A lightning parameterization for numerical cloud models

A new lightning parameterization has been developed to enable cloud models to simulate the location and structure of individual lightning flashes more realistically. To do this, three aspects of previous parameterizations hav e been modified: 1) To account for subgrid-scale variations, the initiation point is chosen randomly from among grid points at which the electric fiel d magnitude is above a threshold value, instead of being assigned always to the grid point having the maximum electric field magnitude. 2) The thresho ld value for initiation can either be constant, as in previous parameteriza tions, or can vary with height to allow different flash initiation hypothes es to be tested. 3) Instead of stopping at larger ambient electric field ma gnitudes, extensive flash development can continue in regions having a weak ambient electric field but a substantial charge density. This behavior is based on lightning observations and conceptual models of lightning physics. However, like previous parameterizations for cloud models, the new paramet erization attempts to mimic only the gross structure of flashes, not the de tailed development of lightning channels, the physics of which is only poor ly understood. Though the choice of parameter values affects the dimensions of a flash, the qualitative features of simulated flash structure are simi lar to those of observed lightning as long as the parameter values are cons istent with the larger electric field magnitudes measured in storms and wit h simulated charge densities produced over reasonably large regions. Initia l simulations show that, by permitting development of flashes in regions of substantial charge density and weak ambient electric field, the new parame terization produces flash structure much like that of observed flashes, as would be expected from the inferred correlation between observed horizontal lightning structure and thunderstorm charge.