Name & Signature Dr. Droegemeier Student ID Meteorology 1004 Introduction to Meteorology Fall, 2001 Examination #3 Wednesday, 28 November 2001 BEFORE YOU BEGIN!! Please be sure to read each question CAREFULLY and, if time permits, check your answers. Answer all questions on the pages provided - do not use additional sheets. Be sure to submit your computer form as well as the exam before leaving the room. This exam is worth 100 points. Part I: Multiple Choice (3 points each) - Indicate the correct answer on the computer form using a #2 pencil. 1. The thermal wind vector is a. The wind associated with temperature gradients b. Something that cannot be "felt" physically c. A fictitious force d. The vector difference between the geostrophic wind at two levels in the atmosphere e. Both b and d. 2. A veering wind with height indicates a. Cold-air advection b. Sinking motion c. Warm-air advection d. Increasing pressure e. Increasing humidity 3. Warm air advection tends to a. Decrease the thickness b. Increase the surface pressure c. Decrease the surface pressure d. Increase the thickness e. Both c and d
4. Baroclinicity refers to a. Wind shear b. Pressure gradients c. Temperature gradients d. Moisture gradients e. None of the above 5. A continental tropical air mass typically originates over a. Washington state b. Florida c. Mexico d. California e. None of the above 6. The optimal location of a surface low with regard to intensification is a. Along the axis of the upper-level trough b. West of the axis of the upper-level trough c. East of the axis of the upper-level trough d. In vertical alignment with the upper-level low e. Insufficient information given to answer the question 7. The favored location for the intensification of a surface cyclone is in which quadrant of a jet streak? a. Right front b. Left front c. Right rear d. Left rear 8. Net convergence acting on a vertically-oriented vortex will a. Spin the vortex down b. Spin the vortex up c. Stretch the vortex d. Compress the vortex e. Both b and c 9. Which of the following is not a condition associated with air mass (single-cell) thunderstorms: a. Conditional instability b. Localized lifting mechanism c. Strong vertical wind shear d. Abundant low-level moisture e. None of the above
10. Microbursts are a hazard to low-flying aircraft because they a. Reduce visibility abruptly b. Create strong temperature gradients c. Create strong wind shear d. Are short-lived and very small scale e. Both c and d Part II: True/False (3 points each) - Indicate the correct answer on the computer form using a #2 pencil. 11. The greatest frequency of thunderstorms is coincident with the greatest frequency of hail 12. Absolute vorticity of an air parcel is the sum of the relative vorticity and the planetary vorticity. 13. A decrease in thickness implies winds backing with height 14. If the upper-level pressure surfaces are purely horizontal, the thermal wind will be zero 15. Rising motion typically is found east of the axis of an upper-level trough Part III: Definitions (5 points each) - Provide a short definition for each term in the space provided. 1. Entrainment The process by which environmental air is mixed into a cloud through turbulent processes. It is a key aspect of downdraft formation in thunderstorms. 2. Relative vorticity The microscopic (at-a-point, or local) spin at a point within a fluid measured relative to the rotating Earth. It is a vector and has three components.
3. Gust front The leading edge of the surface cold pool that spreads out beneath a precipitating thunderstorm or mesoscale precipitation system. Part IV: Short Answer (points as shown) - Provide a concise answer to each of the questions listed below. 1. (10 points) In the context of polar front theory, sketch and briefly describe the stages of a developing mid-latitude cyclone, showing the low, fronts, air flow, pressure patterns, and weather.
2. (10 points) Sketch and briefly describe the three stages of an airmass (single-cell) thunderstorm. Be sure to describe why such storms never last longer than approximately 1 hour. Cumulus Stage -- Cloud consists entirely of updraft and is growing rapidly; significant entrainment occurs. Mature Stage -- Cloud consists of both updraft and downdraft, marking the most intense phase of the storm. The downdraft is driven by entrainment, precipitation drag, and evaporative cooling below clouds base. The resulting cold pool spreads laterally on the ground. Dissipating Stage -- The cloud consists entirely of downdraft, with the gust front spreading in all directions, thereby cutting off the supply of moist, warm air to the updraft. Such storms never last longer than about an hour because, owing to the lack of significant vertical environmental shear, the cloud grows vertically upright. No mechanism exists to separate the updraft and downdraft, and thus precipitation falls back through the updraft, killing it. 3. (20 points) Suppose a strong east-west temperature gradient exists across the southern Great Plains, with temperatures increasing toward the west at 10 C/300 km. Westerly winds of 20 m/s are howling from Amarillo (located 400 km due west of Oklahoma City) to the eastern Oklahoma border. a. At what rate (in degrees per hour) will the temperature change at Oklahoma City? Note that the advection equation is given by, T T T T u, v, w,, t, x, y, z
Given: dt/dx = -10C/300,000 m u = 20 m/s Distance between OKC and Amarillo: 400 km Find the time rate of change of temperature at OKC (that is, find the temperature advection). Using the equation given on the previous page, and noting that only the east-west term is involved, we have, T, T u, t, x Working this problem thus is simply a matter of plugging in the numbers on the RHS, which yields, T 10C 4 (20 m/ s) 6.67x10 C/ sec 2.4 C/ hr, t 300, 000m Does the answer make physical sense? Yes, because warm air is to the west in the presence of a westerly wind -- thus, the temperature should rise at OKC. Note that you didn't need to use the distance between Amarillo and Oklahoma City. b. If the temperature at Oklahoma City is 30C at 11 am, and if the convective temperature is 40C, at approximately what time would you forecast thunderstorms to begin at Oklahoma City? From part a, we have the rate of change of temperature. In this part, we're given the initial and final temperatures and asked to find the time, which will give us the time interval because we're also provided with the starting time. Thus,, T Tfinal Tinitial 40C30C 2.4 C/ hr., t t t, t FromPart a final initial Solving for the t gives 4.17 hours, which is roughly 4 hours and 10 minutes. Thus, convection will begin at 11 am plus 4 hours and 10 minutes, or at approximately 3:10 pm.