An Analysis of Eta Model Forecast Soundings in

Radiation Fog Forecasting

Steve Amburn

National Weather Service, Tulsa, OK

Forecasting Radiation Fog

• Rule of Thumb: clear skies, light winds, abundant moisture

• Mixing Ratio increasing with height (Petterssen, 1940)

• UPS Fog (cross-over) technique

• MAV MOS Guidance

MAV MOS Visibility Forecasts

OBS

MAV

1 2 3 4 5 Wgt

Avg

1 40 15 39 39 49 3.23

2 8 1 12 15 23 3.75

3 19 16 148 249 377 4.17

4 79 42 229 527 1328 4.35

5 236 95 482 2135 37104 4.89

Most Probable Outcome by Categories

MAV/FWC/TAF Forecast Verification

1

2

3

4

5

Fcst Vsby Category (mi.)

Av

g C

ate

go

ry O

bs

erv

ed

(m

i.)

TAF

MAV

FWC

TAF 2.90 2.39 3.57 4.28 4.90

MAV 3.23 3.75 4.17 4.35 4.89

FWC 3.07 3.89 4.37 4.51 4.93

< 1/2 1/2 to 7/8 1 to 2 3/4 3 to 5 > 5

Typical Fog Soundings

• Characterized by:

– Very moist low levels

– Strong temperature inversion

– Very dry above the inversion

– Little or no wind

No-fog Sounding

• Characterized by: – Windy

– Moist aloft (cloudy)

– Dry low levels

Eta Forecast Sounding Series

06 UTC 09 UTC 12 UTC

Purpose of Study

• Can the Eta forecast soundings:– Improve detection of radiation fog– Improve timing radiation fog development– Improve forecast of fog intensity– Improve on MAV MOS guidance

Approach

• Use United Parcel Service forecast technique

• Compare forecasts derived form Eta soundings to MAV and observed data

• Use 03, 06, 09,12, and 15 UTC times for comparisons

• Compute statistics

Modified UPS Technique

• UPS uses previous afternoon surface Td to estimate the Td aloft.

• When surface T drops below the Td aloft, fog is expected

• Modified Richardson number is used in the technique to adjust for wind.

• A table is used to assign fog values

This Study

• Cases selected when winds were light or calm, so UPS Richardson number not a factor

• Forecast sites included TUL, MLC, FSM, FYV

• 125 data samples were collected from both the 12 UTC and 18 UTC Eta run cycles

• Forecasts were for the upcoming night

Conversion Tablefor Fog Intensities

Eta/MAV Ctgry Vsby (mi) Tsfc – Td aloft

1 < ¼ < -3 F

2 ¼ to ½ -3 F

3 ½ to 7/8 -2 F

4 1 to 2 ¾ -1 F

5 3 to 5 0 F

6 6 1 F

7 7 > 1 F

Results

0.000.200.400.600.801.001.201.401.601.802.00

12Z MAV 12Z Eta 18Z MAV 18Z Eta

Sco

res

Run Cycle

Forecast Comparisons MAV -vs- Eta Interpreted Sounding

Bias

MAE

Std Dev

Statistics

Forecast Cycle

Category

Bias

Abs Ctgry

Error/MAE

Standard

Deviation

12Z MAV 1.30 1.53 1.74

12Z Eta 1.52 1.57 1.84

18Z MAV 1.36 1.49 1.75

18Z Eta 1.54 1.70 1.83

Summary

• Both the Eta technique and MAV had a high bias

• MAV mean average category error was lower at both run times

• Eta from 18UTC run cycles had higher error than the 12UTC Eta run

Conclusions

• Modified UPS fog forecasting technique, when applied to Eta model BUFR soundings, did not improve radiation fog forecasts.

• Eta soundings were not able to develop high RH in the low levels in most events.

Considerations

• Eta model surface initialization may not be completely accurate.

• Eta land surface resolution may be too coarse.

• Eta model estimates of terrain, vegetation, soil type may not be completely accurate.

Questions, comments?