Johannes M. L. Dahl

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*

The authors use a high-resolution supercell simulation to investigate the source of near-ground vertical vorticity by decomposing the vorticity vector into barotropic and nonbarotropic parts. This way, the roles of ambient and storm-generated vorticity can be isolated. A new Lagrangian technique is employed in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. Thiscontributionisinterpretedasbarotropicvorticity.Thestorm-generatedvorticityistreatedastheresidual between the known total vorticity and the barotropic vorticity. In the simulation the development of near-ground vertical vorticity is an outflow phenomenon. There are distinct ‘‘rivers’’ of cyclonic shear vorticity originating from the base of downdrafts that feed into the developing near-ground vortex. The origin of these rivers of vertical vorticity is primarily horizontal baroclinic production, which is maximized in the lowest few hundred meters AGL. Subsequently, this horizontal vorticity is tilted upward while the parcels are still descending. The barotropic vorticity remains mostly streamwise along the analyzed trajectories and does not acquire a large vertical component as the parcels reach the ground. Thus, the ambient vorticity that is imported into the storm contributes only a small fraction of the total near-ground vertical vorticity.

Uncertainties in Trajectory Calculations within Near-Surface Mesocyclones of Simulated Supercells

AbstractThis study addresses the sensitivity of backward trajectories within simulated near-surface mesocyclones to the spatiotemporal resolution of the velocity field. These backward trajectories are compared to forward trajectories computed during run time within the numerical model. It is found that the population of backward trajectories becomes increasingly contaminated with “inflow trajectories” that owe their existence to spatiotemporal interpolation errors in time-varying and strongly curved, confluent flow. These erroneous inflow parcels may mistakenly be interpreted as a possible source of air for the near-surface vortex. It is hypothesized that, unlike forward trajectories, backward trajectories are especially susceptible to errors near the strongly confluent intensifying vortex. Although the results are based on model output, dual-Doppler analysis fields may be equally affected by such errors.

Modeling the Flash Rate of Thunderstorms. Part I: Framework

In this study a straightforward theoretical approach to determining the flash rate in thunderstorms is presented. A two-plate capacitor represents the basic dipole charge structure of a thunderstorm, which is charged by the generator current and discharged by lightning. If the geometry of the capacitor plates, the generator-current density, and the lightning charge are known, and if charging and discharging are in equilibrium, then the flash rate is uniquely determined. To diagnose the flash rate of real-world thunderstorms using this framework, estimates of the required relationships between the predictor variables and observable cloud properties are provided. With these estimates, the flash rate can be parameterized. In previous approaches, the lightning rate has been set linearly proportional to the electrification rate (such as the storm’s generator power or generator current), which implies a constant amount of neutralization by lightning discharges (such as lightning energy or lightning charge). This leads to inconsistencies between these approaches. Within the new framework proposed here, the discharge strength is allowed to vary with storm geometry, which remedies the physical inconsistencies of the previous approaches. The new parameterization is compared with observations using polarimetric radar data and measurements from the lightning detection network, LINET. The flash rates of a broad spectrum of discrete thunderstorm cells are accurately diagnosed by the new approach, while the flash rates of mesoscale convective systems are overestimated.

Production of Near-Surface Vertical Vorticity by Idealized Downdrafts

AbstractThis study uses an idealized heat sink to examine the possible roles of the wind profile in modulating the production of surface vertical vorticity by a downdraft. The basic vorticity evolution in these idealized simulations is consistent with previous work: the process is primarily baroclinic and produces near-ground vertical vorticity within the outflow. Sensitivity experiments affirm that the only fundamental requirement for downdrafts to produce surface vertical vorticity is the existence of ambient downdraft-relative flow. Vertical vorticity production increases monotonically as the low-level downdraft-relative flow increases from zero up through intermediate values (in these experiments, 10–15 m s−1), followed by a monotonic decrease for greater values. This sensitivity has to do with the degree of cooling acquired by parcels as they pass through the idealized heat sink as well as the degree to which horizontal vorticity vectors subsequently attain an orientation that is normal to isosurface...

Near-Ground Rotation in Simulated Supercells: On the Robustness of the Baroclinic Mechanism*

AbstractThis study addresses the robustness of the baroclinic mechanism that facilitates the onset of surface rotation in supercells by using two idealized simulations with different microphysics parameterizations and by considering previous results. In particular, the importance of ambient crosswise vorticity relative to baroclinically generated vorticity in the development of near-ground cyclonic vorticity is analyzed. The storms were simulated using the CM1 model in a kinematic base state characterized by a straight-line hodograph. A trajectory analysis spanning about 30 min was performed for a large number of parcels that contribute to near-surface vertical-vorticity maxima. The vorticity along these trajectories was decomposed into barotropic and nonbarotropic parts, where the barotropic vorticity represents the effects of the preexisting, substantially crosswise horizontal storm-relative vorticity. The nonbarotropic part represents the vorticity produced baroclinically within the storm. It was found...

Modeling the Flash Rate of Thunderstorms. Part II: Implementation

AbstractIn Part I of this two-part paper a new method of predicting the total lightning flash rate in thunderstorms was introduced. In this paper, the implementation of this method into the convection-permitting Consortium for Small Scale Modeling (COSMO) model is presented.The new approach is based on a simple theoretical model that consists of a dipole charge structure, which is maintained by a generator current and discharged by lightning and, to a small extent, by a leakage current. This approach yields a set of four predictor variables, which are not amenable to direct observations and consequently need to be parameterized (Part I).Using an algorithm that identifies thunderstorm cells and their properties, this approach is applied to determine the flash frequency of every thunderstorm cell in the model domain. With this information, the number of flashes that are accumulated by each cell and during the interval between the activation of the lightning scheme can be calculated.These flashes are then ra...

Volatility of Tornadogenesis: An Ensemble of Simulated Nontornadic and Tornadic Supercells in VORTEX2 Environments

AbstractDespite an increased understanding of the environments that favor tornado formation, a high false-alarm rate for tornado warnings still exists, suggesting that tornado formation could be a volatile process that is largely internal to each storm. To assess this, an ensemble of 30 supercell simulations was constructed based on small variations to the nontornadic and tornadic environmental profiles composited from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). All simulations produce distinct supercells despite occurring in similar environments. Both the tornadic and nontornadic ensemble members possess ample subtornadic surface vertical vorticity; the determinative factor is whether this vorticity can be converged and stretched by the low-level updraft. Each of the 15 members in the tornadic VORTEX2 ensemble produces a long-track, intense tornado. Although there are notable differences in the precipitation and near-surface buoyancy fields, each storm features s...

Tilting of Horizontal Shear Vorticity and the Development of Updraft Rotation in Supercell Thunderstorms

AbstractThe question of how rotation arises in sheared updrafts is analyzed using the shear and curvature vorticity framework. Local rotation exists where the shear and curvature vorticity have a similar magnitude and the same sign, such that parcels are in near-solid-body rotation. It is shown that the tilting terms of the vertical vorticity equation cannot explain the development of local rotation in the canonical cases where the horizontal vorticity is either purely streamwise or purely crosswise. Rather, vertical shear vorticity develops if crosswise vorticity is tilted, and vertical curvature vorticity develops if streamwise vorticity is tilted. To analyze how local rotation develops, two simulations of updrafts in an environment with crosswise and mostly streamwise vorticity, respectively, are discussed. A trajectory analysis is performed and shear and curvature vorticity budgets are analyzed. It is found that much of the horizontal vorticity near the updraft becomes streamwise, which results from p...