Relationship between lightning-storm characteristics, and both power and rate of lightning-discharge RF emissions observed by FORTE

Prior studies have noted a strongly nonlinear enhancement of lightning flash rates with increasing cloud height. Here we report a related observation, of a tendency for increased intracloud (IC)-discharge radioemission power versus increased height of the electrified cloud. We find that there is a strong statistical increase of effective radiated power (ERP, or isotropic peak power radiated from the source in the receiver bandwidth) of IC discharges, for increasing capping height of the parent storm. Thus a future satellite-based lightning monitor which triggers on only the most intense radio emissions will be strongly selective for electrified storms with very deep vertical development. Such storms are also indicated in severe convective weather. INTRODUCTION Why is thunderstorm height important to monitor? It is now widely accepted that thunderstorm electrification usually requires collisions between graupel (or hail) and ice crystals in the presence of supercooled water [see. e.g, the review in Baker et al., 1999]. Blythe et al [2001] have shown on the basis of scaling relationships that a thundercloud’s lightning flash rate (f) is expected to be “proportional to the product of the downward flux of solid precipitation (i.e, graupel and hail) through the body of the thundercloud and the upward flux of ice crystals into the anvil”. In a useful review on the electrification of severe storms, Williams [2001] (subsequently W2001) shows that having a sustained presence of mixed-phase-hydrometeors together in the same region requires the cloud to have deep vertical development: The cloud must extend upward to the -40 Celsius isotherm or thereabouts, and must implicitly contain an intense core updraft. The latter can lead to tropopause overshoot (see in particular Figure 13.14 of W2001) and thus to water vapor insertion into the normally dry stratosphere. Here we report on initial observations of a relationship between storm height and lightning vigor, with the lightning vigor being sensed by radio emissions. Given that the FORTE satellite carries neither a radar (like the TRMM PR) nor a microwave imager (like the TRMM TMI), our inference of storm height cannot derive from such non-lightning data, at least not on such data provided by FORTE itself. Instead, our inference is based on the distribution of lightning-discharge heights for each storm. In this present paper, we find that the radio ERP is a significant statistical correlate of storm height as inferred from contemporaneous FORTE RF data. Satellite RF lightning monitoring is able to measure the lightning-discharge height, not just the horizontal position (latitude, longitude). This ability is unique to the RF approach: The optical signature of lightning seen from space is a transient cloud-top brightening, regardless of where (in height) the lightning occurs in the atmospheric column underneath the cloud top. Optical photons are elastically Mie-scattered tens to hundreds of times, in what is effectively a diffusion process through the cloud [Koshak et al., 1994; Light et al., 2001] before emerging from the cloud top. By contrast, RF propagates through clouds with neither scattering nor attenuation. a future GPS-based RF monitor would use differential-time-of-arrival (DTOA) methods to retrieve not only the discharge plan location (longitude, latitude) but also the discharge altitude [Suszcynsky et al., 2000].