Note on using H*Wind fields for storm surge and wave modeling

Mark Powell, March 2012

Since the ocean responds to the wind over longer time scales (e.g. ocean waves (period and height) do not immediately respond to wind gusts), its important to use winds with a temporal scale consistent with ocean response. It is recommended that H*Wind fields be converted from a maximum 1 min sustaned wind speed to at least a ten min (or longer) mean wind speed. Below is an excerpt (Powell et al 2010) from our IPET work with OceanWeather on producing wind fields for use with storm surge and wave models applied to Southern Louisiana and Mississippi for better understanding flooding from Hurricane Katrina. In this case H*Wind fields were converted to a 30 min mean wind speed.

"The H*Wind wind speeds are transformed from peak sustained wind speed (which is essentially a stochastic variable) into equivalent 30-minute average wind speeds using the factor 0.81 which is 1/1.235, where 1.235 is a slightly lowered estimate of an adopted gust factor from peak 1-minute wind speed to 30 minute average of 1.24 (Black, 1994). The slight reduction from 1.24 to 1.235 is based experience to compensate for a small energy loss due to the grid transformation. In more recent (i.e. post IPET) hurricane wind field reconstructions we have adopted the combination of spline interpolation and the wind speed dependent gust model of ESDU (1982) which trends at hurricane wind speeds towards a slightly lower gust ratio of 1.215."