1.0 Introduction

1.0 Introduction » 1.1 Heavy Swells and Forecast Challenges

Huge waves may delight surfers, but they can take a large toll on human life, infrastructure, and socio-economic activities, such as fishing, port operations, and recreation management, in coastal areas.

photo showing surfers in big waves with small boats beside

High swell events can develop far from the coast under cyclonic conditions, and take several days to travel to land. During this period, weather conditions in coastal areas often are quite benign, with people going about their activities with little awareness of what’s developing. If early warnings are not issued, heavy swells can take an area by surprise and have a devastating impact. The photographs below show damage from a heavy swell event.

photo showing damage from a high swell event on the Moroccan coast
photo showing damage from a high swell event on the Moroccan coast

Heavy swells are challenging to forecast given the distance at which they can form. Forecasters need to consider many elements of the weather situation and analyze many types of data in order to detect the phenomenon, accurately forecast it, and issue the early warnings needed to minimize its impact.

1.0 Introduction » 1.2 About the Lesson

This lesson aims to improve the ability of marine forecasters to forecast extreme marine events related to high swells. It does so by providing background information on winds and waves, and presenting a process for monitoring and forecasting high swell events using a variety of data. These include ASCAT scatterometer wind data and the ECMWF Extreme Forecast Index (EFI) product, which is useful for verifying model output and improving the quality of heavy swell forecasts. The forecast process is applied to two cases that occurred on the Moroccan Atlantic coast. The first case goes through the forecast process in detail while the second case highlights particular aspects.

Target audience

The lesson is intended for marine operational forecasters, be they beginners or experts. The lesson can be used by forecasters as a self-paced learning program and by instructors as a lecture aid in marine meteorology and forecasting courses, particularly those with a remote sensing component.

Prerequisite knowledge

Before taking the lesson, learners should be familiar with the basics of general and marine forecasting techniques as well as marine forecast models and observational and remote sensing tools.

The lesson uses ASCAT scatterometer wind data, which may be new to some learners. Regardless of whether you have used the data before, it is strongly recommended you take the COMET lesson Using Scatterometer Wind and Altimeter Wave Estimates in Marine Forecasting before starting this lesson.

ASCAT

Lesson objectives

After going through the lesson, learners should be able to:

  • Differentiate high swell and wind waves
  • Describe the characteristics of high swells caused by synoptic events
  • Describe the impact of swell events
  • Monitor and forecast high swell events using a marine forecast process that involves the use of observational data, including ASCAT scatterometer wind data, and model output from WAVEWATCH III and the WAveModel (WAM) along with its Extreme Forecast Index

2.0 About High Swell Events

2.0 About High Swell Events » 2.1 Main Components of a Wave

Wind is the primary cause of wave development. It puts initial stress on the water surface and creates capillary waves, which start the process of transferring energy from the air to the water.

Merchant ship Noble Star in North Pacific storm seas

Waves have three main characteristics:

  • Height: The distance between the trough and crest
  • Length: The distance between two troughs or crests
  • Period: The time that it takes for the wavelength to pass by a point
Water wave anatomy - wave length, height, etc….

Waves are also characterized by their steepness. This is defined as wave height divided by wavelength (S = H/L). So, a steep wave can either have a high wave height or short wave length.

Question

A high, short wave has steepness.

Applying the formula S = H/L, a large height divided by a short length yields high steepness.

2.0 About High Swell Events » 2.2 Wave Growth Factors

There are three basic wave growth factors:

  • Wind speed: The wind blowing on the sea surface creates waves that move in its direction. The wind flow maintains wave growth. The process continues as long as the wind speed is greater than the wave speed.
  • Fetch or fetch length: This is the distance that the wind generally blows over the sea from a constant direction and at a constant speed
  • Duration: This is how long the wind affects the fetch or fetch length

Although each of these factors is important to consider, they do not all have the same impact on wave height. Wave height is most sensitive to changes in wind speed, even when fetch or duration is limited. Therefore, when you’re looking for areas of significant wave generation, consider wind speed first, followed by fetch length and wind duration. This helps explains why accurate wind forecasts are so important to wave forecasts in marine meteorology and numerical modeling.

2.0 About High Swell Events » 2.3 Swell Vs. Wind Waves

During a storm, the sea surface generally looks very chaotic, with lots of short, steep waves of varying heights. Away from very windy areas, the sea surface has long, rolling peaks of a uniform shape. For this reason, we differentiate between two types of surface waves: wind waves and swell.

Wind waves refer to short-period waves that are still being created by winds or are very close to the area in which they were generated. Wind waves are a local manifestation of the energy that has been transferred to the sea from the wind. The energy travels downwind away from the source area, like ripples that form when a stone is dropped in a pond, or from the bow wave from a ship.

photo showing waves affected by wind and current

Swell refers to waves that have moved out of the generating area, far from the influence of the winds that created them. The stronger the winds at the source area, the larger the swell and further it will travel. The longer that the wind blows in the source area, the longer the swell will persist, even long after the wind has ceased or changed direction. The farther swell travels, the more dispersion and nonlinear wave-wave interactions operate, resulting in well-sorted, long-period waves.

photo showing swell

Since wave energy becomes more organized as it travels long distances, swell is more uniform and regular than wind waves. The wave energy leaving the fetch area is concentrated at the center of the fetch exit region and is in the direction of the prevailing wind, although it continues to spread out once it leaves the area.

Changing surface wind variables, such as speed, direction, fetch, length, and duration, tend to create waves of different types. That’s why the sea surface is best described by mathematical functions like wave spectra (the characteristic shape of the spectrum of waves) and statistical parameters like significant wave height (Hs, defined as the average height of the highest one third of all waves). Significant wave height correlates well with average wave height reported by trained observers.

For more information, see the COMET lessons on the MetEd website, Wave Life Cycle I: Generation and Wave Life Cycle II: Propagation & Dispersion.

2.0 About High Swell Events » 2.4 Swell Vs. Wind Wave Exercise

Review the information about the four systems, then answer the question below.

Systems

Wave direction

Wave period (seconds)

Wave height (meters)

Local wind direction

Local wind speed (kt)

1

Northwest

14

3.50

Southwest

15

2

West

04

2.00

West

20

3

Northwest

08

4.50

Northwest

30

4

Southwest

08

3.00

Variable

05

Question

Which are wind waves and which are swells? Select the correct answer.

Systems 1 and 4 have a different wave direction than the local wind and a relatively high period. Since they’re so different from the local wind, they were generated far from the observation zone and have moved via the propagation process.

Systems 2 and 3 have the same direction as the local wind so they are wind waves generated locally.

Please make a selection.

2.0 About High Swell Events » 2.5 Nomograms

Forecasters need to know how to use the nomogram for wave characteristic forecasts so we will introduce it here and refer to it in the cases.

The nomogram illustrates the relationship between wind speed, wind duration, fetch length, and wave growth. The X-axis indicates wind duration in hours; the Y-axis indicates wave height (m); and the contour lines represent fetch and wind speed.

Manual Wave Forecasting Diagram

Question

If a wind blows over a 100-km fetch at 10 m/s over 12 hours, what’s the resulting wave height and period? Use the nomogram to determine this, then select the best answer below.

A 10 m/s wind speed and 100-km fetch length contours intersect at a point of about 9 hours duration. This gives a wave height of 1.8 m and wave period of 5 s, as shown in the nomogram below. Here wave growth is limited by the fetch. Even if the wind continued blowing for 12 hours, the wave would stop growing because it cannot exceed the 100-km fetch length over which the wind blows.

Manual Wave Forecasting Diagram with feedback for an exercise overlaid on nomogram
Please make a selection.

For more information about nomograms, see the COMET lesson Wave Life Cycle I: Generation.

2.0 About High Swell Events » 2.6 Swell Travel

Swell travels along routes that follow great-circles tracks. A great-circle track is the shortest distance between two points on a sphere. When viewed on a two-dimensional map, the track appears curved. For very short travel distances, such as the distance swell might travel in the Gulf of Mexico, a straight line on any map is a good approximation of a great-circle track. However, when swell travels great distances over the ocean, the great-circle track is markedly different than a straight line on a flat map projection.

Polar Stereographic projection of Northern Hemisphere with great circle tracks plotted along with the corresponding straight lines as if drawn on a piece of paper.

2.0 About High Swell Events » 2.7 More About Swell

Here’s some final information about swell.

The highest individual wave that is likely to be encountered in a storm is roughly twice the significant wave height. This height can be greater in rapidly changing conditions.

The energy level carried by waves varies depending on the state of the sea, with heavy storms producing waves of considerable power.

The power (P) of a wave is proportional to its period (T) times the wave height (H) squared, as given by the following formula:

P=0,4 * Hs² * T (kW per m).

The table shows just how much the power of a wave increases with relatively small increases in height. The important issue is that wave height can make a wave really powerful!

Beaufort scale

4

5

6

8

Hs (m)

1

2

4

6

T (s)

6

8

10

14

P (kW/m) (Power)

2.4

12.8

64

201.6

Question

Which system is more powerful?

Swell 1 = 320 kW/m and swell 2 = 640 kW/m. Therefore, swell 2 is more powerful than swell 1. Remember, P=0,4*Hs²*T (kW per m). When (T) doubles, the P (power) doubles. But when Hs doubles, the power (P) quadruples.

Please make a selection.

2.0 About High Swell Events » 2.8 Analyzing Swell: Wave Model Data

Numerical wave models track the location and propagation direction of newly generated waves and existing swell groups. They use this information, along with great-circle tracks and wave physics to forecast their movement.

Several models are operated by national meteorological services, the most widely used being NOAA's WAVEWATCH III and ECMWF’s third generation WAveModel (WAM). Both simulate the rate of change of the two-dimensional wave spectrum in any water depth caused by advection, wind input, and dissipation due to whitecapping and bottom friction, as well as non-linear wave interactions. The models differ principally in the source of the wind fields used - the Global Forecast System (GFS) atmospheric model for WAVEWATCH III and ECMWF’s atmospheric model for the WAM. Here are links to more information on both models.

3.0 About the Marine Forecast Process and ASCAT

3.0 About the Marine Forecast Process and ASCAT » 3.1 Marine Forecast Process

The marine forecast process for high swell events is presented below. We will describe it and then apply it to two cases.

Marine Forecast Process

Step 1: Assess the current atmospheric and marine situation.

  • Determine the meteorological situation by examining satellite (infrared, visible, water vapour, and ASCAT) data and observational (ship and buoy) data
  • Examine the marine model analyses of 10 m winds, sea surface pressure, and significant wave height

Step 2: Compare the marine model analyses with the observations. If they are in agreement, you should be able to use the model forecasts with confidence. Otherwise, you should use them cautiously and mentally adjust the marine situation temporally or spatially.

Step 3: Forecast the marine situation.

  • Interpret the model forecasts of 10 m winds, sea surface pressure, significant wave height, and the Extreme Forecast Index (EFI).
  • What swell heights are forecast? Do they exceed thresholds?
    • Note that Moroccan wave height (Hs) criteria is 4 m or more in the Atlantic Ocean, and 3 m or more on the Mediterranean coast. After the high swell event covered in the case, additional criteria were added that combine wave height and wave period in the Atlantic Ocean: wave height = 6 m or more, and wave period = 16 s or more
  • If thresholds are exceeded, disseminate warnings, then produce and disseminate the marine products. If the values are below threshold values, produce and disseminate the marine products.

Step 4: Continue monitoring the atmosphere and marine environment.

This process is depicted in the flowchart below.

flowchart showing marine forecast process depicted in lesson

3.0 About the Marine Forecast Process and ASCAT » 3.2 About ASCAT

Since we are using ASCAT scatterometer data in the lesson, we will introduce it here. But we won’t go into much depth since it is covered in the prerequisite COMET lesson mentioned earlier (Using Scatterometer Wind and Altimeter Wave Estimates in Marine Forecasting).

ASCAT wind data comes from active radar instruments called scatterometers that emit and receive microwave radiation. The radiation is not affected by clouds, which is why scatterometers can scan the surface in almost all weather conditions - clear, cloudy, or rainy. The data is also useful because it’s available in data-sparse oceanic regions where extreme weather events, such as extratropical cyclones, are generated.

The ASCAT scatterometer flies on EUMETSAT’s MetOp satellites. ASCAT has six antennas that allow for simultaneous coverage of two 550-km wide swaths separated by a 670-km gap. You’ll notice the gap when viewing the data - it’s where little useful wind information is reflected back to the instrument.

ASCAT

Scatterometers are used to monitor a range of parameters - from sea ice and snow cover, to land surface parameters, such as soil moisture and vegetation. However, their primary use is to measure wind speed and direction over the ocean surface (10 m above) for use in weather analysis and forecasting.

4.0 Case Study 1

4.0 Case Study 1 » 4.1 About the Case

This case study examines a strong swell event that occurred on the Moroccan Atlantic coast from 6 to 7 January 2014.

The north and center of the Moroccan Atlantic coast from Larache to Sidi Ifni is an important region for many reasons. It contains the country’s main ports, including the oil port of Mohammedia, the commercial port of Casablanca, and the energy port of Jorf Lasfar. The area also has other important economic activities, including tourism.

map of Morocco with ports and commercial or otherwise important coastal cities labeled

Marine weather forecasts are important for coastal activities as well as for planning operations at sea. Marine weather forecasts are broadcast each morning in bulletins spanning one to five days.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.1 MSG IR and VIS Imagery

We’ll start the case by looking at MSG satellite imagery from several days before the event.

The IR animation from 4 January at 0000 UTC to 5 January at 2100 UTC shows a large low-pressure system offshore moving to the northeast from Newfoundland to Great Britain. On the Moroccan coast, the weather is fair with just a few low clouds and light winds on 5 January.

The visible images of the Moroccan coastal area from 4 to 7 January at midday show some unstable clouds in the northwestern coastal area on 4 January. But they clear up and the area experiences stable conditions thereafter, with clear skies and morning fog patches in some coastal areas.

4 MSG vis 0.6 micrometer images from midday on jan 4, 5, 6, 7 showing coastal Morocco

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.2 Mean Sea Level Pressure

From 4 to 6 January, low pressure (970 hPa) centered south of Newfoundland in the North Atlantic Ocean moves to the northeast and deepens.

ECMWF MSLP analysis for 6 Jan 2014 at 12 H

The ECMWF MSLP animation of the North Atlantic Ocean from 4 January at 0000 UTC to 5 January at 1800 UTC shows a deep depression moving rapidly from eastern Newfoundland to Great Britain. It deepens by about 35 hPa per 24 hours until reaching 936 hPa on 5 January at 0000 UTC. Note the high pressure gradient indicated by the packing of the lines in the south of the low center. The associated winds exceed 50kt locally to the south and southwest of the low where the flow converges.

The maximum fall of pressure occurs between 4 January at 0000 UTC and 5 January at 0000 UTC. The pressure varies from 972 hPa to 936 hPa, corresponding to a deepening of 36 hPa in 24 hours.

This rapidly deepening is called an explosive cyclogenesis or "bomb." Indeed, to be in this category, a depression's central pressure in the mid-latitudes must decrease on average by at least 1 hPa per hour for 24 hours. In our situation, the depression has decreased by 36 hPa in 24 hours. This explosive deepening indicates an exceptional situation.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.3 ASCAT Winds and WV 6.2 µm

Now we’ll look at ASCAT wind data, overlaying it first on satellite imagery and then on MSLP data.

The image shows ASCAT winds on 6 January between 1119 and 1126 UTC overlaid on an MSG WV 6.2 µm image from 6 Jan at 1200 UTC. As you can see, the ASCAT winds highlight the movement of the low-pressure system to the northeast (as evidenced by the strong winds).

ASCAT Wind Between 1119 UTC and 1126 UTC on 6 Jan 2014
with MSG WV 6.2 on 6 Jan 2014 at 1200 UTC

The MSG observation from 1200 UTC confirms that the two ASCAT swaths surround the most active area of low pressure (the cloudy areas).

The ASCAT data indicates the wind speeds associated with the system. They show southwest winds ranging from 40 to 50kt, which correspond to the low pressure moving northeast.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.4 ASCAT Winds and ECMWF MSLP

Now the ASCAT winds from 5 January at 0918 UTC, 1100 UTC and 1239 UTC are overlaid on the ECMWF MSLP chart from 1200 UTC. There are strong westerly winds of about 40 to 50kt southwest of the depression, and anticyclonic conditions with a low wind velocity on the Moroccan coast.

ECMWF MSLP 5 Jan 12 UTC and ASCAT Winds 5 Jan 2014 at 0918, 1100, and 1239 UTC

Question

Focus on the wind southwest of the low pressure. If it continues blowing for 24 hours, will the situation generate high swell off the coast of Morocco?

The strong northwesterly and westerly winds blowing southwest of the low pressure will generate high waves and swells.

Please make a selection.

On the nomogram, a wind of 40 to 50kt blowing for 24 hours would create waves of 8 to 12m height.

Manual Wave Forecasting Diagram

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.5 North Atlantic Ocean Ship and Buoy Observations

Here are ship and buoy observations from the North Atlantic Ocean on 6 January at 1200 UTC.

Ship and Buoy Observations on 6 Jan 2014 at 1200 UTC off the Moroccan coast
Legend for Ship and Buoy Observations

Question 1 of 3

Ship and Buoy Observations on 6 Jan 2014 at 1200 UTC off the Moroccan coast with a line marking the high seas for an exercise

What is the maximum significant wave height reported by ships and buoys on 6 January at 1200 UTC out in the ocean (to the left of the green line)?

Two ships southeast of the Azores Islands report a maximum significant wave height of 8m. The ship to the west is reporting a maximum wind wave height of 8 m, and the ship to the east is reporting a maximum swell wave height of 8 m. Recall that the observations are reported in half meters. The 99 in the eastern ship's wind wave block signifies that period estimation was not possible.

Ship and Buoy Observations on 6 Jan 2014 at 1200 UTC off the Moroccan coast with obs marked for an exercise
Please make a selection.

Question 2 of 3

What is the corresponding wave direction reported by those ships?

The ships report waves moving from the northwest. This is generally in accordance with the wave’s propagation from the generation area.

Please make a selection.

Question 3 of 3

What’s the maximum period that the ships report at 1200 UTC?

The ships report a maximum period of 20 s. This is an exceptional period, and should increase the wave’s power.

Please make a selection.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.6 Observations Off the Moroccan Coast, 1

Now we’ll look at ship and buoy observations taken off the Moroccan coast.

Ship and Buoy Observations on 6 Jan 2014 at 1200 UTC off the Moroccan coast with a box marking observations near the coast for an exercise

Question 1 of 2

What is the maximum significant wave height and highest period reported by ships and buoys off the Moroccan Atlantic coastal areas on 6 January at 1200 UTC?

The ship circled in red reports a significant wave height of about 4.5 m and a period of 18 s. This period value is not usually observed off the Moroccan coast. At the same time, far to the south, two ships observe a significant height of 3 m and 3.5 m.

Ship and Buoy Observations on 6 Jan 2014 at 1200 UTC off the Moroccan coast with obs off of Spain marked for an exercise
Please make a selection.

Question 2 of 2

In this situation, should forecasters take action?

Since this swell height exceeds 4 m, it's above the threshold for issuing a warning. On the Atlantic coast, the warning threshold is a wave height greater than or equal to 4 m, while in the Mediterranean, it is 3 m.

Please make a selection.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.7 Observations Off the Moroccan Coast, 6 Hours Later

Here are ships and buoys observations issued 6 hours later - at 1800 UTC.

Coastal surface obs off Morocco 6 Jan 2014 at 1800 UTC

Question 1 of 2

What are weather conditions at the Moroccan Atlantic coastal stations on 6 January at 1800 UTC?

The weather is stable at the Moroccan coastal stations. The wind is variable, with a low velocity (5kt). High clouds and some fog is observed in the coastal areas.

Please make a selection.

Question 2 of 2

Ship observations near the Canary Islands on 6 Jan 2014 at 1800 UTC

What is the maximum significant wave height reported by the circled ship observation off the Moroccan Atlantic coast at 1800 UTC?

The ship northeast of the Canary Islands reports a maximum significant wave height of 5 m. Note that after 6 hours, the significant wave height has gone from 3 to 3.5 m up to 5 m.
Please make a selection.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.8 Significant Wave Height

This ECMWF significant wave height animation shows a wide area with maximum values of 19 m in the middle of the Atlantic Ocean on 6 January at 1200 UTC. The maximum is moving northeast towards Western Europe and the Great Britain coasts.

4.0 Case Study 1 » 4.2 Step 1: Assess the Current Atmospheric and Marine Situation » 4.2.9 WW111 Wave Field Analysis

The WAVEWATCH III wave field analysis for 6 January at 1200 UTC shows a maximum wave height area moving northeasterly towards the Moroccan coast. Significant wave height values are about 9 to 14 m in the Atlantic Ocean and about 3 to 4 m along the coast of Morocco.

WaveWatch III Significant Wave Height Analysis 6 Jan 2014 1200 UTC

Question

Based on the analysis, how do you think the marine conditions might evolve in the next few hours in the Moroccan coastal areas? The swell heights will reach:

Given the propagation process and wave direction, the maximum wave height will progress from northwest to southeast. The Moroccan coastal areas will experience hazardous marine conditions, with wave heights greater than 6 m. A warning should be issued in accordance with the marine forecast process.

Please make a selection.

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.1 Comparing the Analysis and Observations

One way to see if model analyses are consistent with reality is to overlay them with observational data and see how well they correlate. We’ll do that with the following fields and data:

  • ECMWF MSLP with satellite IR 10.8 µm imagery
  • ECMWF MSLP with surface winds
  • ECMWF 10m wind with ASCAT wind data
  • ECMWF 10m wind with ship observation

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.2 ECMWF MSLP and IR 10.8 µm Imagery

Click the tabs to view the ECMWF MSLP field overlaid on MSG IR 10.8 µm imagery for 6 January at 0000, 0600, and 1200 UTC. When you are ready, answer the question below.


Question

Which statement is correct?

The model MSLP is well correlated with the satellite images at all three times. Indeed, the North Atlantic low pressure (950 hPa) located off the British islands coincides perfectly with the perturbation system seen by the satellite.

Please make a selection.

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.3 ECMWF MSLP and Surface Wind

The overlays of ECMWF MSLP and surface winds for 6 January at 0000 and 0600 UTC show that the maximum wind speed zone of 40 to 50kt matches the areas with the high gradient isobars.

Behind the depression, a ridge has extended to the north from the high pressure system. We see a long fetch of northwest wind of about 40kt southwest of the low pressure system. These strong winds enhance the oceans waves and push them directly to the Moroccan Atlantic coast.

Question

On the image, use the pen to draw the direction of the fetch that’s related to the northwesterly winds blowing southwest of the low. Then click Done.

Tool: Tool Size: Color:

The fetch direction is drawn on the image.

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.4 ECMWF 10m Wind Field and ASCAT Wind Data

We’ve combined the ECMWF 10 m wind field with the ASCAT wind data for 6 January. Let’s see how they correlate.

ECMWF 10 m Wind Speed Valid for 6 Jan 2014 0000 UTC and ASCAT Wind Data
ECMWF 10 m Wind Speed Valid for 6 Jan 2014 1200 UTC and ASCAT Wind Data

At this stage, ASCAT is measuring winds in the 40 to 50kt range for the low pressure system. These are compared to the ECMWF 10 m wind speed for 6 January at 0000 UTC. Taking the wind speed values into account, the ECMWF wind analyses at 0000 and 1200 UTC are very similar to the ASCAT wind data. The colors in the image make this clear. ASCAT and ECMWF use various colors to indicate different wind speeds. The combined image shows that the overlapping areas generally have the same color. This indicates that the ECMWF model wind 10 m analysis is accurate, and that the ASCAT data is likely to have been assimilated into this ECMWF model run.

Question

What are the benefits of assimilating ASCAT data into the ECMWF model? Choose all that apply.

ASCAT wind data are spatial data measured in real time. Since they reflect actual conditions, they improve the data going into the model and therefore improve its forecast results.

Please make a selection.

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.5 Model Wind Speed and Ship Observations

Overlaying the ECMWF 10 m wind speed analysis with ship observations is another way of checking the accuracy of the model forecast. But be aware that ship observations can sometimes be wrong.

ECMWF 10 m Wind With Ship Observations for 6 Jan 2014 0000 UTC

Question 1 of 2

For this situation, how well do the ECMWF 10 m wind speed and ship and buoy observations correlate?

For this situation, we generally see a good correlation between observed and modeled wind direction and speed, especially near the coast. Note the circled 50kt wind speed observation, where the model predicted 30kt.

Please make a selection.

Question 2 of 2

ECMWF 10 m Wind With Ship Observations for 6 Jan 2014 0000 UTC with one ob circled for an exericse

The model 10 m wind results in the chart do not agree with the circled 50kt ship observation. How should it be handled?

The circled observation shows a west wind with a speed of 50kts while the surrounding observations show lower winds. This isolated observation differs from the model observation and does not seem consistent with the model wind field or other buoys and ship observations. Despite this, caution should be exercised before automatically discarding the observation. Rather, you should review the ship’s previous observations, examine its consistency with the MSLP gradient, and then decide whether to trust the data.

Please make a selection.

4.0 Case Study 1 » 4.3 Step 2: Compare the Marine Model Analyses with the Observations » 4.3.6 Step 2 Summary

Here’s a summary of the weather situation during the two days before the event, using the observations and model analysis:

  • The MSG satellite imagery shows a large depression moving northeasterly over the North Atlantic Ocean. On the Moroccan coast, the situation is stable with few low clouds, coastal fog patches, and weak winds
  • The MSLP analysis shows low pressure with an explosive deepening (35 hPa per 24 hours) and a high pressure gradient. The northeasterly movement of the system has led to a large zone of strong northwest winds that blow for more than 24 hours and form a long fetch of about 3000 km.
  • The ASCAT observations show winds of about 40 to 50kts south of the low pressure.
  • Ship observations show high waves (8 m) with a long period (2 s) in the middle of the Atlantic Ocean, and waves of about 3 m increasing rapidly to 5 m offshore the Moroccan coasts.
  • The WW111 and WAM wave model analyses show high significant wave height near the low pressure center that are propagating towards Moroccan coasts.

All of this indicates that the situation will be unusual and that forecasters should be aware and take the appropriate action in accordance with the marine forecast process.

Question

Manual Wave Forecasting Diagram

Using this information and the nomogram, what significant wave height is expected?

A wind of about 40 to 50kt (20 to 25 m/s) blowing over 24 hours will create wave heights of 8 to 12 m.

Manual Wave Forecasting Diagram with feedback for an exercise overlaid on nomogram
Please make a selection.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.1 ECMWF MSPL and Wind

The low is now moving to Great Britain and filling. The pressure in the center rises slowly from 944 hPa on 6 January at 0000 UTC to 968 hPa on 7 January at 1800 UTC.

A strong ridge forms just behind the low, promoting strong northerly flow, which is favorable for the waves moving to the south. The winds are still strong and exceed 40kt near the depression.

The movement of the low pressure generates a long fetch area of about 3000 km with a northwest wind.

The surface wind animation from 6 January at 0000 UTC to 7 January at 1800 UTC shows wind flow moving quickly to Great Britain. A maximum wind value of around 45 to 50kt is located in the center of the North Atlantic Ocean and reaches the European coast.

On the Moroccan coasts, variable and weak winds are recorded during the days of 6 to 7 January.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.2 ECMWF WAM Significant Wave Height

The ECMWF WAM significant wave height field animation from 6 January at 0000 UTC to 7 January at 1800 UTC shows a maximum zone located in the center of the North Atlantic Ocean extending from north of the Azores Islands to Great Britain. The maximum value is around 18 m on 6 January at 0000 UTC, and moves northeasterly towards the West European coast.

The heavy swell is expected to reach the Moroccan coasts by the night of 6 to 7 January, with the significant wave height exceeding 6 m.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.3 WW111 Significant Wave Height

The WAVEWATCH III significant wave height fields from 6 January at 0000 UTC to 8 January at 0000 UTC also show a wave maximum area between 14 and 20 m in the center of the Atlantic Ocean moving straight towards the British Isles and France.

On the Moroccan Atlantic coast, the model predicts significant wave height values between 6 and 9 m during the night of 6 to 7 January and morning of 7 January.

Recall that the significant wave height corresponds to the average of the third highest waves, although individual waves can be higher.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.4 Extreme Forecast Index (EFI)

We’ve looked at both model forecasts. Now we’ll use the Extreme Forecast Index or EFI to evaluate the ECMWF forecasts.

EFI is a predictive parameter used to assess the abnormality of a forecast weather situation based on the difference between the ECMWF ensemble prediction system distribution and the ECMWF model’s climatology. The EFI is produced for maximum significant wave height and other parameters.

The EFI uses values from -1 to +1.

  • If all of the ensemble members forecast values are above the model-climate maximum, EFI = +1
  • If all the forecast values are below the model-climate minimum, EFI = -1

EFI magnitudes of 0.5 to 0.8 (irrespective of sign) generally signify that "unusual" weather is likely. Magnitudes above 0.8 usually signify that "very unusual" weather is likely.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.5 EFI Valid For 7 January 0000 UTC

According to the EFI values valid for 7 January at 0000 UTC (the 24- to 48-h forecast), the marine situation seems to be unusual and related to the extreme event. Indeed, the EFI values on the night of 6 to 7 January are around 0.8 to 0.9. The magnitudes are significant near the Moroccan coast. This implies that the marine situation is abnormal and dangerous, and could generate strong swells.

ECMWF Extreme Forecast Index Issued on 5 January Valid Until 7 Jan 2014 at 0000 UTC

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.6 EFI Valid for 8 January 0000 UTC

ECMWF Extreme Forecast Index Issued on 6 January Valid Until 8 Jan 2014 at 0000 UTC

Question

Based on the EFI chart valid for 8 January at 0000 UTC, where do you think that extreme wave heights will occur? Select all that apply, then click Done.

The EFI of significant wave height has an index value between 0.8 and 0.9 in the North Atlantic coasts and about 0.9 to 1 in the far Central Atlantic coasts.

Please make a selection.

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.7 WW111 Outputs in Selected Points

The tables show the WAVEWATCH III predictions of significant wave height, peak period, and direction of propagation for four points on the Moroccan Atlantic coast: Larache, Casablanca, Safi, and Agadir. The predictions are for 6-hour steps from 6 January at 0000 UTC to 8 January at 1200 UTC (60 hours).

WW3 wave model predictions of significant wave height, peak period, and direction of propagation for Larache, Casablanca, Safi, and Agadir; predictions for 6-hour steps from 6 Jan to 8 Jan 0000 UTC

For the night of 6 January and morning of 7 January, the WW111 model predicts significant wave height values of 6 to 7 m. These values are high. But what’s remarkable are the period values, which are approaching 18 s. The height of long-period swell increases significantly as they shoal and break in shallow water. Depending on the beach steepness, 7 m waves with an 18 s period can easily be 10 m high when they break.

These waves are extremely powerful. Remember the power of a wave is proportional to its period times the significant wave height squared (P=0,4 * Hs² * T (kW per m)).

Here’s the same output in graph form. You can see the predicted evolution of the waves’ characteristics at the four locations.

WW3 wave model predictions of significant wave height, peak period, and direction of propagation in graph form for Larache, Casablanca, Safi, and Agadir; predictions for 6-hour steps from 6 Jan to 8 Jan 0000 UTC

4.0 Case Study 1 » 4.4 Step 3: Forecasting the Marine Situation » 4.4.8 Step 3 Summary

After examining the model forecasts and output for various parameters, including MSLP, significant wave height, EFI, and wave characteristics for four Moroccan coastal points, we can describe the situation as follows:

  • By moving northeasterly, the low pressure has led to a wide zone of strong northwesterly winds blowing on a long fetch for at least 24 hours. This will form and push high swells straight towards the Moroccan coasts.
  • The WW111 and WAM models both show high values of significant wave height and high period propagating towards the Moroccan coasts. These powerful waves will be dangerous for human safety and lead to property loss along the coast and at sea.
  • The waves’ characteristics forecasted for the four coastal points show that the significant wave height will increase rapidly from 3 m up to 6 m during the night of 6 to 7 January
  • The ECMWF extreme forecast index shows a significant magnitude of about 0.8 to 1 on the north and central Atlantic Moroccan coast

Given the unusual situation and exceptional values, forecasters must issue a high level of warning to prevent coastal hazards as quickly as possible.

4.0 Case Study 1 » 4.5 Case Summary

Now, we’ll review what actually happened during the event and some of its impacts.

  • There was explosive cyclogenesis in the North Atlantic, characterized by deepening pressure of 36 hPa in 24 hours
  • There was a strong westerly wind to the south and southwest of the depression
  • The wind velocity reached approximately 40 to 50kt; recall that the wind speed is the first factor to consider in a wave’s growth and propagation
  • There was a long northwest fetch of about 3000 km, which was favorable for the wave’s propagation to the Moroccan Atlantic coasts
  • Ships in the high seas reported high swells of 7 to 8 m, with high periods of 20 s
  • Off the Moroccan coast, ships reported a significant swell height of 4.5 to 5 m, with a period of up to 18 s
  • The WW111 and WAM wave models gave good forecasts for both the significant wave height and period
  • The ECMWF Extreme Forecast Index shows a significant magnitude of about 0.8 to 1 on the north and central Atlantic Moroccan coast

Considering these characteristics, it’s clear that the swell that hit coastal Morocco was exceptionally powerful.

Here are some of the impacts of the event in the coastal areas:

  • In some places, the exceptional swells exceeded the projected swell heights for which the port infrastructures had been sized. This caused major disruptions of port traffic, and closed all of the ports on the Atlantic coast
  • The most significant incidents were reported at the following ports.
    • Mohammedia recorded very strong swells, with peaks exceeding 13 m
    • In the Safi harbor basin, 40 fishing boats and 35 other units were grounded
    • In Casablanca, four people were injured, and many terraced homes along the coast were damaged

Note that the arrival of strong waves coincided with high tides, making them an aggravating factor. The tidal range was about 3.5 m.

5.0 Case Study 2

5.0 Case Study 2 » 5.1 Introduction

This second case involves a potential heavy swell event on the Moroccan coast later the same month - on 18 January 2014. We will not go through the full forecast process - only those parts that let us make our points.

We’ll begin by looking at WW111 wave model output for three points on the Moroccan Atlantic coast: Casablanca, Safi, and Agadir.

5.0 Case Study 2 » 5.2 WW111 Output for 3 Points

The tables show model forecasts of significant wave height, peak period, and direction of propagation from 17 January at 0000 UTC to 19 January at 1200 UTC in 6-hour steps.

Station: CASA_PORT

DATE HEURE

HS (m)

PER (s)

MOY_DIR (deg)

20140117 0:00:00

4,38

12,22

314

20140117 6:00:00

4,52

12,09

314

20140117 12:00:00

5,07

11,83

311

20140117 18:00:00

5,73

11,82

309

20140118 0:00:00

6,23

11,81

311

20140118 6:00:00

6,95

12,31

312

20140118 12:00:00

6,91

11,99

316

20140118 18:00:00

6,20

11,50

318

20140119 0:00:00

5,35

11,21

315

20140119 6:00:00

4,96

10,37

313

20140119 12:00:00

4,31

9,94

312

Station: SAFI

DATE HEURE

HS (m)

PER (s)

MOY_DIR (deg)

20140117 0:00:00

3,69

12,65

320

20140117 6:00:00

4,04

12,81

320

20140117 12:00:00

4,06

12,37

318

20140117 18:00:00

4,99

11,71

312

20140118 0:00:00

5,54

11,65

314

20140118 6:00:00

6,58

12,19

314

20140118 12:00:00

6,69

12,34

317

20140118 18:00:00

6,28

11,85

319

20140119 0:00:00

5,47

11,60

319

20140119 6:00:00

4,84

11,06

318

Station: AGADIR

DATE HEURE

HS (m)

PER (s)

MOY_DIR (deg)

20140117 0:00:00

2,92

12,14

324

20140117 6:00:00

3,57

12,97

324

20140117 12:00:00

3,68

12,81

323

20140117 18:00:00

3,71

12,39

323

20140118 0:00:00

4,65

12,01

322

20140118 6:00:00

5,88

11,75

322

20140118 12:00:00

6,58

12,23

323

20140118 18:00:00

6,27

12,12

324

20140119 0:00:00

5,69

11,72

325

20140119 6:00:00

5,01

11,39

326

20140119 12:00:00

4,39

11,02

325

Question 1 of 2

Based on the wave characteristics forecasted by WW111 model on the coasts of Casablanca, Safi and Agadir, where do you think that wave heights increase first?

Significant wave heights started increasing from the north and progressed south, going from Casablanca to Safi 6 hours later and Agadir 6 hours after that.

Please make a selection.

Question 2 of 2

Focusing on the wave peak period, do you think these values are unusual?

On the Atlantic side, periods of about 12 to 14 s are typical. Contrast this with first case when periods reached 20 s.

Please make a selection.

5.0 Case Study 2 » 5.3 Evolution of Wave Characteristics Near Casablanca

The graph below shows the evolution of wave characteristics for a point near Casablanca. The barbs indicate the wind direction and speed; the arrows indicate the direction of propagation; and the curve shows the evolution of significant wave height.

Notice the gradual increase of significant wave height from 4.5 m in the beginning (17 January at 0000 UTC) to almost 7 m on 18 January at 0600 UTC.

graph showing evolution of wave characteristics near Casablanca, 17 Jan 0000 UTC

5.0 Case Study 2 » 5.4 WW111 SWH, 18 January

The WAVEWATCH III significant wave height field output from 17 January at 0000 UTC to 19 January at 1200 UTC shows an area with maximum wave height values of 9 to 14 m located from north of the Azores Islands to west of the Iberian Peninsula.

On the Moroccan Atlantic coast, the model predicts values between 6 and 9 m during the day of 18 January. These are high compared with climatology.

5.0 Case Study 2 » 5.5 EFI for 18 January

In analysing the Extreme Forecast Index values from 18 January at 0000 UTC to 19 January at 0000 UTC (a 24- to 48-h forecast), we find values of around 0.8 to 0.9 near the center of Moroccan Atlantic coast during the day of 18 January. This indicates the likelihood of an exceptional marine situation.

ECMWF Extreme Forecast Index valid for January 17, 2014 at 0000UTC forecast t+24 to 48

Question

Given the WAVEWATCH III's significant wave height of 6 to 9 m at the Moroccan central coast (particularly Casablanca) and the EFI values of about 0.8 to 0.9 for January 18, can we talk of an extreme wave height yet?

Even with a wave height approaching 7 m and EFI values of 0.8 to 0.9, it’s not yet clear if we’re facing an extreme event. Experience shows that some situations, even with wave heights of 7 m, have no coastal damage or other significant impacts. We need to analyze other information, such as period, before deciding on the magnitude of the situation.

Please make a selection.

5.0 Case Study 2 » 5.6 MSLP

The MSLP animation is for the North Atlantic Ocean from 15 January at 0000 UTC to 17 January at 0000 UTC. It shows that lows are moving zonally north of 60°N, and that the value of the center remains above 960 hPa from 15 to 17 January. These lows are not associated with strong winds and will not generate high waves. So there’s no intense deepening, and the pressure values in the center of the lows are relatively high when compared to the situation in the first case.

The fetch oriented from north-northwest to south-southeast is also relatively short compared to the first case because the storm is closer to the coast.

5.0 Case Study 2 » 5.7 ASCAT

The ASCAT winds shows speeds of 30 to 35kt on the western Iberian Peninsula. These northwest winds are not as strong as in the first case. But they will mainly generate wind waves since the generation area is closer to the coast. The fetch is not long enough to develop high swells.

ASCAT image over the Atlantic Ocean 17 Jan 2014 at 0000 UTC

Question

Given all these conditions, does the 17 January situation require issuance of a warning?

Forecasters should issue a warning since the forecasted significant wave height may constitute a risk for mariners.

Please make a selection.

5.0 Case Study 2 » 5.8 Comparing the Cases

There are similarities between the 6 and 18 January situations. For example, both produced high significant wave heights of about 7 m, and high magnitudes of the Extreme Forecast Index of about 0.8 to 0.9. But there are differences.

On 6 January, low pressure evolved rapidly, deepening to a value of 936 hPa. The deepening reached the stage of explosive cyclogenesis with an intensity of 36 hPa in 24 hours, while the low pressure on 18 January never exceeded 965 hPa at the center. Plus, there wasn’t any intense deepening or strong winds associated with it.

The fetch in the 6 January situation was very long - around 3000 km. In contrast, the fetch in the 18 January situation was relatively short.

The periods recorded during the 6 January situation were exceptional, 18 s. In contrast, the periods did not reach 13 s on 18 January. The shorter period waves will not increase in height nearly as much as the longer period waves when shoaling and breaking in shallow water.

The 18 January situation impacted human activities but did not result in any infrastructure damage or loss of human life.

6.0 Summary and References

6.0 Summary and References » 6.1 Summary

This lesson aims to improve the ability of marine forecasters to predict extreme high swell events and make more accurate forecasts by:

  • Recognizing the characteristics of weather situations that can generate heavy swells
  • Applying a marine forecast process that integrates various types of observational and model data, including ASCAT wind data and the Extreme Forecast Index

The lesson’s cases both involved significantly high wave heights. But they evolved in ways that led to different outcomes. The cases highlight the importance of looking at a range of observational and model data and not making assumptions based on limited data or assessments.

6.0 Summary and References » 6.2 References

Aurélien BABARIT, Jean-Marc ROUSSET, Hakim MOUSLIM, Judicaël AUBRY, Hamid BEN AHMED et Bernard MULTON La récupération de l’énergie de la houle, Partie 1 : Caractérisation de la ressource et bases de l’hydrodynamique. Manuscrit auteur, publié dans "revue 3EI (2009) pp.17-25.

COMET lessons on the MetEd website:

ECMWF web site

Guide to Wave Analysis and Forecasting, 1998, second edition - WMO-No. 702 (.pdf)

Frederick Sanders and John R. Gyakum, 1980: Synoptic-Dynamic Climatology of the “Bomb”. Mon. Wea. Rev., 108, 1589–1606.

7.0 Contributors

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