Heavy Weather Image Credit: Karsten Petersen
When weather events at sea cause damage to ships and cargo or injure crew members, just how do you determine what happened? All ship masters make frequent log entries regarding the ship’s position, vessel condition, fuel and water reserves, as well as the weather encountered. When tens of thousands to millions of dollars are at stake, the various parties involved in a marine venture need to know what actual weather conditions were encountered. This is when a forensic marine weather expert can be employed to make an independent assessment.
Real Example
In July of 2008, a cargo vessel was transiting the northern Arabian Sea, east of Socotra Island, and allegedly sustained significant cargo damage due to bad weather conditions. The cargo interests in the venture, however, claimed that the weather encountered was “normal and expected” for the summer monsoon season and that the vessel operator was negligent in conducting proper ship handling and navigation. The month of July is well-known for rough weather conditions in this area of the Arabian Sea due to persistent southwesterly monsoon winds. To that end, they presented a forensic weather analysis as evidence.
Arabian Sea
Ocean Weather Services was asked to review the weather analysis report that was presented by the weather consulting company. This report claimed that wind conditions were from the southwest between Beaufort forces 6 and 7 (22-32 knots) and produced waves in the range of 4 to 5.5 meters which normally occurs in this area during July. The report concluded that winds of Beaufort Force 9 (41-47 knots) with waves up to 8 meters did not occur as was reported by the master of the vessel.
Beaufort Force 9 Sea State
The Review
Our review of the report showed that the conclusions regarding wind and wave conditions were based primarily on output from 3 US weather computer forecast models and from derived surface winds from the NOAA Satellite data. What was not included in the report were any actual weather observations from buoys or other ships in the same area at that time.
The most accurate source of surface marine weather data are instrumented weather buoys. From our research, we discovered that there were no such buoys near the location of the incident, so to establish the prevailing wind and sea conditions, other direct observations, such as ship weather reports, were considered along with any pertinent computer weather model data.
NOAA Ocean Weather Buoy
Ship Weather Observations
Ship observations are usually taken by experienced mariners with wind speed and direction either measured directly with allowances made for ship motion or by estimating those conditions by viewing the sea state. Wave observations taken from onboard ships are, for the most part, estimated by observing the sea state. Ship observations are, however, actual observations and are an essential source of information to be considered along with other data sources.
Computer Models Vs. Actual Data
The model data outputs, like those used in the report, depend on the accuracy of the initial conditions and the grid spacing used in each model. The initial conditions will include all available weather observations and satellite data available. The model will then derive initial conditions across the grid. Since the model grid points at that time varied from 12.5 km up to 50 km, they sometimes miss small atmospheric features. To reconstruct this event, we obtained and plotted all the ship weather observations and plotted those reports that were at the time and near the location of the incident.
NOAA GFS Model 10-Meter Surface Wind
Conclusion
The list of actual ship observations obtained from NOAA’s National Climatic Data Center ( NCDC) database included a total of 4 observations of Beaufort force 8 (34-40 kt) wind conditions and 6 observations of ships reporting Beaufort force 9 (41-47 kt) wind conditions in the general vicinity of the vessel at that time, with significant wave heights reported between 4 and 6 meters. The significant wave height is the average of the highest one-third of all waves.
Location of observed gale force (beaufort Force 8-9) Winds
Waves
The full spectrum of wave heights and wave periods in the open sea can be extremely complex with a mix of individual waves interfering with each other so that you can get wave peaks and troughs adding or subtracting to the height of individual waves. The wave heights reported by ships and buoys and also in ocean wave models are presented as significant wave heights.
Since the significant wave height represents the average of the 1/3 highest waves, then some waves will be higher. Based on past statistical studies, a significant wave height of 6 meters means that, on average, about 1 in 10 waves will reach a height of about 7-8 meters, and about 1 wave in 100 will reach a height of about 10 meters.
Statistical Distribution of Wave Heights
It was the opinion of Ocean Weather Services that the vessel did encounter winds up to Beaufort force 9 (41-47 knots) and highly likely encountered some waves in the range of 7 to 10 meters. In addition, over open ocean, wind gusts can briefly exceed the reported wind speed by 20-25 percent, and in rare cases, up to 40 percent.
Fred Pickhardt
Ocean Weather Services
