There has been a lengthy recent discussion on the verification of Irene’s maximum sustained winds near the time and immediately following landfall. The participants were discussing how at the time of landfall, NHC had the system with maximum sustained winds of 75 kts, yet the highest “official” observed sustained wind speed was 58 knots (Cape Lookout). Here is a snippet from Chris Landsea at NHC:
“Trying to analyze the intensity (maximum 1 min winds) by using only
“official” observations without taking into account sampling issues is
a fool’s pursuit. These stations are on the synoptic-scale, while the intensity
is on the mesoscale. As a simple example, attached is the experimental
HWind product from HRD around the time of landfall with the three closest
official stations of Hatteras, Beaufort, and Cape Lookout. The RMW with
the peak winds occurs between the synoptic-scale stations. One does not
have much hope of capturing the true landfall intensity just based upon
Secondarily, once the peak 1-min winds go from an oceanic exposure to
open terrain exposure over land, they drop by roughly 15% (Vickery et al. 2009).
For Irene, with the intensity at landfall being 75 kt, the most that any
station not right at the coast would likely experience would be 60-65 kt.”
I think this has significant relevance to the discussions we have been having in our conference calls as well as verification of NDFD wind forecasts. Due in part to the suggestions of collaborators, we have been broadening our NDFD verification to HWIND analyses for recent landfalling systems. The NDFD over forecasting of wind speeds has decreased somewhat when looking at HWIND analyses, which include much more than in situ ASOS/AWOS observations. It is key to understand these biases when using ASOS/AWOS data.
I am currently working on verification of NDFD forecasts for Irene and will have images to follow soon!