Leah D. Grant

Cold Pool and Precipitation Responses to Aerosol Loading: Modulation by Dry Layers

AbstractThe 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 precipitati...

Aerosol-cloud-land surface interactions within tropical sea breeze convection

In this study, the influence of aerosols, surface roughness length, soil moisture, and synergistic interactions among these factors on tropical convective rainfall focused along a sea breeze front are explored within idealized cloud-resolving modeling simulations using the Regional Atmospheric Modeling System (RAMS). The idealized RAMS domain setup is representative of the coastal Cameroon rainforest in equatorial Africa. In order to assess the potential sensitivity of sea breeze convection to increasing anthropogenic activity and deforestation occurring in such regions, 27 total simulations are performed in which combinations of enhanced aerosol concentrations, reduced surface roughness length, and reduced soil moisture are included. Both enhanced aerosols and reduced soil moisture are found to individually reduce the precipitation due to reductions in downwelling shortwave radiation and surface latent heat fluxes, respectively, while perturbations to the roughness length do not have a large impact on the precipitation. The largest soil moisture perturbations dominate the precipitation changes due to reduced low-level moisture available to the convection, but if the soil moisture perturbation is more moderate, synergistic interactions between soil moisture and aerosols enhance the sea breeze precipitation. This is found to result from evening convection that forms ahead of the sea breeze only when both effects are present. Interactions between the resulting gust fronts and the sea breeze front locally enhance convergence and therefore the rainfall. The results of this study underscore the importance of considering the aerosol-cloud-land surface system responses to perturbations in aerosol loading and land surface characteristics.

Microphysical and Dynamical Characteristics of Low-Precipitation and Classic Supercells

AbstractThe sensitivity of supercell morphology to the vertical distribution of moisture is investigated in this study using a cloud-resolving model with 300-m horizontal grid spacing. Simulated storms are found to transition from classic (CL) to low-precipitation (LP) supercells when the strength of elevated dry layers in the environmental moisture profile is increased. Resulting differences in the microphysical and dynamical characteristics of the CL and LPs are analyzed.The LPs produce approximately half of the accumulated surface precipitation as the CL supercell. The precipitating area in the LPs is spatially smaller and overall less intense, especially in the rear-flank downdraft region. The LPs have smaller deviant rightward storm motion compared to the CL supercell, and updrafts are narrower and more tilted, in agreement with observations. Lower relative humidities within the dry layers enhance evaporation and erode the upshear cloud edge in the LPs. This combination favors a downshear distributio...

Cold pool dissipation

The mechanisms by which sensible heat fluxes (SHFs) alter cold pool characteristics and dissipation rates are investigated in this study using idealized two-dimensional numerical simulations and an environment representative of daytime, dry, continental conditions. Simulations are performed with no SHFs, SHFs calculated using a bulk formula, and constant SHFs for model resolutions with horizontal (vertical) grid spacings ranging from 50m (25m) to 400m (200m). In the highest resolution simulations, turbulent entrainment of environmental air into the cold pool is an important mechanism for dissipation in the absence of SHFs. Including SHFs enhances cold pool dissipation rates, but the processes responsible for the enhanced dissipation differ depending on the SHF formulation. The bulk SHFs increase the near-surface cold pool temperatures, but their effects on the overall cold pool characteristics are small, while the constant SHFs influence the near-surface environmental stability and the turbulent entrainment rates into the cold pool. The changes to the entrainment rates are found to be the most significant of the SHF effects on cold pool dissipation. SHFsmay also influence the timing of cold pool-induced convective initiation by altering the environmental stability and the cold pool intensity. As themodel resolution is coarsened, cold pool dissipation is found to be less sensitive to SHFs. Furthermore, the coarser resolution simulations not only poorly but sometimes wrongly represent the SHF impacts on the cold pools. Recommendations are made regarding simulating the interaction of cold pools with convection and the land surface in cloud-resolving models.

Aerosol-cloud-land surface interactions within tropical convection simulations

Mutual impacts of enhanced aerosol concentrations and reductions to surface roughness length on convective rainfall patterns are investigated within idealized simulations of tropical convection focused along a sea-breeze convergence zone. Results indicate that enhanced aerosol concentrations lead to more intense convective precipitation that occurs over a smaller area. Reduced surface roughness length leads to a temporal delay in enhanced precipitation via two competing mechanisms: reduced surface latent heat fluxes and enhanced sea breeze convergence. Mutual impacts of enhanced aerosol concentrations and reduced surface roughness length on convective rainfall patterns lead to a further temporal delay in increased precipitation, which appears to be due to a negative feedback on the strength of the sea breeze convergence. These results indicate that it is important to consider the mutual aerosol-cloud-land surface interactions as part of a complex, nonlinear system in order to correctly represent the spati...

The challenges of representing vertical motion in numerical models

Even though vertical motion is resolved within convection-permitting models, recent studies have demonstrated significant departures in predicted storm updrafts and downdrafts when compared with Doppler observations of the same events. Several previous studies have attributed these departures to shortfalls in the representation of microphysical processes, in particular those pertaining to ice processes. Others have suggested that our inabilities to properly represent processes such as entrainment are responsible. Wrapped up in these issues are aspects such as the model grid resolution, as well as accuracy of models to correctly simulate the environmental conditions. Four primary terms comprise the vertical momentum equation: advection, pressure gradient forcing, thermodynamics and turbulence. Microphysical processes including their impacts on latent heating and their contributions to condensate loading strongly impact the thermodynamic term. The focus of this study is on the thermodynamic contributions to vertical motion, the shortfalls that arise when modeling this term, and the observations that might be made to improve the representation of those thermodynamical processes driving convective updrafts and downdrafts.

The Role of Cold Pools in Tropical Oceanic Convective Systems

AbstractThe processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particul...

The Life Cycles of Ice Crystals Detrained From the Tops of Deep Convection: ANVIL ICE LIFECYCLE

Extensive anvil cirrus clouds generated by deep convection have important impacts on the Earth’s radiation budget and climate. We use growth-sedimentation trajectory calculations to investigate the life cycles of anvil ice crystals as they are advected downwind from their convective source. Temperature, water vapor, and wind fields from a cloud-resolving model simulation of an isolated cumulonimbus cloud are used to drive the calculations. Ice crystals are initialized in the main upper-level detrainment zone with a size distribution based on in situ measurements made in a convective core at about 12 km. Advection, deposition growth, and sedimentation of thousands of sample ice crystals are tracked over about 2.5 hr; neither aggregation of ice crystals nor radiative effects are included. Results support the importance of deposition growth and gravitational size sorting in the evolution of the anvil cirrus. Most ice crystals initialized with maximum dimensions larger than about 200 μm fall out of the anvil and sublimate in subsaturated air below within about 2 hr. Few small ice crystals are present in the lower part of the mature anvil. Vapor deposition growth accounts for about 50% of the ice mass remaining after about 2 hr. Ice crystals larger than about 50 μm in the mature anvil have grown substantially by deposition of vapor. This result is consistent with the predominance of bullet rosette habits observed in mature anvils. Variations in ice crystal fall speeds and growth rates associated with ice crystal habit assumptions have little impact on the ice crystal life cycles. Plain Language Summary Numerical simulations of anvil cirrus ice crystal life cycles demonstrate the importance of deposition growth and sedimentation. Ice crystals with maximum dimensions larger than about 200 μm fall out of anvil cirrus within a few hours. Most ice crystals persisting in the anvils grow by deposition of vapor, and ice crystal growth is important for maintenance of the anvil ice mass. These results are consistent with the predominance of bullet rosette habits observed in mature anvil cirrus.

Cold Pool‐Land Surface Interactions in a Dry Continental Environment

Abstract Cold pools influence convective initiation and organization, dust lofting, and boundary layer properties, but little is known about their interactions with the land surface, particularly in dry continental environments. In this study, two‐way cold pool‐land surface interactions are investigated using high‐resolution idealized simulations of an isolated, transient cold pool evolving in a dry convective boundary layer. Results using a fully interactive land surface demonstrate that sensible heat fluxes are suppressed at the center of the cold pool but enhanced at the edge due to the spatial patterns of land surface cooling and the air temperature and wind speed perturbations. This leads to cold pool dissipation from the edge inward. Latent heat fluxes are primarily suppressed within the cold pool, and the magnitude of this suppression is controlled by competition between atmospheric and land surface effects. By comparing the fully interactive land surface simulation to a simulation with imposed surface fluxes, the land surface‐cold pool feedbacks are shown to reduce the cold pool lifetime, extent, and intensity by up to 50% and influence the pattern of boundary layer turbulent kinetic energy recovery, which have significant implications for cold pool‐induced convective initiation.