The small space and time scales associated with the coastal zone place severe demands on measurement systems. Space-borne remote sensing systems have the potential to measure phenomena both over the coastal ocean and over the land; however, besides routine photogrammetry of clouds and infrared imagery, even satellite data are sparse and of inadequate spatial resolution. High-resolution wind measurements over the entire coastal zone are essential. Scatterometry and synthetic aperture radar (SAR) can provide surface wind and wave data; however, while the spatial resolution of synthetic aperture radar is adequate, obtaining high-resolution winds remains elusive. From the perspective of global change, long-term monitoring of coastal environments, particularly vegetation, water levels, soil moisture and coastal ocean biological productivity can be conducted with sufficient resolution using SAR, imaging spectrometers, and microwave scanning radiometers [ Leatherman, 1993], and satellite ocean color techniques [ McClain et al., 1993].
Current operational coastal meteorological observations are obtained from a variety of systems operated primarily by the National Weather Service (NWS) of the National Oceanographic and Atmospheric Administration (NOAA). NOAA operates 33 rawinsonde sites within 10 km of the coast. These measure pressure, air temperature, relative humidity and wind. NOAA also operates 26 instrumented buoys within 50 km of the coast or Great Lakes shores. These are equipped with sensors for pressure, wind, air temperature, and humidity at 3 or 10 m above the sea surface. In addition volunteer observing ships (VOS) provide pressure, temperature, humidity, wind, sea state and cloud visual observations every three hours within 200 km of land. These data are blended by an objective analysis scheme at the NWS to provide input to forecast models. It is interesting to note that the reporting times for synoptic measurements, 00 and 12 UTC, do not coincide with maxima or minima in coastal phenomena, such as the sea-breeze. Also balloon soundings do not often resolve the boundary layer, reflecting more the local urban boundary layer than that over the nearby coast.
The Doppler radar determines the velocity of falling
precipitation either toward or away from
radar by measuring the Doppler shift. Data from this system will
improve coastal wind
measurements with 24 operational NOAA radars providing some over
water coverage.
Reflectivity data will be available up to 400 km offshore and
Doppler winds up to 150 km.
Coverage is not universal, however. Various research quality
surface-based remote sensing
devices have been used successfully to study coastal phenomena.
For example, dual Doppler
techniques (multiple radars used together to investigate smaller
volumes of air) have been used to
investigate coastal eddies and lidar has been used to study the
diurnal land-sea breeze. Lidar is
an instrument that uses a laser to generate pulses that are
reflected from atmospheric particles.
New surface-based radar remote-sensing techniques (e.g., the
ocean surface current radar, OSCR)
to measure surface currents are also being tested [e.g.,
Shay et al., 1994]. These have the
potential to obtain currents with an accuracy of about 4 cm
s
and a spatial resolution of about 1
km within about 40 km of the coast, and a spatial resolution of
250 m with 12 km of the coast.
Coastal experiments to explore specific phenomena will continue
to use a combination of these
techniques with additional instruments mounted on ships,
aircraft, specialized buoys and other
platforms. Mobility or high spatial resolution, or both are
critical to satisfactory measurements in
the coastal zone. Most of the phenomena of interest propagate
either along or across the
coastline and vary on small space scales. Coastally-trapped
events and topographically-modified
flows can best be sampled by aircraft that combine remote-sensing
and in-situ techniques.
Overland and Bond [1993] have shown that when synoptic-scale
flow encounters coastal
topography, mesoscale features develop that have a scale of the
half-width of broad coastal
mountains, or the Rossby radius for steep topography. These
scales of 50 to 150 km are much
smaller than operational observing networks.