Back

How accounting for instability in the lower atmosphere improves forecasts of extreme waves

Sofar Ocean

Stakeholders across the blue economy — from shipping companies to nonprofits removing plastic from the ocean — trust Sofar’s global ocean weather forecasts to help them operate efficiently and safely. These forecasts, which assimilate the ground truth ocean observations collected by our global network of Spotter buoys each day, outperform government models by up to 40-50%, and give Sofar customers a critical accuracy advantage enabled by real-time data.

Access to highly accurate forecasts is particularly critical when metocean conditions are at their most extreme. Operating in inclement weather hampers efficiency and jeopardizes the safety of crews, cargo, and infrastructure. Recent updates to Sofar’s global ocean weather forecast improve the accuracy of its predictions of moderate (5-10m) to extreme (>10m) wave activity. Specifically, by accounting for instability in the lower atmosphere — namely, air density variability and local wind gustiness — our forecasts of moderate waves improve by over 40% and our forecasts of extreme waves improve by over 20%.

These improvements impact two of Sofar’s product offerings:

  • Wayfinder Voyage Optimization: Better forecasts and hindcasts of moderate to extreme waves improve the accuracy of AdaptiveVPM™’s speed and fuel predictions, as well as the reliability of Wayfinder’s continuously optimized voyage guidance.
  • Data Services: Any customer or researcher that uses the Sofar API to access ocean weather insights will reap the benefits of improved forecasts of moderate to extreme waves.

Forecasts traditionally do not account for air density variability and local wind gustiness

Air density and local wind gustiness both play a role in wave generation, yet many forecasts do not account for either dynamic.

  • Air density: Denser air exerts a stronger force on the ocean surface, which contributes to wave growth. Air density, however, is historically treated as uniform over the global ocean and constant in time.
  • Local wind gustiness: In unstable atmospheric conditions, local wind gusts can be temporarily stronger than the averaged wind measurement that most wave models use. These local wind gusts can generate sea waves that are substantially larger than the wave heights derived from this averaged wind measurement, which doesn't vary rapidly over short time scales or small space scales. Local wind gustiness is not explicitly forecasted by numerical weather models because of limitations imposed by model spatial and temporal resolution.

Visualizing air density variability and local wind gustiness in the North Pacific Ocean

During the series of atmospheric rivers that battered the California coast in January 2023, strong storms traversed the North Pacific Ocean over relatively warm ocean waters. Below, we visualize the instability in the lower atmosphere that was present as one of these storms passed through the Pacific on January 21st.

  • Figure 1 shows air density in the Pacific. Warm colors represent areas with higher air density, which exerts a stronger force on the ocean surface and contributes to wave growth.
  • Figure 2 shows air-sea temperature differences, which are a proxy for atmospheric instability. Warm colors represent areas where sea surface temperatures are warmer than surface air temperatures; in these regions, wind gusts are likely large and important for wave growth. Most of the time, wind gustiness has a larger effect on wave growth than air density.
Figure 1. Air density in the North Pacific Ocean on January 21st, 2023 00:00Z. Warm colors represent areas with higher air density, which exerts a stronger force on the ocean surface and contributes to wave growth. The yellow pentagon indicates the position of Spotter 010455 (see Figure 4).
Figure 2. Air-sea temperature differences in the North Pacific Ocean on January 21st, 2023 00:00Z. Warm colors represent warmer sea surface temperatures relative to air temperatures; in these areas, wind gusts are likely large and important for wave growth. Black barbs represent the average 10 meter wind direction and magnitude. Increased length and number of pennants on the barbs represent higher values for wind speed. Average 10 meter wind speed is dominated by larger scale dynamics, but local wind gusts can introduce variability around that average. The yellow pentagon indicates the position of Spotter 010455 (see Figure 4).

Accounting for air density variability and local wind gustiness improves forecasts of extreme waves

Figure 1 and Figure 2 make clear that air density variability and gusty wind conditions were present as a storm traversed the North Pacific Ocean on January 21st. In Figure 3, we assess the effect of these lower atmosphere dynamics on wave generation from January 20th, 2023 00:00Z to January 22nd, 2023 12:00Z by measuring the differences between:

  • The wave heights forecasted by Sofar’s global ocean weather forecast when accounting for air density variability and air-sea temperature differences (Updated forecast).
  • The wave heights forecasted by Sofar’s global ocean weather forecast when not accounting for air density variability and air-sea temperature differences (Original forecast).

In multiple areas, the Updated forecast predicts wave heights that are more than 1m larger than those predicted by the Original forecast. This is a direct result of the Updated forecast accounting for air density variability and air-sea temperature differences. In particular, there is spatial correlation between the areas where the Updated forecast predicts larger wave heights (Figure 3, bottom, warm colors) and the areas with positive air-sea temperature differences (Figure 3, top, warm colors). The wave height differences persist longer than the air-sea temperature differences because waves propagate away from storms and this storm is weakening.

Figure 3. Top to bottom, for the period of January 20th, 2023 00:00Z to January 22nd, 2023 in the North Pacific Ocean: air density; air-sea temperature differences; differences in wave height forecasted by 1) Sofar’s global ocean weather forecast when accounting for air density variability and air-sea temperature differences (Updated forecast) and 2) Sofar’s global ocean weather forecast when not accounting for air density variability and air-sea temperature differences (Original forecast).
By accounting for air density variability and air-sea temperature differences — a proxy for local wind gustiness — the
Updated forecast predicts wave heights in certain areas that are more than 1m larger than those predicted by the Original forecast. In particular, there is spatial correlation between the areas where the Updated forecast predicts larger wave heights and the areas with positive air-sea temperature differences. The yellow pentagon indicates the position of Spotter 010455 (see Figure 4).

In Figure 4, we validate the accuracy of the Updated forecast’s predictions of wave height by comparing them to the ground truth observations of wave height made by Spotter buoy 010455 during the atmospheric rivers that traversed the North Pacific Ocean. Spotter buoy 010455’s position is represented by the yellow pentagon in Figures 1, 2, and 3.

From January 1st to January 14th, the Updated forecast’s predictions of wave height were closely aligned with the ground truth observations of wave height made by Spotter buoy 010455. Forecasted and observed wave heights were frequently moderate (5-10m) to extreme (>10m). Overall, by accounting for differences in stability and density in the lower atmosphere, the Updated forecast more accurately predicts moderate to extreme wave heights than the Original Forecast.

Figure 4. Time series of observed (Spotter 010455) and forecasted (Updated forecast) significant wave height at Spotter 010455’s position for the two-week period between 1/1/2023 and 1/14/2023. Observed and forecasted significant wave height are closely aligned.

By accounting for air density variability and local wind gustiness, Sofar’s forecasts of moderate to extreme waves during strong winds improve dramatically. These improvements help ensure that Captains and Operators using our Wayfinder platform and customers using the Sofar API have access to wave forecasts that are as accurate as possible as they operate in inclement ocean weather.

To schedule a demo with the Wayfinder team, please fill out this short form. To schedule a demo with the Data Services team, please fill out this short form.

Sofar in the News
In the News
October 23, 2023
Wayfinder delivers 8% total equivalent fuel savings on Brazil-China (BDI C14) voyages arriving in early October
In the News
October 2, 2023
What was different about the latest Advanced Naval Technology Exercise
In the News
September 25, 2023
Sofar Ocean's Superior Weather Forecasts Minimize Voyage Costs

Related Stories