Lab 6 – Waves Generated by Large Storms
This exercise is a case study of a very quickly developing low pressure center (a bomb cyclone) and the response of the sea surface to the rapidly changing pressure and winds. While students may understand that hurricanes generate large waves, winter storms can also produce high seas very quickly. The waves generated by these intense storms travel to the shoreline at different speeds and are responsible for large amounts of coastal erosion put infrastructure, ecosystems, and communities at risk . They also produce potentially dangerous conditions for ships at sea.
This lab is divided into four scaffolded activities that should be completed in order, although the instructor can stop the lab after any particular activity if the necessary learning outcomes have been met. Lab 6.3, for instance, may be reserved for a slightly more advanced class or special topic activity. The activities are intended for undergraduate students in Introductory Oceanography courses (for either marine science majors or non-science majors) with Activity 6.3 progressing towards more quantitative skills and Activity 6.4 providing additional practice. This exercise, particularly the concepts in Activity 6.3 could also be extended to upper level marine science courses.
Approximate time involved: Activity 1 – 15 minutes, Activities 2 & 3 – 30 minutes each.
- LO1. Students will demonstrate basic data literacy in graph interpretation by identifying values of meteorological and oceanographic variables (with appropriate units) at specified times and describe their evolution over time.
- LO2. Students will identify specific values of barometric pressure in a time series and describe the changes occurring in barometric pressure during a specific period of time.
- LO3. Students will identify trends in wind speed in a time series and relate changes in barometric pressure to changes in wind speed to develop and establish the relationship between the two variables.
- LO4. Students will identify specific values of wind speed and significant wave height in a time series and relate changes in wind speed to changes in wave height to develop and establish the relationship between the two variables.
- LO5. Identify specific values of wave period and wave height and establish a relationship between the two variables.
- LO6 Students will use wave period to calculate wavelength and wave speed and then examine the relationship between wavelength and wave speed.
|Learning outcome||Activity 6.1||Activity 6.2||Activity 6.3||Activity 6.4|
|Outcome 1||Guided practice||Guided practice||Applied||Applied|
|Outcome 5||Introduced and guided practice|
|Outcome 6||Introduced and guided practice|
What students should know before this activity
- Data literacy: Students should have completed Lab 2.1 and be familiar with how to read and interpret time series graphs. This includes identifying the minimum and maximum of a time series, identifying values at specific times, and understanding the units of different variables.
- Content knowledge: Storms at mid-latitudes can be as large and devastating as tropical storms. They are created by the merging of cold and warm air masses and can have strong winds related to the changes in pressure across the front. Changes in the atmosphere above the ocean also change the ocean surface. In particular, winds generate surface gravity waves. Ocean surface waves are often described by three characteristics – wave height, wave period, and wavelength. Ocean surface gravity waves will travel across the ocean at different speeds depending on these characteristics.
What instructors should know before this activity
- Mid-latitude storms, sometimes referred to as extratropical cyclones, can be associated with extreme changes in pressure and winds. In some cases there can be very large changes in pressure that results in a ‘bomb cyclone’ which is defined by the national weather service as a drop in pressure of at least 24mb in a 24 hour period. At the air-sea boundary, these changes in pressure and wind generate waves on the ocean surface. Wind speed, wind duration, and fetch are the primary variables responsible for the generation of surface waves in the ocean. Winds that blow faster and longer over a larger area (i.e., fetch) will generate larger waves.
- After waves are generated, the travel away from the storm at a speed dependent on their wavelength. This is why surface gravity waves are called ‘dispersive’ — they travel at different speeds. The speed at which a wave travels is the wavelength divided by the wave period (distance divided by time).
Optional pre-lab activities:
Introductory lecture on storms, particularly changes in pressure and winds as well as an introductory lesson on ocean wave generation and characteristics. A brief introduction in how ocean waves are measured may also be useful (e.g. https://cdip.ucsd.edu/m/documents/wave_measurement.html).
- Procedure: Explain that this exercise looks at changes in the atmosphere related to a storm and then at ocean surface waves and local winds as a strong cyclonic storm passes over the instrument array. Emphasize that changes can occur quickly and that the atmosphere and the ocean are interconnected — changes in one will result in changes in the other.
- Consider also using the news reports and the satellite video below to further motivate the activities.
- Commonly encountered issues/questions/misconceptions:
- Be sure students are using the proper axis for their variable of interest and recognize the units on each axis.
- Emphasize that all answers should have the proper units.
- Activity 6.3 may require more guidance for introductory and non-majors who are not used to quantitative questions. The calculations are not difficult but guidance to avoid any initial reactions to this type of question may be needed. Hopefully by doing the calculations they arrive at an understanding that the waves they see at the ocean traveled far distances and at different speeds.
- Adaptations for different course levels and duration of activity (lab vs. lecture period): Activity is stand alone and may be used in as a lab activity, in-class exercise, or homework exercise.
- Reflection questions are included in each activity.
- Activity can be used in an upper-level course as a refresher in ocean wave characteristics.
- Interested in more data and investigation? Check out an 18-month time series of atmospheric and wave conditions from the Endurance Array in the Pacific Northwest: https://datareview.marine.rutgers.edu/nuggets/view/22. Data nuggets are particularly useful in courses in which students are prepared to graph data on their own.
- Sneed, A. 2018. What Is This “Bomb Cyclone” Threatening the U.S.? Scientific American retrieved from https://www.scientificamerican.com/article/what-is-this-ldquo-bomb-cyclone-rdquo-threatening-the-u-s/
- NOAA GOES-East Captures East Coast ‘Bomb Cyclone’ (Video) from NOAA Environmental Visualization Laboratory, NOAA National Environmental Satellite, Data, and Information Service (NESDIS) : https://youtu.be/ApwsdTrtm3g or https://www.nesdis.noaa.gov/content/noaa-goes-east-captures-east-coast-bomb-cyclone-video
- The Historic “Bomb Cyclone” of January 2018 Source: Coastal Processes, Hazards, and Society by Barlow, T, Maygarden, D,Yarnal, B. licensed by Penn State’s College of Earth and Mineral Sciences (http:// e-education.psu.edu/oer/), Creative Commons license (cc By-NC-SA 4.0), https://creativecommons.org/licenses/by-nc-sa/4.0/, accessed (07/15/2020) Penn State University.