Meteorology 5140/6140

Assignment 2. Due Friday January 30. 20 points
This assignment will begin to familiarize you with some of the mesoscale and synoptic aspects of the Christmas Day 2008 storm. Later we'll begin to relate the radar, upper air, and satellite info to these surface reports. You will submit all the graphics and text in webCT as discussed in class.

• 1. Using MesoWest, display the tabular data and time series on campus (WBB) as follows: (a) metric units; (b) UTC; (c) 48 hour period ending at 0 UTC 27 December. Save and submit the graphics for (1) temperature and moisture; (2) pressure; (3) scalar wind; (4) precipitation.
• 2. Answer the following using the graphs and tables. You will need to set the time to 0 UTC 26 December to see the tabular values prior to that time.
  • (a) when did the surface pressure trough pass WBB and what was the minimum surface pressure (mb)? (Answer: 2:15 UTC 827.8 mb).
  • (b) what was the total drop in pressure from 0 UTC 25 December until the passage of the surface trough (in mb)? (Answer: 22 mb)
  • (c) What was the total rise in pressure after the passage of the pressure troughuntil 0 UTC 27 December (in mb)? (Answer: 24 mb).
  • (d) There are two time periods after the passage of the surface pressure trough when there are noticeably sharp increases in surface pressure. When were those two periods and roughly by how much did the pressure increase during those periods? (Answer: immediately following the surface pressure trough until 3:20 UTC a rise of about 3 mb and from 5:25-6:25 rise of over 2 mb)
  • (e)What was the time of the passage of the cold front and how does that relate to the time of the passage of the surface pressure trough? (Answers: 2:15 UTC and coincident)
  • (f)There is a very sharp temperature drop at the leading edge of the cold front and then the temperature leveled off for a while. When was this period, what was the total temperature drop, and rate of temperature decrease (degrees C per hour). (Answers: 2:15 to 3:30 UTC 3.3C to -2.9C so -5C/hr)
  • (g) How did the temperature change around the time of the second sharp increase in surface pressure following the passage of the surface pressure trough? (Answer: temperature increases immediately prior to and then dropped sharply)
  • (h)At what time did the winds begin to veer from sustained southerlies and was this before, conncurrent with, or after the passage of the surface trough? (Answer: 2:10 UTC immediately before)
  • (i)After the initial frontal passage, when did the winds stop veering? (Answer: 3:30 UTC)
  • (j)At what time did the winds finish backing after your time in (i)? (Answer: 4:45 UTC)
  • (k)How do the winds change after that time and how does that relate to the features previously identified in the pressure and temperature traces? (Answer: winds veer with time reaching wind speeds of 10 m/s)
  • (l) So, given the changes in pressure, temperature, and wind associated with the secondary feature identified in (k) and earlier, discuss what this feature might be? (Answer: secondary trough)
  • (m) What and when was the peak precipitation rate (cm/5 min)? Was this rain or snow? Hint: consider the wet bulb temperature. (Answer: this is a bit of a trick question as there are 2 obs with .15 cm/5 min immediately after the front at 2:35 and 2:40. The first may be accumulated over an interval greater than 5 min, as the sensor has to "wake up". So, the second, would be the peak. Snow, given the wet bulb temperature around -1C)
• (3) Look at the command line for the web browser and toggle back and forth from WBB to KSLC.
  • (a) What is the 1500 m pressure (in mb) at WBB and KSLC at 2:15 UTC 26 Dec? (Answer 824.22 and 826.24 mb. )
  • (b) Determine the distance between the campus and the airport. What is the magnitude of the pressure gradient in mb/km at 1500 m? (Answer: 12 km. .166 mb/km)
  • (c) If the only force acting on the air was the pressure gradient force, and if the winds started from rest, how fast would the air be flowing from KSLC toward WBB after 10 minutes? Assume the density of the air is 1 kg/m3. (dv/dt = 1/rho * .166 mb/km. Sort thru units and get about 10 m/s)
• (4) Now use the observations at KSLC to evaluate the intensity and timing of the heavier precipitation. Be aware that this is a blend of automated observations every 5 minutes combined with human observer reports a few minutes before the top of the hour and when conditions warrant a special report. The human reports are repeated in subsequent automated reports until the next human observation.
  • (a) When did the surface pressure trough pass KSLC and what weather conditions were observed at that time? (Answer: 1:45 UTC and light rain)
  • (b) What was the air temperature and the wet bulb temperature when the precipitation type changed from rain to snow? (Answer: air 1C, wet bulb .1C)
  • (c) At what time did the heaviest precipitation begin to be reported and what visibility was reported at that time (km)? (Answer: 2:27 UTC heavy snow and .4 km visibility)
  • (d) When was thunder first reported? (Answer: 2:35 UTC)
• (5) Now toggle to get graphics and tabular displays for KWMC (Winnemuca).
  • (a) At what time did the surface pressure trough pass KWMC. What was the lowest pressure at 1500 m at that time? (Answer: 1300 UTC 25 December and 828 mb)
  • (b) Determine the distance beteen KWMC and KSLC. (Answer: 500 km)
  • (c) What is the speed of the surface pressure trough between KWMC and KSLC (m/s)? (Answer: 500 km/13 hr = about 10 m/s)
• (6) Not let's get a sense of the connections between the passage of the surface trough and what must be happening aloft.
  • (a) Beginning from the continuity equation in pressure coordinates, derive an expression for the change in surface pressure as a function of the mean divergence in the layer from 850 mb to the top of the atmosphere (p=0). Remember that omega is the change of pressure with time. Assume that the vertical motion at the top of the atmosphere (p=0) is 0. You should end up with dp850/dt = - mean div throughout entire atmosphere * 850 mb (answer: d(omega)/dp = -divergence. d(omega) = - div dp. integrate from 850 to 0: -omega850 = - mean div (-850) substitute in dp850/dt = - mean div (850)
  • (b) Prior to the passage of the surface trough at WBB, was there mean divergence or mean convergence in the column above? Why? After the passage of the surface trough at WBB, was there mean divergence or mean convergence in the column above? Why? (Answer: prior mean div, since surface pressure falling. After, mean convergence since surface pressure rising)
  • (c) From the time of the frontal passage at WBB until 0 UTC 27 Dec, you found in 2c that the pressure increase was X mb/Y hr. Using that number what was the magnitude of the mean divergence in the column over this entire period? How does this compare to typical instantaneous values of divergence in the midtroposphere commonly associated with rising motion in synoptic-scale systems? Why? (24 mb/22 hr) (1/850) * (1/3600 s) = 4x10-7 sec-1 This is smaller than the order 10-6 per sec observed for large-scale systems, but this is averaged over the entire column.
  • (d) A definition of a synoptic "bomb" is that the central pressure of a surface low at sea level deepens by 24 mb in 24 hr. Comparing the change in the surface pressure from KWMC to KSLC at the times of the passage of the surface trough, would you put this in the category of a rapidly deepening storm? (Answer: no only a drop in central pressure of a few mb at most over 13 hours)
  • (e) Using the same approach as that developed in part (a) and (c), estimate what the mean divergence would be in a column from sea level (order 1000 mb) to the top of the atmosphere, if a synoptic bomb were to take place. In this case, we are computing more correctly in a lagrangian framework. (Answer: (24 mb/24 hr) (1/1000) (1/3600)= 2.7x10-7 sec-1.)
• 7. What other feature of interest did you notice in the tabular summairies or time series at KWMC, KSLC, or WBB?