Notes on Wind Scaling and Wind Stress Parameterizations in Ocean and Atmospheric Models
June 13, 2011.
Surface wave models:
1. Wave Watch 3 (WW3): uses winds from the NECP GFS operational output to drive
WW3 with no “gustiness” scaling factor. [Hendrik Tolman and Arun Chawla (NCEP)
per Will Perrie.]
2. SWAN: The SWAN manual states not to use a “gustiness” scaling factor since gustiness
is already taken account of inside the model. [Will has queried Marcel Zijlema (Delft T
U).]
3. FVCOM-SWAVE: similar to SWAN. No scaling factor. [Changsheng Chen]
Bulk parameterizations of surface air-sea fluxes:
1. COARE 3: Algorithm to compute surface wind stress, sensible and latent heat fluxes
(Fairall et al., 2003). While developed for open ocean application, frequently used in
shelf and coastal regions. Inputs are bulk wind speed relative to water, air temperature,
relative humidity, air pressure at their measurement heights and SST typically averaged
over 20 min to 1 hour. (Algorithm developed using 20-min averages of the measured uw,
uT, and uq covariance fluxes (which include gustiness) and the bulk variables.)
2. COARE 4: Improved version of COARE 3 based on more recent data. Fig. 1 shows
comparison of COARE 3 and COARE 4 drag coefficients as a function on the 10 m
neutral relative wind speed (Edson, 2009). Note the clear increase in COARE 4 drag
coefficient over COARE 3 coefficient at wind speeds at higher wind speeds (above 15
m/s). Like COARE 3, COARE 4 was developed for open ocean applications, but will be
used for open shelf and coastal applications (with caution). COARE4 includes onshore
wind events measured at the Martha’s Vineyard Coastal Observatory air-sea tower, so
hopefully be more accurate in the onshore wind conditions in the stronger storms over the
New England/Gulf of Maine region. [Edson, J., 2009. Review of air-sea transfer
processes. Proceedings, ECMWF Workshop on Ocean-Atmosphere Interactions, 7-10-12
November 2008, 7-24.]
Fig. 1 Binned data and the COARE 3 (green), COARE 4 (red), and a
ECMWF model as a function of 10-n neutral relative wind speed.
3. Mark Powell wind stress parameterization used with ADCIRC+SWAN [Corbitt Kerr]:
ADCIRC uses Garrett (1977) drag coefficient Cd = 0.001(0.75 + 0.067*U10) where U10
is the scalar wind speed relative to the ocean surface. SWAN uses Wu (1982) draft
coefficient Cd = 0.001(0.08 + 0.065*U10). To account for the change in drag coefficient
depending on the relative orientation of U10 and the direction of the dominant surface
waves, UND uses a close variant of the Powell (2006) two step approach for
ADCIRC+SWAN hurricane simulations: 1) divide wind field into sectors relative to
direction of storm center (Fig. 2); 2) use different drag coefficients in different sections
(Fig. 3). Max Cd = 0.003.
Fig. 2
Fig. 3
See http://caseydietrich.com/index.html?p=SWAN+ADCIRC/index100801.html for
description of method and results.
4.
5.
Ocean Weather Inc (OWI) wind stress parameterization (?)
SLOSH wind stress parameterization (?)
Weather models:
1. Weather Research and Forecast (WRF) mesoscale weather model: [Houshuo Jiang]
WRF output data files include U10 and V10(surface wind) interpolated to the 10 m level
(a fixed height above the sea surface), but air temp (called T2) is interpolated to the 2 m
level (also a fixed height above the sea surface). The WRF output data files also include
3-D data (u, v, w and thermal dynamic variables) directly output at the (x, y, eta or halfeta) grid points. The first eta level used by a common WRF simulation is usually 100200 m above the sea surface and the heights corresponding to each eta level can vary with
time and space.
WRF output data is output at an instantaneous time (say every one hour). But if the grid
resolution is reasonably coarse [say 30 - 50 km (i.e. roughly more than 10 km)], the
output data can be regarded as a time mean over the interval between successive output
files. In these situations, the output winds are relatively smooth, therefore it should be ok
to apply a wind scaling factor like 1.09 to take into account "gustiness". But, if the
resolution is fine enough [say 1 km (less than 10 km)], the output data should (since it is
instantaneous data) should already include some of the "gustiness" (varying greatly in
time and space). In this case, I am not sure about the method to account for the
"gustiness".
WRF has the ability to reflect wind gustiness, but need high spatial (both horizontal and
vertical) resolution, and the WRF output data file has to be output at much higher
frequency than hourly. Alternatively, we can output data at individual points at the
frequency of the time resolution of the WRF simulation (say every 10 s).
2. Mark Powell (MP), Ocean Weather Inc (OWI), SLOSH hurricane surface wind models.
Hurricane wind “gustiness” scaling factor (WSF): [note from Joannes Westerink]
1. The wind “gustiness” factors that I have are as follows:
- MP's recommended factors that we have been using for all hindcasts over the past 3
years (i.e. model validation experience is based on these)
2.
3.
4.
5.
Time shift
MP
OWI
30 to 10
1.04
1.09
10 to 1
1.12
1.138
30 to 1
1.165
1.24
Note that the 10 to 1 minute values are similar b/w MP and OWI. Since we use 10 min
winds for our air-sea drag laws, this is important when using peak winds or the OWI
product.
One thing that is pertinent for the simulations is how the OWI wind fields are converted
from MP's 1-minute winds to the 30 minute winds that are produced. I assume that a
factor of 1.24 is used. This means that in the ADCIRC simulations we should use 1.09 to
produce 10 minute winds. On the other hand when using the H*WIND fields directly we
simply should use 1/1.12
Assuming 1 minute winds of 100 knots. OWI would convert these to 80.6 knots for 30
min avg and we would convert them to 87.9 knots 10 min avg (using 1.09). MP's 1
minute 100 knot wind would convert directly to 89.3 knots. The difference b/w these two
winds results in a 4.8% difference in wind stress.
However if we take an original 100 knot wind from H*wind, OWI converts it to 80.6
knots 30 min wind, and then we use 1.04 to convert it to a 10 minute wind, we would get
83.8 knots. This results in a 21% difference in wind stress.
6. We have been using 1/1.12 for H*Wind 1 min to 10 min and 1.09 for the OWI combined
product. For our own 5 layer pure PBL with Ed's implementation of Jelesnianski's radius
to max wind, we continue to use 1.04 to go from 30 to 10 min avg.