1. Multi-Year Examination of Dense Fog at Burlington International Airport John M. Goff NOAA/NWS Burlington, VT

2. Emphasis of Research To examine the long-term occurrence of dense fog at Burlington International Airport (BTV) in an effort to understand synoptic and mesoscale signals that favor its formation. To improve short term low instrument flight rule (LIFR) forecasts at BTV. To examine the long-term occurrence of dense fog at Burlington International Airport (BTV) in an effort to understand synoptic and mesoscale signals that favor its formation. To improve short term low instrument flight rule (LIFR) forecasts at BTV.

3. Data Specifics Hourly weather data at BTV from January 1979 to December 2003 (24 yrs.) Data coincidental with NCEP North American Regional Reanalysis (NARR) data Criteria for dense fog occurrence: visibility 0.5 km Hourly weather data at BTV from January 1979 to December 2003 (24 yrs.) Data coincidental with NCEP North American Regional Reanalysis (NARR) data Criteria for dense fog occurrence: visibility 0.5 km

4. Data Specifics Contd. Fog classification similar to Tardy 2000. Six types used, including: - radiation fog (type RF) (wind speed 5 knots under mainly clear skies at fog onset) - advection fog (type AF) (wind speed must be > 5 knots with sudden drop in vis.) - fog produced by precipitation (type PF) (precipitation must fall within 3 hours of fog onset) - fog resulting from the lowering of cloud base (type LCB) - fog resulting from the evaporation of surface moisture at sunrise (type EF) - indeterminate (type IF) Fog classification similar to Tardy 2000. Six types used, including: - radiation fog (type RF) (wind speed 5 knots under mainly clear skies at fog onset) - advection fog (type AF) (wind speed must be > 5 knots with sudden drop in vis.) - fog produced by precipitation (type PF) (precipitation must fall within 3 hours of fog onset) - fog resulting from the lowering of cloud base (type LCB) - fog resulting from the evaporation of surface moisture at sunrise (type EF) - indeterminate (type IF)

5. Preliminary Findings Fog types RF, PF, and LCB comprise 94% of all events About 5 RF and 9 PF or LCB events per year Fog types RF, PF, and LCB comprise 94% of all events About 5 RF and 9 PF or LCB events per year

6. Preliminary Findings Contd. Frequency distribution plots of dominant fog types: - type RF maximum in late summer/early fall - combined types PF/LCB maximum in cold season (Nov – Mar) Frequency distribution plots of dominant fog types: - type RF maximum in late summer/early fall - combined types PF/LCB maximum in cold season (Nov – Mar)

7. Wind Rose Data Wind rose plots were compiled for all type RF, and combined type PF/LCB events Distinct directional trends in the data are evident in the plots - Type RF events – light northeast to east flow - Combined type PF/LCB events – variable wind speeds predominately from the north or northwest Wind rose plots were compiled for all type RF, and combined type PF/LCB events Distinct directional trends in the data are evident in the plots - Type RF events – light northeast to east flow - Combined type PF/LCB events – variable wind speeds predominately from the north or northwest

8. Wind Rose Data Type RF Events Type RF (34% of all events ) – drainage wind from northeast to east - Strong mesoscale signal that radiation fog drifts across runway from Winooski River valley to immediate northeast and east - Few events with onset wind directions outside of the 045 to 135 sector Type RF (34% of all events ) – drainage wind from northeast to east - Strong mesoscale signal that radiation fog drifts across runway from Winooski River valley to immediate northeast and east - Few events with onset wind directions outside of the 045 to 135 sector

9. Wind Rose Plot for all RF Events

10. BTV ASOS Site Location and Surrounding Topography

11. Wind Rose Data for Combined Type PF/LCB Events Combined types PF/LCB (60% of all events) – variable wind speeds predominantly from the north and northwest Strong north/northwest signal supports prior evidence that this flow regime enhances low level mesoscale convergence in the northern Champlain Valley Combined types PF/LCB (60% of all events) – variable wind speeds predominantly from the north and northwest Strong north/northwest signal supports prior evidence that this flow regime enhances low level mesoscale convergence in the northern Champlain Valley

12. Wind Rose Plot for All Type PF and LCB Events

13. Mesoscale Convergent Signature in Northern Champlain Valley

14. NARR Data Analysis Mean sea level pressure plots compiled across the eastern U.S. at time of onset of each type RF, PF and LCB event Several synoptic patterns identified favoring each dominant fog type Mean sea level pressure plots compiled across the eastern U.S. at time of onset of each type RF, PF and LCB event Several synoptic patterns identified favoring each dominant fog type

15. NARR Analysis of Type RF Events Anticyclone building into northern Vermont from the north or northwest Anticyclone building into northern Vermont from the west or southwest Anomalous/indeterminate events Many events appear to be preceded by a weak frontal passage some 6 to 18 hours in advance Anticyclone building into northern Vermont from the north or northwest Anticyclone building into northern Vermont from the west or southwest Anomalous/indeterminate events Many events appear to be preceded by a weak frontal passage some 6 to 18 hours in advance

16. Frequency Distribution of Identified Synoptic Patterns

17. NARR Analysis of Type RF Events Contd. Anticyclone building into northern Vermont from north or northwest

18. NARR Analysis of Type RF Events Contd. Anticyclone building into northern Vermont from west or southwest

19. NARR Analysis of Combined Type PF/LCB Events Cold or occluded frontal passage Approach of warm front Convergent northerly flow north or west of surface cyclone Cold or occluded frontal passage Approach of warm front Convergent northerly flow north or west of surface cyclone

20. Frequency Distribution of Identified Synoptic Patterns

21. NARR Analysis of Combined Type PF/LCB Events Contd. Cold or occluded frontal passage

22. NARR Analysis of Combined Type PF/LCB Events Contd. Approach or passage of warm front

23. NARR Analysis of Combined Type PF/LCB Events Contd. Convergent northwest flow on west to northwest side of surface cyclone

24. Other Findings Did antecedent precipitation affect the likelihood of RF events? - most likely no Did antecedent precipitation affect the likelihood of RF events? - most likely no

25. Future Initiatives Focus on long-duration RF, PF and LCB events per importance to aviation Composite analysis of long-duration events using NARR data (McGill U.) Focus on long-duration RF, PF and LCB events per importance to aviation Composite analysis of long-duration events using NARR data (McGill U.)

26. Limitations Differences in hourly data (pre-ASOS vs. human observer) Study addresses low visibility/dense fog events only. Do signatures identified pertain to all IFR events? Differences in hourly data (pre-ASOS vs. human observer) Study addresses low visibility/dense fog events only. Do signatures identified pertain to all IFR events?

27. Conclusions 24 years of dense fog climatology examined Majority of events were either radiation fog, or fog resulting from precipitation or lowering of cloud base Clear directional trends in wind data Several synoptic mean sea level pressure patterns favor the dominant events 24 years of dense fog climatology examined Majority of events were either radiation fog, or fog resulting from precipitation or lowering of cloud base Clear directional trends in wind data Several synoptic mean sea level pressure patterns favor the dominant events

28. Acknowledgements The author would like to thank Paul Sisson (SOO WFO BTV) for overall guidance and assistance with this project Thanks is also given to Eyad Atallah of McGill University for work on the composite analysis, and to Conor Lahiff of WFO BTV for help with the wind rose plotting software The author would like to thank Paul Sisson (SOO WFO BTV) for overall guidance and assistance with this project Thanks is also given to Eyad Atallah of McGill University for work on the composite analysis, and to Conor Lahiff of WFO BTV for help with the wind rose plotting software