Fall 2015 Laboratory 7 Quasigeostrophic Forcing Functions 300 points. You are provided with the following for 12 UTC 12/4/12 (a) WRF-NAM

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1 ERTH 465 Fall 2015 Laboratory 7 NAME Quasigeostrophic Forcing Functions 300 points. You are provided with the following for 12 UTC 12/4/12 (a) WRF-NAM initialization of SLP and mb thickness; (b) WRF-NAM initialization of 500 mb heights and absolute vorticity; (c) Location map. Note A, B and C. (d) Visible and infrared satellite images for 1901 UTC 12/4/12 (e) Charts I produced from mod_grib: (i) 850 mb temperature advection; (ii) 500 mb absolute vorticity advection; and (iii) 700 mb vertical velocity. You will be QUALITATIVELY evaluating the contributions to upward or downward motion at these locations on the basis of the descriptive form of the quasigeostrophic omega equation, as discussed in class. 1

2 Part A. Evaluations (100 pts) 1. Your first estimate for the sign of these two terms will come from an evaluation of their qualitative nature using (a) and (b) above, assuming that the temperature advection from nam_thick is proportional to the 850 mb temperature advection. (50 points) 850 mb Temperature 500 mb Absolute Vorticity Net Effect Zero Indeterminate to Slight Indeterminate Table 1: Forcing for upward or downward synoptic scale motion determined from nam_thick and nam_vort. (Insert Up or Down in each box for the forcing terms and either Up, Down or Indeterminate in the row labeled Net Effect.) 2. Your second estimate for the sign of these terms will come from looking at their values at the three locations on the actual values on the mod_grib charts (50 points) 850 mb Temperature 500 mb Absolute Vorticity Net Effect Strong No temperature Warm cold=downward advection motion Indeterminate = Zero Indeterminate 2

3 Table 2: Forcing for upward or downward synoptic scale motion determined from mod_grib plots. (Insert Up or Down in each box for the forcing terms and either Up, Down or Indeterminate in the row labeled Net Effect.) Part B. Comparison with Actual Vertical Fields (30 points) Complete Table 3, which is simply the qualitative nature of the vertical motion at 700 mb, and Table 4, which is simply the nature of the 300 mb convergence field. (15 points) 700 mb Vertical Velocity Weak Table 3: Insert the actual 700 mb vertical velocity for each location (in microbars per second) (15 points) 300 mb DIV/CONV 6.0 X 10-5 s-1 Strong Divergence Zero to weak Convergence 0.5 X 10-5 s-1 Weak Divergence Table 4: Insert the actual 300 mb divergence or convergence for each location (in units of 10-5 s -1 ) Part C. Synthesis (40 points each for 160 points) 1. Perform a careful frontal analysis on (a). 3

2. Each student should compare their results in Tables 1 and 2 with the actual values of vertical velocity and upper tropospheric divergence. Write a few paragraphs in discussion.

4 2. Each student should compare their results in Tables 1 and 2 with the actual values of vertical velocity and upper tropospheric divergence. Write a few paragraphs in discussion. Were your results consistent or not? Discuss carefully. My values were consistent with my estimates from nam_thick and nam_vort. The take home message is that the vorticity advection itself cannot fully describe or diagnose the vertical velocity field. For example, at Location C the warm advection is occurring in an environment of negative vorticity advection. Since the vertical velocity was upward, it is clear that the vertical motion associated with warm advection exceeded the forcing for subsidence diagnosed by the negative vorticity advection. 3. Each student should compare their frontal analysis with the two satellite mages. Were your results consistent or not? Discuss carefully. The frontal analysis generally corresponds to what I see on the two satellite images provided, one visible and one infrared. However, the two satellite 4

5 images are for a time about 7 hours after the map time for the frontal analysis so some progression of the cloud pattern should be evident. The cloud presentation is consistent with an occlusion, with the occluded cloud band dissipated in the areas downslope between the coastal ranges and Rocky Mountains in British Columbia. The triple point on the satellite images seems to be in north central Washington, and seems reasonable given the progression. The warm front is difficult to locate on the two satellite images due to the amount of high cloudiness, but projecting southward from the triple point seems reasonable. The cold front appears to be in the correct position on the satellite images and is comparable to the position shown on the frontal analysis. A second cloud feature is the comma cloud west of Vancouver Island. That seems to be collocated with the troughline shown on the surface analysis. 4. Each student should compare Tables 3 and 4 and should determine whether Dines Compensation was illustrated by this case. There was good correspondence between my values shown in Tables 3 and 4 and Dines Compensation. The most ambiguous values were at Location A, but even for that location, my inferred weak convergence at 300 mb is consistent with weak downward motion. For locations B and C, the divergence in the upper troposphere was collocated with upward motion at 500 mb with the strongest values near the triple point, at location B. 5

Example of the "Vorticity Advection" Pitfall

Example of the Vorticity Advection Pitfall Example of the "Vorticity Advection" Pitfall NOGAPS 60 h Forecast for 500 mb Heights and Absolute Vorticity The 500 mb chart with absolute vorticity overlain is the first chart most operational meteorologists