an analysis of eta model forecast soundings in radiation fog forecasting steve amburn national...

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?