84th AMS Annual Meeting

Monday, 12 January 2004: 2:15 PM
On the relationship between lightning and rainfall over the north Pacific Ocean
Room 618
Antti Pessi, University of Hawaii, Honolulu, HI; and S. Businger
The wave guide between the Ionosphere and the Earth’s surface allows VLF noise generated by lightning between 5 and 15 kHz (called sferics) to propagate over very long distances. The goal of the new Pacific Lightning Detection Network (Pacnet) is to provide long-range lightning detection over the central and northern Pacific Ocean. Two hybrid broadband receivers have been installed on Kauai and Unalaska. A third will soon be operational at Kwajalein. Sites for Kiritibati and Midway are planned, but logistics suggest that they may take longer to install to complete the network. However, with three sites and supplementary data from mainland U.S. detectors, a useful data stream and products are available. Because of its long-range detection capability, Pacnet provides inexpensive, accurate monitoring of convective storms that represent a threat to ocean shipping and airborne carriers and are beyond the range of weather radars. Although GOES and GMS provide continuous IR imagery, convective activity is often obscured by cirrus anvils. Some of the cirrus-covered clouds pose little hazard to aircraft operations, whereas cold clouds that produce lightning have significant updrafts, increasing the threat of wind shear. Low orbiting satellites that carry microwave radiometers such as Tropical Rainfall Measuring Mission (TRMM) with its TRMM Microwave Imager (TMI), provide intermittent glimpses of convective precipitation. Unfortunately, they do not permit the evolution of convective weather systems to be monitored continuously from space. However, TRMM's Lightning Imaging Sensor is used to assess the performance of Pacnet. The distribution of deep moisture, and in particular, latent heating in deep convection, is critical for accurate forecasts of cyclogenesis. A promising application of Pacnet is to derive estimates of the rainfall rate from the lightning data. In this study the ratio of rainfall to lightning over the Pacific is investigated by comparing the number of lightning strokes and convective rainfall obtained from TRMM's microwave sensor for a variety of storm systems. The relationship between lightning and rainfall may vary significantly, depending on air-mass characteristics and cloud microphysics. Preliminary results indicate that the ratio of lightning to rainfall rate shows a stable pattern over the Pacific Ocean. This result holds promise that lightning data over the Pacific can be assimilated into numerical models as a proxy for latent heating.

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