About Profiler Data
Wind Profiles
The wind profilers in the NOAA Profiler Network (NPN) operate
continuously, alternating sampling modes every 1 minute between
a low or high mode, and switch beam positions (eastward,
northward, or vertical) every 2 minutes. Each mode contains 36
range gates (sampling heights), spaced every 250 m in the
vertical. The low mode samples the lower atmosphere, beginning
at 500 m above ground level (AGL) and continues to 9.25 km AGL.
The high mode slightly overlaps the top of the low mode,
beginning at 7.5 km AGL and extends to a maximum height of 16.25
km AGL.
The winds displayed at each height are an hour average from
the preceding hour of data, e.g., 6-minute data acquired
between 1600-1700 UTC would be averaged and displayed with
the time stamp of 1700 UTC. The displayed winds have all
"passed" our single- station quality control (continuity)
requirements and typically represent high quality data. In
the overlap region between modes (7.5-9.25 km AGL), wind data
from the high mode are displayed (unless they have failed QC,
in which case low mode data that passed QC will be displayed,
if available). The hourly winds and RASS temperatures from
the NPN are available here on the Web about 15 minutes
after the hour.
See our NPN/CAP Real-Time Data Displays.
Spectral Moments
Wind Profilers detect minute fluctuations in atmospheric
density, caused by the turbulent mixing of volumes of air with
slightly different temperature and moisture content. The
6-minute spectral moments data are from the vertical beam, with
no QC applied. Data from the high mode are displayed in the
overlap region.
The vertical velocity of the atmosphere is directly measured
by each profiler, and reported in the range interval of about
+/- 12 m/s. The velocity is positive when the motion is
toward the ground (as during precipitation). Most
meteorological motions are in the range of -3 to +8 m/s, with
the vast majority near 0 m/s. For this reason, and to improve
the visual aspect of the velocity resolution available on the
Web, we have chosen to display velocity data only in the
range of -3 to +8 m/s. Velocities outside this range are
typically not valid, e.g., random velocity estimates from
weak scattering regions of the atmosphere.
The signal power is a measure of the amount of backscattered
power received from the atmosphere. High signal power values
(greater than 60 dB) are typically associated with high
moisture content or the presence of precipitation particles,
while low power values (less than 40 dB) usually indicate a
dry or stable atmosphere. Meteorological features such as
moisture advection and cloud layers, vertical extent of
convection, and bright bands (inferring the 0 degree C level)
are all visible at times in the signal power displays.
For further reading, and examples of precipitation
(stratiform and convective), the bright band, and jet stream
cirrus, please refer to the cover article from the October
1995 issue of the Bulletin of the American Meteorological
Society titled, "Using Spectral Moment Data from NOAA's
404-MHz Radar Wind Profilers to Observe Precipitation", by
F.M. Ralph, P.J. Neiman, D.W. van de Kamp, and D.C. Law.
When the wind profilers operate in the high mode, a longer
transmitted pulse (increased average power) is used.
Therefore, sensitivity in the high mode is increased (by a
factor of about 40 (16 dB)). This accounts for the horizontal
boundary that is visible in all signal power displays at 7.5
km AGL. Signal power is not corrected for the loss due to
range in our displays. The 6- minute spectral moments
displays become available here on the Web about 30-45 minutes
after the hour.
Other features that may be observed in the spectral moments
displays are a result of non-meteorological causes:
1. Vertical strips of low signal power and (usually) random
velocity estimates. Wind profilers have the potential to
cause radio interference to six satellites that carry
sensitive receivers used to locate low-power emergency
beacons from downed aircraft and ships in distress. To
minimize the possibility of interference, the transmitter at
each profiler site is turned off whenever any of the six
polar-orbiting satellites are scheduled to pass nearby. The
transmitter inhibit duration is 6 minutes, and occurs about 7
times daily (varying between 4 and 10 times) for each site in
the network.
2. Isolated points or vertical strips of high signal power.
Discontinuous high signal power in time are usually caused by
aircraft flying through the main beam and/or a sidelobe
sometime during the 1 minute dwell time in each mode.
3. Low signal power in the lowest few gates. Sensitivity Time
Control (STC) is used to attenuate the very strong signals
returned from the lower atmosphere and close-by ground
clutter targets in the first few range gates. Without this
signal attenuation, the profiler's receiver would routinely
saturate due to the strong signals and produce incorrect wind
measurements.
4. Nearly continuous vertical or horizontal strips of
relatively high signal power. Vertical strips are usually
associated with internally generated interference. Horizontal
strips are usually associated with a ground clutter target,
or an artifact generated by the transmitter (and sometimes
used in an attempt to "break up" patterns of internal
coherent interference).
5. Three horizontal strips in radial velocity. This indicates
use of our backup communications (via GOES) with its limited
capability. Hourly winds will be available, along with
minimal 6- minute radial velocity data, and no signal power
data will be available when the backup communications are
used.
RASS
- Radio Acoustic Sounding System
|
The displayed hourly averaged RASS virtual temperatures
have all "passed" our single-station QC requirements
and typically represent high quality data. The
low-level wind velocity and acoustic attenuation
typically limit the height coverage of RASS data.
Information about the Bird Identification Algorithm
used to mark winds that may be contaminated by
migrating birds.
|
|
| 
