Sfo approach fog study 07 09
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SFO APPROACH FOG STUDY 07-09. Noel Keene ZOA CWSU Fremont, CA. SFO APPROACH FOG STUDY 07-09. Reason for study: Fog causes significant air traffic impacts at San Francisco International Airport(SFO) Fog is difficult to forecast Overforecast by models

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SFO APPROACH FOG STUDY 07-09

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Sfo approach fog study 07 09

SFO APPROACH FOG STUDY07-09

Noel Keene

ZOA CWSU

Fremont, CA


Sfo approach fog study 07 091

SFO APPROACH FOG STUDY07-09

  • Reason for study:

    • Fog causes significant air traffic impacts at San Francisco International Airport(SFO)

    • Fog is difficult to forecast

      • Overforecast by models

      • CWSU often forecasts earlier clearing than actual

    • Scenario for fog likely follows a pattern

    • Increased lead-time for fog events and better accuracy for total clearing will benefit air traffic operations


Sfo approach fog study 07 09

Data

  • Analyzed fall/winter data for 2007/2008 and 2008/2009

  • Parsed out days where 12Z 500mb height was greater than 5690DM and 12Z SFO-SAC pressure gradient > -1mb and < +1mb

    • All non-frontal fog events occurred in these conditions

    • Wanted to evaluate conditions affecting fog other than the typical light winds and high pressure


Sfo approach fog study 07 09

Data

  • 67 days met initial criteria

    • 11 fog events

    • 56 mornings with no fog

  • Fog event considered to occur when visibility <1SM affected SFO approach zone

    • Did not use 5/8SM because archived SMB observations do not indicate visibilities <1SM


Sfo approach fog study 07 09

Data

  • Evaluated several elements

    • 12Z OAK 500mb height

    • Onshore pressure gradient (SFO-SAC)

    • Offshore gradients (SFO-ACV and SFO-TVL)

    • 10Z dewpoint at SFO approach zone (SMB)

    • 12Z OAK 925mb wind direction and speed

    • Time that visual approaches occurred at SFO

      • Typically SCT or less coverage of clds of below 3500ft

      • Typically visibility > 3sm

    • Forecaster notes taken day of event


Findings

Findings

  • Clearing time later than expected

  • Some correlation between fog occurrence and:

    • 10Z SMB Dewpoint

    • 12Z OAK 925mb wind direction

  • Weaker correlation between fog occurrence and:

    • 12Z SFO-ACV gradient

    • 12Z SFO-TVL gradient


Findings1

Findings

  • Many variables contribute to fog formation over Bay Area

    • Therefore…correlations for single variables are low

  • Multiple Linear Regression using both 10Z dewpoint and H9 Wind Direction from 180 yielded higher R values

  • Graphical representation yields some insight into important parameters that contribute to fog formation


Clearing time

Clearing Time

  • When fog occurred…airport arrival rate never reached optimum(45/60) before 18Z

  • Average clearing time 21:31

  • Two all-day events where fog transitioned to low ceilings


Clearing time1

Clearing Time

Avg


10z smb dewpoint correlation

10Z SMB Dewpoint Correlation

  • Correlation between 10Z SMB Dewpoint and Fog Occurrence at SMB

    • 0.26

  • Dewpoint values for fog occurrence

    • Average…………………..46.8

    • Standard Deviation….2.1

  • Dewpoint values for mornings with no fog

    • Average…………………..43.4

    • Standard Deviation….5.0


10z smb dewpoint correlation1

10Z SMB Dewpoint Correlation

Avg


12z oak 925mb wind direction

Subtracted 180 from H9 winds and took absolute value

Larger numbers represent more northerly winds

Correlation between H9 wind direction and fog occurrence at SMB

0.20

H9 wind (FM 180) direction values for fog occurrence

Average…………………..147

Standard Deviation….35

H9 wind (FM 180) direction values for mornings with no fog

Average…………………..124

Standard Deviation….45

12Z OAK 925mb Wind Direction


12z oak 925mb wind direction1

12Z OAK 925mb Wind Direction

Avg


Multiple linear regression

Multiple Linear Regression

  • Low P-Value and Significance F

    • Indicates both variables (dwpt and wind dir) have some effect on outcome (if fog occurs or not)

  • Still fairly low correlation (multiple R)

    • Indicates that using only dewpoint and wind dir to forecast fog would likely yield poor results…this is fairly obvious


Northerly flow

Northerly Flow

  • Northerly flow is only direction where no nearby downsloping will occur

  • Source region of northerly flow is North Bay Valleys

    • Average lower minimum temperatures

    • SF Bay can achieve saturation more readily

SFO APPROACH


Conclusions

Preconditions for fog during recent fall/winter seasons at SFO approach have been:

H5 Hgt > 5690DM

SFO-SAC pressure gradient < 1mb and > -1mb

Northerly flow enhances fog formation

Negative SFO-ACV pressure gradient

Significant northerly component to wind at 925mb

High dewpoint temp increases chance for fog formation

Fog did not form when 10Z dewpoint temp < 45F

SF Bay does not cool as readily as inland valleys so higher dewpoint is needed

Keying in on secondary elements that aid in fog formation should provide better lead-time for fog events

On fog event days…clearing at SFO approach not likely before 20Z

Conclusions


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