Winter-time Lightning in the North Pacific

Lightning activity is relatively infrequent during the winter over the midlatitude oceans, such as the North Pacific. However, bursts of activity do occur often associated with frontal systems, or convective systems within cold air outbreaks. The climatology of lightning activity using the World Wide Lightning Location Network (WLLNN) over the North Pacific, defined as north of 20°N to the Alaskan coastline and from 180°W to the North American west coast, is explored for 2 winter seasons, Oct – Mar 2009/2010 and 2010/2011. It was found that lightning activity increased significantly whenever cold upper-level troughs or lows migrated southwards over relatively warmer water. The cold-air aloft associated with these lows destabilized the atmosphere as they plunged southward and sufficiently strong convection would result and produce lightning.

Significant differences in the lightning distribution and frequency were found between the two winter seasons. The 2009/2010 winter was an El Nino winter where the midlatitude jet is typically further south than normal (around 40°N) and much of the storm activity is directed into California. The 2009/2010 winter exhibited frequent lightning events all the way across the Pacific, from the dateline to the California coastline usually just south of 40°N. Most of these lightning events were comma clouds that formed within cold-air outbreaks and cold upper-level lows. There were several periods where these comma clouds impacted the west coast of California with strong winds, intense precipitation and lightning. The 2010/2011 was a La Nina winter and the midlatitude jet had a more northwesterly trajectory and was aimed towards the Washington/Oregon and southern British Columbia. The 2010/2011 winter season had much lower frequency of lightning activity across the North Pacific, even in the southern latitudes of the region (i.e. south of 40°N). The only region with significant activity was to the northwest of Hawaii near the dateline. During one of the only periods with comma cloud development across the Pacific (March 2010), the comma clouds did not produce very much lightning, unlike the comma clouds in the previous winter.

One event that occurred in December 2009 is explored in detail. A comma cloud formed when a secondary upper-level trough developed behind and west of a major midlatitude cyclone. Lightning frequency increase substantially during the formation of the comma cloud and preceded the amplification of the upper-level trough. The comma cloud eventually interacted with the midlatitude cyclone to its east and formed an instant occlusion. Numerical model forecasts made with the Weather Research and Forecasting (WRF) mesoscale model are used to explore the structure and development of the comma cloud. Numerical experiments where the lightning data is assimilated are performed to assess the feasibility of using the WWLLN data as a data source over the ocean region and to explore if improvements in forecasts are found when the lightning data is included.