Spatio-temporal Scale and Optical Radiance of Flashes over East Asia and Western Pacific Areas

Distributions and correlations of flash properties including duration, length, footprint and radiance are investigated in the east Asia and western Pacific areas of 18°-36°N, 70°-160°E and six specially chosen regions (Region 1-6) within it, by analyzing data of lightning imaging sensor (LIS) aboard Tropical Rainfall Measuring Mission (TRMM) satellite from 2002 to 2014. While the flash density over land is generally greatest, followed by offshore waters and deep ocean, the spatial scale and radiance of flash over the deep ocean is the greatest, followed by offshore waters and land, and the duration of flash over offshore waters is the longest, followed by the deep ocean and land. Regions with the largest flash density, duration, spatial extent and radiance are the southern Himalayan front, offshore waters near the east coast of China, deep Pacific Ocean in the southern part of study area and ocean to the east of Japan, respectively. Meanwhile, the flash duration, spatial extent and radiance always have the smallest values over the Tibet Plateau and the southern Himalayan front. In most regions, based on samples during periods of 0900-1400 LT and 1800-0600 LT, the monthly variation of the flash spatial size and radiance is roughly unanimous, except for the ocean to the east of Japan. Inverse correlations of flash activity with flash spatial scale and radiance in the monthly variation is relatively obvious over land. In addition, it is found that some flash properties over some regions in monthly variation are different between periods of 0900-1400 LT and 1800-0600 LT. The flash spatial-temporal scale and the radiance follow lognormal distributions. Relative to the flash over ocean, properties of flash over land tend to concentrate toward smaller values. During 1800-0600 LT when the LIS is of relatively better performance, the median range of flash properties in 6 chosen regions are:Flash duration from 0.18 to 0.29 s, length from 12 to 21 km, and radiance from 0.11 to 0.52 J·m^-2·sr^-1·μm^-1. Correlation analysis between different properties of flash show that relationships between flash properties during 1800-0600 LT are better than those during 0900-1400 LT, and the best correlation is between length and footprint, because they both represent the spatial scale of flash. Relationships between flash spatial scale and radiance are also strong, but the flash duration has weak correlations with flash spatial scale or radiance.