Abstract

Haboobs are dust storms formed by strong surface winds in convective storm outflow boundaries, or cold pools, which can loft large quantities of mineral dust as they propagate. Both cold pools and the dust they loft are impacted by land surface properties resulting in complex surface interactions on haboobs. As a result of these additional complexities brought about by surface interactions, it is unclear which surface parameters and physical processes are important for predicting haboob intensity and dust concentrations. Here we applied the Morris one-at-a-time (MOAT) global sensitivity statistical method to an ensemble of 120 idealized simulations of daytime and nighttime haboobs to investigate the land surface properties that affect both dust mobilization and cold pool dynamics. MOAT identifies and ranks the importance of different input factors, which for the prediction of haboob strength and dust concentrations are 1) initial cold pool temperature, 2) surface type (vegetation), 3) soil type (clay content), and 4) soil moisture. The underlying physical mechanisms driving these feedbacks were then analyzed using a traditional one-at-a-time factor analysis. Time of day is significant for determining boundary layer height and dissipation via surface fluxes, leading to shallower, more intense cold pools/haboobs at night. Most of the land parameters modify the cold pool through impacts on surface fluxes, while surface type is dominated by roughness length effects. By ranking the importance of these surface factors, we have identified which variables are most sensitive and must be constrained via observations and data assimilation in numerical dust prediction models.

Abstract

The Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS) has undergone development focused on improving the treatment of aerosols in the microphysics model, with the goal of examining the impacts of aerosol characteristics, scavenging, and regeneration processes, among others, on precipitation processes in clouds ranging from stratocumulus to deep convection and mixed-phase orographic clouds. Improvements in the representation of aerosols allow for more comprehensive studies of aerosol effects on cloud systems across scales. In RAMS there are now sub- and supermicrometer modes of sulfate, mineral dust, sea salt, and regenerated aerosol. All aerosol species can compete for cloud droplet nucleation, and they are regenerated via hydrometeor evaporation. A newly applied heterogeneous ice nuclei parameterization accounts for deposition nucleation and condensation and immersion freezing of aerosols greater than 0.5-μm diameter. There are also schemes for trimodal sea salt emissions and bimodal dust lofting that are functions of wind speed and surface properties. Aerosol wet and dry deposition accounts for collection by falling hydrometeors as well as gravitational settling of aerosols on water, soil, and vegetation. Aerosol radiative effects are parameterized via the Mie theory. An examination of the simulated impact of aerosol characteristics, sources, and sinks reveals mixed sensitivity among cloud types. For example, reduced aerosol solubility has little impact on deep convection since supersaturations are large and nearly all accumulation-mode aerosols activate. In contrast, reduced solubility results in reduced aerosol activation in precipitating stratocumulus. This leads to lower cloud droplet concentration, larger droplet size, and more efficient warm rain processes.

Abstract

Variations in storm microstructure due to updraft strength, liquid water content, and the presence of dry layers, wind shear, and cloud nucleating aerosol concentrations are likely to lead to changes in hail sizes within deep convective storms. The focus of this paper is to determine how the overall dynamics and microphysical structure of deep convective storms are affected if hail sizes are somehow altered in a storm environment that is otherwise the same. The sensitivity of simulated supercell storms to hail size distributions is investigated by systematically varying the mean hail diameter from 3 mm to 1 cm using the Regional Atmospheric Modeling System (RAMS) model. Increasing the mean hail diameter results in a hail size distribution in which the number concentration of smaller hailstones is decreased, while that of the larger hailstones is increased. This shift in the hail size distribution as a result of increasing the mean hail diameter leads to an increase in the mean terminal fall speed of the hail species and to reduced melting and evaporation rates. The sensitivity simulations demonstrate that the low-level downdrafts are stronger, the cold pools are deeper and more intense, the left-moving updraft is shorter-lived, the right-moving storm is stronger but not as steady, and the low-level vertical vorticity is greater in the cases with smaller hail stones. The maximum hail mixing ratios are greater in the larger hail simulations, but they are located higher in the storm and farther away from the updraft core in the smaller hail runs. Changes in the hail size distribution also appear to influence the type of supercell that develops.

Abstract

Recent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' hot tower hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation that uses a sophisticated two-moment microphysical scheme. A line of tropical convective cells develops within a finer nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature (θ _e ) are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in θ _e along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. The simulations demonstrate that mixing with dry environmental air decreases θ _e along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase θ _e above the freezing level. Latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified. Much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, but the strongest updrafts are composed of air from closer to the surface. The importance of both boundary layer and midlevel inflow in moist environments is underscored in this study.

Abstract

Cumulus congestus is the middle mode of tropical convection, with cloud tops around or exceeding the 0°C level (∼5 km AGL). While some congestus are terminal, meaning capped by the 0°C stable layer, others are transient and may develop into deep convection. Although this distinction impacts convective transport of water vapor and aerosols into the midtroposphere and the congestus to deep convection transition, there is still much to be understood about the processes causing congestus to overshoot the 0°C level and continue growing. We simulate a field of tropical congestus using high-resolution idealized model simulations, identify and track the updrafts, and composite congestus properties. Terminal and transient congestus updrafts are characterized by a similar overturning circulation between the updraft and subsiding shell. However, transient congestus have stronger updrafts, and the downward branch of their corresponding circulations are constrained by the 0°C level. The balance between buoyancy and perturbation pressure gradient accelerations predominantly determines the shape of the vertical velocity profile, though vertical advection through bulk and subplume fluctuations are also shown to be important near and above the 0°C level. Our findings support previous results suggesting buoyancy as a control on congestus height. We find that congestus developing in more humid midlevel environments are more likely to be transient. Finally, we explore how congestus updrafts influence their near moisture and aerosol environments: terminal congestus return more aerosol to the atmosphere through evaporation along their edges, while transient congestus create stronger midlevel detrainment layers of aerosol and water vapor due to the trapping of the regenerated aerosol above the 0°C level.

Abstract

Precipitation processes play a critical role in the longevity and spatial distribution of stratocumulus clouds through their interaction with the vertical profiles of humidity and temperature within the atmospheric boundary layer. One of the difficulties in understanding these processes is the limited amount of observational data. In this study, robust relations among liquid water path (LWP), cloud droplet number concentration (N _d ), and cloud-base rain rate (R _cb) from three subtropical stratocumulus decks are obtained from A-Train satellite observations in order to obtain a broad perspective on warm rain processes. The cloud-base rain rate R _cb has a positive correlation with LWP/N _d , and the increase of R _cb becomes larger as LWP/N _d increases. However, the increase of R _cb with respect to LWP/N _d becomes more gradual in regions with larger N _d , which indicates the relation is moderated by N _d . These results are consistent with our theoretical understanding of warm rain processes and suggest that satellite observations are capable of elucidating the average manner of how precipitation processes are modulated by LWP and N _d . The sensitivity of the autoconversion rate to N _d is investigated by examining pixels with small LWP in which the accretion process is assumed to have little influence on R _cb. The upper limit of the dependency of autoconversion rate on N _d is assessed from the relation between R _cb and N _d , since the sensitivity is exaggerated by the accretion process, and was found to be a cloud droplet number concentration to the power of −1.44 ± 0.12.

Abstract

Recent research pertaining to aerosol impacts on cloud microphysics has shown a need for understanding mineral dust entrainment into moist convection. The goal of this study is to examine the pathways in which nonmicrophysically active mineral dust is entrained into supercell storms within three commonly observed dust regimes. The Regional Atmospheric Modeling System (RAMS) with an interactive dust model that allows for surface emission was used to achieve this goal.

First, a supercell is simulated within an already dusty environment (EXP-BACKGROUND) to investigate ingestion purely from a background source. Second, the supercell is simulated within a clean background environment and lofts its own dust via the interactive dust model (EXP-STORM) to investigate the regime in which the only source of dust in the atmosphere is due to the storm itself. Finally, the supercell is simulated with a low-level convergence boundary introduced ahead of the supercell to investigate dust lofting by outflow boundary interactions (EXP-BOUNDARY). Results indicate that the supercell in EXP-BACKGROUND ingests large dust concentrations ahead of the rear flank downdraft (RFD) cold pool. Conversely, dust lofted by the cold pool in EXP-STORM is ingested by the supercell in relatively small amounts via a narrow corridor generated by turbulent mixing of the RFD cold pool and ambient air. The addition of a convergence boundary in EXP-BOUNDARY is found to act as an additional source of dust for the supercell. Results demonstrate the importance of an appropriate dust representation for numerical modeling.

Abstract

In this two-part study, relationships between the cloud gamma size distribution shape parameter, microphysical processes, and cloud characteristics of nonprecipitating shallow cumulus clouds are investigated using large-eddy simulations. In Part I, the dependence of the shape parameter (which is closely related to the distribution width) on cloud properties and processes was investigated. However, the distribution width also impacts cloud process rates and in turn cloud properties, and it is this aspect of the relationship that is explored in Part II and is discussed in the context of aerosol–cloud interactions. In simulations with a bulk microphysics scheme, it is found that the evaporation rates are much more sensitive to the value of the shape parameter than to the condensation rates. This is due to changes in both the rate of removal of mass and the rate of removal of fully evaporated droplets. As a result, cloud properties such as droplet number concentration, mean droplet diameter, and cloud fraction are strongly impacted by the value of the shape parameter, particularly in the subsaturated regions of the clouds. These changes can be on the same order of magnitude as changes due to increasing or decreasing the aerosol concentration by a factor of 16. Particular attention is paid to the impact of the shape parameter on cloud albedo. The cloud albedo increases as the shape parameter is increased as a result of the changes in evaporation. The magnitude of the increase is about 4 times larger than previous estimates. However, this increase in cloud albedo is largely offset by a decrease in the cloud fraction, which results in only small increases to the domain-average albedo. Implications for the aerosol relative dispersion effect are discussed.

Abstract

The relative sensitivity of midlatitude deep convective precipitation to aerosols and midlevel dry layers has been investigated in this study using high-resolution cloud-resolving model simulations. Nine simulations, including combinations of three moisture profiles and three aerosol number concentration profiles, were performed. Because of the veering wind profile of the initial sounding, the convection splits into a left-moving storm that is multicellular in nature and a right-moving storm, a supercell, which are analyzed separately.

The results demonstrate that while changes to the moisture profile always induce larger changes in precipitation than do variations in aerosol concentrations, multicells are sensitive to aerosol perturbations whereas supercells are less so. The multicellular precipitation sensitivity arises through aerosol impacts on the cold pool forcing. It is shown that the altitude of the dry layer influences whether cold pools are stronger or weaker and hence whether precipitation increases or decreases with increasing aerosol concentrations. When the dry-layer altitude is located near cloud base, cloud droplet evaporation rates and hence latent cooling rates are greater with higher aerosol loading, which results in stronger low-level downdrafts and cold pools. However, when the dry-layer altitude is located higher above cloud base, the low-level downdrafts and cold pools are weaker with higher aerosol loading because of reduced raindrop evaporation rates. The changes to the cold pool strength initiate positive feedbacks that further modify the cold pool strength and subsequent precipitation totals. Aerosol impacts on deep convection are therefore found to be modulated by the altitude of the dry layer and to vary inversely with the storm organization.

Abstract

The goal of this research is to investigate the impacts of a stably stratified layer embedded within a neutrally stratified environment on the behavior of density currents in an effort to extend the environmental regimes examined by Liu and Moncrieff. Such environments frequently support severe weather events. To accomplish this goal, nonhydrostatic numerical model experiments are performed in which the strength and height of the embedded stably stratified layer within a neutrally stratified environment are varied. The 1-km-deep stable layer base is varied between 1, 2, and 3 km AGL. Additionally, the strength of the stable layer is systematically varied between Brunt–Väisälä frequencies of 0.006, 0.012, and 0.018 s⁻¹, following the methodology of Liu and Moncrieff. The model and grid setup are also similar to that of Liu and Moncrieff, utilizing the Arakawa C grid, leapfrog advection, a Robert–Asselin filter, and grid spacing of 100 and 50 m in the horizontal and vertical directions, respectively. Results show that the height of the density current decreases and the propagation speed increases with stronger and lower stable layers, provided that the stable layer is sufficiently thin so as to not act as a gravity wave ducting layer. As the strength of the stable layer increases and the height of this layer decreases, the horizontal pressure gradient driving the density current increases, resulting in faster propagation speeds. Such results have implications for cold pool propagation into more stable environments.