Observing Systems by Technology
Contact: Al Bedard
Infrasonics is the study of sound below the range of human hearing. These low-frequency signals are produced by a variety of geophysical processes including earthquakes, severe weather, volcanic activity, geomagnetic activity, ocean waves, avalanches, turbulence aloft, and meteors and by some man-made sources such as aircraft and explosions. Infrasonic and near-infrasonic signatures may provide advanced warning and monitoring of these extreme events. Infrasonic sensors invented at PSD are the basis for a demonstration network which has detected avalanches, tornados, and sprites. They are also used to study the propagation of sound which can be used to address such issues as airport and industrial noise.
The University of Colorado and NOAA PSD* Air-Sea Interaction Group have spent more than a decade developing techniques for accurate direct covariance ship-based measurements of the turbulent heat, mass, and momentum fluxes (Fairall et al., 1996a; 1997). Improved measurement techniques, studies of fundamental physical processes, and development of simplified representations of flux processes are essental to improving analyses and forecasts of weather, climate, and environmental conditions over the oceans and in coastal zones.
Flux instrument packages are configured to meet operational and experimental goals. Observations generally include the following.
Radar wind profilers transmit pulses of radio energy which scatter off refractive index inhomogeneities in the atmosphere. In the clear air these inhomogeneities are typically caused by variations in temperature or humidity of atmosphere. By measuring the Doppler shift, the wind speed and direction can be calculated as a function of height.Profilers designed at different frequencies can observe regions of the atmosphere from the boundary-layer to the stratosphere. Combining profiler data with measurements taken by an acoustic sensor call a RASS (radio acoustic sounding system), temperature profiles of the atmosphere can also be derived.
Platteville 50-MHz Rardar Wind Profiler with RASS
This PSD* radar was built in the mid 1980's and is part of the first "operational" network of wind profilers (the "Colorado Network") that predates the present Wind Profiler Demonstration Network (WPDN) in ESRL's Global Systems Divison (GSD). It is collocated with the 404-MHz WPDN profiler at Platteville, Colorado, and it continues to operate unattended, providing wind profiles to l8-km ASL and temperature profiles to 6-km AGL. The radar is often used by meteorologists as part of local research projects and by the University of Colorado in meteor trail studies. It is also used by SDID staff as a test bed to validate and intercompare with other sensors as part of ongoing advancement of technology.
915-MHz Radar Wind Profiler with RASS
This instrument is nearly identical to the Platteville 50-MHz instrument in cost of maintenance and operation. Its height coverage is not as great as the 50-MHz system, but it is used more frequently by the National Weather Service (NWS) and FSL for regional forecasts. It is the most sensitive 915-MHz system in existence. Because it is located next to the NWS rawinsonde release point, it is frequently used for sensor intercomparisons by a number of agencies.
915-MHz Profiler Network
ETL has developed a network consisting of 14 portable 915-MHz radar wind profilers with RASS. Data transmission and communications are accomplished via phone lines and satellite systems to a central data hub in Boulder, Colorado. These profilers are used in a variety of ways including individual installations to extend the technology in new environments, deployment as a mesoscale research network, and acquiring data for initialization of test models for air-quality transport studies.
Radar Acquisition and Display System (RADS)
The Radar Acquisition and Display System (RADS) is a configurable system designed to record and display data from a research Doppler radar with polarization diversity. Constructed primarily from commerically available components, the VME-based processor controls radar operations, data acquisition and display through a graphical interface. Incorporating GPS navigation directly into the data stream, it can be configured for ground, aircraft and ship based radars. The programmable processor allows for real-time calculation and display of custom data fields and monitoring of data quality.
ETL designs and operates high-performance, transportable, Doppler radars for atmospheric and oceanic research. These radars use higher frequencies (shorter wavelengths) and are smaller than storm surveillance radars. Our radars have been used extensively in field research projects at dozens of locations in the United States and other countries. The radars were designed and built in-house and are in a continuous state of development as latest state-of-the-art capabilities are added. The Ka-band "cloud" radars have superb sensitivity that allows them to detect tiny cloud particles in addition to precipitation. In combination with radiometers, these cloud radars are used to estimate microphysical features of the nearby clouds, including ice crystal and water droplet sizes and mass contents. Polarization measurements from the scanning cloud radar allow the particle shapes and types (plate crystals, column crystals, droplets, etc.) to be identified in the clouds. Polarization capabilities recently implemented in the scanning X-band "hydro-radar" allow new possibilities for more accurate estimation of rainfall and snowfall rates. Both scanning radars have also been used for studies of the ocean surface and both are equipped with the new RADS processor that was designed at ETL.
NOAA D 9.3-GHz Atmosphere & Ocean Radar
ETL developed this state-of-the-art X-band radar primarily for observations of the ocean surface, rain, snow, storm airflow patterns, and for hydrological applications. It has Doppler, dual- polarization, and full scanning capability, including the ability to scan downward beneath the horizon for ocean work. Fine-scale measurements are possible with selectable range resolution from 7.5 to 150 meters. Polarization options include switching between H and V, or using the "split" H/V, configuration that has been proposed for future NEXRAD upgrades. The polarization measurements include differential phase (Kdp), and differential reflectivity (ZDR), which can used for more accurate estimates of rainfall rate and identification of precipitation particle types. The radar uses ETL's new Radar Acquisition and Display System (RADS), which allows various options for scan control and computing derived parameters in realtime. The radar is transportable in North America on its own trailer bed or it can be shipped overseas in standard sea containers. ETL engineers are working toward implementing fully automated, unattended operation and remote control of this system.
NOAA/K 35GHz Scanning Cloud Radar
ETL has developed a Ka-band (8.7-mm wavelength) system designed primarily for observations of non-precipitating and weakly precipitating clouds. By virtue of its short wavelength, it has excellent sensitivity to very small hydrometeors and is insensitive to ground clutter. The radar has been used extensively for research of the radiative effects of clouds for climate change programs and for observations of winter storms. A rotating quarter-wave plate allows transmission of a continuous sequence of polarizations from circular to elliptical to linear to examine hydrometeor types. The radar transmits 85-kW of peak power in a 0.5-degree conical beam width using a 1-m parabolic antenna with an offset Cassegrain feed. Radial velocity, reflectivity, and depolarization are measured at 256 gates with 37.5m resolution using PPI and RHI or fixed beam scans. Sensitivity is about -30 dBZ at 10-km range.
In another configuration H,V, or slant-linear dual-polarization is available with a 1.8 meter antenna for even greater sensitivity. The radar uses ETL's new Radar Acquisition and Display (RADS) system.
NOAA Portable Cloud Observatory (NPCO)
The NPCO is a unified single unit that combines active and passive remote sensinginstruments for observing clouds overhead. It was formerly called the MMCR package. This observatory is housed in a standard 20-foot sea container for automated operations on land or on research ships at sea. The heart of the NPCO is a 35-GHz cloud radar that is identical to the MMCR cloud radar built by ETL for the U.S. Department of Energy. This radar is able to detect extremely weak clouds overhead with resolution as fine as 45 m. Dual-channel microwave radiometers and a narrow-band infrared radiometer also point vertically along side of the radar s beam. Standard surface meteorological instruments complete the suite of sensors in the container. Estimates of cloud microphysical properties, such as vertical profiles of hydrometeor median size, total concentration, and mass content are computed from the combined radar and radiometer data using retrieval techniques developed at ETL.
ARM Millimeter-Wave Cloud Radar (MMCR)
ETL designed and built the MMCR in the 1990's for the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program to monitor cloud conditions over Cloud and Radiation Testbed (CART) sites. Using data from many instruments at these sites, scientists are examining how clouds affect climate and climate change through their interactions with radiant energy in the atmosphere. The MMCR is an unattended 35-GHz Doppler radar that has the ability to detect extremely weak clouds as well as precipitation overhead. It uses a 6-ft or 10-ft antenna and sophisticated signal processing methods to attain exceptionally good sensitivity and a low-power transmitter for dependable long-term operations. Height resolution of the data is as good as 45 m. The first MMCR was installed at the site in Oklahoma in 1996, followed by others in Alaska and the tropical western Pacific. An upgraded data system and dual-polarization hardware are now being constructed at ETL for the ARM MMCRs.
Ron Brown Scanning Precipitation Radar
NOAA/ETL serves as instrument mentor for the Doppler C-band radar which was built and installed by Radtec Engineering, Inc.. The instrument is available to principle investigators for a wide variety of marine studies sponsored by NOAA and other agencies. C-band represents a compromise between more heavily attenuated higher radar frequencies, such as X-band, and the larger size and weight requirements of lower frequency S-band weather radars, such those used for the land-based WSR-88D (NEXRAD) systems. In many respects, this radar is the equivalent of an oceangoing NEXRAD that can provide research-quality observations, in addition to routine storm surveillance. Rain statistics at sea derived from its data are well suited for evaluating assumptions used in satellite precipitation algorithms.
S-band Precipitation Profiler
The S-band vertical profiler is based on existing S-band and UHF profiler technology which has been modified for research. It's dynamic range has been extended to study moderate to heavy precipitation which would not be otherwise possible. The S-band has been calibrated through a side-by-side comparison with the Ka-band radar. In a typical cloud profiling mode of operation, the sensitivity is -14 dBZ at 10 km. Examples taken from a recent field campaign illustrate the profiler's ability to measure vertical velocity and radar reflectivity profiles in clouds and precipitation.
Radiometers are passive instruments which receive energy signals that are naturally emitted from objects within the instrument's viewing angle. A radiometer antenna pointed upward into the air receives infrared and radio frequency emissions from the atmosphere's various chemical constituents. Each constituent possesses a unique emission spectrum that corresponds exactly to its absorption spectrum. Radiometers "listen" at selected frequencies to best sort out the constituents and measure their abundances.
Airborne Microwave Radiometers
Ground-Based Microwave Radiometers
Ground-Based Infrared Radiometers in Operation