Meteorology 6210: Assignment 2

Purpose: To apply dynamics in the context of the isentropic vertical coordinate to evaluate 3-dimensional atmospheric motion. This exercise focusses on a couple of weak storms that travelled across British Columbia and the Pacific Northwest.

• On a workstation in the lab, bring up the Garp program from the program menu

• Select the Model Plan Projection Option

• Select under Area: North America (you will want to zoom in later; but to begin with, look at the situation on the large-scale)

• Select all Eta-Isen model runs (991022/1200F000-991026/1200F06). This covers the period at 6 h intervals from October 22, 12z until October 26, 18z.

• Select Vertical Coordinate "Isentropic" (thta will appear in the box)

• Select for Level 1 "290" for the 290K surface

• Select from Scalar: Isen and then select Pres.

• Select Display and Close

• Questions
  • Focus initially over the Pacific Northwest and offshore of the state of Wasahington. What are the major pressure features at the initial time, i.e., where is the 290 K surface near the ground and where is it in the mid-upper troposphere?
  • Briefly, how does the distribution of pressure on the 290K surface vary with time at Quillayute WA (extreme northwest tip of WA)? 300K surface?

• Select Temp_C in the scalar list and Display

• Question

  • How does the distribution of temperature compare to the distribution of pressure at the initial time and subsequent times? Why do they (pressure and temperature) have this relationship on isentropic surfaces?

• Select Pres on the 350 K level. What is the slope of the pressure surface on the 350K surface relative to that of the 290 K surface? What is the relationship for selected features between the pressure of the 290 K and 350K surfaces?

• Clear Display. For the 290K surface, Select Mont_sfunc (Montgomery Streamfunction) and overlay Pressure. Overlay the GeoWind_kts form the Vector list: Isen. This is the geostrophic wind computed from the Montgomery streamfunction

• Questions
  • Is the geostrophic wind on an isentropic surface parallel to Montgomery streamfunction?
  • Is there any relationship between geostrophic wind and isobars on an isentropic surface? geostrophic wind and isotherms?
  • If the flow is adiabatic and in geostrophic balance and steady, which contours define trajectories of the fluid motion?

• Clear display. Select for 290K surface from "Vector: Isen" Wind_kts and display. Overlay pressure.

• Questions
  • For the initial time only, assume that the flow is adiabatic and that the distribution of pressure on the 290K surface remains constant with time. Locate at least one region where adiabatic ascent is taking place, at least one region where little ascent is taking place, and at least one region where subsidence is taking place. (Because of the lack of lat-lon lines available in this version of garp and since most of the locations will be offshore, print out a map and label the locations on the map.)
  • Assuming the same as before, identify regions where a parcel would be adiabatically cooling or warming.
  • Evaluate whether adiabatic ascent or descent is occurring over Vancouver, BC at the level of the 290 K surface as a function of time ignoring the motion of the surface.

• To be more accurate for estimates of vertical motion, the speed at which the isentropic surface should be considered. The speed of the surface will be estimated here to be 0, 10, or 20 kts from west to east.

• Successively plot the advection of pressure at 0, 10, and 20 kts from the Scalar:Isen menu.

• Questions

  • Focus initially on frames 13 (25/12) and 14 (25/18) immediately offshore the state of Washington. Estimate the times when ascent would be found over MA.
  • How does the magnitude of the ascent and descent change as the speed of the surface is varied?
  • Is there any disagreement between the estimates of rising motion (i.e., rising instead of sinking or vice versa) due to consideration of the movement of the isentropic surface?
  • Which speed of the surface do you feel is most representative of this system?

• Final Questions
  • Why is relative humidity on an isentropic surface a poor indication of clouds?
  • Describe briefly the evolution of the weather that you would expect at Quillayute, WA during this period based on the information available here.