ERTH 365 Homework #2: Hurricane Harvey. 100 points

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1 ERTH 365 Homework #2: Hurricane Harvey 100 points Due by 6pm, Tuesday 30 October 2018, ELECTRONIC SUBMISSON BY ONLY BY 6PM (send to (acceptable formats: MS Word, Google Doc, plain text, or answers via regular ) NO LATE SUBMISSONS ACEPTED Student Learning Outcomes: Upper Division Science A. C, E,F, and G; Environmental Interconnection, A, C, D, and E; Ethical Reasoning and Action, A and B. Note: When written answers are required, please respond with a complete sentence or sentences, each with a subject, verb and object. You do not need to print the graphics, though you will need to look at them. Use this link to see a web version of this homework including the graphics, which you will need to answer these questions: ork_2_f18.html Look at all the links associated with Harvey on the supplementary class notes page to assist you with this homework. Part 1. Hurricane Harvey, August 25, 2017 Satellite Imagery (20pts)

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3 Compare the two images, which are for 2237 UTC 8/25/17. Note the portions of the images labeled A and B (the same locations at the same times for both images). Answer the following questions. Questions: 1 What evidence do you see on the INFRARED image that the clouds at the two locations are probably cumulonimbus? (6 pts) The coldest cloud tops are associated with the two locations. The red and yellow color indicates temperatures of around -70C and -80C. Cold temperatures can be associated with towering clouds, such as cumulonimbus. 2 What evidence do you see on the VISIBLE image that the cloud at A is cumulonimbus? (6 pts) The area labeled A is towering so high above the surrounding overcast that it is casting a shadow. This means that the top of the cloud is much taller than the surrounding cloud mass, and is probably associated with a cumulonimbus.

4 3 In general, with respect to the center of the storm, where would you expect the heaviest precipitation to be occurring and why? use the images to justify your answer (8 pts) The coldest cloud tops appear to be around the center of the storm, as mentioned in answer to (1) above. Thus, towering cloud forms, usually associated with heavy precipitation, should be occurring around the center of the storm. Part 2. Hurricane Harvey, Surface Weather Map (25pts) Surface pressure map at 06UTC on Friday, 25 th August Isobars every 4hPa. Questions: 1. What time was this weather chart created for in local time for Corpus Christie? (2pts) 01 AM CDT FRI, 25 th August What is the approximate lowest pressure seen in Hurricane Harvey? (3pts) 992 hpa. 3. Assuming that there no actual data on the weather chart, but only pressure lines or isobars are visible, describe what you see on the surface weather map that would suggest that strong winds would be occurring at the location of SHIP A in the north-

5 west Gulf of Mexico coastal sections (5 pts) The isobars are very close together. This suggests that the wind speeds would be very high in that location. 4. Examine the weather information for SHIP A and write down the temperature, dewpoint, pressure and wind speed. Describe how the (a) dew point temperature; (b) wind speed; and, (c) atmospheric pressure were unusual (compared to the kind of values we have seen so far this semester on the surface weather maps). (10 pts) The temperature is 85F. The dewpoint temperature of 78F is higher than any dewpoint we have seen on a weather map in this class, and much greater than the 60F dew point we've discussed as being a threshold for thunderstorms. The sustained wind speed is 35 mph, which is also the highest sustained wind speed we have seen on a surface weather map, as well as being strong for a sustained wind speed at sea-level. Finally, the atmospheric pressure is hpa, which is also lower than average pressure of hPa and also lower than we have seen on a weather map in this class, but not as low as some pressures we have seen in some of the hurricanes discussed this semester, which were well below 1000hPa. 5. Examine the weather information for SHIP A and Ship B and write down the wind speeds at each location and why the wind speed at SHIP A is much higher than the wind speeds at SHIP B. (5pts) The wind speed at SHIP A is approximately 35mph, while at SHIP B it is about 10mph. SHIP A is much closer to the center of Hurricane Harvey and the isobars are much closer together a SHIP A than at SHIP B, therefore accounting for the higher wind speed. Part 3. Hurricane Harvey, Corpus Christie Meteogram (15pts) CDT = 5hrs behind UTC CST = 6hrs behind UTC. It s AUG, so CDT. (answers in either local or UTC are acceptable)

6 Meteogram for Corpus Christie from 00UTC 23UTC 25 August Questions: 1. Give the date and time at which the center of Harvey was closest to Corpus Christie and how did you tell. (7 pts) The lowest pressure was observed around 23 UTC (6:00 PM CDT) on 25 August. This appears to be the time that the storm was closest to Corpus Christie. Also, the strongest winds occurred at that time. 2. What was the date and time of the peak wind gust, and what was its strength and direction? (5 pts) The peak wind gust was 54 mph, and that was observed at 23 UTC (6 PM CDT) on 25 August. 3. One of the chief problems in the Corpus Christie region with the passage of Harvey was

7 something called the "storm surge" and not the wind, or the rainfall associated with the storm. However, what was the strongest sustained wind (include date and time), and how much rainfall occurred at Corpus Christie over the 24hour period shown? (3pts) The strongest sustained wind was northwest at 35 mph, and that was observed at 23 UTC (6:00 PM CDT) on 25 August. 0.98" of rainfall occurred during the 24hr period. Part 4. Hurricane Harvey, Corpus Christie Radar Animation (20pts) Radar showing Hurricane 00:25UTC on 26 th August Questions: 1. In general, in what direction was Harvey itself moving as it was affecting the city of Corpus Christie? (5 pts) Harvey appeared to be moving towards the Northwest as it was affecting Corpus Christie. 2. In general, how were the precipitation areas associated with Harvey moving with respect to the storm itself (clockwise or counterclockwise)? (5 pts) The rain areas appear to be moving counterclockwise with respect to the center of the storm.

8 3. In general, where was the heaviest precipitation/strongest thunderstorm activity located with respect to the center of the storm? (5 pts) The heaviest precipitation, indicated by the oranges and red colors, are found around the center of the storm, with a greater concentration suggested west of the center, east to northeast of Corpus Christie (offshore). The colors indicate heaviness of precipitation, with reds, yellows and purples indicating the heaviest precipitation. This is consistent with my answer in Part 1 above. 4. How does the radar animation corroborate the fact that hurricanes are cyclones? (5 pts) Cyclones are low pressure areas with the center completely enclosed by at least one isobar. Wind should move counterclockwise around such pressure systems. The radar animation shows that the rain bands are moving counterclockwise. Assuming that the rainfall areas are associated with clouds whose motion is directed by the winds, then the winds are moving counterclockwise. In addition, the rainfall areas are making a complete circuit around the center of the disturbance, consistent with the fact that at least one isobar encircles the center. Part 5. Hurricane Harvey and resulting heavy rainfall (20pts) It is a well-known fact that the highest hurricane fatalities arise from fresh water flooding. Harvey produced unprecedented rainfall totals. First, examine the diagrams below showing the forecast tracks (pay VERY CAREFULL attention to the FORWARD speed of the hurricane), the actual sea-surface temperature (SST) and sea-surface departure from average plots and finally the forecast (predicted) 7-day rainfall, the estimated observed (actually occurred) 7-day rainfall totals and answer the following questions based on your knowledge of hurricanes and the information provided by the diagrams below: (click to make larger)

9 Hurricane Harvey Forecast tracks issued: 10PM CDT WED AUG 23 rd, 7PM CDT SAT AUG 26 th and 10PM CDT SUN Aug 27 th Actual sea-surface temperatures ( C) and track (yellow arrow) of Harvey (left) and sea-surface temperature departures from average ( C) (right).

10 Forecast (predicted) 7-day rainfall totals (inches) on the left and estimated observed (actually occurred) 7-day rainfall totals on the right for roughly the same period ~ 11PM CDT 23 rd AUG to ~11PM 30 th AUG. Questions: 1. Look carefully at the three track plots above and note roughly the locations where Harvey was forecast to be and the forward speed on each of the three advisory plots. What can you say about the forward speed and location of Harvey by looking at all three track plots? (5pts). On Wednesday 23 rd AUG at 10PM CDT Harvey was moving slowly at 2mph towards the NW and forecast to become a hurricane by 7PM on FRI evening, covering some distance before making landfall, however, thereafter very little movement of Harvey was expected between 7PM SAT night and 7PM SUN night. The 2 nd advisory shown at 7PM CDT SAT night shows very little movement of Harvey was forecast and the final advisory at 10PM CDT on SUN continued to show very little movement of Harvey until at least 7PM, THU. This means Harvey was near stationary from at least 7PM SAT until 7PM WED over almost the same area. 2. Look at the sea-surface temperature and the sea-surface temperature anomaly map. The yellow arrow on the sea-surface temperature map shows the rough path of Harvey towards the Texas coast. What where the sea-surface temperatures encountered by Harvey? Where these temperatures normal? What effect do you think these sea-surface temperatures had on Harvey? (5pts) Sea-surface temperatures were mostly above 30 C (86 F) and were about 1-2 C (2-4 F) above average. We know that for hurricanes to form and intensify, sea-surface

11 temperatures must be above 28 C (82 F), so Harvey was bound to intensify rather rapidly as it was also moving very slowly over these extremely warm waters. 3. Finally, look at the 7-DAY FORECAST rainfall chart produced on the 23 rd of AUG (i.e. predicted/expected rainfall between AUG) and the estimated total 7-DAY OBSERVED rainfall ending on the 30 th of AUG (i.e. what actually occurred between AUG). Compare the rainfall totals FORECAST in the black rectangle (they cover the same areas in both plots) with those in the estimated OBSERVED total-7day rainfall totals and discuss the amounts, why they are different and what could have been the cause for such high rainfall totals that actually occurred versus what was forecast. The previous two questions should provide you with some of the answers. (10pts). The highest forecast rainfall in the forecast chart was southwest of Houston with forecast totals of between 16 and 20, and in Houston, between 4 to 8 was forecast. However, the estimated 7-day rainfall totals show the highest values of over 50 to the east of Houston, with the bulk of the heaviest rain (40-50 ) to the east of Houston. Possible reasons why the forecast is so different from the estimated observed rainfall: The forecast models are updated every 6 hours and it is possible that forecasts on THU through SAT may have predicted substantially more rainfall on those days compared to the forecast on WED when Harvey was still a tropical depression a few hundred miles away from the coast. The forecast model did not accurately take into account the stalling of Harvey over the same area; The fact that the tracks show that Harvey essentially stalled out over the same areas for a long period of time accounts for the very high rainfall totals seen. Another reason so much rain fell was that Harvey moved over very warm seasurface temperatures. Very high sea-surface temperatures pump a lot more water into the air above it, so Harvey had a lot more water vapor to absorb and then release as rainfall over the same area for days on end.