Subject: Re: questions for presenters Date: Tue, 06 Aug 2002 06:32:16 -0400 From: "James Franklin" Organization: NOAA/Tropical Prediction Center To: Colin.J.McAdie@noaa.gov CC: "Edward.N.Rappaport" , "John.L.Beven" , "James.M.Gross" , "Richard.J.Pasch" , "Brian.R.Jarvinen" , Christopher Landsea , Mark Powell , Peter Black Colin J Mcadie wrote: > > 2) To James: > Based on your preliminary analysis of dropsonde profiles in coastal > regions versus those in a purely marine environment, do you now have > some uncertainty in the use of a constant reduction factor for flight > level winds in near-coastal regions? > > Do I have "some" uncertainty? Yes. Mark's raising the issue in and of itself created some uncertainty. And the results thus far certainly are consistent with his notion. But both Bonnie and Georges were weakening at landfall. And the wind speeds in these storms were very different than at Andrew's landfall. I am not knowledgeable enough about boundary layer processes to know whether Mark's new take on roughness lengths makes sense or not, or how the result might apply to Andrew. So there is "some" uncertainty. However, the small sample of sondes (near shore vs offshore) currently available is not sufficient, in my view, to warrant altering any current practices. It has been the presumption at NHC for years and years that oceanic winds impinged on the immediate coastline. My take on Mark's published work is that he thought so too. The sonde results are solid for the open ocean. I don't think we should change a long-standing practice without good evidence to do so. The conclusion to be drawn from the results so far, I think, is that we need to collect more data, and in stronger storms, and as soon as possible. On the other hand, clearly, it would be awkward to upgrade Andrew now, based on the current understanding, and in several years (because that's what it would take, I think, to acquire the necessary data) adjust it and a whole bunch of other storms downward if it turns out he and Peter Sparks are right. > > > 3) To all: How should the apparent strengthening seen in the surface > pressure and satellite data after the 0810 UTC 162 kt > flight-level wind be interpreted in terms of the wind field? Also, > what effect, if any, should the strong cells forming along the coast > in the northern eyewall have had on the surface wind field at the > shoreline? I indicated in my presentation that I thought my estimates were conservative, because of the apparent post-last-fix strengthening. I can't speculate on the effect of the strong cells on the cyclone-scale wind field. -- James L. Franklin Hurricane Specialist, National Hurricane Center NOAA/NWS/Tropical Prediction Center 11691 SW 17th Street, Miami FL 33165 Email: James.Franklin@noaa.gov Ph: 305-229-4475 Fax: 305-553-1901