cory behnke meteorologist noaa/nws detroit-pontiac

ADDRESSING THE NEARSHORE WAVE FORECASTING DILEMMA: OBSERVATIONS FROM RECENT MODIFICATIONS TO THE GLERL WAVE MODEL

March 24 2010

18th Annual Canada/US Great Lakes Operational Meteorology Workshop (GLOMW)Toronto, Canada

Despite more reliable wave height forecasts, the sparsity of real-time marine observations fosters an uncertainty, particularly within the nearshore environment. Recent feedback from the marine community in southeastern Michigan and Saginaw Bay suggests further improvements in wave height forecasts are needed. Given the motivation, limitations of the baseline Great Lakes Environmental Laboratory wave model were addressed for much of 2009 by including a shallow water consideration and an increased grid resolution to 2.5 km. A basic sensitivity examination between the multiple setups was conducted using steady state comparisons and results will be provided. With anecdotal accounts remaining the sole means of "ground truth", maintenance trips into the central waters of Saginaw Bay on 9-19-2008 and 7-27-2009 provided for a great opportunity in qualitatively assessing forecast performance.

Abstract

3/24/2010

2

18th Annual Canada/US Great Lakes Operational Meteorology Workshop (GLOMW)

Motivation

Recent feedback out of Saginaw Bay and Lake St Clair is that wave height forecasts need further improvement.

Areal coverage of the Great Lakes system is approximately 95,000 sq miles/246,000 sq km.

Roughly 9,900 miles/15,900 km of shoreline. 20 total buoys. 9 U.S. 11 Canada. All located outside of 5nm from

shoreline.

3/24/2010

3


Modifications to Operational GLERL Wave Model• Shallow Water Considerations1

• Increased Grid Resolution to 2.5 km2

09/19/2008 Saginaw Light #1 07/27/2009 Gravelly Shoal

Outline

1) Implemented October, 2008 – Greg Mann 2) Implemented July, 2009

3/24/2010

4


Available Wave Forecast Guidance

• Inputs include: NDFD 10m winds, NDFD 2m air temperatures, GLERL LSTs and GLERL Ice Analysis.

• Run on-demand for specific domain within WFO (~5-10 min)

• Horizontal resolution is same as the host GFE grids. NDFD currently supports 5km.

• Runs to 156 hrs

• Inputs include: NDFD 10m winds, NDFD 2m air temperatures, MMAB SSTs and Ice Analysis.

• Runs for the Great Lakes occur 4 times daily at NCEP (~40 min)

• Horizontal resolution is roughly 4 km (latitude-longitude grid)

• Runs to 156 hrs.

Great Lakes Environmental Lab (GLERL) Wave Model WaveWatch III

A main limitation of the baseline GLERL model is that

shallow water effects are ignored.

Source terms include: bottom friction, depth-induced

breaking, and scattering due to wave-bottom interactions.

3/24/201018th Annual Canada/US Great Lakes Operational Meteorology Workshop (GLOMW)

5

Shallow water physics approximation Strictly Bottom Effects - does not include direct reflection or

refraction effects (although refraction may result from bottom drag effects on wave phase vector).

Phase speed of wave is incrementally lowered when Wavelength >= Depth and limited by the breaking steepness.

Adjustment is a proportional comparison between the depth limited wavelength and deep water wavelength acting on the deep water phase speed.

When wavelengths are shortened to breaking steepness (amplitude * 7 ~= wavelength) or the wave height exceeds 40% of the depth the phase is relaxed to a value greater than the deep phase effectively removing the stress and allowing amplitude dampening – mimicking breaking.

Wave Model Modifications


6

A total of 3600 simulations were run to steady state for the western Great Lakes.

Horizontal resolution: 2.5 km and 5 km. Shallow, Deep (baseline) water physics. 15 wind directions. [040, 090, 130, 180, 210,

220, 230, 240, 250, 260, 270, 280, 290, 320, 360]

12 wind speeds. [6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50 knots]

5 stability classes. [stable, marginally stable, neutral, marginally unstable, unstable]

Steady State Flow Simulations


7

A random sample was gathered to understand the changes to the model

output brought on by the introduction of the shallow water physics and increased resolution. Total sample size was 500

conditions.

On average, the shallow water adjustment increases gridded wave heights.• 5.0 km Deep to 5.0 km Shallow results in an average increase of around 7.8%• 2.5 km Deep to 2.5 km Shallow results in an average increase of around 5.0%• 5.0 km Deep to 2.5 km Shallow results in an average increase of around 3.4%Shallow water adjustment has the greatest impact on nearshore grids and under moderate to strong flows.• Nearshore environment: average increase of around 10.4% for 5km Deep to 5km Shallow.

average increase of around 6.5% for 2.5km Deep to 2.5km Shallow. average increase of around 4.8% for 5km Deep to 2.5km Shallow.

• >=18 knots: average increase of around 10% for 5km Deep to 5km Shallow. average increase of around 7.1% for 2.5km Deep to 2.5km Shallow. average increase of around 6.2% for 5km Deep to 2.5km Shallow.

Comparison Results


8

Shallow 270 22kt 2.5km Deep 270 22kt 2.5km Deep 270 22kt 5km Shallow 270 22kt 5km

Shallow water adjustment also outputs greater wave heights on average for the deeper open lake environment. 5.0 km Deep to 5.0 km Shallow results in an average increase of around 4.0% 2.5 km Deep to 2.5 km Shallow results in an average increase of around 2.9%

Comparison Results cont…


9

Shallow 320 42kt 2.5km

2003 11 13 8 21.1 26.1 283 3.782003 11 13 9 21.3 26.7 281 5.442003 11 13 10 22.8 29.3 286 6.012003 11 13 11 21.7 27.0 290 5.912003 11 13 12 21.7 27.2 292 7.17

2003 11 13 9 21.9 26.7 291 4.242003 11 13 10 20.7 26.3 297 5.382003 11 13 11 21.3 27.3 302 5.322003 11 13 12 21.0 26.5 312 5.772003 11 13 13 21.1 27.4 308 5.78

Southern Lake Huron ~24 ft

Northern Lake Huron ~19 ftDeep 320 42kt

2.5km

Deep 290 42kt 2.5km Shallow 290 42kt 2.5km

Nov 13, 2003

Solely increasing resolution results in a reduction of forecasted wave heights. This difference is attributed to coarse grid averaging. The 5km to 2.5km disparity is most pronounced in nearshore regions where

bathymetrical gradients are greatest . In general, increased resolution leads to negligible differences for the open

lake or where fetch remains relatively constant.

Comparison Results cont…


10

Shallow 220 22kt 5km Shallow 220 22kt 2.5km

Deep 220 22kt 5km

Deep 220 22kt 2.5km

Greater wave heights in the shallows (subject to breaking - so the height is depth limited

Introduction of wave height variation in shallow water associated with bottom structure (e.g. shoals, island slopes)

Higher wind conditions result in increased wave heights in deep water from wave trains that originate in extensive shallow regions (e.g., wave generated in Saginaw Bay translating to Lake Huron)

• Initial waves are steeper producing shorter period wave trains for the given amplitude allowing for greater wind stress.

• Furthermore, slower dispersion of the wave phase in the deep water results in prolonged surface stress resulting in maintenance greater amplitude.

Effects of Shallow Water Approximations


11

Short Case Studies

3/24/2010

12


• 2.5 km delta-x• 40 levels• No convective scheme• Goddard Microphysics• Mellor-Yamada-Janjic PBL with Eta Similarity

Scheme• NOAH land surface model• NAM initial and boundary conditions

(Replicate real-time forecast setup)• High Resolution RTG SST

WRF Configuration


13

NWS DTX electronics staff left Essexville CG aboard vessel at approximately 14Z.

Approximate 1 hr journeys to and from Saginaw Bay Light #1, SBLM4 Waves of 4 feet prevented personnel from landing at site!

19 September 2008


14

19 September 2008 - Nearshore Forecast


15

Inner Saginaw Bay322 PM EDT THU SEP 18 2008

.FRIDAY...SOUTH WINDS 5 TO 10 KNOTS...INCREASING TO 10 TO 15 KNOTS. MOSTLY SUNNY. WAVES 1 TO 2 FEET.

1000 PM EDT THU SEP 18 2008

.FRIDAY...SOUTH WINDS 5 TO 10 KNOTS...INCREASING TO 10 TO 15 KNOTS. MOSTLY SUNNY. WAVES 1 TO 3 FEET.

353 AM EDT FRI SEP 19 2008

.TODAY...SOUTHEAST WINDS 10 TO 15 KNOTS...TURNING TO SOUTH. PARTLY SUNNY. WAVES 1 TO 2 FEET.

3/24/201018th Annual Canada/US Great Lakes Operational Meteorology Workshop (GLOMW)16

GSLM4 2008 09 19 07 13 130 9.8

GSLM4 2008 09 19 12 13 160 5.1

GSLM4 2008 09 19 15 14 180 2.6

GLERL Wave Model Shallow


17

07Z 2.5 km

07Z 5 km



18

14Z 2.5 km

14Z 5 km

Steady State Example


19

Steady State example in much better agreement with observations. 130 18 kts unstable.

Embarked East Tawas Coast Guard vessel at approximately 14Z. Approximate 1 hr journeys to and from Gravelly Shoals, GSLM4 Return trip to East Tawas Coast guard station at 1630Z

27 July 2009


20

27 July 2009 - Nearshore Forecast


21

Outer Saginaw Bay355 PM EDT SUN JUL 26 2009

.MONDAY...WEST WINDS 10 TO 15 KNOTS...TURNING TO SOUTHWEST. PARTLY SUNNY. WAVES 1 TO 3 FEET...SUBSIDING TO AROUND 1 FOOT LATE.

940 PM EDT SUN JUL 26 2009

.MONDAY...WEST WINDS 10 TO 15 KNOTS...TURNING TO SOUTHWEST. PARTLY SUNNY. WAVES 1 TO 3 FEET.

351 AM EDT MON JUL 27 2009

.TODAY...WEST WINDS 10 TO 15 KNOTS...BECOMING SOUTHWEST 5 TO 10 KNOTS...THEN INCREASING TO 10 TO 15 KNOTS DURING THE AFTERNOON. PARTLY SUNNY. WAVES 2 TO 4 FEET.

27 July 2009 - What Happened?


22

Tawas Bay ~ 1 foot Tawas Pt. to Alabaster ~ 1 to 3 feet Alabaster to Pt. Lookout ~ 3 to 4 feet Pt. Lookout to Gravelly Shoal ~ 5 feet occasional 6 feet

3/24/201018th Annual Canada/US Great Lakes Operational Meteorology Workshop (GLOMW)23

SBLM4 2009 07 27 09 00 277 5.6

SBLM4 2009 07 27 12 00 241 6.5

SBLM4 2009 07 27 14 00 205 6.3 SBLM4 2009 07 27 15 00 222 6.3



24

15Z 2.5 km

15Z 5 km



25

16Z 2.5 km

16Z 5 km

Steady State Example


26

Steady State example in much better agreement with observations. 220 18 kts unstable.

19 September 2008 – Models did not resolve stronger wind field from the night before. This led to lower wave heights overnight and less residual wave activity during the morning.

19 September 2008 – Steady state example in better agreement with observations provides 4 ft wave heights near Saginaw Bay Light #1.

27 July 2009 – Again, weaker modeled wind field led to under-forecasted wave heights.

27 July 2009 – Steady state example in better agreement with observations provides for 5 ft wave heights near Gravelly Shoals.

Short Case Study Summary


27

No substitute for correct wind field and stability!

Seemingly uneventful days can be far from it on the Lakes.

With higher resolution gridded wave height forecasts, what is the best way to convey this information?

Wave height comparisons with new GLERL shallow water modification, WaveWatchIII and Swan.

Three Dimensional Atmospheric Idealized Flow over the Great Lakes

Future work

3/24/2010

28


cory behnke meteorologist noaa/nws detroit-pontiac

Documents