| Description |
We explore the physical reasons behind the pronounced contrast in the probability of lightning over land and ocean. For decades, from a multitude of platforms, it has been observed that lightning is much rarer over the ocean, and throughout the literature, many have o↵ered physical and environmental hypotheses to explain this dichotomy. Using Tropical Rainfall Measuring Mission (TRMM) precipitation features and oceanic ground-based radar features, we observed that over the tropical oceans, lightning tends to occur in large (>1000 km2), mature storms and develops after periods of rapid growth of large hydrometeors in the charging zone. The probability of lightning over the ocean increases steeply with increasing size of the feature and is extremely low for small, isolated features. This result is in stark contrast with the small, isolated convection with lightning commonly found over land. This size spectrum dependency changes both seasonally and geographically, although it remains fairly strong, and whatever deviations occur we can find strong synoptic and environmental evidence as to why. We fail to find strong relationships between the probability of lightning and environmental variables, such as convective available potential energy (CAPE) or total column water vapor, frequently examined throughout the literature. Rather, we find that the development of large, mature, strong convection, in which we are more likely to detect lightning, occurs after several episodes of increasingly large and strong convection, usually in the presence of large-scale forcing, such as easterly waves. Smaller features with lightning, in contrast, can be found in regimes with higher CAPE. We speculate that the likelihood of lightning in the tropical oceans depends on the growth of the convection into large, organized features, and we examine the conditions in which organization occurs, the understanding of which is an ongoing challenge to the meteorological field. In this dissertation, we argue that the likelihood of developing lightning over the tropical oceans is tied to its organization, and this result has far-reaching implications for the distributions and trends of deep convection throughout the globe. |
