Journal of the Atmospheric Sciences

Tropical cyclone (TC)-radiation interaction is important in TC development in which the structure of moisture and cloud modulate radiation. This study validates the representation of moisture, cloud and TC-radiation interaction in the products of the Goddard Earth Observing System atmospheric model, version 5.12.4 (GEOS5) by utilizing a satellite-based observational estimation of TC-radiation interaction from the CloudSat Tropical Cyclone (CSTC) dataset. The GEOS5 products include the Modern-Era Retrospective Analysis for Research and Application, Version 2 (MERRA-2), and the MERRA-2 Atmospheric Model Intercomparison Project (AMIP) set of simulations (M2AMIP). Under similar TC intensity, GEOS5 TCs experience a comparable longwave (LW)-cloud feedback to that in CSTC despite having at least an order of magnitude less cloud ice mass. This results from the presence of upper-tropospheric ice clouds which are efficient at reducing the outgoing LW radiation. The clear-sky LW feedback is greater in GEOS5 TCs which results from a greater occurrence frequency of dry conditions in the TC’s outer region. The LW radiatively-driven circulation favors TC development while the SW-driven circulation is confined to the upper troposphere and modulates the circulation at the outflow layer. TCs in CSTC experience a deep-layer LW radiatively-driven circulation, while that in GEOS5 TCs is confined to the upper troposphere and is less effective at supporting TC development, which might contribute to the low intensity bias of GEOS5 TCs. The top-heavy distribution of ice in GEOS5 TC restricts the radiatively-driven circulation to the upper troposphere. The small discrepancies between GEOS5 outputs indicates that the data assimilation process for MERRA-2 does not substantially alter the TC-radiation interaction in GEOS5.

To illuminate the nature of convective organization, a competition for uniformly supplied destabilization is staged in a nonrotating cyclic cloud model. Scattered convection in a control-run equilibrium is driven to aggregate in a belt covering half the domain, by prescribing vertically sheared wind or warm-rain process denial. The equilibrium response is a belt-scale ascent which transports moisture toward the perturbed area. Larger and longer-lived rainstorms occur, whose competitive success earns the title of “organized”. Such success is revealed by the resulting domain-wise circulations and suppressed rainfall over the unperturbed areas. Although shear is adverse to columnar updrafts, it has beneficial effects later, fostering wider (less-entraining) updrafts and perhaps another kinetic energy source besides buoyancy. Warm-rain process denial (more purely adverse to columnar precipitation processes) reveals similar responses in equilibrant circulations and precipitation as the shear experiments, overall suggesting a “hardship breeds hardiness” interpretation. Sudden denial in a belt illustrates the multi-timescale succession: cloud water builds up (few hours), a pulse of rain (hours 5-10) initiates the belt-scale circulation, and some continuing refinement (up to 20 hours) warms the upper levels everywhere, reducing all updrafts’ buoyancy. Parameterization of organization in terms of such controlling factors is discussed.

Various microphysical models attempt to explain the occurrence of broad droplet size distributions (DSD) in clouds through approximate representations of the stochastic droplet growth by condensation in a turbulent environment. This work analyzes specific idealized models, where the variability of droplet growth conditions arises primarily from variability in the turbulent vertical velocity of the air carrying these droplets. Examples are the stochastic eddy hopping model operating in adiabatic parcels and certain types of DNS-like models. We show that such models produce droplet size statistics that are spatially inhomogeneous along the vertical direction, causing the predicted DSD to depend on the DSD spatial sampling scale Δ. In these models, Δ is implicitly related to the spatial extent of the droplets turbulent diffusion (approximated by Brownian-like excursions) and thus grows like t 1/2 . This leads to spurious continuous DSD broadening, as the growth in time (also like t 1/2 ) of the standard deviation of droplet squared radius arises essentially from the growth of the sampling scale Δ. Also, the DSDs predicted by the models discussed here are non-locally broad (in the sense that large and small droplets are not well-mixed in sufficiently small volumes) and thus do not necessarily indicate enhanced probabilities of gravity-induced droplet coagulation. In the effort to build a firm physical basis for subgrid parametrizations, this study presents a framework to explain the merits and limitations of idealized models, indicating how to assess and use them wisely as a subgrid representation of turbulent condensation in large-eddy simulations of clouds.

Low-level mixed-phase clouds occur frequently and persistently in the central Arctic and thus play a key role in climate feedback mechanisms, airmass transformations, and sea ice melt. Turbulent entrainment at cloud top driven by radiative cooling modulates these clouds by affecting the boundary layer heat budget. However, reliable measurements of this small-scale process are scarce. This study presents new insights into entrainment in radiatively driven cloudy mixed layers at high latitudes based on a library of daily large-eddy simulations covering the full Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) drift. The simulations are based on measurements, cover a periodic and homogeneously forced small domain representing conditions observed at the Polarstern research vessel, and resolve Arctic turbulence and clouds to a high degree. Approximately 1 out of 3 simulated days contains cloud mass in the liquid phase. A drift-average heat budget analysis shows that the bulk cloud-topped mixed layer is dominated by radiative cooling. Warming by top entrainment partially counters this cooling, at efficiencies of about 21%. While this compensation is significant, such efficiencies are also much lower compared to previous findings for subtropical warm marine stratocumulus. Interestingly, a few outlying MOSAiC cases show similarly high efficiencies. Analysis of turbulence energetics and dedicated sensitivity experiments reveals that high entrainment efficiency can be achieved in two ways: surface coupling and strong local wind shear. The former explains the high efficiencies in the subtropics, while the latter drives the highest efficiencies encountered during MOSAiC. In general, these findings emphasize the important role played by wind shear in boosting entrainment efficiency. Significance Statement Stratocumulus cloud layers in the high Arctic often contain liquid water at subzero temperatures. Such cloud layers cool rapidly through radiation, a process that locally creates turbulence. This turbulence causes mixing in the atmosphere. Using many high-resolution atmospheric model simulations on a supercomputer, we investigate how this mixing process causes warming in the cloud layer. The simulations are closely based on measurements made during the recent yearlong Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) drift experiment. We find that the warming only partially counteracts the radiative cooling and that a strong change in the wind with height can affect this balance.

The evolution of micron-sized droplets in clouds is studied with focus on the ’sizegap’ regime of 15-40 μm radius, where condensation and differential sedimentation are least effective in promoting growth. This bottleneck leads to inaccurate growth models and turbulence can potentially rectify disagreement with in-situ cloud measurements. The role of turbulent collisions, mixing of droplets, and water vapour fluctuations in crossing the ’size-gap’ has been analysed in detail. Collisions driven by the coupled effects of turbulent shear and differential sedimentation are shown to grow drizzle sized droplets. Growth is also promoted by turbulence-induced water vapour fluctuations, which maintain polydispersity during the initial condensation driven growth and facilitate subsequent growth by differential sedimentation driven coalescence. The collision rate of droplets is strongly influenced by non-continuum hydrodynamics and so the size evolution beyond the condensation regime is found to be very sensitive to the mean free path of air. Turbulence-induced inertial clustering leads to a moderate enhancement in the growth rate but the intermittency of the turbulent shear rate does not change the coalescence rate significantly. The coupled influence of all these phenomena is evaluated by evolving a large number of droplets within an adiabatically rising parcel of air using a Monte Carlo scheme that captures turbulent intermittency and mixing.

The Kuo-Eliassen equation provides the mean meridional circulation that must be present for the axisymmetric component of a flow forced by heat and momentum sources to remain balanced as it evolves. It does not tell us whether or not the flow is steady. Using this equation to explain how the mean meridional circulation is perturbed due to a change in thermal or momentum forcing, including the forcing due to large-scale eddies, requires a division of the forcing into prescribed and reactive parts and, most importantly, a physical theory for the latter. It should not be used without explicit discussion of the assumptions being made about the reactive component of the forcing and justification for the choice being made.

Anomalies in the stratospheric polar vortex (SPV), such as sudden stratospheric warming (SSW) events, significantly impact surface weather patterns. While the influence of SSWs on the troposphere is robust on average, individual events exhibit large variability, partly due to the substantial difference in dynamics and SPV evolution across events. Understanding the physical processes driving SSWs is therefore important. In this study, we investigate SPV dynamics, focusing on non-linear coupling between planetary wave modes. We use potential enstrophy and eddy total energy budget analyses to quantify the contributions of different physical processes to SPV evolution. These budget analysis frameworks are unique in being able to study the contribution of non-linear wave–wave interactions to the dynamical evolution of the SPV. When applying this framework to both an idealized simulation and re-analysis data of the 2003 SSW, we find that non-linear wave–wave interactions can play a crucial role during SSWs. In the idealized simulation, wave-2 structures emerge in the stratosphere without a prescribed wave-2 source, attributed to non-linear transfer of enstrophy and energy from wave-1 to wave-2. In the 2003 case study, interactions between wave-1 and wave-2 contribute to a displacement-to-split transition. We also find indications of quasi-linear coupling and upscale enstrophy transfer from wave-2 to wave-1 during this period. The use of the enstrophy budget analyses highlights the significant impact of non-linear wave–wave interactions in SPV transitions. These complex interactions contribute to the uniqueness of each SSW event and may help explain the variability observed across different SSWs.

Lying at the heart of the North Atlantic Oscillation (NAO) is the interaction between the low-frequency processes and synoptic eddies. Though extensively studied, a consensus is still elusive. Particularly, a localized Lorenz energy cycle is lacked, which may provide a scenario with details disappearing in the global cycles as obtained. Using a functional analysis apparatus specifically for this purpose, namely, multiscale window transform (MWT), and the MWT-based theory of canonical transfer, for the first time such a cycle is constructed here. The result clearly shows that the energetic processes occur in different regions—a characteristic overlooked in previous studies. Specifically, we find that the kinetic energy (KE) transferred from the synoptic eddies to the low-frequency flow (over the ocean southwest of British Isles) is firstly transported to the rising branch of the secondary circulation at the exit region of the NAO jet stream, and converted into available potential energy (APE) there. The gained APE is then transported to the sinking branch of the secondary circulation at the entrance region of the jet stream, enhancing its ambient baroclinicity. More synoptic eddies are hence generated, and their energy is transported to the southwest of British Isles to compensate the energy consumption there. These processes together form a localized energy loop, in which the low-frequency flow and the synoptic eddies feed each other, generating a positive feedback. The implications here are that the positive feedback mechanism proposed in a zonally uniform framework with only one secondary circulation may need to be updated to incorporate two secondary circulations, together with their localized spatial variations.

Turbulence has long been considered an important source of droplet spectral broadening in adiabatic volumes of warm convective clouds. The key idea is that, in a turbulent environment, droplets follow different trajectories, and this leads to wide droplet spectra for droplets arriving at a given location inside a cloud. This has been referred to as eddy hopping. Past theoretical studies and idealized turbulence simulations applying direct numerical simulation (DNS)-like approaches suggested that eddy hopping can potentially explain the difference between observed droplet spectra and those predicted from adiabatic ascent in a non-turbulent volume. This paper considers droplet spectra in an adiabatic volume not far from the cloud base in an unprecedented high-resolution (7.5 m grid length) three-dimensional (3D) simulation of a warm turbulent cumulus congestus cloud applying Lagrangian particle-based microphysics. The spectral width approaches several tenths of 1 µm in the 3D simulation versus only up to 0.2 µm in a reference non-turbulent adiabatic parcel. We apply an idealized one-dimensional stochastic cloud updraft model that either excludes or includes turbulent vertical velocity fluctuations to show how the fluctuations affect cloud condensation nuclei (CCN) activation and subsequent growth of cloud droplets. Droplet spectra are significantly wider when effects of turbulence are included. The more complete droplet growth equation that includes kinetic, surface tension, and solute effects above the cloud base significantly adds to the variability of cloud droplet growth in the turbulent flow and thus to the adiabatic spectral width at a given height within the simulated cloud.

It is well known that African easterly waves (AEWs) can develop into tropical cyclones. However, the processes leading to development are not well understood. To this end, we examine a 38-yr climatology of AEW tracks sorted into developing AEWs (DAEWs) and strong nondeveloping AEWs (SNDAEWs). Wave-centered composites for tracks in the eastern Atlantic (40°–10°W, 5°S–30°N) and West African monsoon regions (10°W–20°E, 5°S–30°N) reveal that DAEWs occur over a more humid background state in both regions. The more humid environment causes DAEWs to exhibit heavier precipitation and wave amplification via vortex stretching. Examination of the column moist static energy (MSE) budget reveals that DAEWs exhibit stronger radiative heating and more moistening via horizontal MSE advection than SNDAEWs. The stronger horizontal MSE advection in DAEWs is due to a northeast shift in the maximum MSE relative to the wave axis, causing the northerlies in the wave to advect a higher MSE into the maximum precipitation. In contrast, MSE is maximum near the center of NDAEWs, making the moistening of the rainfall by horizontal MSE advection weaker. DAEWs exhibit stronger radiative heating per unit of rainfall relative to NDAEWs, suggesting that cloud-radiative feedbacks are stronger in these systems. The sum of horizontal MSE advection and radiative heating explains the buildup in MSE seen over the rainy region of the DAEWs that is not seen in SNDAEWs. These results underscore the importance of moisture, cloud–radiation interactions, and horizontal MSE advection in tropical cyclone (TC) development over these regions. Significance Statement African easterly waves are the most common precursors of tropical cyclones in the Atlantic basin. Despite significant progress in understanding the processes that distinguish waves that develop into tropical cyclones versus those that do not, important gaps in knowledge remain. In this study, we employed a wave-centered compositing scheme and the moist static energy budget to understand the differences between easterly waves that develop and the strongest nondeveloping waves. Our results show that waves that develop into tropical cyclones occur in a more humid environment where less dry air is transported toward the wave’s rainy region. The more humid environment is also associated with stronger rainfall as well as stronger radiative heating in developing waves, the latter which favors the buildup of moisture in developing waves. Our results underscore the importance of water vapor and its horizontal distribution in determining the development of African easterly waves.

The life cycle of multibanded precipitation structures is closely examined using an idealized baroclinic wave simulation down to 4-km grid spacing. The model develops a wedge-shaped region of multibanded precipitation east of the near-surface low center within a region of 700–500-hPa potential instability and 600–500-hPa southwesterly vertical wind shear. Cells that develop near the southern tip of the wedge elongate into southwest–northeast-oriented bands as they move northward with the mean flow and then dissipate several hours later as they move further northward within the cyclone comma head. Using a band-following framework and a potential vorticity (PV) budget, the processes resulting in band genesis, growth, and decay are investigated. First, the cell’s moist updraft from below 600 hPa redistributes the horizontal vorticity within the 600–500-hPa layer into the vertical, which combined with latent heat release results in a horizontal PV dipole around the cell. This PV is advected northeastward at midlevels, causing the dipole to extend from the cell. Flow perturbations between the two PV anomalies result in 600–500-hPa divergence northeast of the cell and an elongated region of upward motion and the genesis of the precipitation band. The PV dipole and band continue to intensify primarily from latent heating. As the band moves northward away from the 700–500-hPa potential instability, diffusion and turbulent mixing weaken the PV dipole and the circulations maintaining the band. As the updraft subsequently weakens, snow fallout persists for about 1–2 h before the band fully dissipates. Significance Statement Multibanded precipitation structures are difficult to predict and can greatly affect snowfall accumulations. The small-scale (20–40 km) nature of these features makes them difficult to sample with observations, thereby impacting how accurately they are conveyed at the start of model forecasts. This study investigates the mechanisms that cause snowbands to grow and decay in a numerical weather model. The initial updraft of the developing band creates circulations that perturb the ambient flow aloft, which in turn induces additional updrafts that cause the band to develop. After several hours, the band dissipates as processes related to turbulent mixing disrupt these circulations.

Recent studies based on modeling frameworks have shown that cloud radiative feedback can accelerate the early stage development of tropical cyclones (TCs). In this study, we utilize satellite data from the Clouds and the Earth’s Radiant Energy System (CERES) and CloudSat, along with the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) (ERA5), to explore the role of cloud radiative feedback in TC genesis by comparing developing (DEV) and nondeveloping (NDEV) TC seeds in their early stages. Results show that cloud longwave heating dominates the positive feedback of convective organization and drives a transverse circulation, which assists in moistening the inner core of the incipient vortex. In contrast, shortwave plays a minor role and modulates the diurnal structure variation. The comparison between DEV and NDEV systems reveals that the DEV group exhibits more vigorous convection near the vortex center, generating stronger cloud longwave feedback and circulation response, which further favors TC genesis. The examination of environmental factors further demonstrates that vigorous convection in the DEV system is associated with less ventilation, primarily attributed to weaker vertical wind shear (VWS) and lower midlevel entropy deficit.

The Spectral Weak Temperature Gradient (SWTG) method uses the weakness of horizontal temperature gradients in the tropics to parameterize the large-scale dynamics in limited-area or single column models. In its original configuration, the SWTG method applies to the full depth of the troposphere without explicit consideration of the greater horizontal temperature gradients in the planetary boundary layer (PBL) that may be caused, for example, by sea surface temperature (SST) gradients. To account for convergence in the PBL induced by these stronger gradients, we modify the SWTG method to include an externally-specified vertical mass flux at the PBL top. We implement this “Forced SWTG” method in NCAR’s Single Column Atmospheric Model. The Forced SWTG method produces comparable rainfall to the Unforced SWTG method when forced by the same SST and PBL mass flux that the Unforced method induces over that SST. At a given SST, upward PBL mass fluxes strengthen precipitation, while downward PBL mass fluxes suppress precipitation. When forced using the climatological SST and 850 hPa mass flux taken from observation-based reanalysis data, the Forced SWTG method reproduces most features of the observed annual mean tropical rainfall climatology. Moreover, its predictions remain largely unchanged when it is forced by a spatially uniform SST field if the same observation-based 850hPa mass flux is imposed. This suggests precipitation is relatively insensitive to SST when near-surface convergence is held fixed, yet does not resolve the underlying causality since PBL convergence is not entirely external to convection (as specified here) but coupled to it.

The observed surface response to stratospheric warming largely resembles the negative phase of the North Atlantic Oscillation (NAO), a regional mode of the tropospheric intrinsic variability. We use an idealized general circulation model to show that zonal asymmetries in the tropospheric response to stratospheric variability are closely linked to zonal asymmetries in the tropospheric intrinsic variability. A SYMMETRIC model configuration with zonally homogeneous forcing and annular tropospheric variability is compared against a LOCALIZED configuration with stationary wave sources and localized NAO-like variability. Warming the stratosphere in both configurations produces a surface response that resembles the corresponding leading mode of tropospheric intrinsic variability. This suggests that the same eddy feedbacks responsible for the intrinsic variability also play a central role in zonally localizing the tropospheric response to stratospheric forcing. We rule out the zonally averaged secondary circulation (“downward control”) as the primary pathway for transmitting dynamical signals from the stratosphere down to the troposphere. In SYMMETRIC, downward control is responsible for less than a third of the total tropospheric response; we infer that transient planetary waves account for the remaining fraction. In LOCALIZED, stationary planetary waves produce strong two-way coupling between the NAO and the stratosphere that dominates any coupling mediated by downward control. We find evidence that stationary planetary waves also mediate two-way coupling in reanalysis output, although it suggests the upward coupling in LOCALIZED is too strong. In light of these results, we propose “Planetary Waves with Eddy Feedbacks (PWEF)” as an alternative paradigm to “Downward Control With Eddy Feedback (DCWEF)” to understand stratosphere-troposphere coupling.

Two idealized numerical experiments are conducted to address secondary eyewall formation (SEF) of tropical cyclones (TCs) in positive and negative TC-relative environmental helicity and the underlying dynamics. No SEF is observed in the positive environmental helicity experiment, whereas SEF arises in the negative environmental helicity experiment. The results indicate that an asymmetric SEF process is observed in the negative environmental helicity, with the upshear-left development of convection initially arising near the edge of the inner core and subsequently becoming azimuthally symmetric to form an outer eyewall. However, contrasting with previous findings, a confluent region in the lower layers gives rise to the upshear-left convection initiation. The mid-level dry intrusion, linked to the shear, triggers an intensified descending inflow below the stratiform sector of the principal rainband. Conversely, no such inflow occurs in the positive environmental helicity experiment due to the misalignment between the mid-level dry intrusion and the stratiform portion of the principal rainband. The descending inflow confluences with the supergradient wind-forced outflow from the inner core. A new finding of the study is that tropospheric humidification in the upshear-right quadrant serves to promote the convective axisymmetrization process prior to the onset of SEF in the negative environmental helicity. In particular, the “showerhead effect” markedly contributes to mid-level humidification in the upshear-right quadrant. As the upshear-right troposphere near the edge of the inner core becomes moist, the axisymmetrization process is rapidly accomplished, ultimately leading to the formation of the secondary eyewall in the negative environmental helicity.

High supersaturation on the order of 10% in unpolluted moist convection is a prerequisite for substantial warm-phase aerosol invigoration. However, such high supersaturation has not yet been confidently observed. To accurately detect high supersaturation, the analysis method must avoid generating spuriously high values, and one such method is presented here. Applied to aircraft data from GoAmazon’s relatively unpolluted rainy season, the method finds only low supersaturation: the observed values have a median of 0.5% and are all less than about 1%. Combining both rainy-season and dry-season measurements, the convective supersaturation during GoAmazon is found to scale as the boundary-layer aerosol concentration to the −2/3 power, as previously predicted. For moist convection of any type, a 10% supersaturation would require a very high vertical velocity, a very low sum of droplet diameters per volume, or some high/low combination of both.

This study evaluates a popular density current propagation speed equation using a large, novel set of radiosonde and dropsonde observations. Data from pairs of sondes launched inside and outside of cold pools along with the theoretical density current propagation speed equation are used to calculate sonde-based propagation speeds. Radar/satellite-based propagation speeds, assumed to be the truth, are calculated by manually tracking the propagation of cold pools and correcting for advection due to the background wind. Several results arise from the comparisons of the theoretical sonde-based speeds with the radar/satellite-based speeds. First, sonde-based and radar-based propagation speeds are strongly correlated for US High Plains cold pools, suggesting the density current propagation speed equation is appropriate for use in midlatitude continental environments. Second, cold pool Froude numbers found in this study are in agreement with previous studies. Third, sonde-based propagation speeds are insensitive to how cold pool depth is defined, since the preponderance of negative buoyancy is near the surface in cold pools. Fourth, assuming an infinite channel depth and assuming an incompressible atmosphere when deriving the density current propagation speed equation can increase sonde-based propagation speeds by up to 20% and 11%, respectively. Finally, sonde-based propagation speeds can vary by ~300% based on where and when the sondes were launched, suggesting sub-mesoscale variability could be a major influence on cold pool propagation.

The mesoscale and microphysical structure of a cloud system associated with an Arctic front is analyzed using data from two research aircraft, two WSR-88D radars, the HYSPLIT model, and initialization fields from the RAP model. The flights, conducted during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign, collected in situ and remote sensing data as the cloud system moved across Illinois. The system developed within an air mass that, based on back trajectory analysis, originated over the subtropical eastern Pacific before being lifted over the Arctic front. This led to a region of potential instability extending upward over the frontal zone. The ascending flow triggered the release of the instability that manifested as elevated convection in the storm’s southern sector. In the convective region, supercooled water was found in cloud towers, leading to saturated conditions that supported growth of a range of particle habits and growth by riming. Within this region, and in shallower clouds between convective towers, needle particle habits, supercooled water, and high ice particle concentrations implied active secondary ice processes. Two snowbands formed north of the convective region, with radar evidence suggesting that precipitation within these bands originated in cloud towers at altitudes of 4–6 km in a near-neutral to weakly unstable region. Water saturated conditions, evidenced by supercooled water at the sampling level, permitted the growth of a range of particle habits. Despite ice particle concentrations < 15 L⁻¹ within the bands, some aggregated particles exceeding a centimeter in maximum dimension were observed at −5°C, likely contributing to the 21–27 dBZe reflectivity characteristic of the bands.

Squall lines consist of a buoyancy discontinuity with positive buoyancy extending hundreds of kilometers behind their leading edge. Because of this structure, conceptual models for isolated deep convective updrafts, which have a comparatively limited horizontal extent, fail to explain squall line thermodynamics. The present article addresses this knowledge gap by forming analytic solutions for effective buoyancy using simplified density distributions that mimic squall line structure, and by examining accompanying numerical simulations. It is shown with both analytical analysis and simulations that effective buoyancy along most squall line updraft trajectories is less than half of the value predicted by parcel theory, implying that squall lines are fundamentally incapable of realizing all of their convective available potential energy as kinetic energy. This scaling factor is generally unaffected by the system’s horizontal extent, the width of the deep convective updrafts along the system’s leading edge, and the curvature of bowing segments. When the cold pool and low-level shear are close to balanced, there is an increased prevalence of “hot towers,” whose local buoyancy exceeds their immediate surroundings, allowing the effective buoyancy to buoyancy ratio along some trajectories to slightly exceed one half. As the rearward slant of updrafts increases, the dilution of updraft buoyancy increases, the ratio of effective buoyancy to buoyancy decreases, and hot towers become less prevalent, leading to weaker updrafts. Dilution of updrafts, along with system slant and the associated reduction in effective buoyancy, are the primary controls on updraft intensity. These results provide an underlying foundation for future theories that predict squall line updraft speeds.

Modeled hail trajectories have previously been studied in individual observed supercells or in simulated supercells with similar background environments. To explore the impact of changing updraft structure on hail formation from a different perspective, this study analyzes detailed hail trajectories in a large ensemble of time-averaged supercell-like updrafts. The updrafts are created with an idealized heat source, which allows systematic investigation of the full range of updraft widths and intensities reported in the literature. The simulations exhibit a dominant trajectory pathway with a single ascent and following a curved horizontal trace. However, a systematic shift in the trajectories and in their start and end locations is found with increasing updraft intensity and updraft width. Furthermore, wider updrafts but with only moderate intensity provide optimal conditions for hail of most sizes. The exception is giant hail, which requires both wide and intense updrafts. This result is partially linked to the occurrence of an alternative trajectory pathway characterized by the recycling of hailstones (1–4 cm) in the back-sheared anvil region, which then grew to giant size after reentering the updraft.

Caribbean easterly waves (CEWs) propagate in an environment that is distinct from that of other easterly waves since it exhibits substantial westerly vertical wind shear. In spite of this distinction, their structure, propagation and growth have not received much attention. A linear regression analysis reveals that these systems exhibit features consistent with moisture modes that are destabilized by moisture-vortex instability. They exhibit large moisture fluctuations, are in weak temperature gradient (WTG) balance, and moist static energy (MSE) growth is partly driven by meridional mean MSE advection by the anomalous winds. However, its circulation tilts vertically against the mean shear, a feature that is often associated with baroclinic instability. To reconcile these differences, a linear stability analysis employing a moist two-layer model is performed using a basic state that resembles the Caribbean Sea during boreal summer. The unstable wave solution from this analysis exhibits a structure that resembles observed CEWs. Excluding the upper troposphere from the stability analysis has little impact on the propagation and growth of the wave, and its circulation still exhibits a westward tilt in height. Thus, baroclinic instability is not the main growth mechanism of CEWs despite their structural similarity to baroclinic waves. Instead, the instability is largely rooted in how the lower tropospheric circulation interacts with water vapor, as expected from moisture mode theory. These results suggest that tilting against the shear should not be used as the sole diagnostic for baroclinic instability. Baroclinic instability is unlikely to be a primary driver of growth for most oceanic tropical depression-type waves, in agreement with previous work.

Laboratory experiments and theoretical modelling are conducted to determine the raindrop size distribution (DSD) resulting from distinct fragmentation processes under various upward airstreams. Since weather radar echoes are proportional to the sixth power of the average droplet diameter, understanding the fragmentation mechanisms that lead to different breakup sizes is crucial for accurate rainfall predictions. We utilize a two-parameter gamma distribution for theoretical modelling and estimate the average droplet diameter from the theoretically obtained characteristic sizes, often treated as assumed input parameters for different rain conditions in rainfall modelling. Our experimental and theoretical findings demonstrate a close agreement with the DSD predicted by the Marshall and Palmer relationship for steady rain conditions. Additionally, in situ DSD measurements at different altitudes were obtained through research flights equipped with advanced sensors, further validating our rainfall model. This study underscores the effectiveness of laboratory-scale experiments and the critical importance of accurately characterizing DSD to enhance rainfall predictions.

Mesoscale convective systems (MCSs) are the primary rainfall contributors over the Bay of Bengal (BoB) during the South Asian summer monsoon. Previous studies have established a strong connection between MCS initiation over the BoB and diurnal gravity waves propagating from India. However, the precise role these waves play in triggering offshore MCSs remains unquantified. In this study, we analyze a typical MCS event, representative of the climatological spatiotemporal characteristics of MCS initiation in the region, to investigate the relative roles of diurnal gravity waves and other mesoscale processes in offshore MCS initiation. An ensemble-based satellite data assimilation (DA) experiment is conducted, assimilating all-sky infrared radiances from Meteosat-8 into the WRF model. The ensemble forecast, initialized from DA analyses, shows that many ensemble members accurately capture both the timing and location of MCS initiation. Analysis of the “successful” members reveals diurnal gravity waves play a significant role in enhancing lower-tropospheric moisture and destabilizing the offshore environment. Surprisingly, similar gravity waves and destabilization are also present in members that failed to capture MCS initiation. Further analysis indicates that land-breeze front from northern Sri Lanka is a key factor distinguishing “successful” from “unsuccessful” members, which, in successful members, is strong enough to lift air above the LFC and lead to MCS initiation. Accurately simulating the land-breeze front depends on the correct representation of pre-MCS clouds and surface winds. This suggests that while diurnal gravity waves contribute to environmental destabilization, surface and boundary-layer processes are crucial for the practical predictability of offshore MCS initiation.

Realistically representing deep atmospheric convection is important for accurate numerical weather and climate simulations. However, parameterizing where and when deep convection occurs (“triggering”) is a well-known source of model uncertainty. Most triggers parameterize convection deterministically, without considering the uncertainty in the convective state as a stochastic process. In this study, we develop a machine learning model, a random forest, that predicts the probability of deep convection, and then apply clustering of Shapley additive explanations (SHAP) values, an explainable machine learning method, to characterize the uncertainty of convective events. The model uses observed large-scale atmospheric variables from the Atmospheric Radiation Measurement constrained variational analysis dataset over the Southern Great Plains, United States. The analysis of feature importance shows which mechanisms driving convection are most important, with large-scale vertical velocity providing the highest predictive power for more certain, or easier to predict, convective events, followed by the dynamic generation rate of dilute convective available potential energy. Predictions of uncertain, or harder to predict, convective events instead rely more on other features such as precipitable water or low-level temperature. The model outperforms conventional convective triggers. This suggests that probabilistic machine learning models can be used as stochastic parameterizations to improve the occurrence of convection in weather and climate models in the future. Significance Statement Convective storms, which produce clouds and precipitation, are difficult to represent in models since they occur at scales smaller than a model grid box. The purpose of this study is to better understand why convection is sometimes easier or harder to predict with certainty. This is important because predicting where and when convection occurs in atmospheric models affects the energy, moisture, and momentum processes in these models, which is known to lead to errors in weather forecasts and climate projections. This work highlights the importance of representing uncertainty in processes like convection.

Tropical cyclone precursor vortices are convectively coupled vortices. They undergo significant changes in size and intensity before transitioning to a mature hurricane or typhoon. This paper designs a stochastic quasi-2D model to study the vortices’ formation and interaction. Based on the diagnostic result of a cloud-permitting simulation, we parameterize deep convection as random pulses whose probability of occurrence depends on the spatially smoothed vorticity. The dependence of convective probability on the vorticity field represents the mesoscale feedback. The smoothing represents the spontaneous spreading of convective activity by cold pools and other processes. Simulations show that the system exhibits two stages: the vortex formation stage and the vortex interaction stage. The vortex formation stage features the stochastic nucleation of vortices and their subsequent growth via the mesoscale feedback. The growth of mesoscale vorticity magnitude undergoes a power law growth and then transitions to exponential growth. An analytical theory is proposed to capture this transition. The vortex interaction stage features vortex merging. The vortex size grows due to merging and spontaneous spreading of convective activity. When the vortex size grows sufficiently large, it is squeezed by the convection-induced convergent flow, which converts the growth in size to the growth in vorticity magnitude. This adjustment process corresponds to a bidirectional kinetic energy transfer, with the rotational wind producing an upscale energy transfer and the convergent wind producing a downscale energy transfer. This quasi-2D model provides a simple framework for understanding the multiscale interaction in tropical cyclogenesis.