Vandemark
D.,
P.D. Mourad, S.A.Bailey, T.L. Crawford, C.A. Vogel, J. Sun, B. Chapron,
2001: Measured changes in ocean surface roughness due to atmospheric
boundary layer rolls, JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS,
106 (C3), 4639-4654.
One new and key tool in climate
prediction and weather monitoring is the global open-ocean surface wind
speed data that come from satellite radar and radiometers. These
microwave systems measure the variations in short wind waves and use wave
intensity to estimate the surface wind, creating a global product with a
horizontal resolution on the order of 25-50 km. Wind gustiness, local
strong variations in the surface wind, is not resolved by these sensors.
But this information is of great interest to boaters, especially in the
coastal regions, and to climate modelers where model accuracy is often
critically-dependent upon precise knowledge of the variability in wind
forcing. This study presents a simple aircraft experiment that documents
the change in short wind waves in response to well-organized secondary
wind circulations over the ocean. The wind circulation pattern measured
in this case is often termed as "wind rows" or "cloud streets" and one can
view the ocean as being covered, over a large 100-200 km square region,
with long rows (or streaks). The repeating undulations are associated
with changes in the near-surface wind if one traverses across this "wind
row" plane. The changes are one of many sources of wind gusts found on
the open-ocean.
The aircraft study here used a high-fidelity wind probe
and surface-roughness radar sensors to directly measure the changes in
both air flow and surface waves as the plane flew a long 50 km transect
across the 'wind row' field. The data provide, for the first time, a
nearly instantaneous look at the very close correlation between surface
wind speed changes and the resulting surface wave modification. The roll
cell wind modulation widths were measured to be about 1.5 km across and
the wind speed varied by 10%. Our measurements indicate that the
surface waves react almost immediately and that wave pattern changes
mirroring the wind should be readily identified by a satellite with
sufficient (km-scale in this case) spatial resolution. One such sensor,
the satellite synthetic aperture radar (SAR), can collect ocean surface
images having 10 m pixel resolution. Our study also shows that the
Canadian Space Agency's RADARSAT SAR, collecting data at the same time as
the aircraft, was able to cleanly resolve the "wind row" features. The
use of SAR for high-resolution ocean wind mapping in coastal zones is an
emerging remote sensing application. The present study serves as one
validation of the technique's promise.
