Analysis of Cloud-to-Ground Lightning on the Olympic Peninsula

Estberg, Gerald; Hadziomerspahic, Amila

A 25 year record from 1985-2009 of cloud-to-ground lightning strikes on the Olympic Peninsula from the National Lightning Detection Network was analyzed in order to develop a first lightning climatology of this region. Additional strike data from Western North America was also used to develop this climatology. The primary source of atmospheric data used to develop this climatology was from the North American Regional Reanalysis. Statistical distributions of the annual cycle were developed revealing a primary warm season maximum dominated by negative polarity strikes in August and a secondary wet season maximum dominated by positive polarity strikes in November. Statistical distributions of the warm season diurnal cycle revealed a primary afternoon maximum and additional secondary maximum associated with singular intraseasonal events whose timing was dictated by their spatial and temporal origins. Statistical elevation distributions revealed a primary warm season elevation maximum at 1350m related to cloud base heights. Several secondary maximums and minimums related to orography and the temperature field were also revealed. Positive and negative polarity strikes had distinctly different distributions attributed to the vertical charge structure within the thunderstorm cell. The wet season distribution differed from that of the warm season; these differences were related to the contrasting winter temperature field and thunderstorm origins. A case study and statistical analysis of the major warm season intraseasonal events revealed the importance of tropical moisture origins and subsequent dynamic evolution. The 700mb behavior appears to be an important indicator of this evolution. The influence of a cyclonic vortex typically in the mid latitude eastern Pacific Ocean was a critical factor in determining the likelihood of the development of an Olympic Peninsula lightning event. Storm motion was examined through case studies of major warm season events, and warm season events which occurred in the afternoon. Major events generally entered the Olympic Peninsula from the south although some differences in direction and entry location were influenced by details of the tropical moisture source dynamics responsible for the vertical instability. Strike motion through the Olympic Peninsula was well represented by the 700mb wind direction and speed. The cloud base ceiling was also a factor in determining the progression of events through the Peninsula. Some of the afternoon events progressed across the Peninsula in different directions, but all were consistent with the 700mb wind direction. On shore 700mb cyclonic vortices were responsible for two events moving from north to south. The two largest wet season events were also examined. Storm motion was from the southwest and associated with the progression of wet season frontal boundaries on shore. Interannual behavior on the Olympic Peninsula was also examined without attempting rigorous statistical analysis because of the limited length of the study period. The beginning dates for the warm season lightning showed no definite trend over the 25 years of observations. The annual strike count of afternoon warm season events appeared to be correlated with the total annual strike count of all warm season events. The time series of these totals was correlated with the warm season tropical ocean temperature averages used as a measure of El Nino activity. The annual average near surface specific humidity was correlated with the total annual wet season strike count. The warm season strike totals were correlated with the wet season totals a year later.

Analysis of Cloud-to-Ground Lightning on the Olympic Peninsula

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