Basic Convective and Mesoscale Research
NSSL Project 6 – Investigation of Synoptic and Mesoscale Meteorological Processes Associated with Hazardous Weather: Thunderstorm Electrification Modeling
Funding Type: CIMMS Task II
Objectives
Gain insight into electrification and microphysical processes
and lightning behavior of thunderstorms through numerical simulation.
Accomplishments
Idealized space-varying storm environment. A successful simulation of
the 22 May 1981 "Binger" supercell has been achieved with
an idealized inhomogeneous environment (see figure). A complete kinematic
and electrical lifecycle is simulated, from initiation to maturity
to decay, as the storm moves from a low-CIN environment into increasingly
cooler boundary layer air. Multiple observed soundings are used to
parametrically generate time invariant fields of potential temperature,
water vapor mixing ratio, and horizontal winds on a fixed mesoscale
parent grid that contains the moving model grid. The effects of the
scheme are two-fold: 1) the introduction of an environment with more
inhibition suppresses secondary convection that previously caused upscale
growth of the system, whereas the observed storm remained fairly isolated;
2) the storm first grows in a low-inhibition environment and is able
to sustain itself in moderate inhibition, but as it enters the most
hostile environment it weakens from a supercell into a small complex
of multicell storms or completely dies out, depending on the severity
of the introduced inhibition.
Microphysics effects on electrification. Prediction of number concentration as well as mass of cloud droplets and ice crystals has modified the model results concerning the relative importance of noninductive (graupel-ice) and inductive (graupel-droplet) charge separation. Previously, droplet concentrations were assumed to be constant throughout a storm or modulated by air density as a function of height. Predicted droplet concentrations, however, tend to be lower than these assumptions because of parcel expansion and collection by precipitation, resulting in reduced inductive charge separation by virtue of fewer particles to experience rebounding collisions. [Single-moment schemes have the unrealistic feature of reducing the average droplet size in response to collection by larger particles, but maintaining a constant concentration.] Noninductive charge separation, on the other hand can become enhanced at higher temperatures (0 to -20º C) because the production of crystals by ice multiplication can be properly tracked in terms of crystal size and concentration. Previous work suggested that the inductive charging mechanism was needed for generating strong lower charge regions (i.e., at higher temperatures), but that noninductive charge separation could also be responsible if the crystal concentrations were sufficiently high. Previously this could only be achieved by artificially forcing higher assumed concentrations. Results are now supporting the noninductive graupel-ice interaction as a possible primary mechanism for lower charge regions.
This project is ongoing.
Publications
Mansell, E. R., C. L. Ziegler, and D. R. MacGorman, 2007: A lightning
data assimilation technique for mesoscale forecast models. Mon. Wea.
Rev., 135, 1732-1748.
Fierro, A. O., L. Leslie, E. R. Mansell, J. M. Straka, D. R. MacGorman, and C. L. Ziegler, 2007: A high-resolution simulation of microphysics and electrification in an idealized hurricane-like vortex. Met. Atmos. Phys., submitted.
Mansell, E. R., 2006: A numerical perspective on storm electrification. Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract AE24A- 02.
A severe supercell in its mature stage at 5 hours of simulation in an idealized non-homogeneous environment. The surface blue colors indicate perturbation temperature compared to the far western surface values. Simulated lightning is shown by the red and light blue volume surfaces, with negative cloud-to-ground lightning flashes occurring in the storm core. As the storm moves eastward, it encounters increasingly cooler boundary layer air, which reduces CAPE and increases CIN. The capping inversion suppresses secondary convection, so that the storm remains isolated for almost 7 hours before dissipating. In a homogeneous environment, the storm would have grown into a squall line by about 3-4 hours.