Subject: Re: questions for presenters Date: Wed, 7 Aug 2002 23:40:49 -0400 From: "Peter Black" To: Colin.J.McAdie@noaa.gov CC: "Edward.N.Rappaport" , "John.L.Beven" , "James.M.Gross" , "Richard.J.Pasch" , "Brian.R.Jarvinen" , "Colin.J.McAdie" , Christopher Landsea , James Franklin , Mark Powell Colin and all- Sorry for this late reply- my final 2¢ from Tampa: Regarding your question Colin: 3) To Pete: Why did you feel that you could quantitatively convert (or interpret) a surface wind over land from marine observations, but Mark could or would not? That is, your document says "multiplying this by the mean over land to over water ratio of 0.92 +/-0.8..." There is clearly going to be an internal boundary layer set up downwind from any roughness change, whether it is at the coast or at the reef or at the barrier islands. My factors are simply the best guess available to my knowledge as to what the reduction of the mean wind and increase in gust factor is likely to be for open terrain conditions compared to open ocean conditions. It is my attempt to guess what might be the peak 1-min wind conditions that an anemometer over land might have measured. To answer one of James' comments: Last time I looked, coastal and inland wind measurements were still being considered when estimating a storm's peak winds and in fact as Colin refers to in question 1), Mark even did it in his first Andrew paper. I think the reference by James to Peter Sparks is out of context for this discussion and serves to muddy the waters. We should be attempting to clarify, not sling mud. I have a general comment on the interpretation of James' sonde profile comparisons. There is another, and in my opinion, much more likely cause for the difference in PBL profiles observed in Georges and Bonnie. SFMR data in storms like Emily 1985, Michael 2000, Humberto 2001, and damage survey results for landfalling Celia in Corpus Christi all show that a strong rotation in wind asymmetries can occur due to shear in the PBL. This shear can produce a 180 deg rotation in the location of the max wind in the lowest 5,000 ft of the storm. If This is the case for Geroges or Bonnie, then the PBL sonde profiles in the left eyewall should show very little change with height while those on the right would show abnormally large change with height. The change between off shore and nearshore profiles could be explained simply as achange in the low-level shear as the storm approaches the coast, and may have little to do with a roughness change. Which do I believe? It's an open question. But I do have several dramatic cases from recent SFMR data that show the rotation in the wind max. Regarding your other third question: "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 believe the convective cells which formed at the coast had two effects: First they probably caused mixing the strong winds at the top of the PBL downward to the surface and extending the zone of strong coastal winds much further inland than would otherwise have occurred. Second they probably enhanced the surface inflow throughout the PBL which would have forced stronger downward motion in the PBL as parcels accelerated across the strong eyewall pressure gradient. This may have resulted in super gradient flow penetrating into the clear eye near the surface and caused the implied CAT 5 winds from the SE over Naranja Lakes area. So even if the easterly winds near BK Corp were considered sub 135 kt, I believe that the winds in Naranja, when integrated over the subdivision, should be on order 145 kt. If I had a vote, I'd vote for Chris' definition of hurricane impacts and on this basis, for a best guess with probablility of 0.73, that Andrew was a CAT 5. Re your question: "5) To Mark: It is noted that the pressure at the time of the 109-111 kt wind at Fowey Rocks was 968 mb. The pressure analysis appearing in Fig 4 of Ed's prelim would seem to indicate that the tightest gradient (about 1 hour later), and presumably the maximum wind, was in the vicinity of BK Corp, at about 945 mb. Even given the fact that the system was deeping over this hour, it seems that the 968 mb is too high to be associated with the maximum wind. This is a concern because our recollection of your presentation (Mark correct us if this is not correct) is that you questioned your analysis because of the apparent mismatch with the Fowey Rock observation - taking it as representative of the maximum wind. In summary, the 968 mb pressure would seem to support some increase in wind after the failure, and perhaps a better match with your analysis." I would agree with your line of reasoning, Colin. Regarding James comment: "From Ed's map, it should also be possible to estimate the difference in gradient wind speed at a pressure of 968 compared to that at 945 mb. How much more wind does the increased gradient get you?" I did the gradient wind calculation for the steepest pressure gradient I could estimate from Ed's over-land pressures, and that is what gets you the 190 mph gradient wind estimate resulting in a 150 mph 1-min wind over land. Good luck in your deliberations tomorrow. I have enjoyed the exchanges on this topic immensely. Regards- Pete. Peter G. Black Hurricane Research Division NOAA/AOML