diurnal cycle in 100 hPa σ PV was found, with a development phase (days 1–3) and a mature phase (days 4–6). From days 1–3 to days 4–6 there is a reduction in mean σ PV by ~ 25% and in diurnal range by ~ 45% for C1–C2 and ~ 60% for C3–C5, and elimination of the preference for a 6 am peak. These results are consistent with cloud-radiative forcing theories of TCs.
Abstract There is strong evidence that the atmospheric moisture content of several solar system planets, including Earth, has varied over their lifetimes. A growing body of work also documents a range of atmospheric water vapor content on exoplanets. An improved understanding of the coupling between atmospheric moisture availability and convection could yield greater intuition about the past and current states of planetary atmospheres, including Earth’s atmosphere. In this work, we investigate the changing heat engine behavior of localized radiative-convective equilibrium convection in a suite of moist-to-nearly-dry numerical simulations. Each simulation has a constant surface relative humidity, with values ranging from saturated to nearly dry surface conditions. We observe a deepening of the planetary boundary layer and a corresponding lifting of the cloud base under surface drying, in agreement with previous numerical and observational studies. The primary factor contributing to this is the reduction in the lifting condensation level temperature implied by the Clausius-Clapeyron relationship. Additionally, a diagnostic of the overall mass transport by atmospheric convection increases in drier conditions, consistent with prior work. This mainly results from an increase in planetary boundary layer convective mass transport. In contrast, free-tropospheric convective mass transport decreases, in agreement with previous studies. Finally, we find that surface evaporation is associated with less irreversible entropy production under surface drying and transitions from a spontaneous process to a non-spontaneous process. This occurs because near-surface air is more humid than the surface in the drier experiments, whereas in moister conditions the boundary layer is drier than the surface.
Abstract Convective aggregation, often leading to organized convective systems, is ubiquitous over tropical oceans and contributes substantially to both total and extreme precipitation. Previous theoretical studies have emphasized the crucial role of convection–moisture feedback in convective aggregation, yet the additional contributions of other dynamical processes remain poorly understood. Here, we develop a two-layer stochastic dynamic model to isolate and quantify how compensating subsidence, cold pools, and vertical shear influence convective aggregation beyond the convection–moisture feedback. The novelty of this model lies in three key aspects: adding compensating subsidence as a dynamic mechanism to maintain the total number of individual convective cells, coupling the boundary layer moist static energy (MSE) budget to represent both suppressing and triggering effects of cold pools, and incorporating the effect of vertical shear on convection initiation through convection triggering probability. The results show that the suppressing effect of cold pools is essential for keeping individual convective cells separated, while their triggering effect promotes larger cluster sizes and faster aggregation; this triggering effect is reduced in the presence of interactive compensating subsidence. The model is then applied to investigate how convective aggregation responds to changes in mean convection lifetime and mean boundary layer MSE under warming. Besides, the model reveals a crucial role of vertical shear in controlling cluster morphology and propagation. The model is expected to serve as a theoretical tool for investigating the impact of various physical processes on convective aggregation and may potentially serve as a prototype for convection parameterization that incorporates these dynamical processes. Significance Statement Understanding how tropical rainstorms organize and evolve is critical for improving weather and climate predictions. This study introduces a simplified model that captures how key dynamical processes influence the formation and clustering of storms over tropical oceans. The model shows that compensating subsidence regulates the total number of convective cells, cold pools influence cell distribution, cluster size, and aggregation period, and vertical shear shapes cluster morphology. The findings also provide new insights into how storm systems may respond in a warming climate. The model offers a valuable theoretical framework for understanding key physical processes and is simple enough to serve as a foundation for improving the representation of storm behaviors in larger-scale weather and climate models.
