Mime-Version: 1.0 Date: Wed, 25 Oct 2000 14:34:28 +0000 To: David Schultz , map@atmos.albany.edu, map_disco@nssl.noaa.gov, ddowell@ou.edu, hblue@ou.edu, wicker@nssl.noaa.gov From: "Lance F. Bosart" Subject: Re: map discussion items: The Kicker and the Kickee Hi Dave, I hope that everyone gets a "kick" out of the map discussion war that you have launched.... Funny you should mention it, but Henry's rule of thumb came up in our map discussion last Friday too in conjunction with the movement of cutoff cyclones in the SW US. I checked out your home page. The 500 hPa height loops suggest that the important feature was the strengthening downstream ridge. Your calculation of vorticity advection at 500 hPa at 60 h suggests broad, but weak, AVA in the NW flow upstream of the "kickee" with CVA more concentrated downstream of there from KS to TX. I suggest that this AVA was the crucial feature in response to the strengthening short-wave ridge. At issue is how this field evolved over the previous 60 h. At the level of QG theory (height tendency equation) it doesn't matter whether you use the sum of the vorticity advection and differential thermal advection or the QGPV advection to determine the height tendency. When it comes to computing vorticity advection the strength of the geostrophic wind field also matters. Downstream of the building ridge the geostrophic NW flow strengthens over a regional area. I suspect that this also contributes to the broad, but weak, AVA upstream of the kickee at 60 h. A computational note. Our experience here is that the Eta model height and temperature fields contain abundant two- and four-delta wavelength noise (for whatever reasons) that make QG calculations (especially Q-vectors) look "funny" in many places. Suggest you use the AVN grids or the coarse resolution (2.5 deg) NCEP/NCAR reanalysis gridded fields for QG calculations. The old LFM grids worked beautifully for QG diagnostics because the output height and temperature fields on isobaric surfaces available to users were very smooth. Lance