ANSWER KEY. Part I: Synoptic Scale Composite Map. Lab 12 Answer Key. Explorations in Meteorology 54

Transcription

1 ANSWER KEY Part I: Synoptic Scale Composite Map 1. Using Figure 2, locate and highlight, with a black dashed line, the 500-mb trough axis. Also, locate and highlight, with a black zigzag line, the 500-mb ridge axis located downstream from the trough. Transpose both of these axes to Figure 3 your synoptic-scale composite chart. Explorations in Meteorology 54

2 Figure 3 Synoptic Scale Composite Chart Valid at 1200 UTC on 2 April 1994 Explorations in Meteorology 55

3 2. The area between the axes of an upper-level trough and the next downstream ridge often is a preferred region for large-scale rising motion. Outline this area on your composite chart (Figure 3) using a blue pencil. 3. Examine the 850-mb temperature field on Figure 4 and the sounding data in Table 1. In a short paragraph, describe the temperature pattern at 850 mb over western Oklahoma, the Texas Panhandle, and southwest Texas. Include answers to the following questions in your paragraph: (a) Are the temperatures at 850 mb in this area relatively warm or relatively cool? (b) How do the temperatures change with height over this area (i.e., do they increase or decrease with height)? (c) Is this temperature change with height a stable or unstable configuration? (d) Based upon the wind and moisture data (Table 1), from where did the air at 850 mb in this region likely come? (Hint: You may want to compare the 850-mb temperatures and winds with the surface temperatures and winds. You also may want to calculate the temperature change with height from the sounding data in Table 1.) At 850 mb, warm air was present over western Oklahoma and the Texas Panhandle to the Texas-Mexico border between El Paso and the Big Bend. This area had the warmest air over the United States at this analysis time. These 850-mb temperatures were considerably warmer than corresponding surface temperatures; hence, a significant temperature increase with height occurred across this area. For example, the surface temperatures at Amarillo, TX and Midland, TX were about ~8-9ºC cooler than their associated 850-mb temperatures. Thus, this area was covered by a strong inversion, also known as a cap, which is a stable pattern. The warm air at 850 mb also was very dry. Surface dew points were 3.5ºC less than the corresponding temperatures, whereas 850 mb dewpoint depressions were 20-30ºC. Because of the dryness of the air at 850 mb and the southerly component of the winds, the air at this level likely came from the arid region known as the Mexican Plateau. 4. On your composite chart (Figure 3), outline in red the significant temperature pattern discussed in question 3. (Hint: You may want to sketch a 15 C or 16 C isotherm to represent the pattern in this case.) See Figure 3 (composite chart). 5. Examine the 850-mb dewpoint depression maps (Figures 5 and 6). Compare the moisture pattern at 1200 UTC on 2 April 1994 (Figure 5) with the one 12 hours later at 0000 UTC on 3 April (Figure 6). What happened to the moisture over Oklahoma and Texas? Scallop in green the location of the significant moisture at 850 mb at 1200 UTC on your composite chart (Figure 3). Use a green arrow to denote the movement of the moistest air (also known as a moisture tongue) during the following 12-hour period (to 0000 UTC on 3 April). Abundant moisture advected northward over eastern Texas and eastern Oklahoma from southern Texas and the Gulf of Mexico during the 12-hour period. Dewpoint depressions decreased over central Oklahoma from 25ºC to 7ºC, while 2ºC dew point depressions were as close as northeastern Texas. Strong southerly winds prevailed across the entire area of significant moisture advection. 6. Examine the Skew-T diagrams for Norman, OK (OUN) at 1200 UTC on 2 April 1994 (Figure 7) and 0000 UTC on 3 April 1994 (Figure 8). On the morning sounding, describe the structure of the temperature plot beneath 500 mb. List the levels where the temperature decreases with height and the layers where temperature increases with height. What is the technical name for the significant feature or pattern below 850 mb? Layer(s) where temperature decreases with height at 1200 UTC From 850 mb to 500 mb (and higher) Layer(s) where temperature increases with height at 1200 UTC Name of feature below 850 mb at 1200 UTC From the surface to 850 mb An inversion Explorations in Meteorology 56

4 7. (Advanced Students/Meteorology Majors) Does the temperature pattern below 850 mb aid or oppose storm development? Explain your answer. The inversion prevents storms from forming immediately because it prevents surface air parcels from rising to form deep, thunderstorm clouds. However, some amount of inversion is necessary to keep the surface instability confined so that explosive development could occur later. Thus, the inversion can be a positive factor for thunderstorm development. Sometimes, other factors exist and overcome the inversion. 8. Look at the winds on the morning sounding (Figure 7). Comment on how the winds speed and direction change with height. Often, meteorologists consider 500-mb winds to be the steering currents for storms. Based on this concept, what direction should the storms move across central Oklahoma on the afternoon on 2 April 1994? To indicate your forecasted storm motion, plot and label a blue arrow on your composite map across central Oklahoma. Forecasted direction of storm movement across central OK-West to east. The winds basically increased in speed with height. They also turned from southerly at the surface, to southwesterly up to 850 mb, and westerly above 850 mb. The 500-mb winds were westerly. 9. Compare the morning sounding (Figure 7) to the evening sounding (Figure 8). Describe how the winds changed, in general, throughout the day. The winds increased in speed at nearly all levels from the morning to the evening sounding, perhaps indicating an increasing intensity of the storm system as it organized over Oklahoma. The surface winds changed during the day to be southeasterly, indicating that more moisture advected into central Oklahoma (e.g., Norman surface dew point increased from 9 C to 13 C). Part II: Mesoscale Composite Map County-level data from the Oklahoma Mesonet ( was available for the mesoscale analysis on 2 April Mesonet maps are displayed in local time. You may want to refer to the map of Oklahoma counties in Figure 9 to use county names in your answers to the questions that follow. 10. Examine the temperature, dew point, and wind data for 2:30 PM CST on 2 April 1994 (Figures 10, 11, and 12). Identify any fronts, drylines, and high- or low-pressure centers using the customary symbols and colors. Transpose those symbols onto your mesoscale composite map (Figure 13). Figure 10 Temperature (in F) for 2 April 1994 at 2:30 PM CST (2030 UTC), as Measured by the Oklahoma Mesonet Explorations in Meteorology 57

5 Figure 11 Dew Point (in F) for 2 April 1994 at 2:30 PM CST (2030 UTC), as Measured by the Oklahoma Mesonet Figure 12 Winds Speed (in kts) and Direction for 2 April 1994 at 2:30 PM CST (2030 UTC), as Measured by the Oklahoma Mesonet Explorations in Meteorology 58

6 Figure 13 Mesoscale Composite Chart Valid at 2:30 PM (2030 UTC) on 2 April 1994 The preferred area for storm development is generally east of the dryline (in the moist tongue) and near the thermal ridge. This location maximizes surface instability (i.e., the area that is the warmest and most moist) which act to help overcome any cap. 11. Using Figure 10, identify the location of the warmest air (also known as a thermal ridge). Shade this warm air region in red pencil on your mesoscale composite map (Figure 13). [Hint: To best identify this feature, you may want to locate and sketch an isotherm in the 80º 83ºF range.] See Figure 13 (composite chart). 12. Using Figure 11, identify the location of the moisture tongue. Draw a green scalloped line around this moist air region on your mesoscale composite map (Figure 13). Shade the area within the scalloped line with a green pencil. [Hint: To best identify this feature, you may want to locate and sketch the 55ºF isodrosotherm. Surface dewpoint temperatures in the mid-50s often are considered as the threshold needed for severe storm development.] 13. Inspect the base reflectivity image from 2:42 PM CST (2042 UTC) shown by Figure 14. At this time, the NEXRAD radar was operating in clear-air mode a very sensitive configuration used to detect atmospheric boundaries such as fronts or drylines. In clear-air mode, the radar detects changes in atmospheric density, typically resulting from either temperature or moisture gradients. When two dissimilar air masses are adjacent, a discontinuity in atmospheric density may show up as a thin line on the radar display. On Figure 14, a thin line is evident to the west of the radar s location (near Oklahoma City). How does the thin line correspond with patterns observed by the Oklahoma Mesonet (Figures 10, 11, and 12)? The thin line over Kiowa, Caddo, and Blaine counties was associated with the dryline. Explorations in Meteorology 59

7 Describe the density differences between the west and east sides of the thin line (i.e., Is the atmospheric density higher or lower on the west side as compared to the east?). How did you arrive at your answer? The west side of the dryline was hot and dry, while the east side was warm and moist. Water vapor molecules have less mass than do air molecules. Thus, moist air is less dense than dry air and causes the dryline to show up as a density discontinuity between the more dense hot, dry air on the west side and the less dense warm, moist air on the east side. Explain why weather radar may have an advantage over a network of surface weather stations in detecting these low-level discontinuities. The radar has one primary advantage compared to a surface-based mesonet: it can detect the location of the dryline with higher resolution, mostly in space but sometimes also in time (e.g., 6-minute updates). 14. By 2:30 PM CST, thunderstorms had not developed in Oklahoma. Using both your knowledge of the environment represented by your composite charts (Figures 3 and 13) and your knowledge of the ingredients necessary for thunderstorm development, where will thunderstorms most likely develop first? Outline your forecasted area of several counties (~5-10) in black on your mesoscale composite chart (Figure 13) where you think the storms will form. Counties where storms will form initially (Note: County names are on Figure 9.) Jefferson, Cotton, Stephens, Comanche, Caddo, Grady, Canadian, Blaine, and Kingfisher. See Figure 13 (composite chart). 15. When you finish your lab, your instructor will show you a radar loop of the base reflectivity (in precipitation mode) from 3:15 PM to 8:00 PM CST. Use this information to verify qualitatively your forecasts of storm development and motion. Describe how well you forecasted the storm motion (question 8) and the location(s) of storm initiation (question 14). If you did not forecast the event well, what additional information may have helped you to make a better forecast? The general flow of the echoes was from west to east across Oklahoma, agreeing with the forecasted storm motion. 16. (Advanced Students/Meteorology Majors) Use the radar loop (displayed by your instructor) to comment on the evolution and motion of the storms that formed in both Comanche County and Major County (see Figure 9 for county names). In considering your answer, compare the motion of these two storms with the motion of other storms across Oklahoma. A few convective cells deviated from the general flow. The storm that formed in Major County turned to the southeast; it became a right-moving storm. The storm in Comanche County split into two storms. First, one storm from the split made a slight turn to the right (i.e., southeastward). This storm passed through Pontotoc County and produced hail damage there. The second storm from the split turned to the northward. This left-moving storm eventually merged with the northern complex of storms. This left-mover also accelerated and persisted for an unusually long time compared to most left moving storms. Explorations in Meteorology 60