Influence of sea salt aerosols on the development of Mediterranean tropical-like cyclones

Medicanes are mesoscale tropical-like cyclones that develop in the Mediterranean basin and represent a great hazard for the coastal population. The skill to accurately simulate them is of utmost importance to prevent economical and personal damage. Medicanes are fueled by the latent heat released in the condensation process associated with convective activity, which is regulated by the presence and activation of cloud condensation nuclei, mainly originating from sea salt aerosols (SSAs) for marine environments. Henceforth, the purpose of this contribution is twofold: assessing the effects of an interactive calculation of SSA on the strengthening and persistence of medicanes, and providing insight into the casuistry and sensitivities around their simulation processes. To this end, a set of simulations have been conducted with a chemistry–meteorology coupled model considering prescribed aerosol (PA) and interactive aerosol (IA) concentrations. The results indicate that IA produces longer-lasting and more intense medicanes. Further, the role of the initialization time and nudging strategies for medicane simulations has been explored. Overall, the results suggest that (1) the application of spectral nudging dampens the effects of IA, (2) the initialization time introduces a strong variability in the storm dynamics, and (3) wind–SSA feedback is crucial and should be considered when studying medicanes.

The Evolution of Hail Production in Simulated Supercell Storms

Hailstorms pose a significant socioeconomic risk, necessitating detailed assessments of how the hail threat changes throughout their lifetimes. Hail production involves the favorable juxtaposition of ingredients, but how storm evolution affects these ingredients is unknown, limiting understanding of how hail production evolves. Unfortunately, neither surface hail reports nor radar-based swath estimates have adequate resolution or details needed to assess evolving hail production. Instead, we use a novel approach of coupling a detailed hail trajectory model to idealized convective storm simulations to better understand storm evolution’s influence on hail production. Hail production varies substantially throughout storms’ mature phases: maximum sizes vary by a factor of two, and the concentration of severe hail more than fivefold during 45-60-min periods. This variability arises from changes in updraft properties, which come from (i) changes in low-level convergence, and (ii) internal storm dynamics, including anticyclonic vortex shedding/storm splitting, and the response of the updraft’s airflow and supercooled liquid water content to these events. Hodograph shape strongly affects such behaviors. Straighter hodographs lead to more prolific hail production through wider updrafts and weaker mesocyclones, and a periodicity in hail size metrics associated with anticyclonic vortex shedding and/or storm splitting. In contrast, a curved hodograph (favorable for tornadoes) led to a storm with a stronger but more compact updraft, which occasionally produced giant (10-cm) hail, but that was a less-prolific severe hail producer overall. Unless storms are adequately sampled throughout their lifecycles, snapshots from ground reports will insufficiently resolve the true nature of hail production.

Analysis of sheet erosion component variability on four complex hillslopes and consecutive storms under laboratory conditions

Understanding of rainfall-runoff processes in arid and semi-arid regions, such as runoff discharge (Q) and sediment concentration (SC) in hillslopes with different geometries, can offer better insights into hydrological processes. Consequently, considering intra- and inter-storm dynamics of Q and SC during consecutive storms (CSs), on hillslopes of various geometric shapes, has not been accurately and scientifically studied. The current research was planned to study the response of the sheet erosion components. The experiments were performed on four complex hillslopes (CHs) including straight-parallel, straight-convergent, concave-convergent, and convex-convergent under five CSs with rainfall intensity of 45 mm/h on a sandy loam soil in a 1 × 2 m2 plot under laboratory conditions. The results showed that the individual effects of the CSs and CHs and their interactive effects on Q and SC were significant (P ≤ 0.00). However, Q was more influenced by the CSs (ηp2 = 0.65) and SC was more affected by the CHs (ηp2 = 0.77). Moreover, analysis of the hydrographs (HGs), sedigraphs (SGs), and sediment rating loops (SRLs) observed in four CHs during five CSs indicated the diversity in the behavior of the SC (from 2.32 to 68.68 g/L) in comparison with variations in Q (from 14.68 to 38.38 mL/s).

Changes in Vertical Velocity and Precipitation Mode in Flash Flood-Producing Storms in the Mississippi River Basin in a Future Climate

<p>The Mississippi River Basin (MRB) is a flash flood hotspot in the United States, receiving the most frequent floods and highest rainfall accumulations across the country. In a future warmer climate, this region exhibits some of the greatest increases in rainfall associated with storms that produce flash floods. In order to better understand these future changes, convection-permitting simulations of a current and future climate are utilized to study changes to storm dynamics and precipitation in these convectively-driven flash flood-producing storms. </p><p>First, nearly 500 flash flood-producing storms in the MRB are examined under a pseudo-global warming framework to examine the role of vertical velocity in modulating future rainfall changes. Three different categories of storms are designated based on their vertical velocity magnitude in the current climate–weak, moderate, and strong. While all storm categories display an increase in future rainfall accumulation, the amount of increase varies by the storm’s vertical velocity magnitude, which also changes in the future. </p><p>Second, idealized WRF simulations are run based on a composite sounding of the flash flood-producing storms in the MRB that occurred during the warm season. Future temperature, moisture, and horizontal wind perturbations are added to the initial sounding using the CESM Large Ensemble Data Set under the RCP 8.5 emissions scenario. In these idealized simulations, the contribution of different precipitation modes to future changes in rainfall are examined. The relationship between changes in future precipitation mode and storm dynamics provides a better understanding of how storm processes influence future changes in rainfall in a flash flood prone region in the United States. </p><p> </p>

Influence of sea salt aerosols on the development of Mediterranean tropical-like cyclones

Medicanes are mesoscale tropical-like cyclones that develop in the Mediterranean basin and represent a great hazard for the coastal population. The skill to accurately simulate them is of utmost importance to prevent economical and personal damages. Medicanes are fuelled by the latent heat released in the condensation process associated to convective activity, which is regulated by the presence and activation of cloud condensation nuclei, originated mainly from sea salt aerosols (SSA) for marine environments. Henceforth, the purpose of this contribution is twofold: assessing the effects of an interactive calculation of SSA on the strengthening and persistence of medicanes; and providing insight on the casuistry and sensitivities around their simulation processes. To this end, a set of simulations has been conducted with a chemistry/meteorology coupled model considering prescribed aerosols (PA) and interactive aerosol concentrations (IA). The results indicate that IA produces longer-lasting and more intense medicanes. Further, the role of the initialization time and nudging strategies for medicane simulations has been explored. Overall, the results suggest that (1) the application of spectral nudging dampens the effects of IA; (2) the initialization time introduces a strong variability on the storm dynamics; and (3) wind-SSA feedback is crucial and should be considered when studying medicanes.

Updraft Vertical Velocity Observations and Uncertainties in High Plains Supercells Using Radiosondes and Radars

Observations of the air vertical velocities (wair) in supercell updrafts are presented, including uncertainty estimates, from radiosonde GPS measurements in two supercells. These in situ observations were collected during the Colorado State University Convective Cloud Outflows and Updrafts Experiment (C3LOUD-Ex) in moderately unstable environments in Colorado and Wyoming. Based on the radiosonde accelerations, instances when the radiosonde balloon likely bursts within the updraft are determined, and adjustments are made to account for the subsequent reduction in radiosonde buoyancy. Before and after these adjustments, the maximum estimated wair values are 36.2 and 49.9 m s−1, respectively. Radar data are used to contextualize the in situ observations and suggest that most of the radiosonde observations were located several kilometers away from the most intense vertical motions. Therefore, the radiosonde-based wair values presented likely underestimate the maximum values within these storms due to these sampling biases, as well as the impacts from hydrometeors, which are not accounted for. When possible, radiosonde-based wair values were compared to estimates from dual-Doppler methods and from parcel theory. When the radiosondes observed their highest wair values, dual-Doppler methods generally produced 15–20 m s−1 lower wair for the same location, which could be related to the differences in the observing systems’ resolutions. In situ observations within supercell updrafts, which have been limited in recent decades, can be used to improve our understanding and modeling of storm dynamics. This study provides new in situ observations, as well as methods and lessons that could be applied to future field campaigns.

C-Band Dual-Doppler Retrievals in Complex Terrain: Improving the Knowledge of Severe Storm Dynamics in Catalonia

Convective activity in Catalonia (northeastern Spain) mainly occurs during summer and autumn, with severe weather occurring 33 days per year on average. In some cases, the storms have unexpected propagation characteristics, likely due to a combination of the complex topography and the thunderstorms’ propagation mechanisms. Partly due to the local nature of the events, numerical weather prediction models are not able to accurately nowcast the complex mesoscale mechanisms (i.e., local influence of topography). This directly impacts the retrieved position and motion of the storms, and consequently, the likely associated storm severity. Although a successful warning system based on lightning and radar observations has been developed, there remains a lack of knowledge of storm dynamics that could lead to forecast improvements. The present study explores the capabilities of the radar network at the Meteorological Service of Catalonia to retrieve dual-Doppler wind fields to study the dynamics of Catalan thunderstorms. A severe thunderstorm that splits and a tornado-producing supercell that is channeled through a valley are used to demonstrate the capabilities of an advanced open source technique that retrieves dynamical variables from C-band operational radars in complex terrain. For the first time in the Iberian Peninsula, complete 3D storm-relative winds are obtained, providing information about the internal dynamics of the storms. This aids in the analyses of the interaction between different storm cells within a system and/or the interaction of the cells with the local topography.

Inertial Waves in Axisymmetric Tropical Cyclones

The heat engine model of tropical cyclones describes a thermally direct overturning circulation. Outflowing air slowly subsides as radiative cooling to space balances adiabatic warming, a process that does not consume any work. However, we show here that the lateral spread of the outflow is limited by the environmental deformation radius, which at high latitudes can be rather small. In such cases, the outflowing air is radially constrained, which limits how far downward it can subside via radiative cooling alone. Some literature has invoked the possibility of “mechanical subsidence” or “forced descent” in the storm outflow region in the presence of high inertial stability, which would be a thermally indirect circulation. Mechanical subsidence in the subsiding branch of a tropical cyclone has not before been observed or characterized. A series of axisymmetric tropical cyclone simulations at different latitudes and domain sizes is conducted to study the impact of environmental inertial stability on storm dynamics. In higher-latitude storms in large axisymmetric domains, the outflow acts as a wavemaker to excite an inertial wave at the environmental inertial (Coriolis) frequency. This inertial wave periodically ventilates the core of a high-latitude storm with its own low-entropy exhaust air. The wave response is in contrast to the presumed forced descent model, and we hypothesize that this is because inertial stability provides less resistance than buoyant stability, even in highly inertially stable environments.

Dynamic Modeling of Power Outages Caused by Thunderstorms

Thunderstorms are complex weather phenomena that cause substantial power outages in a short period. This makes thunderstorm outage prediction challenging using eventwise outage prediction models (OPMs), which summarize the storm dynamics over the entire course of the storm into a limited number of parameters. We developed a new, temporally sensitive outage prediction framework designed for models to learn the hourly dynamics of thunderstorm-caused outages directly from weather forecasts. Validation of several models built on this hour-by-hour prediction framework and comparison with a baseline model show abilities to accurately report temporal and storm-wide outage characteristics, which are vital for planning utility responses to storm-caused power grid damage.

What Drives the Intensification of Mesoscale Convective Systems over the West African Sahel under Climate Change?

Extreme rainfall is expected to increase under climate change, carrying potential socioeconomic risks. However, the magnitude of increase is uncertain. Over recent decades, extreme storms over the West African Sahel have increased in frequency, with increased vertical wind shear shown to be a cause. Drier midlevels, stronger cold pools, and increased storm organization have also been observed. Global models do not capture the potential effects of lower- to midtropospheric wind shear or cold pools on storm organization since they parameterize convection. Here we use the first convection-permitting simulations of African climate change to understand how changes in thermodynamics and storm dynamics affect future extreme Sahelian rainfall. The model, which simulates warming associated with representative concentration pathway 8.5 (RCP8.5) until the end of the twenty-first century, projects a 28% increase of the extreme rain rate of MCSs. The Sahel moisture change on average follows Clausius–Clapeyron scaling, but has regional heterogeneity. Rain rates scale with the product of time-of-storm total column water (TCW) and in-storm vertical velocity. Additionally, prestorm wind shear and convective available potential energy both modulate in-storm vertical velocity. Although wind shear affects cloud-top temperatures within our model, it has no direct correlation with precipitation rates. In our model, projected future increase in TCW is the primary explanation for increased rain rates. Finally, although colder cold pools are modeled in the future climate, we see no significant change in near-surface winds, highlighting avenues for future research on convection-permitting modeling of storm dynamics.