Abstract

Idealized 3D numerical simulations are used to quantify the impact of moving marine atmospheric boundary layers (MABLs) on squall lines in an environment representative of the U.S. mid-Atlantic coastal plain. Characteristics of the MABL, including depth and potential temperature, are varied. Squall lines are most intense while moving over the deepest MABLs, while the storm encountering no MABL is the weakest. Storm intensity is only sensitive to MABL temperature when the MABL is sufficiently deep. Collisions between the storm cold pools and MABLs transition storm lift from surface-based cold pools to wavelike features, with the resulting ascent mechanism dependent on MABL density, not depth. Bores form when the MABL is denser than the cold pool and hybrid cold pool–bores form when the densities are similar. While these features support storms over the MABL, the type of lifting mechanism does not control storm intensity alone. Storm intensity depends on the amplification and maintenance of these features, which is determined by the ambient conditions. Isolated convective cells form ahead of squall lines prior to the cold pool–MABL collision, resulting in a rain peak and the eventual discrete propagation of the storms. Cells form as storm-generated high-frequency gravity waves interact with gravity waves generated by the moving marine layers, in the presence of reduced stability by the squall line itself. No cells form in the presence of the storm or the MABL alone.

Abstract

Inland squall lines respond to the stable marine atmospheric boundary layer (MABL) as they move toward a coastline and offshore. As a storm’s cold pool collides with the marine layer, characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics. Idealized numerical experiments exploring a parameter space of MABL characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the MABL. When the outflow is less buoyant, storms are forced by a cold pool within the marine environment. When the buoyancies are equivalent, a hybrid cold pool–internal gravity wave develops after the collision. The collision between a cold pool and less buoyant MABL initiates internal waves along the stable layer, regardless of MABL depth. These waves are inefficient at lifting air into the storm, and ascent from the trailing cold pool is needed to support deep convection. Storm intensity decreases with deeper and less buoyant MABLs, in part due to the reduction in elevated instability. Precipitation is enhanced just prior to the collision between a storm and the deepest marine layers. Storms modify their environment downstream, leading to the development of a moist adiabatic unstable layer and a lowering of the level of free convection (LFC) to below the top of the deepest marine layer. An MABL moving as a sea breeze into the storm-modified air successfully lifts parcels to the new LFC, generating convective towers ahead of the squall line. This mechanism may contribute to increased coastal flash flooding risks during observed events.

Abstract

A mechanism for precipitation enhancement in squall lines moving over mountainous coastal regions is quantified through idealized numerical simulations. Storm intensity and precipitation peak over the sloping terrain as storms descend from an elevated plateau toward the coastline and encounter the marine atmospheric boundary layer (MABL). Storms are most intense as they encounter the deepest MABLs. As the descending storm outflow collides with a moving MABL (sea breeze), surface and low-level air parcels initially accelerate upward, though their ultimate trajectory is governed by the magnitude of the negative nonhydrostatic inertial pressure perturbation behind the cold pool leading edge. For shallow MABLs, the baroclinic gradient across the gust front generates large horizontal vorticity, a low-level negative pressure perturbation, and thus a downward acceleration of air parcels following their initial ascent. A deep MABL reduces the baroclinically generated vorticity, leading to a weaker pressure perturbation and minimal downward acceleration, allowing air to accelerate into a storm’s updraft. Once storms move away from the terrain base and over the full depth of the MABLs, storms over the deepest MABLs decay most rapidly, while those over the shallowest MABLs initially intensify. Though elevated ascent exists above all MABLs, the deepest MABLs substantially reduce the depth of the high-θ _e layer above the MABLs and limit instability. This relationship is insensitive to MABL temperature, even though surface-based ascent is present for the less cold MABLs, the MABL thermal deficit is smaller, and convective available potential energy (CAPE) is higher.

Abstract

A packet of equatorial Rossby (ER) waves that lasted 2.5 months is identified in the lower troposphere of the northwest Pacific. Waves within the packet had a period of 22 days, a wavelength of 3600 km, a westward phase speed of 1.9 m s⁻¹, and a near-zero group speed. The wave properties followed the ER wave dispersion relation with an equivalent depth near 25 m. The packet was associated with the development of at least 8 of the 13 tropical cyclones that formed during the period. A composite was constructed around the genesis locations. Tropical cyclones formed east of the center of the composite ER wave low in a region of strong convection and a separate 850-hPa vorticity maximum.

The background flow during the life of the packet was characterized by 850-hPa zonal wind convergence and easterly vertical wind shear. Wave amplitude peaked at the west end of the convergent region, suggesting that wave accumulation played a significant role in the growth of the packet. The presence of easterly vertical wind shear provided an environment that trapped energy in the lower troposphere. Each of these processes increases wave amplitude and thus the likelihood of tropical cyclone formation within the waves.

The initial low pressure region within the wave packet met Lander’s definition of a monsoon gyre. It developed to the west of persistent localized convection that followed the penetration of an upper-tropospheric trough into the subtropics. It is argued that the monsoon gyre represented the initial ER wave low within the packet.

Abstract

A detailed microphysical model of hail growth is developed and applied to idealized numerical simulations of deep convective storms. Hailstone embryos of various sizes and densities may be initialized in and around the simulated convective storm updraft, and then are tracked as they are advected and grow through various microphysical processes. Application to an idealized squall line and supercell storm results in a plausibly realistic distribution of maximum hailstone sizes for each. Simulated hail growth trajectories through idealized supercell storms exhibit many consistencies with previous hail trajectory work that used observed storms. Systematic tests of uncertain model parameters and parameterizations are performed, with results highlighting the sensitivity of hail size distributions to these changes. A set of idealized simulations is performed for supercells in environments with varying vertical wind shear to extend and clarify our prior work. The trajectory calculations reveal that, with increased zonal deep-layer shear, broader updrafts lead to increased residence time and thus larger maximum hail sizes. For cases with increased meridional low-level shear, updraft width is also increased, but hailstone sizes are smaller. This is a result of decreased residence time in the updraft, owing to faster northward flow within the updraft that advects hailstones through the growth region more rapidly. The results suggest that environments leading to weakened horizontal flow within supercell updrafts may lead to larger maximum hailstone sizes.

Abstract

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 2 and the concentration of severe hail by 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 life cycles, snapshots from ground reports will insufficiently resolve the true nature of hail production.