Forecasting Surface Wind Gusts in Positively Stable
Environments
Stas Speransky
Florida State University
James E. Lee
National Weather Service, Sterling, VA
Brian LaSorsa
National Weather Service, Sterling, VA
Outline
• Problem Statement• Background• Unstable/Stable Cases• Hypothesis + Goal• Methodology• Results• Testing Model Equation• Demo• Conclusions• Limitations• Future Research
Problem Statement
• Wind maxima between 0.5 km and 3 km above the earth’s surface frequently bring in warm air over a colder earth’s surface in the mid-Atlantic region, producing a positively static stable environment.
• This environment inhibits mixing• However, it has been observed that a portion of the low
level wind max does mix to the surface in the form of Wind Gusts
• Meteorologists are in need of a predictive technique to assist them in forecasting wind gusts in this environment
Typical Set Up
Nose of inversion
Background
• Prior research (Lee and Girodo, 1997) showed that in environments exhibiting negative static stability, determining the depth of the mixed layer is important in forecasting surface wind gusts.
• It was found that the wind at the top of the mixed layer frequently mixes down to the earth’s surface
• This research led to the inclusion of the “Momentum Transfer” function in BUFKIT
• There is currently no known technique to determine the surface wind gusts in stable environments
Unstable/Stable
Unstable Stable
Hypothesis + Goal
• Correlations can be developed between surface Wind Gust and:
1. Depth of Stable Layer» Surface to nose of the inversion
2. Magnitude of Low Level Jet» Wind at nose of the inversion
3. Stability Index» Lapse rate between surface and nose of the
inversion
• Apply correlations to develop a predictive technique to forecast surface wind gusts in stable environments
Methodology
• Use the LWX Upper Air Retrieval System (UARS) for KIAD to determine events when 850 mb wind > 50 kt from 1996-2014.
• Analyze each event’s Skew-T to determine if it qualifies as positively stable. Eliminate unstable, NW flow, and no inversion cases.
• In addition, extract the following 4 statistics from the sounding text file:
1. Depth of the stable layer (m)
2. Wind at the nose of the inversion (kt)
3. Temperature at the surface (C)
4. Temperature at the nose of the inversion (C)
Lapse Rate:
Methodology (cont.)
• Using the LWX Climate Data Retrieval System (CDRS), go through each event to retrieve surface gust measurements from KIAD ASOS system at +/- 2 hours from upper air release (5 hour window)
• If no wind gusts reported, eliminate event from study
• Calculate and plot results using Excel
Results (50+ kt)
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f(x) = 28.4923494662324 x^0.110736129487963R² = 0.238935513593806
f(x) = 0.121356249664426 x² + 1.68361533308724 x + 26.8488956508227R² = 0.501792892776731
Lapse Rate vs Gust
Lapse Rate (-°C/km)
Win
d G
ust
(kt
)
Correlation Coefficient: 0.708
Quadratic Regression
Good correlation!
• However, many events did not have reported gusts within the 5 hour timeframe, so in 18 years – only 30 events
• So…lets lower the LLJ criteria to 45 kt to have more data more confidence in the model equation
Results (45+ kt)
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f(x) = 28.4923494662324 x^0.110736129487963R² = 0.238935513593806
f(x) = 0.121356249664426 x² + 1.68361533308724 x + 26.8488956508227R² = 0.501792892776731
Lapse Rate vs Gust
Lapse Rate (-°C/km)
Win
d G
ust
(kt
)
26 events added (red) – still fits curve nicely
Results (45+ kt combined)
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Lapse Rate vs Gust
Lapse Rate (-°C/km)
Win
d G
ust
(kt
)
Correlation Coefficient: 0.712
56 events
Results (other parameters)
Correlation Coefficient: 0.265 Correlation Coefficient: 0.301
Poor correlations
Testing Model Equation
y = 0.079x2 + 1.4272x + 25.475
y = wind gust
x = lapse rate
Average forecast error = 3.13 kt
% +- 10 kt % +- 5 kt % +- 4 kt % +- 3 kt % +- 2 kt
100 82.1429 73.2143 51.7857 33.9286
Demo “The Stas Stabilizer”
Conclusions
Under a 45+ kt Low Level Jet (850 mb), strong positive correlation (Correlation Coefficient of 0.712) was found between the Lapse Rate and surface Wind Gusts.
Weak correlations were found between the Stable Layer Depth and surface Wind Gusts, as well as Wind at Nose of Inversion and surface Wind Gusts.
An equation derived from quadratic regression of the Lapse Rate vs Wind Gust relationship does a respectable job in forecasting Wind Gusts in stable environments. For the analyzed events, 82% of the cases are within 5 kt of the forecast value. The average forecast surface Wind Gust error is 3.13 kt.
Limitations
• 45+ kts @ 850 mb• Absolutely stable environment• Average lapse rate doesn’t account for the
“kinks”
Future Research
• Extend study to other regions of the country in order to see if the equation could be used as a prognostic tool there as well
• Explore conditionally unstable environments
• Integrate into BUFKIT
Acknowledgements
• Jim Lee • Steve Zubrick • Diana Norgaard• Jared Klein• Brian LaSorsa
Thank You!Questions/Comments?