Post on 30-Dec-2015
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?