Subject: Re: Offshore vs nearshore sonde composite Date: Sun, 04 Aug 2002 13:57:02 -0400 From: James Franklin To: Peter Black CC: Colin McAdie , Ed Rappaport , Richard Pasch , John L Beven , "James M. Gross" , Brian R Jarvinen , Mark Powell , Max Mayfield , Chris Landsea Pete, Thanks for the kind words, but I am really not sure what the results mean. In particular, I do not see how the ratio I computed relates to over water vs over land gust factors. This ratio, as computed from the sondes, to the extent that it is representative and means anything at all, represents the ratio of the mean wind near shore (but still over water) to the mean wind well out to sea. Land doesn't have anything to do with it. My suggestion is to follow the line of thinking I outlined for estimating a 1-min sustained wind over land. By the way, the definition of intensity has no mention of land in it, so one need not, indeed, should not, be trying to convert actual surface winds occurring over one surface to some other surface for the purpose of estimating the intensity. As desirable as this might be for certain applications, that simply is not part of the definition at this time. The intensity of a tropical cyclone is the maximum sustained wind present at 10 m at a particular point in time and attributable to the cyclone circulation. The underlying surface can be water, land, or crazy glue, for that matter - whatever the hurricane happens to be over at the time. Cheers, James ----------------------------------------------------------- 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 > From: "Peter Black" > Date: Sun, 4 Aug 2002 12:39:33 -0400 > To: "James Franklin" > Cc: Colin J Mcadie , Edward N Rappaport > , Richard J Pasch , > John L Beven , James M Gross , > Brian R Jarvinen , Mark Powell > , Max Mayfield , Christopher > Landsea > Subject: Re: Offshore vs nearshore sonde composite > > James and all- > > Great detective work James, and an excellent composite. This is very > revealing to me. Take it for what it is worth, but your > nearshore/offshore sonde ratio is exactly equal to the ratio I > estimated from composite ratios of hurricane over land gust factors > to hurricane over water gust factors, as per the first paragraph in > my Andrew Cat 5? report: > > " Powell shows how this average (the the peak 1-minute average flight > level wind) represents a 10-minute average over-water wind at 10 m at > a fixed point. To estimate a 10-minute average surface wind over land > at 10 m, the average ratio of the 10-min overland (open terrain) wind > to the 10-min over water wind first needs to be multiplied. This > value is 0.92 ± 0.08 according to Black." > > So where do we go from here? 145 kt X .92 = 133 kt = CAT 4. My > suggestion is to follow the line of thinking I outlined for > estimating a 1-min sustained wind over land. That would still result > in a CAT 5 conclusion, but maybe another line of thinking needs to be > worked out. > > Congrats to Mark for raising this issue and to sticking to his guns > in the face of a withering cross fire. And congrats to James for so > quickly digging into the data base and taking an objective look at > the evidence. This is the way consensus ought to work. I think > everyone will be satisfied that no stone was left unturned, whatever > the final decision turns out to be. > > Regards- Pete. > > > At 5:58 PM -0400 8/3/02, James Franklin wrote: >> After scouring the sonde database for nearshore and comparable offshore >> sondes, I was only able to come up with a sample of 10 nearshore sondes >> and 9 "comparable" offshore sondes, the 6 each that I identified >> previously for Georges, and a few more for Bonnie. All 19 sondes were >> dropped on the right hand side of their respective storms' eyewall over >> a relatively short period of time. >> >> I normalized the wind speed in each profile by what I call the boundary >> layer top wind speed, which I define simply as the average wind speed in >> the 300-700 m layer. I then composited the normalized profiles for each >> sample, and the resulting composite profiles are given in the >> attachment. The profiles only go down to 30 m, because too many sondes >> start dropping out of the sample below that point. For whatever it's >> worth, the nearshore sondes wind is about 92% of the offshore sondes >> wind speed, at 30 m. >> >> This is about all I can think of to do by way of additional research, >> given the existing data, unless someone has a specific request or idea. >> >> James >> -- >> 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 >> >> >> Attachment converted: Gladys2:Nearshore vs offshore.GIF (GIFf/JVWR) >> (000445EF) > > ********************************************************************** > ********** > > Peter G. Black > Hurricane Research Division > NOAA/AOML > 4301 Rickenbacker Cswy > Miami, FL 33249 > > PH: 305-361-4320 > FX: 305-361-4402 > > Email: Peter.Black@noaa.gov > > ********************************************************************** > **********