Ocean surface winds are one of the key components of the Earth system. By using Scatterometer
observations, we can understand the effect of ocean surface winds on ocean circulation and on the
air-sea interactions that fuel weather systems.
Understanding these interactions is critical for improving weather forecasting — from isolated
storms to organized squalls, and from hurricanes to seasonal and intraseasonal phenomena such as El
Niño and monsoon rains. Indeed, operational meteorologists have grown accustomed to make their
forecast of hurricane formation and evolution based on scatterometer wind estimates.
Scatterometers bounce pulses of microwave energy off the ocean surface. The returned energy gives us
information on the size and orientation of waves, which change depending on wind speed and
direction. Scatterometer data helps us monitor surface wind around the world’s oceans.
Scatterometer missions can be broadly classified in two categories, defined by the electromagnetic
frequency used and the scanning strategy. Different frequencies have different sensitivities to
atmospheric parameters like rain, and to ocean surface parameters like sea surface temperature and
wind speed.
C-band Push-Broom Instruments (5.25 GHz)
The observing geometry provides better accuracy across the measurement swath, but at the expense
of a large gap in coverage. C-band instruments provide better retrievals in rain than Ku-band
instruments because of the lower impact of rain at these lower frequencies.
C-band
C-band Scan Characteristics
Employed by EUMETSAT. Three fan-beam antennae in a single or dual swath configuration transmit
high radar frequency pulses and receive backscattered energy from the ocean surface. One example
of this type of scatterometer is ASCAT.
Ku-band
Ku-band Scan Characteristics
Employed by NASA and the Indian Space Research Organization (ISRO). The antenna spins and emits
two continuous beams of high radar frequency pulses. The antenna receives backscattered energy
from the ocean surface. One example of this type of scatterometer is RapidScat.
The scanning strategy provides wind estimates over a much larger swath but with lower accuracy
near the center of the swath. These instruments provide very accurate winds in rain-free
conditions and in some rainy conditions, but are negatively impacted in stronger rain events.
Meet the Scatterometers
Launched between 1999 and 2016, these NASA, NOAA, ISRO and ESA scatterometers create a full,
continuous picture of ocean surface winds that help improve weather forecasting.
The size and orientation of waves changes depending on wind speed and direction, giving us a way
to monitor the surface wind vector around the world's oceans using satellite scatterometer
observations of the wind-modulate ocean surface.
Derivitives: curl and divergence of the wind
Divergence and convergence (negative divergence) describe whether there is an outflow from a
region (divergence) or inflow into the region (convergence). The tropical atmospheric circulation
is largely driven by the near-surface convergence of momentum and moisture that on some scales can
be estimated from the scatterometer-derived wind convergence.
Surface wind stress magnitude and direction
Wind stress is the drag force exerted on the ocean surface by adjacent (near-surface) layer of
moving air. Wind stress is the most important forcing on the upper ocean circulating, driving the
surface currents.
Derivatives: curl and divergence of the wind stress
The curl of the wind stress is very important in deep ocean circulation, a primary driver of the
upper-ocean circulation and has been shown to modulate the air-sea coupling in regions of strong
fronts in sea surface temperature.
Data Portals
Through use of consistent records, MEaSUREs expands understanding of the Earth system by linking
multiple satellites into a constellation and by facilitating use of data in developing comprehensive
Earth system models.
Worldwide Ocean Wind (WOW) Portal
Dedicated to the visualization and online analysis of the ocean surface winds as observed by a
number of scatterometers since 2014. The WOW portal will be augmented to include the entire data
record of scatterometer winds, stress sand spatial derivatives, starting in 1999.
Launched between 1999 and 2016, these NASA, NOAA, ISRO and ESA scatterometers create a full,
continuous picture of ocean surface winds that help improve weather forecasting.
