SFO APPROACH FOG STUDY07-09

Noel KeeneZOA CWSU

Fremont, CA

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

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

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

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

• 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

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

• 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 TimeTime When Visual Approaches Occurred at SFO

0:00:00

2:24:00

4:48:00

7:12:00

9:36:00

12:00:00

14:24:00

16:48:00

19:12:00

21:36:00

24:00:00

26:24:00

9/25/070:00

11/14/070:00

1/3/080:00

2/22/080:00

4/12/080:00

6/1/080:00

7/21/080:00

9/9/080:00

10/29/080:00

12/18/080:00

2/6/090:00

Date

Tim

e(G

MT

)

Avg

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 Correlation10Z SMB Dewpoint

0

10

20

30

40

50

60

9/25/070:00

11/14/070:00

1/3/08 0:00 2/22/080:00

4/12/080:00

6/1/08 0:00 7/21/080:00

9/9/08 0:00 10/29/080:00

12/18/080:00

2/6/09 0:00

Date

Dew

po

int(

F)

No Fog

Fog

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

925mb Wind Direction From 180

0

20

40

60

80

100

120

140

160

180

200

8/6/07 0:00 11/14/070:00

2/22/080:00

6/1/08 0:00 9/9/08 0:00 12/18/080:00

3/28/090:00

Date

Ab

s(18

0-H

9 W

ind

Dir

)

No Fog

Fog

Avg

Multiple Linear RegressionSUMMARY OUTPUT

Regression StatisticsMultiple R 0.3835931R Square 0.1471437Adjusted R Square 0.1204919Standard Error 0.3500265Observations 67

ANOVAdf SS MS F Significance F

Regression 2 1.352843591 0.676422 5.520975 0.006138Residual 64 7.841186259 0.122519Total 66 9.194029851

Coefficients Standard Error t Stat P-value Lower 95% Upper 95%Lower 95.0%Upper 95.0%Intercept -1.3045508 0.460550877 -2.832588 0.006167 -2.224607 -0.384495 -2.224607 -0.384495Dewpoint Temp 0.0263832 0.009263159 2.848189 0.005906 0.0078779 0.044889 0.00787794 0.0448885H9 WndDirFm180 0.0024207 0.00100169 2.416601 0.018528 0.0004196 0.004422 0.00041958 0.0044218

• 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

SFO APPROACH

• 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

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