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Synoptic Meteorology Lab 6

26 March, 2014 – due Fri 4 April

Isentropic maps, isentropic vertical velocity, and IPV

First, we run gdvint on the gempakmodel output files to redistribute the data from isobaric to isentropic levels.

GDFILE your gempakgrid filesomething like yyyymmdd_hh _f00_f36.gem

GDOUTF your choice of output file nameeg. yyyymmdd_hh _f12_isentropic.gem

GDATTIM Grid date/time: 18 Z on 19 Dec 2012

GVCORDPRES/THTA

GLEVEL 285;290;295;300;305;310;315;320;325

MAXGRD2000number of 2D or 3D variables to be transferred

GAREA leave blank same as your model domain

VCOORDnone;mslv;esfcthis transfers all 2D grids as well, such as surface precipitation

Please repeat this for 6 times in total, from 18Z on 19 Dec to 00Z on 21 Dec 2012, at 6 hour intervals (your forecast hour 06 to 36)

Notes:

  1. Experience testifies that the isentropic maps will not look good and may have strange discontinuities if your input isobaric data is coarse in the vertical, e.g. 100 mb. A 25 mb vertical interval is better. You may have to rerun wrf2gem, and set num=41 or higher. (The default has been num=41)
  2. Your daytime WRF output may have slightly superadiabatic lapse rates in the southern tier of your domain, esp. at 18-00Z. I doubt it, because it is mid-winter, anyway, if it happens then this leads to an ambiguity (two theta levels at a given location x,y). gdvint may be able to handle this: the algorithm chooses the highest level in this case. But this too may lead to a discontinuity in the mapped height of an isentropic surface. If you see such discontinuity, I suggest you take a time 3 hours earlier or later (15Z or 03Z), by which time the sun is lower in the sky and the surface sensible heat flux in the model has reduced to near-zero.

Next, you open gdplot2. Make sure your new input file is the isentropic one you just created. Plot winds (barbs), pressure (contours) and relative humidity (color fill) on the 290K and 310K surfacesat 12Z on 20 Dec 2012 – similar to the real-time maps found on the College of Dupage site (2 maps, 10% of this lab’s grade).

Questions (7 in total, 7% each)

  1. analyze vertical velocity from the flow crossing isobars, and pencil-mark regions of ascent and descent on the 2 plots
  2. estimate the motion (cx,cy) (units: m/s) of the main trof centered at 12Z on 20 Dec 2012, from an animation of 500 mb heights (generated in a previous lab), or an animation of the 310K isentropic map over the 36 hours of your WRF run.

Generate a new wind field, (u-cx,v-cy), the storm-relative wind:

VSUB(WND,VECN(cx,cy)) where WND=(u,v) is the horizontal wind field

Plot the storm-relativewinds (barbs), pressure (contours) and mixing ratio (color fill) on the 290K and 310K surfacesat the same time (2 maps, 10%).

  1. Again analyze vertical velocity from the storm-relative flow crossing isobars, and pencil-mark regions of ascent and descent on the 2 plots

Plot the storm-relative winds (barbs), pressure (contours) and vertical velocity w (color fill) on the 290K and 310K surfacesat the same time (2 maps, 10%).

  1. Compare the estimated updraft (& downdraft) regions, from your isentropic analysisusing actual and storm-relative motion, to the actual large-scale updrafts/downdrafts in the last two maps. Discuss differences. Jim Moore, in his COMET module “isentropic analysis” ( argues that in principle, storm-relative motion on isentropic surfaces is the better diagnostic of large-scale vertical motion. Is that true in this case?

Generate isentropic potential vorticity (IPV): PVOR (PRES, WND). Multiply by 106to obtain standard PV units: MUL(PVOR(PRES,WND),1000000)

Plot the storm-relative winds (barbs),pressure (contours), and IPV (color fill) on the 290K and 310K surfaces at 6 hour intervals between 18Z on 19 Dec to 00Z on 21 Dec 2012 (12 maps, 21%of this lab’s grade).PVOR is computed over a layer (the stability term), so set GLEVEL=285:295(low levels) and GLEVEL=305:315 (upper levels). For other variables, GLEVEL is simply 290K or 310K.

Suggestion: try to put the earliest 6 maps in one six-panel figure, 290 K on the left and 310 K on the right, and the following 6 maps in a second six-panel figure, to facilitate analysis and improve presentation. This makes it easier to compare low-level to upper-level maps, and to see evolution.

Note: if the 290K level is too low (no data in the warmer south), you could try 295K, and the GLEVEL range would have to be lifted to 290:300

  1. How large are the main PV anomalies, relative to stratospheric PV values starting at 2 PV units?
  2. Examine the conservation of IPV at low levels and upper levels over time: You will see transport and deformation, but do you also see generation/decay of area-integrated IPV? If so, is there a possible relationship with latent heating, or proximity to the terrain (friction)?
  3. Discuss the relative position of the main low-level & upper-level positive PV anomalies in the context of the storm’s evolution. Is this consistent with theory? (transition from a tilted open-wave system to a vertically stacked occluded system)