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RADIATION FOG STUDY. Office Stats. 00z ELM TAF POD for FG – 0.53 FAR – 0.55 06z ELM TAF POD for FG – 0.68 FAR – 0.50. Crossover Technique. This method, by itself, showed skill in determining when at least BR would occur
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Office Stats • 00z ELM TAF • POD for FG – 0.53 • FAR – 0.55 • 06z ELM TAF • POD for FG – 0.68 • FAR – 0.50
Crossover Technique • This method, by itself, showed skill in determining when at least BR would occur • When FG/BR occurred, the low temperature at ELM reached the crossover temperature about 80% of the time • However, it was less able to distinguish between FG and BR • When the “dense fog” threshold was reached (Crossover – 3 deg. F) or exceeded, FG only occurred about 55% of the time
Water Temperature vs. Low Temperature • No data available at this time for temp. of Chemung River near ELM…closest gages are on the Tioga River in PA (Mansfield) • For this study, compared early morning low at ELM to climatological value of water temp. on Cayuga Lake near ITH • For June into August of this year, this compared favorably to Mansfield (+/- 3 deg. F) • Need to look at this more closely over a longer time period
More Water Temperature vs. Low Temperature • August through December • When Delta T was 23 deg. F or more, FG occurred the vast majority of the time (20 FG/only 3 BR) • When Delta T was between 14 deg. F and 23 deg. F, FG and BR occurrences were evenly split (20 each) • When Delta T was less than 14 deg. F, BR took place the vast majority of the time (13 BR/only 4 FG)
Yet More Water vs. Low Temperatures • April through July • When Delta T was less than 7 deg. F, BR occurred the vast majority of the time (30 BR/only 7 FG) • When Delta T was 8 deg. F or more, FG and BR events were about even (9 FG/6 BR)
RADIATION EVENTS Average Wind Speed (knots) 59 FG CASES 73 BR CASES
PRELIMINARY CONCLUSIONS • Between sigma layers 5 through 8, the average flow in BR events was twice as strong as FG events. • FG – 10 knots • BR – 20 knots • FG Events • Vast majority of events occur when the wind speeds at sigma layers 5 and 8 are 13 knots or less. • A few extreme outliers but these came from two minor fog events.
BR Events • Majority of events occur when the wind speeds at sigma layers 5 and 8 are greater than 12 knots and less than 25 knots. • There were several events that occurred with winds greater than 25 knots but mainly from short lived 5-6sm BR events.
FG Composites • Sea level pressure - indicates surface high pressure directly over region or ridge axis just to our east. • H85 – Ridge axis generally to the west of ELM. Composites show a range from Erie PA to the central southern tier.
BR Composites • Sea level pressure – indicates surface high pressure over the mid Atlantic region or moving off the coast. (Increased moisture advection but also more mixing) • H85 – A flatter ridge and axis generally to the east of ELM. • Late season events occurred with ridge axis further east.
GENERAL OBSERVATIONS • Null fog events depict that a temp/dewpoint spread of 8 degrees or greater from sigma 3 through 8, will prevent fog formation. • Fog events that do occur with a NE flow are generally short lived events or have a late onset time. This due to greater mixing and dry advection as surface high pressure builds over region.
FWC MOS Prediction of Fog (FG) and Light Fog (BR) at ELM • Shows some seasonal talent in predicting FG events • Shows better talent in predicting BR events
FG prediction • FWC MOS able to predict FG events better than 50% of the time in June, August, and September. • Accuracy increased slightly in 00Z forecast over the 12Z forecast, for the following morning. • Little ability shown other months of the year.
Explanation of Charts: • Hit: MOS forecast visibility exactly matched ELM observed visibility. • Near: MOS forecast visibility was within two categories of ELM observed visibility. • Over: MOS forecast visibility was more than two categories lower than observed. • Under: MOS forecast visibility was more than two categories higher than observed. • FG or BR must have occurred within the three hour MOS forecast time window.
BR Prediction • Both 12Z and 00Z FWC MOS forecasts showed year round ability in forecasting BR events. • Very little seasonal variation in ability
FWC MOS Summary • Has skill in predicting BR events in ELM year round. • Has skill during June, August and September in predicting FG events in ELM, but little skill the remainder of the year • The 00Z MOS is somewhat better than the 12Z at predicting both FG and BR.
LAMP MOS FG Forecasts at ELM • LAMP MOS showed similar skill to that of the FWC MOS. • Skill was directly related to month. • June, August and September showed reasonable skill, while other months showed little skill • Skill improved closer to event, e.g. the 23Z LAMP showed better skill than the 17Z