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].
FORTE observations of lightning radio‐frequency signatures: Capabilities and basic results
The FORTE satellite, launched on August 29, 1997, carries both radio‐frequency‐receiver and optical (imaging and photometric) payloads for the study of lightning. The radio‐frequency (RF) data for the first 7 months of operation are described, both to illustrate the satellite's capabilities and to explain the basic statistical findings so far. FORTE's multichannel RF trigger system represents a significant advance in space‐based monitoring of lightning emissions. We are able to observe even rather weak and diffuse RF emissions from lightning and are no longer limited to the brightest known events, “transionospheric pulse pairs,” or TIPPs. We do see TIPPs, and we show that the FORTE observations of TIPPs are consistent only with the second pulse's being due to a ground reflection. We find that TIPPs are basically bimodal in character, one type having a steep roll‐off of power from 38 to 130 MHz and the other being essentially flat‐spectrum in that range. The steep‐spectrum TIPPs cluster together in the manner of most RF emissions from lightning, while the flat‐spectrum events tend to maintain a wider spacing (>0.1 s) between recurrent emissions.
Atmospheric drag effects on the KOMPSAT-1 satellite during geomagnetic superstorms
We examine the atmospheric drag on the low earth-orbiting satellite, KOMPSAT-1 in a sun-synchronous orbit at ~685-km altitude starting in 1999, during a 3-month (October-December) period in 2003. This 3-month interval includes the October 29-30 and November 20 magnetic superstorms and weak to moderate storms. We observed that the daily KOMPSAT-1 drag acceleration transiently responses to transient storm-time disturbances. That is, there is an one-to-one correspondence between the KOMPSAT-1 drag accelerations and the storm events. We find that the drag acceleration correlates strongly with the level of geomagnetic disturbance. This indicates that the trajectory of KOMPSAT-1 is significantly perturbed during extremely disturbed intervals because of atmospheric density increase. The main contributor to the density increase is Joule heating associated with the geomagnetic activity rather than the solar EUV radiation, as reported by previous studies. We suggest that understanding how the upper atmosphere responses to the geomagnetic-associated heating is important to predict space weather impacts on low earth-orbiting satellites.