Browsing by Author "Falk, Nicholas Michael, author"

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Open Access
Cold pool propagation and cold pool-land surface interactions
(Colorado State University. Libraries, 2025) Falk, Nicholas Michael, author; van den Heever, Susan C., advisor; Grant, Leah D., advisor; Bell, Michael M., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
Convective cold pools are important components of the Earth system as they influence processes such as deep convective initiation, storm longevity and intensity, surface energy fluxes, and aerosol transport. The overarching goal of the research outlined in this dissertation is to investigate the propagation characteristics of cold pools, as well as the interactions between cold pools and the land surface. The three studies comprising this dissertation use field campaign observations and high-resolution numerical simulations to investigate these cold pool processes. The first study evaluates a popular density current propagation speed equation using a large, novel set of radiosonde and dropsonde observations. First, 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. Second, radar/satellite- based propagation speeds are calculated by manually tracking the propagation of cold pools and correcting for advection due to the background wind. Comparisons of the propagation speeds calculated in these different ways demonstrate that 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. Sonde-based propagation speeds are largely insensitive to how cold pool depth is defined, since the preponderance of negative buoyancy is near the surface in cold pools. Sonde-based propagation speeds can vary by ~300% based on where and when the sondes were launched, suggesting sub-mesoscale variability could have a major influence on cold pool propagation. The impacts of topographic slope on daytime haboob propagation speeds and dust lofting are examined in the second study comprising this dissertation, along with how these impacts are modulated by surface roughness length. A suite of 40 idealized, large-eddy simulations are conducted with varied linear topographic slopes and surface roughness lengths. It is found that on flat ground, greater surface roughness increases drag on haboobs and causes haboobs to dissipate faster, thereby decreasing both haboob propagation speeds and associated dust lofting. As the topographic slope is increased, an upslope anabatic wind forms which causes downslope haboob propagation speeds to decrease and upslope haboob propagation speeds to be mostly unchanged. Anabatic winds act to loft dust as well, leading to increased masses of dust being lofted jointly by the haboob and anabatic wind as topographic slopes are increased. The third study investigates the individual and synergistic impacts of cold pools and land surface heterogeneity on convection initiation. Idealized large eddy simulations of deep convection over the Amazon rainforest are conducted testing realistic and homogenized vegetation, along with realistic and eliminated cold pools. Convection initiation is more frequent over forested than deforested areas due to more favorable thermodynamics. Heterogeneous vegetation aggregates storm initiation locations compared with homogeneous vegetation. Over heterogeneous vegetation, cold pools propagate into deforested regions, thereby initiating storms and disaggregating storm initiation locations. Convection initiation locations are randomly distributed over homogeneous vegetation, with or without cold pools, demonstrating that cold pools have minimal impacts on convection initiation locations over homogeneous vegetation. The findings of this dissertation research shed new light on fundamental cold pool processes and should be helpful for improving the representation of cold pools in forecast and climate models. Several avenues for future research are discussed.
Open Access
Strong and weak cold pool collisions
(Colorado State University. Libraries, 2022) Falk, Nicholas Michael, author; van den Heever, Susan C., advisor; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee member
Collisions between convective cold pools commonly initiate new convective storms. This occurs through enhancements to the vertical velocity through mechanical forcing, and increased water vapor content via thermodynamic forcing. The goal of this study is to investigate the impact of the following four parameters on the mechanical and thermodynamic forcing associated with cold pool collisions: (1) the initial temperature perturbation of cold pools, (2) the initial distance between cold pools, (3) the environment in which cold pools exist, and (4) the strength of atmospheric diffusion. To achieve this goal, the dynamical and thermodynamical processes of colliding pairs of cold pools is investigated using a two-dimensional, high- resolution non-hydrostatic anelastic model. The four parameters of interest were varied across a wide range of values in a model suite comprised of 11,200 large eddy simulations in total. To facilitate our analysis, a classification of cold pool collisions into categories of "mechanically strong" and "mechanically weak" is proposed. "Mechanically strong" cold pool collisions occur when the updraft velocities resulting from the collisions are greater than those produced by the flow of air forced up the leading edges of individual cold pools. In "mechanically weak" collisions, the updraft velocities produced by individual cold pools are greater than those from cold pool collisions. An analogous classification of "thermodynamically strong/weak" collisions is also proposed. The results of this analysis show that the initial temperature perturbation of the cold pools has the largest impact on mechanical and thermodynamic forcing from cold pool collisions. Colder cold pools have greater horizontal wind velocities at their heads, leading to greater near- surface horizontal convergence when they collide. This in turn leads to greater updraft velocities which are also more effective at advecting water vapor upwards. The second largest impact on mechanical and thermodynamic cold pool forcing is from the environment in which the cold pools exist. Due to a decreased vertical gradient of potential temperature, weaker low-level static stability increases mechanical forcing as the air lofted by the collisions is decelerated less by negative buoyancy. Environments with larger low-level vertical moisture gradients are associated with increased thermodynamic forcing through enhanced vertical moisture advection. The initial edge-to-edge distance between the cold pools has the third largest impact on the proxies for convective initiation. Mechanical forcing is found to peak at an optimal initial distance between cold pools of ~2.5 km due to a balance between the creation and dissipation of kinetic energy. Thermodynamic forcing, on the other hand, peaks for much greater initial cold pool distances than those associated with the mechanical forcing. This is likely a result of the faster updraft winds generated during collisions for closely spaced initial cold pools also being more effective at advecting moisture away during the collision, thereby decreasing the thermodynamic forcing. The smallest impact on the proxies for convective initiation comes from the atmospheric diffusion rate which impacts cold pool strength through mixing. Thus, this work finds that convective initiation becomes increasingly likely from a cold pool collision when the cold pools are colder, the environment is less stable and has a greater vertical water vapor gradient, the cold pools start close to some optimal separation distance, and the atmospheric diffusion rate is low.