Part C: Heat Energy for Katrina
The rapid intensification of tropical cyclones is an important area of current research. Rapid intensification is defined as wind speeds that increase by 30 knots or more in less than 24 hours. Scientists are actively investigating how the amount of heat energy available in the upper ocean affects the intensity of storms. They want to use this information to improve predictions about how intense a storm will be when it makes landfall.
Heat content of the ocean is important because the warmer the ocean water is, the more easily it evaporates into water vapor. Water vapor takes heat energy from the ocean into the atmosphere. When the water vapor recondenses into liquid water, it releases the heat energy and makes it available as power for further wind and rain. In the right atmospheric conditions, heat energy contained in the ocean can be considered as fuel for hurricanes.
2005's devastating Hurricane Katrina is one example of a storm that experienced rapid intensification. Hurricane Katrina encountered the Gulf Loop Current in the Gulf of Mexico and intensified overnight from a Category 3 to a Category 5 hurricane. In this Part of the lab, you'll use ImageJ to examine several data products (images) that depict the amount of heat energy available in the Gulf of Mexico just before Hurricane Katrina moved through it. You'll also examine data to check whether Hurricane Katrina was able to use the heat energy to power the storm.
Stop and Think2. Read the summary above. Use your own words to describe how heat in the ocean provides power for tropical storms.
- Right-click (or ctrl-click with a one-button mouse) this link to download a .zip archive that contains a stack of data images ( 1.1MB Jan13 08). Your browser may expand the file as soon as the download is complete, or you may need to double-click the zipped file in order to expand it. Save the expanded file in an obvious place where you can find it again.
- Launch or activate ImageJ again. Choose File > Open then navigate to where you saved the expanded 26August2005.tif file.
- Use the arrow buttons at the lower right of the image window to view each of the 5 images. Read the explanations below to interpret the data images.
Interpreting the Data Images
Each data image shows land in green. Various colors over the waters of the Gulf of Mexico indicate heat content of the water. Examine the color scale on the right of each image to interpret what the colors mean.
Sea Surface Temperature
Satellite-based sensors record the amount of radiation emitted in different wavelengths over our whole planet every day. Scientists apply mathematical algorithms to these data to produce images that show the temperature of water surfaces. The indicated temperatures are regularly checked against measurements recorded by ships and buoys.
Sea Height Anomaly
As the temperature of water increases, it experiences an increase in volume. At the molecular level, the faster motion associated with higher temperatures means that individual molecules push each other further apart, resulting in a larger volume. Though this increase in volume can't be seen in small amounts of water, warm areas of the ocean exhibit a "swelling" that can be detected by satellite instruments. Similarly, colder water decreases in volume and may result in an area that is lower than average sea level. Altimetry sensors on satellites collect data that show areas where sea level is higher or lower than average. Areas where the ocean surface is above or below mean sea level show up as sea height anomalies in these images; areas that are higher than average represent areas of warm water, areas with lower than average sea levels indicate cold water.
Depth to 26°C Isotherm
By comparing sea surface temperature, sea height anomalies, and thermometers at varying depths in the ocean, scientists have developed an algorithm that allows them to estimate the depth to which the water temperature is at or above 26°C. The 26°C isotherm (iso = same; therm = temperature) is at some depth below the surface for all areas where the surface temperature is 26°C or above. The deeper the depth to this isotherm, the higher the heat energy that is available to transfer to the surface.
Tropical Cyclone Heat Potential
The Tropical Cyclone Heat Potential (TCHP) is a estimate of the amount of heat energy that is available to power tropical cyclones. The Atlantic Oceanographic and Meteorological Laboratory (AOML) has defined TCHP as the integrated vertical temperature from the sea surface to the depth of the 26°C isotherm. This essentially means that higher sea surface temperatures and deeper depths of the 26°C isotherm indicate that more heat energy is available to power storms.
Analyzing the images
You can use the straight line selection tool to make distance measurements on any of these images. Results are reported in kilometers.
- Choose the Straight Line Selection tool from the ImageJ toolbar, then click and drag across any feature in the image. While your mouse button is still down, you can read the length= (reported in kilometers) in ImageJ's status bar, just below the tools.
Stop and Think3. On the Sea Height Anomaly image, what are the dimensions (length and width) of the area that is over 25 cm above mean sea level? 4. Describe the dimensions (length, width, and depth) of the water depicted by the orange/red pixels in the center portion of the Depth to 26°C Isotherm image. What is the temperature of that water at the surface?
Plotting Katrina's Path
To see if Katrina crossed the areas where the most heat energy was available, you'll mark the storm's path with a segmented line selection. The path you mark on the last slice of the stack will be viewable as an "overlay" on all the data image slices.
- Click the arrow buttons on the lower right of the image window to display the last image of the 26August2005 stack.
- In the ImageJ toolbar, right-click (ctrl-click with a one-button mouse) the Straight line selections tool. Pull down to choose the Segmented Lines tool. Click several spots on the image to get familiar with how the tool works. If you want to start a new line, double-click to end the current line, then start over.
- Use the list of coordinates and the light gray latitude/longitude grid to generate a series of line segments that represent Katrina's path. Click the cursor once at each set of coordinates, then double-click at the end. NOTE: You'll have to interpolate the location of the last point.
- Once you've generated the yellow selection showing Hurricane Katrina's path, click the arrow buttons on the image stack to see the storm's path as an overlay on the heat energy images.
- You can draw the line onto any of the images in the stack.
- Set the line width by double-clicking the line selections tool.
- Set the color of the line by double-clicking on the eye dropper tool.
- If the selection line has disappeared, choose Edit > Selection > Restore Selection to retrieve it.
- Choose Edit > Draw.
Tapping in to the Potential Heat
As Hurricane Katrina moved over the Gulf of Mexico, did it take the stored heat energy from the ocean? You'll generate the storm's path again and view it over a series of TCHP images to check if the heat potential was utilized by the storm.
- Right-click (ctrl-click with a one-button mouse) to download an animation showing changes in the TCHP through Hurricane Katrina ( 3.5MB Jan14 08). Your browser may expand the file as soon as the download is complete, or you may need to double-click the zipped file in order to expand it. Save the expanded file in an obvious place where you can find it again.
- Launch or activate ImageJ again. Choose File > Open then navigate to where you saved the expanded TCHPseries.tif file.
- Use the Segmented line selection tool and the coordinates to generate the storm's path on the first image of the TCHPseries stack.
- Use the arrow buttons at the lower right of the image window to view each of the images with the storm-path overlay.
Stop and Think5. Describe how the animation supports the idea that heat energy in the upper ocean is available to power tropical storms.
What's the Tropical Cyclone Heat Potential for Today?
You can access the same types of heat energy images you just explored for other locations and dates. You may want to examine data for other storms that have occurred since January 1, 2005, or just check current conditions.
- Access the list of dates for which images are available (will open in a new window).
- When you choose a date of interest, you'll find 4 images available for each of 11 different geographic areas. The four types of images are THCP (Tropical Cyclone Heat Potential), D26 (Depth to the 25°C Isotherm), SHA (Sea Height Anomaly) and SST (Sea Surface Temperature).
- Take a look at the most recent images for the Gulf of Mexico to compare them with August of 2005.
- Access the current Global TCHP image to see what locations have the highest TCHP.
- Where on Earth is the Tropical Cyclone Heat Potential highest today?
- Describe today's conditions in the Gulf of Mexico with respect to heat energy.