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

Emphasis of Research

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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.

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To improve short term low instrument flight rule (LIFR) forecasts at BTV.

Data Specifics

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Hourly weather data at BTV from January 1979 to December 2003 (24 yrs.)

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Data coincidental with NCEP North American Regional Reanalysis (NARR) data

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Criteria for dense fog occurrence: visibility

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0.5 km

Data Specifics Contd.

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Fog classification similar to Tardy 2000.

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Six types used, including: - radiation fog (type RF) (wind speed

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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)

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Fog types RF, PF, and LCB comprise 94% of all events About 5 RF and 9 PF or LCB events per year Preliminary Findings Fog Type Percentages (January 1979 - December 2003) 1% 2% 8% 34% 52% 3% RF AF PF LCB EF IF

Preliminary Findings Contd.

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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) FOG TYPE MONTHLY DISTRIBUTION JANUARY 1979 - DECEMBER 2003 40 35 30 25 20 15 10 5 0 JAN MAR MAY JUL SEP NOV RF PF/LCB

Wind Rose Data

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Wind rose plots were compiled for all type RF, and combined type PF/LCB events

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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 Data Type RF Events

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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

Wind Rose Plot for all RF Events

BTV ASOS Site Location and Surrounding Topography

Wind Rose Data for Combined Type PF/LCB Events

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Combined types PF/LCB (60% of all events) – variable wind speeds predominantly from the north and northwest

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Strong north/northwest signal supports prior evidence that this flow regime enhances low level mesoscale convergence in the northern Champlain Valley

Wind Rose Plot for All Type PF and LCB Events

Mesoscale Convergent Signature in Northern Champlain Valley

NARR Data Analysis

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Mean sea level pressure plots compiled across the eastern U.S. at time of onset of each type RF, PF and LCB event

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Several synoptic patterns identified favoring each dominant fog type

NARR Analysis of Type RF Events

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Anticyclone building into northern Vermont from the north or northwest

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Anticyclone building into northern Vermont from the west or southwest

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Anomalous/indeterminate events

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Many events appear to be preceded by a weak frontal passage some 6 to 18 hours in advance

Frequency Distribution of Identified Synoptic Patterns Percentages of Favored Synoptic Patterns for all RF Events 38% 8% 54% High N or NW High W or SW Indeterminate

NARR Analysis of Type RF Events Contd.

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Anticyclone building into northern Vermont from north or northwest

NARR Analysis of Type RF Events Contd.

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Anticyclone building into northern Vermont from west or southwest

NARR Analysis of Combined Type PF/LCB Events

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Cold or occluded frontal passage

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Approach of warm front

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Convergent northerly flow north or west of surface cyclone

Frequency Distribution of Identified Synoptic Patterns Percentages of Favored Synoptic Patterns for All PF/LCB Events 47% 32% 21% Cold/Occld Fropa Warm Front NW of Sfc Cyclone

NARR Analysis of Combined Type PF/LCB Events Contd.

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Cold or occluded frontal passage

NARR Analysis of Combined Type PF/LCB Events Contd.

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Approach or passage of warm front

NARR Analysis of Combined Type PF/LCB Events Contd.

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Convergent northwest flow on west to northwest side of surface cyclone

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Did antecedent precipitation affect the likelihood of RF events?

- most likely no Other Findings Frequency Distribution of all RF Events as Related to Antecedent Precipitation 42% 31% 27% ANTECEDENT PCPN <= 12 HR ANTECEDENT PCPN 12-24 HR ANTECEDENT PCPN > 24 HR

Future Initiatives

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Focus on long-duration RF, PF and LCB events per importance to aviation

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Composite analysis of long-duration events using NARR data (McGill U.)

Limitations

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Differences in hourly data (pre-ASOS vs. human observer)

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Study addresses low visibility/dense fog events only. Do signatures identified pertain to all IFR events?

Conclusions

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24 years of dense fog climatology examined

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Majority of events were either radiation fog, or fog resulting from precipitation or lowering of cloud base

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Clear directional trends in wind data

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Several synoptic mean sea level pressure patterns favor the dominant events

Acknowledgements

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The author would like to thank Paul Sisson (SOO WFO BTV) for overall guidance and assistance with this project

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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