Numerical simulations of the evolution of tropical cyclone electrification, lightning, microphysics, and dynamics at landfall: preliminary results
Alexandre Fierro, Univ. of Oklahoma, Norman, OK; and L. Leslie, E. R. Mansell, G. J. Holland, and J. M. Straka
Providing accurate and timely forecasts of the intensity and location of landfalling tropical cyclones (TCs) is a major meteorological challenge, and is increasingly important as coastal regions affected become more populated. A major unsolved problem is why TCs vary so much in their electrical activity. Some storms have little lightning activity, while others are extremely active, especially in their spiral cloud bands or within their eyewall as they intensify or weaken. At present, little is known about the evolution of charge and subsequent electrification in hurricanes, so our early results are a guideline for future studies. The findings are expected to have major implications for TC predictions and lightning observation strategies at landfall. We suggest that they may also lead to improved understanding of TC structure in general.
Toward this goal, a sophisticated cloud model featuring a 10-ICE microphysics scheme and a 3D branched lightning module explores the utility of a systematic monitoring of lightning activity such as flash rate, cloud to ground polarity and stroke multiplicity within TCs, as they strengthen or weaken over the ocean, especially when they make landfall. Of interest is how the microphysical and subsequent charge structure differs from, or resembles, that of electrically active continental convective systems such as supercells or mesoscale convective systems. A preliminary set of high-resolution numerical simulations were performed on a fine grid having a horizontal grid spacing of 3km and a vertical mean spacing of 600 m (45 height levels). The environmental initial conditions were from a composite sounding from TC Charley (2004), which showed a clear increase in lightning activity before intensifying from a borderline Category 3 to a high-end Category 4 storm on the Saffir-Simpson scale 8 hours before landfall on the west Florida coast. A meridionally orientated horizontal slab moving towards the TC at a fixed constant speed (of 8 m/s) was used as an initial simulation of landfall. More sophisticated landfall representations are being developed.
Preliminary results show that the highest total lightning flash rate are found within the stronger cells forming the outer rainbands and within the eyewall, where updraft speeds seldom exceed 15 m/s, consistent with observations. Significant charging capable to produce lightning flashes are collocated with regions having moderate graupel mixing ratio (> 0.5 g/kg) and moderate LWC (> 1 g/kg), namely within the eyewall and the strongest outer band cells. Using the Gardiner non-inductive scheme and weak inductive charging settings, the eyewall exhibits a normal tripole charge structure while a normal dipole is observed in the outer eyewall startiform region as induction responsible for the formation/enhancement of the lowest charge region becomes negligible there. The charges forming the dipole in the outer eyewall are generated within the eyewall via non-inductive collisional charging between graupel pellets and lighter ice crystals in the mixed-phase region at midlevels (~-15C isotherm level at 7km AGL) and are ejected radially outward by the centripetal force induced by the storm intense circulation at and near its center.
Extended Abstract (1.7M)
Poster Session 1, Advances in Technology and Operational Utility of Lightning Data
Monday, 30 January 2006, 2:30 PM-4:00 PM, Exhibit Hall A2
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