| 5.
Field Observing Systems 5.1
Radar Systems 5.1.1
NCAR S-Pol Research Radar The NCAR S-Pol research radar is a polarimetric
Doppler radar developed at NCAR/ATD. The radar system is designed to be
transported to the research field campaign location. The entire system
is packaged into standard sized sea containers that are used as the
antenna support structure at the research site. The structure and
pedestal are designed to work properly in winds up to 30 m/s. If needed,
a CP-2 inflatable radome can be used to enclose the radar. The S-Pol radar uses a high performance modern
transmitter, low sidelobe parabolic antenna, modern dual channel
receiver, and a NCAR designed VME data system. The transmitter, built by
Westinghouse, is an ASR-9 based unit using an air-cooled klystron and
produces a one-Megawatt, one-microsecond pulse. Polarization switching
is accomplished using a NCAR built mechanical switch that provides 49 dB
transmit isolation, which is comparable to a dual transmitter
configuration. Dual receivers, each providing 40 dB isolation is
required for cross-polar measurements. The 8.5 m reflector is a high
compliance aluminum structure providing –30 dB first sidelobes and at
least –35 dB integrated cross-polar isolation. Pulse pair and dual
polarization processing is performed by a NCAR designed VME-based
Integrated Radar Acquisition system (VIRAQ). The VIRAQ processor uses a
dual range 90 dB digital IF system with multiple C44 DSP chips that are
controlled by a 486 CPU host running DOS. Refer to Table 5.1 for a
detailed list of component specifications. Between 100-128 samples are need to obtain accurate,
reliable polarimetric measurements. Thus, it is recommended to use a PRF
of 960 Hz at a scan rate of 6 deg/sec.
This scan rate is needed to utilize the ground clutter filter. Table
5.1.
Technical specifications for the NCAR S-Pol radar.
The CSU-CHILL research radar is a portable
polarimetric, Doppler radar system stationed at Colorado State
University. The system is packaged into shipping crates and assembled
similarly to the S-Pol radar. The antenna for the system is housed
inside a CP-2 inflatable radome. The CSU-CHILL radar utilizes modern dual
transmitters, high performance parabolic antenna, dual receivers, and
programmable signal processor. The transmitting system consists of two
klystron-based transmitters, each producing ~1.0 megawatt, one
microsecond pulse. The antenna is an 8.5 meter, parabolic antenna with a
half-power beamwidth of one degree. The main beam and sidelobe patterns
at horizontal and vertical polarizations are well matched. The matched
dual receivers have noise power levels of approximately -115 dBm.
Information processing is handled by a programmable Lassen SP20 signal
processor and recorded to 8 mm Exabyte cassette tapes. See Table 5.2 for
a detailed list of specifications. Between 100-128 samples are need to obtain accurate, reliable polarimetric measurements. Thus, it is recommended to use a PRF of 960 Hz at a scan rate of 6 deg/sec. This scan rate is needed to utilize the ground clutter filter and match well with the S-Pol radar measurements. Table
5.2.
Technical specifications for the CSU CHILL radar.
5.1.3 NWS
WSR-88D Operational Weather Radar The WSR-88D (KHGX) radar stationed in League City, TX
will provide continuous surveillance scans of the research area. These
surveillance scans will consist of scans at a sequence of constant
elevation angles of radar reflectivity and radial velocities. Because
the radar is an operational radar, the scanning strategies for this
radar are not controllable by the research team and limited to three
Volume Coverage Patterns (VCPs). See Fig. 5.1 for a graphical
representation of scanning levels for the three main VCPs. A data
recorder will be installed at this facility during the field campaign. VCP-11 is a precipitation mode that uses a short
radar pulse giving a nominal 250 m range resolution. This mode has two
surveillance scans for reflectivity and Doppler velocities. The slow
rate of pulsing gives a 460 km unambiguous range and the fast rate of
pulsing for a 150 km unambiguous range and high Nyquist or folding
velocity exceeding 25 m/s. The fast pulse rate is used only in the low
scan angles. The precipitation mode consists of 360-deg-in-azimuth scans
at 14 elevation angles to obtain 5 minute updates of full volume scans.
The lowest seven elevation angles are contiguous and the remaining steps
between elevation angles exceed the nominal radar beamwidth of 0.95 deg,
leaving some gaps in the vertical coverage. VCP-21 is a very similar precipitation mode to the
VCP-11 mode. This mode uses a short radar pulse giving a nominal 250 m
range resolution. This mode has two surveillance scans for reflectivity
and Doppler velocities. The slow rate of pulsing gives a 460 km
unambiguous range and the fast rate of pulsing for a 150 km unambiguous
range and high Nyquist or folding velocity exceeding 25 m/s. The fast
pulse rate is used only in the low scan angles. The precipitation mode
consists of 360-deg-in-azimuth scans at 9 elevation angles to obtain 6
minute updates of full volume scans. The lowest five elevation angles
are contiguous and the remaining steps between elevation angles exceed
the nominal radar beamwidth of 0.95 deg, leaving some gaps in the
vertical coverage. VCP-31 is a clear air mode and is used to detect
early formation of convective precipitation, air mass discontinuities,
and to obtain wind profiles. It uses a long pulse transmission at 5
elevation angles to obtain a 10 minute update rate. The long pulse is
used to provide a greater sensitivity. There are separate surveillance
and Doppler scans at the lowest three elevation angles.
Fig. 5.1.
Graphs depicting elevation versus distance from radar for different scan
angles of the three different VCPs. 5.1.4
Radar Scanning Strategies The main priority is for the two research
radars to conduct dual polarimetric, dual Doppler radar sampling. Both
radars will be performing similar scans and integration times to obtain
high quality polarimetric and Doppler measurements. The scanning
strategy used by the radars needs to have both broad scale and fine
scale measurements of the storm dynamics and microphysics. To achieve
this goal a PPI-RHI scanning strategy, which toggles between PPI and RHI
mode, will be used. This strategy consists of a ~5 minute storm volume
scan across azimuthal sectors at a sequence of constant elevation angles
(PPI). Following this, a ~2 min storm volume scan in the elevation angle
direction at a sequence of constant azimuth angles (RHI) will be
conducted. This will give a repeat cycle of ~7 min for a complete
PPI-RHI scan sequence. This scan sequence will be repeated continuously
except during aircraft penetrations. The RHI scan will be focused on the storm
section that is of primary interest. In this case, the primary interests
would be updrafts and electrically active regions. This can be
determined from radar, aircraft, electrical measurements, and field
reports. Specific placement of the PPIs and RHIs will be determined with
the help of scan optimizers and some "canned" scans determined
before the start of the field study. The NWS Doppler radar (KHGX) will be used
for surveillance scans to keep track of the larger scale development of
storms in the region. The data from KHGX will also be incorporated into
the data set to allow triple Doppler analysis in certain areas of the
study region. Additional near real time data from nearby NWS radars will
also be available in the operations center for planning purposes. 5.2 Surface Mesonet Systems [Table of Contents] 5.2.1 TNRCC Measurement Stations The Texas Natural Resource Conservation
Commission (TNRCC) maintains a set of surface air monitoring stations in
southeast Texas. These stations are mainly for atmospheric pollution
monitoring. There are approximately 40 stations with a majority of them
in the Houston metro region. Figure 5.2 shows the location of most of
the sites in southeast Texas. Figure 5.3 is the zoomed map of Houston
showing the location of the various TNRCC monitoring stations. Each
monitoring station does not measure the same atmospheric variables, but
many measure trace gas concentrations or aerosol concentrations. The
three variables that each station does measure include temperature,
moisture, and horizontal wind. To obtain information on the variables
monitored at each station, please refer to the following web address:
http://www.tnrcc.state.tx.us/cgi-bin/monops/select_month?region12.gif. TNRCC performs regular maintenance of all
monitoring stations and performs thorough quality checks on all data.
The data from the stations will be available from TNRCC as an archived
set after the finish of the HEAT field campaign. The operations center
can obtain access to rough data through TNRCC’s website if needed.
Fig.
5.2.
Map of southeast Texas showing the location of TNRCC air monitoring
stations. Houston area denoted in yellow.
Fig.
5.3.
Close up map of Houston depicting the location of air monitoring
stations. Houston urban area denoted in yellow. 5.3
Aircraft The University of Wyoming King Air aircraft is a research craft with several meteorological instrument packages for boundary-layer, turbulence/flux, cloud physics, and atmospheric chemistry studies. This craft is not armored like the T-28, thus is not capable of penetrating strong convective regions with hail. This craft is still capable of penetrating storms and taking microphysical measurements of various cloud and precipitation particles. The main measurements that will be taken are: pollution concentrations near sea breeze and over Houston and supercooled liquid water content. Table 5.4 is a listing of the instruments that will be included on the aircraft. Table 5.4. Wyoming King Air instrument packages with instrument measurement range, accuracy, and resolution.
5.3.2 WMI LearJet [Table of Contents] WMI's (Weather Modification Inc.) Learjet is a high performance aircraft used for weather modification and atmospheric research. The craft's maximum altitude of 45,000 ft and speed of Mach 0.83 allow the craft to operate near thunderstorm tops and respond quickly to targeted storms. Fitted with strengthened engine nacelle inlets to minimize damage due to severe weather, the aircraft is capable of storm penetration. Below is a list of sensors that are currently on (regular font) and need to be added (bold font) to the aircraft.
5.3.3 Airborne Chemistry Instrumentation Chemistry instrumentation will be placed on board the University of Wyoming's King Air Research Aircraft (UWKA) and the Weather Modification, Inc. (WMI) Lear jet. Below is a table of the proposed instrumentation used to measure the particular chemical species. Table 5.5. Chemical instrumentation to be placed on board the UWKA and WMI Lear jet.
5.4.1
MGLASS Units (2) The NCAR Mobile GLASS (MGLASS) facility is
a completely self-contained unit that can be carried by any full-size
pickup truck. The basic system contains all the hardware required to
make up to four simultaneous atmospheric soundings, including equipment
to make supporting surface meteorological observations. The mobility
gives the project planner the option to deploy to a specific site, make
a sounding, and if required move to another site for the next sounding.
The first sounding can be active and in the air while the truck is
mobile, although this does affect sounding quality. Sounding site
station elevation values are typically taken from a topographic map. If
that is not available or if the location is not absolutely certain, a
calibrated aircraft pressure altimeter can be used. The sondes used by the MGLASS units are Vaisala brand
rawinsondes. The rawinsonde package includes a 403 MHz band transmitter,
GPS receiver, and pressure, temperature, and relative humidity sensors.
Table 5.6 lists the specifications for the devices in the rawinsonde
package. Both the thermodynamic and navigation signals are transmitted
roughly every 1.5 seconds to the MGLASS data system to be processed and
archived. In addition to the standard rawinsonde package, a CCN
concentration-measuring package will be added to obtain CCN
concentration profiles in storm environments. The instrument was
developed at the University of Wyoming Laramie to obtain improved
vertical profiles of CCN. The CCN counter is similar to other static
thermal-gradient diffusion chamber instruments, except that it utilizes
a lightweight photodetector, instead of a charged coupled device (CCD).
The instrument uses voltage measurements from the photodetector to
relate the amount of scattered light to the CCN concentration
approximately every 30 seconds. The actual value of CCN concentration is
determined during post processing. From calibration studies, the
estimated accuracy is estimated to be +/- 15% for a supersaturation of
1%. The standard GLASS data system consists of a power
supply, an RS-232 Multiplexer (MUX), a rack controller, a 403 MHz
receiver, a MWG201 GPS navigation data processor, a NCAR RS-80
meteorological data processor, and a personal computer. Information is
transferred to and from the GLASS personal computer through RS-232
connections using the MUX, which switches between the navigator, the Met
processor and the Campbell data logger to gather the data required to
process and display the atmospheric soundings. Table 5.6. Instrument specifications for the MGLASS
rawinsonde package.
Mobile electrical balloon units will be modeled after
the MGLASS units. The mobile facilities will consist of a mobile balloon
facility housed in a standard conversion van and a minivan for
additional crew members. The balloon facility will contain all the
necessary equipment to conduct atmospheric soundings and an electric
field mill to measure the static electric field. If possible, sequential
soundings will be made from a fixed location as the storm moves, in part
because data reception is best when the mobile laboratory remains
stationary. However, data can be acquired as the vehicle moves, if
necessary either for safety or for targeting another part of the storm. The crew will have nine members, one to remain
available for communication with the operations center during launch,
one to monitor balloon data reception, five to launch the balloon and
instruments, and two to begin inflating the next balloon. Balloons will
be launched from a portable launch tube, which has been used for
launches in wind with speeds up to 75 mph (65 knots). Each flight will carry one electric field
meter and a rawinsonde. Forty electric field meters will be available
for flights to measure the electric field inside storms. The mobile
laboratory will house receivers to acquire data from up to four electric
field meters simultaneously. Balloon tracking and standard thermodynamic
measurements will be similar to the NCAR MGLASS system, which can
process data from up to four sondes simultaneously. The data produced
from each flight are the electric field, pressure, temperature, dew
point, horizontal wind speed and direction, ascent rate, time, latitude,
longitude, and altitude at approximately 1-s intervals along the balloon
track. In addition, the electric field will be measured at the ground
during periods of interest. 5.4.3
TAOS Units The NCAR Tethered Atmospheric Observing
System (TAOS) is a balloon born, lower atmospheric sounding system. This
setup allows a balloon to be deployed up to height of 1 km and brought
back down to ground when observations are finished. This permits the
sensor packages to be used repeatedly, unlike normal rawinsonde setups.
The basic instrument package, Vaisala rawinsonde, installed on the
tether measures temperature, relative humidity, wind speed, and wind
direction (See Table 5.6 for more information). Each sensor reports at a
one second sampling rate. The sensor package is powered by a common cell
phone battery, which typically last for five hours. With a two balloon /
winch system, operations in wind speed up to 45 mph are possible. 5.4.4
Upper-Air Sounding Station Due to the lack of an Upper Air sounding station near
Houston, a temporary station will be established for the field campaign.
The tentative location for the balloon launch facility will be the NWS
office in League City, TX (~15 miles southeast of downtown Houston). The
instrument package will be the Vaisala GPS rawinsonde that is also used
in the mobile balloon units. See Table 5.6 for a detailed list of
rawinsonde specifications. The facility will conduct soundings at the
standard twelve-hour intervals (00Z and 12Z). During intensive
operations periods, the Operations Director may request soundings to be
conducted at three or six hour intervals. The data will be immediately
accessible to both the League City Weather Forecast Office and the HEAT
Operations Center. The data will be useful for both real-time
forecasting and post analysis. 5.5
Wind Profiler System Doppler Sodar systems, built by Remtech, Inc., will
be deployed around the Houston area to obtain lower tropospheric wind
profiles. These profilers are capable of reliably measuring horizontal
wind speed and direction and vertical motions. In ideal atmospheric
conditions, the sodars can obtain measurements up to 1.5 km, but the
normal range is approximately 1 km. These measurements are accomplished
by emitting a strong acoustic pulse vertically. The system then detects
the Doppler frequency shifts in the backscattered echo, due to thermal
turbulence in the atmosphere. These frequency shifts and relative return
strengths are then processed to obtain a vertical wind profile. A list
of the systems specifications can be found in Table 5.7 below. The system only requires one phased array antenna to
operate. A total of three electronically steered beams are used to
achieve a three dimensional wind. Two of the beams are tilted 30 degrees
from vertical and turned 90 degrees from each other to provide the
horizontal component of wind velocity. The third beam is pointed
vertically and provides that component of the wind. The system software
controls the sequence and rate of operation for each beam. Table 5.7. Technical Specifications of the Remtech Sodar.
5.6.1 Lightning Detection and Ranging (LDAR II) Network The LDAR II network will be deployed over a 100-km diameter area both within and around Houston, Texas and will consist of twelve time-of-arrival Lightning Detection and Ranging (LDAR-II) stations. The network, manufactured by Vaisala-Global Atmospherics, Inc., will be capable of mapping the lightning in three spatial dimensions and time. Figure 5.5 displays the approximate locations of the LDAR II stations relative to the area under investigation. The electric field instruments will measure the overall charge structure of storms and the amount and sign of the charge transferred from lightning. For detecting and locating ground strokes and intracloud discharges, the 'fast' electric field change waveform recording stations, CSU three-station flat plate antenna network, will be used. The LDAR II network operates by detecting RF radiation in the VHF electromagnetic field signature of lightning events in a bandwidth of 5 MHz in the 50 to 150 MHz frequency range. Discharges from initial breakdown processes to subsequent charge transfer are measured with an accuracy of 110 nanoseconds relative to the other sensors in the network using a clock that is synchronized via a Global Positioning System (GPS) receiver. To provide redundancy and provide a larger area of measurements, twelve stations will be used in the LDAR II network. The LDAR network will be capable of producing large amounts of data in a short period of time due to extremely fast electronics. High-resolution systems provide the position of all possible events in real time and require a maximum data rate of 300 kbps. All time-related information is sent to the Location Processor in real time and the solutions are displayed. Detailed position and density maps of lighting channels will be generated, resulting in up to 10,000 source locations per second. If we choose to require amplitude and diagnostic information then the maximum data rate required will be 600 kbps.
Fig. 5.5. Approximate location of LDAR II stations (green circles) in the HEAT campaign area. Purple box denotes the area of the study considered as the 'urban area' 5.6.2
National Lightning Detection Network The National Lightning Detection Network (NLDN) managed by Vaisala-Global Atmospherics, Incorporated will be utilized to detect and record cloud-to-ground (CG) lightning activity. The network consists of 106 Time-Of-Arrival (TOA) sensors with approximately half containing Direction Finder (DF) sensors. This nationwide network monitors location, polarity, current, and multiplicity of each flash. The NLDN detection efficiency for SE Texas is typically 80-90% of all CG flashes, with a location error of less than 500 m. Fig. 5.6 is a map of the NLDN sensor type and locations. Real time NLDN information will be available at the Operations Center to assist in identifying and tracking electrical activity within storms. Additionally, an archive of lightning data for the region will be obtained from Global Atmospherics, Inc. for research purposes.
Fig 5.6.
Map of NLDN sensor locations and type (IMPACT -
Improved Performance from Combined Technology; TOA - Time Of Arrival)
for the continental United States. 5.7
Additional Instrument Information Below is a list of web addresses for further
information on the various instruments used in the HEAT field campaign. NCAR S-Pol
Research Radar: http://www.atd.ucar.edu/rsf/spol/spol.html CSU-CHILL
Research Radar: http://chill.colostate.edu/ TNRCC
Measurement Stations: http://www.tnrcc.state.tx.us/air/monops/index.html CCN
Concentration Instrument: http://www-das.uwyo.edu/ccp/documents/ccn_instrument.pdf
SDSMT
Armored T-28: http://www.ias.sdsmt.edu/institute/t28/ University
of Wyoming King Air: http://flights.uwyo.edu/ WMI Lear Jet 35A: http://www.weathermod.com/aircraft_lear.htm MGLASS
Units: http://www.atd.ucar.edu/sssf/facilities/class/class.html TAOS Units:
http://www.atd.ucar.edu/sssf/facilities/taos/taos.pdf Wind
Profiler System: http://www.remtechinc.com/sodidx.htm
Lightning Mapping Array: http://www.glatmos.com |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
