Preliminary Results from a Study on the Environments of Thunderstorms Over the Northeastern Pacific Ocean and Gulf of Alaska Jonathan Garner National Weather Service Juneau, AK

Marine-related lightning fatalities consistently rank highest in the U.S. (Holle 2007b, Jensenius 2015) The purpose of this study is to evaluate thunderstorm forecasting parameters specific to cool marine environments in order to provide better decision support to mariners Introduction Image courtesy of Ron Holle

Thunderstorm Electrification Bright et al. (2005) Equilibrium Level Temperature ≤ −20° C: Ensures updraft extends through the charge reversal temperature zone CHARGE REVERSAL TEMPERATURE ZONE WARM CLOUD LAYER Temperature at the Lifting Condensation Level ≥ −10° C: Ensures the presence of supercooled water CAPE in the 0° to −20° C layer ≥ 100 to 200 J kg -1 : Ensures sufficient upward vertical motion exists through the charge reversal temperature zone

Methodology Lightning strikes (null-events) observed during operational shifts – GLD360 lightning data 40-km GFS proximity soundings used to analyze thermodynamic parameters – GFS utilized due to large model domain – Model surface temperature/dewpoint compared with closest fixed buoy…modified if difference exceeds 1° C Model error analysis performed from the surface to 300 mb for PAYA, PANT, and KUIL

Methodology Results presented today span from August 2014 to September 2015 Lightning and null events were classified by warm season (WS; May-Oct) and cool season (CS; Nov-April) occurrence Events also classified based on synoptic location

Domain 40° N 150° W

Synoptic Classification Post Frontal (63 Lightning Events / 40 Null Events) Warm Conveyor Belt (6 Lightning Events / 4 Null Events) Frontal (25 Lightning Events / 16 Null Events)

40-km GFS Error Analysis TemperatureDewpoint mb ° C

Thermodynamic Parameters PARAMETERLIGHTNING WARM SEASON (mean) NULL WARM SEASON (mean) LIGHTNING COOL SEASON (mean) NULL COOL SEASON (mean) PW (GFS)0.96 in.0.98 in.0.55 in.0.42 in. SST (Satellite)15.9° C12.8° C11.3° C8.4° C 925-mb T (GFS)10.5° C8.1° C4.5° C1.3° C 500-mb T (GFS)-20.7° C-20.5° C-29.4° C-32.4° C 925―600-mb Lapse Rate (GFS) 6.3° C km ° C km ° C km -1

MUCAPE J kg -1 LGTNG-WS MEAN451 J kg -1 (n=42) NULL-WS MEAN91 J kg -1 (n=26) STATISTICALLY SIGNIFICANT >99% Confidence Level LGTNG-CS MEAN354 J kg -1 (n=52) NULL-CS MEAN199 J kg -1 (n=34) STATISTICALLY SIGNIFICANT >99% Confidence Level

EL Temperature ° C LGTNG-WS MEAN-35° C (n=42) NULL-WS MEAN-16° C (n=26) STATISTICALLY SIGNIFICANT >99% Confidence Level LGTNG-CS MEAN-35° C (n=52) NULL-CS MEAN-32° C (n=34) NOT STATISTICALLY SIGNIFICANT 15% Confidence Level

LCL Temperature ° C LGTNG-WS MEAN11° C (n=42) NULL-WS MEAN8° C (n=26) STATISTICALLY SIGNIFICANT >99% Confidence Level LGTNG-CS MEAN6° C (n=52) NULL-CS MEAN2° C (n=34) STATISTICALLY SIGNIFICANT >99% Confidence Level

Warm Cloud Depth H FZLVL -H LCL (meters) m LGTNG-WS MEAN1757 m (n=42) NULL-WS MEAN1329 m (n=26) NOT STATISTICALLY SIGNIFICANT 91% Confidence Level LGTNG-CS MEAN828 m (n=52) NULL-CS MEAN334 m (n=34) STATISTICALLY SIGNIFICANT >99% Confidence Level

Thunderstorms failed to develop without lifting mechanism: -Upper level shortwave trough -Surface frontal boundary -Ascent windward of elevated terrain

Summary Warm Season: – Differential advection of cold mid/upper-level airmass is necessary for steep lapse rates and sufficient MUCAPE – Upper-level environment must be cold enough for EL temperature ≤ -20° C Otherwise updraft will not extend through the charge reversal zone and lightning will fail to occur Cool Season: – Cold mid/upper-level airmass is usually present for steep lapse rates and sufficient MUCAPE – However, boundary layer is often cold Inadequate for supercooled liquid water in the updraft – Only those cool season environments with relatively warm boundary layers (deep WCD/warm LCL temperature) will yield graupel, charge separation, and lightning