TITLE: Profiler/Sodar ABLE Radio Acoustic Sounding System (RASS) [ABLE]
CONTACTS:
Richard L. Coulter - ER 203
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
Voice: 630 252-5833
Fax: 630 252-5498
Email: rl_coulter@anl.gov
1.0 DATA SET OVERVIEW
This data set contains hourly profiles of virtual temperature from
the Atmospheric Boundary Layer Experiments (ABLE) Radio Acoustic Sounding
System (RASS) operated by the Argonne National Laboratory in the Walnut
River Watershed in Butler County Kansas (east of Wichita). During the
IHOP period three RASS were in operation. Data cover the period from 13
May to 25 June 2002 The data are in ASCII format.
2.0 INSTRUMENT DESCRIPTION
2.1 Instrumentation
The ABLE 915 MHz radar wind profiler with RASS: Three of these profiler
systems are deployed within the ABLE region. Profiler locations are
Whitewater, Oxford, and Beaumont, KS.
General Purpose
The radar wind profiler/RASS (RWP) measures wind profiles from (nominally)
0.1 km to 5 km and virtual temperature profiles from 0.1 km to 2.5 km. It
operates by transmitting electromagnetic energy into the atmosphere and
measuring the strength and frequency of backscattered energy. Virtual
temperatures are recovered by transmitting an acoustic signal vertically and
measuring the electromagnetic energy scattered from the acoustic wavefront.
The propogation speed of the acoustic wave is proportional to the square
root of the virtual temperature.
Primary Quantities Measured with System
The primary quantities measured with the system are the intensity and Doppler
frequency of backscattered radiation. The wind speed is determined from the
Doppler frequency of energy scattered from refractive index fluctuations
(caused primarily by moisture fluctuations but also by temperature
fluctuations) embedded within the atmosphere; the virtual temperature is
determined from the Doppler frequency of microwave energy scattered from
acoustic energy propogating through the atmosphere.
Detailed Description
List of Components
The 915 MHz radar wind profiler is manufactured by Radian Corp. It consists
of a single-phased microstrip antenna array consisting of either four
(Whitewater and Oxford) or nine (Beaumont) "panels". The antenna is
approximately 2 or 4 m square and is oriented in a horizontal plane so the
"in-phase" beam travels vertically. Other components in the system include
four stationary acoustic sources located at the corners of the antenna, a
receiver, an interface module, and a computer for data analysis and processing.
Description of System Configuration and Measurement Methods
The radar wind profilers operate by transmitting in two different vertical
planes and receiving backscattered energy from refractive index fluctuations
that are moving with the mean wind. By sampling in the vertical direction and
in two tilted planes, the three components of motion can be determined. The
Beaumont and Whitewater systems consist of a single phased array antenna that
transmits alternately along five pointing directions: one vertical, two in the
north-south vertical plane (one south of vertical, one north of vertical), and
two in the east-west vertical plane (one east of vertical, one west of
vertical). The non-vertical beams are tilted at about 23 degrees from vertical.
The Oxford system has only three pointing directions (one south of vertical,
one west). Radial components of motion along each pointing direction are
determined sequentially. It takes, nominally, 30 - 45 seconds (dwell time) to
determine the radial components from a single pointing direction. Thus, at
the Beaumont and Whitewater sites the system cycles through five beams (South,
North, East, West, and vertical) at low power, and then cycles the five beams
again at a high power (longer pulse length) setting. Then the whole process
is repeated. About five minutes elapse before the system returns to the
beginning of its sequence. Within an averaging interval, the estimates from
each beam-power combination are saved (11-12 in a 1-hr period) and these
values are examined and compared at the end of the period to determine the
consensus-averaged radial components of motion. The Oxford system is slightly
different. When operating in multipower mode it averages in the low power mode
for 25 minutes, then in the high power mode for 25 minutes.
Briefly, consensus averaging consists of determining if a certain percentage
(e.g., 50%) of the values fall within a certain range of each other
(e.g., 2 m/s). If they do, the median of those values is used to produce the
radial wind estimate. The radial values are then combined to produce the wind
profile. The results of this averaging process are what are reported in the
".cnw" data files produced by the ABLE data system. Included in these files
are height, speed, direction, radial components, # values in consensus, and
signal-to-noise ratio (SNR).
During a single time period during which the system operates in a single
pointing direction (dwell time), the data that is produced in the ".mod" and
".spd" files is created. The system transmits pulses at about 1-10 kHz rate
into the atmosphere. The backscatter from each transmit pulse is sampled at,
for example, a 1 MHz rate. This results in 1 sample every 150 m in range. The
samples at each range gate are averaged together (time domain integration)
over some number (e.g., 100) of pulses to produce a phase value for input
into a FFT. After (e.g., 64) values are produced, the FFT is performed (one
for each range gate). This process takes on the order of 1 sec. A number
(about 30) of these spectra are then averaged together during the dwell time.
At the end of the dwell time we have produced a single averaged spectrum from
each range gate along the designated pointing direction. The spectra
themselves are placed in the ".spd" data files.
The spectra are analysed by the system before moving to the next pointing
direction. This analysis produces estimates of the snr, the noise, the mean
velocity (proportional to frequency), and the first moment (spectral width)
at each range gate. This is the information that is stored in the ".mod" data
files. Both the ".mod" and ".spd" data files thus have information at about
(dwell time) intervals; however, the data sequences among pointing directions
and output powers.
A note of warning about the mean values in the ".mod" files. The values are
in % of full scale times 100, where full scale is the nyquist velocity of
the spectrum. Thus, velocity estimates are determined by multiplying the
"mdf" column times the full-scale bandwidth and dividing by 10,000.
RASS operation is essentially the same, except that the averaging time is
about 10 minutes and only a single pointing direction (vertical) is used.
Also, the atmosphere is "seeded" with a sound wave; the index of refraction
changes created by the sound wave are the signal source. In order to sample
both the sound wave (speed about 340 m/s) and the atmosphere (to remove air
velocity from temperature estimates) a larger FFT is required (2048 points).
This requires a smaller number of points for each time domain integration
and increases the processor time required to calculate the FFT. The ".spd"
files again are spectra; however, only a portion of the spectra are
reported, namely a region near 0 Doppler shift to account for atmospheric
motions and a region around the expected speed of sound. The ".mod" files
now consist of moments and widths from both the atmospheric portion of the
spectrum and from the acoustic portion (the main contributor to the
temperature calculation). The ".cnt" files consist of profiles of
temperature and number of consensus values.
In normal operation, temperature profiles are determined during the first
10 minutes of every hour and the wind profile is averaged over the remaining
50 minutes.
2.2 Station Locations
UTM km (Zone 14)
Site 99 deg Meridan DEG dddmmmsss ddmm.mm Alt (m)
---------------------------------------------------------------------------
Beaumont, KS 4167.1 UTMN km 37.627 37 37' 38" 37 37.64' 478
717.2 UTME km 96.538 96 32' 19" 96 32.32'
Oxford, KS 4126.7 UTMN km 37.273 37 16' 25" 37 16.41' 360
668.8 UTME km 97.095 97 05' 45" 97 05.75'
Whitewater, KS 4189.5 UTMN km 37.841 37 50' 26" 37 51.43' 416
659.6 UTME km 97.186 97 11' 10" 97 11.16'
For file names the station IDs are as follows:
Beaumont be
Oxford ox
Whitewater wh
Topo maps and aerial photos are available at:
gonzalo.er.anl.gov/ABLE/sitelatlon.html
3.0 DATA COLLECTION AND PROCESSING
UCAR/JOSS conducted no processing or quality control on these data.
4.0 DATA FORMAT AND FILE NAMING
4.1 Data Format
These data are in ASCII format.
Temperature Profile File Format:
Line Description
---- -----------
1 Station
2 Version
3 N_ Lat W_Long Site_elevation_meters
4 yr mo da hr mi sc minutes_to_add_for_UT
5 Ave_time_min Number_of_beams Number_of_range_gates
6 Samples_required_for_consensus:Total_samples (consensus_window_size-m/s)
7 Number_of_coherent_averages
Number_of_spectral_averages
Pulse_width_ns
Inter-pulse_period_ms
8 Full-scale_Doppler_velocity_m/s
Delay_to_first_range_gate_ns
Number_of_range_gates
Spacing_between_range_gates_ns
9 Azimuth elevation
10
11 Range_gate_height_AGL_km
Uncorrected_RASS_temperature_consensus_degC
Corrected_RASS_temperature_consensus_degC
Vertical_Wind_speed_consensus
Number_of_samples_returned_for_T_Tc_W
Mean_SNR_db_for_T_Tc_W
End $
All times are UTC.
4.2 File Naming conventions
wh_rass020602.cnt
where:
wh is the station ID (here Whitewater)
rass is the data type (RASS)
02 is the year (2002)
06 is the month (June)
02 is the day of the month
5.0 DATA REMARKS
None.
6.0 REFERNCES
ABLE Home Page: gonzalo.er.anl.gov/ABLE/index.html