4.0 The ASTER Research Facility

4.1 Introduction

The Atmosphere/Surface Turbulent Exchange Research (ASTER) facility is a portable micrometeorological facility designed to support research in the atmospheric surface layer. State-of-the-art sensors coupled with advanced data acquisition, analysis, and display provide the ability to measure and analyze the turbulent structure of the atmospheric surface layer. The facility provides capability for measurement of surface fluxes of momentum, sensible heat, and water vapor. By incorporating user-supplied chemical analyses, ASTER can also provide measurement of surface fluxes of trace chemical species. In addition to the flux measurements, ASTER can provide low level profiles of wind, temperature, and humidity. Radiation and soil measurements are also available to complete the surface energy budget.

The ASTER facility has been designed to aid scientists in the investigation of the interactions between the atmosphere and the surface. Such interactions include: emission and deposition fluxes, including deposition of atmospheric acidity and oxidants; transfer processes to and from specific plants or plant canopies; cycling of nutrients; and the mechanisms by which and the extent to which the biosphere controls or affects the atmospheric composition and structure.

The ASTER facility has been designed to be extremely flexible with modular components that allow each deployment to be specifically adapted to the needs of that particular project. The system is easily expandable and allows for interfacing user equipment (e.g. chemical sensors and instrumentation) with a minimal amount of effort.

4.2 ASTER Description

The current ASTER configuration consists of several tower-mounted sensor arrays, each supported by an independent data acquisition system, the ASTER data acquisition module, or ADAM. The ADAMs are rugged field computers that sample and format the data measured by the sensors. Each ADAM can be located up to a kilometer from the ASTER operations field base and is connected to the field base via an ethernet link using fiber optic cable.

The ASTER operations field base houses a central data-archiving computer, workstations for data analysis and display, and the standard array of computer peripherals. It also provides space for sensor maintenance and repair as well as space for a simple chemistry laboratory. The field base itself is fabricated from two modified 3.5 meter by 6 meter "seatainers" and provides a relatively comfortable working environment.

The facility is self-contained with little external support required. The only external requirements for the ASTER installation are a telephone connection and a power drop to the ASTER field base. At present, the facility does not have independent power generation capability.

4.3 ASTER Tower Configurations

The components of the ASTER facility are modular and thus there is the possibility for a wide range of instrumented tower configurations to suit each individual research application. One tower might be configured to measure fluxes of heat and momentum, while another could be configured to obtain a wind or scalar profile. Currently, up to six ten-meter towers supporting sensor arrays are available in the NCAR ASTER facility. The sensors can be mounted at 0.5 meter intervals on the towers.

4.4 ASTER Measurements and Instrumentation

Standard ASTER sensors are available for the following measurements: momentum, sensible heat, and water vapor fluxes by eddy-correlation; vertical profiles of wind, temperature, and humidity; radiation fluxes; and soil temperature, soil moisture, and soil heat fluxes. The ASTER sensors are a mix of commercial instruments and in-house developments. Whenever possible, each sensor has a microprocessor that provides sensor output in calibrated engineering units through a serial communications link.

User-supplied or non-standard sensors can be readily accommodated by ASTER. Power and mounting are provided and the various data streams are easily integrated into the system.

Table 23 lists the sensors currently available with the ASTER facility.

TABLE  14  Sensors Available with ASTER
Sensor                            Manufacturer                  Parameter                 Rate   #   
3-D sonic anemometer              Applied Technologies, Inc.    u,v,w - m/s; Tvs - deg C   10Hz  2   
3-D sonic anemometer              Gill Instrumentation, Inc.    u,v,w - m/s; Tvs - deg C  18Hz   1   
3-D sonic anemometer              University of Washington      u,v,w - m/s; Tvs - deg C  20Hz   2   
1-D sonic anemometer              Applied Technologies, Inc.    w - m/s; Tvs - deg C      20Hz   1   
Platinum resistance thermometer   Atmospheric Instrumentation   Temperature - deg C        20Hz  3   
                                  Research, Inc.                                                     
UV Hygrometer                     CSI                           vapor density - gm/m3     20Hz   2   
CO2 Concentration                 LICOR                         density - gm/m3           20Hz   1   
Prop-vane anemometer              NCAR/SSSF - R.M. Young        u,v - m/s                 1Hz    6   
Hygrothermometer                  NCAR/SSSF - Vaisala           T - deg C; RH -%          1Hz    6   
Dew Point                         EG&G                      Dew point - deg C         1Hz    6   
Pressure sensor                   NCAR                          Pressure - mb             1Hz    6   
Rain gauge (Optical - ORG)        Scientific Technology, Inc.   Rainfall rate - mm/hr     1Hz    1   
Net Radiometer                    Micromet Systems              Net radiation - W/m2      1Hz    1   
Precision Spectral Pyranometer    Eppley                        Global shortwave 
1Hz 1 radiation - W/m2 Precision Infrared Pygeometer Eppley Global longwave
1Hz 1 radiation - W/m2 Soil temperature sensor NCAR Soil temperature - deg C 1Hz 12 Heat flux plate Micromet Systems Soil heat flux - W/m2 1Hz 1 Surface temperature sensor Everest Surface temp. - deg C 1Hz 1 Ultraviolet radiometer Eppley photometer Ultraviolet radiation
1Hz 1 - W/m2 Dual beam ozone analyzer Thermo Electron 49 O3 1Hz 1 Carbon monoxide analyzer Thermo Electron 48 CO 1Hz 1 Condensation nucleus counter TSI 3760 CNC Condensation nuclei 1Hz 1 ---------------------------------------------------------------------------------------------------
Flux measurements made by the ASTER facility utilize the eddy-correlation technique. Momentum flux is determined from the covariance between the vertical and horizontal velocity fluctuations measured by the 3-dimensional sonic anemometers. Sensible heat flux is determined from the covariance between measured vertical velocity and temperature fluctuations. The temperature fluctuations are obtained with an Atmospheric Instrumentation Research, Inc. platinum resistance fast-response temperature sensor. Latent heat flux is determined from the covariance between measured vertical velocity and humidity fluctuations. Those humidity fluctuations are measured with a fast-response ultra-violet absorption hygrometer.

Measurement of fluxes of trace chemical species are obtained from correlating fluctuations from chemical species sensors with those from the ASTER velocity sensors. The ASTER facility has a CO2 sensor for this purpose. Other chemical specie measurement requires user-supplied sensors which typically can be readily interfaced to ASTER. In chemical flux determination, delay times through inlet tubes can be measured and compensated for in the processing.

Tilt corrections for the sonic anemometer are applied in the software. Over uniformly flat terrain, the tilt correction puts the anemometers into a coordinate system parallel to the surface. In complex terrain, signals from two-axis level sensors are used to orient the sonic anemometers to gravity. Note also that the sonic anemometers make a virtual temperature measurement, Tvs, reported in degrees Celsius.

Mean temperature and humidity measurements are made with a hygrothermometer developed at NCAR and previously used with the NCAR Portable Automated Mesonet (PAM). These hygrothermometers are integrated sensors which utilize a platinum resistance thermometer and solid state relative humidity sensor inside an aspirated radiation shield enclosure.

Wind profiles are measured with commercial propeller-vane wind sensors which have been modified at NCAR to obtain a high resolution vane azimuth using an optical encoder. These sensors have also been modified to output a serial data string giving output in either windspeed and direction or orthogonal components.

Measurement of the surface energy balance is achieved using up-looking and down-looking pairs of short and long-wave radiometers in tandem with soil heat flux plates and soil temperature sensors. The chemical environment around ASTER sites can be monitored with sensors which measure ozone, carbon monoxide, as well as condensation nuclei.

4.5 The ASTER Data Acquisition Module, ADAM

ASTER currently has four data acquisition modules, or ADAMs. The ADAM is the interface between an individual sensor array and the facility base station. The ADAM samples and processes both analog and serial data from the sensors in real time. The data are formatted and transferred to the base station for archival and display.

Two-way communication exists between the ADAM and the base station computer through a fiber-optic internet connection. This allows for real time changes in the operation of the ADAM and its associated sensors through commands issued by the user at the base station computer. In addition, solenoids and motors can be controlled remotely through the ADAM.

The ADAM provides considerable flexibility to the deployment of any given array of sensors. The ADAM electronic design is based on a VME backplane supporting several digital processor cards. Each card is an independent network node running VxWorks, a real time Unix-like operating system. This allows for a flexible, configurable data system. The processing cards include: a master CPU board for control of data collection and other processors on the bus, time synchronization of data samples, and communications with the base station computer; a serial communications card which handles up to 16 channels of ASCII input data; and an analog-to-digital converter with 14-bit accuracy, available with front end amplification and filtering as needed. The analog-to-digital converter card currently handles up to 48 channels. Typical per-channel analog sample rates are 1 and 20 samples per second, although channels can be configured for higher sampling rates. Typical data rates handled by the ADAM might be 320 samples per second analog (16 channels) and 3600 characters per second serial.

The ADAM software controls data sampling, synchronizes the data, and transmits the data to the base computer. The serial and analog data input to the ADAM arrive asynchronously and thus the data are time tagged using the base computer clock as a reference. The incoming data are then buffered, ordered in time sequence, and transferred to the base station computer. Sensor modifications or additions are made to the ADAM configuration by editing a configuration file in the base computer and transferring that file to the ADAM.

Physically, the ADAM is a small, self-contained unit, about 1.0 meter by 1.0 meter by 0.6 meters. The unit requires standard 110V 60Hz power. The power drop to the ADAM typically powers the sensors for a given array, as well. The size and placement of the ADAM are designed to cause minimal environmental disturbance to the area being sampled. An integrated radiation/precipitation shield protects the ADAM from the elements. Temperature control in the ADAM is obtained with a solid-state Peltier cooling system. The fiber-optic ethernet link between the ADAM and the base station provides lightning protection for the computer systems.

4.6 The ASTER Operations Field Base Computing Capability

The ASTER facility operations field base (two modified "seatainers") houses the ASTER base station computing facility. This is a general purpose computing facility which provides data acquisition, archival, analysis, and display capabilities. Routine processing performed includes raw data archival, data display, and continuous calculation of first and second order statistics. Software is available for traditional micrometeorological analyses, and a framework is available to incorporate additional project specific analyses into the system. A mechanism is also provided to allow the user to easily incorporate specialized or user-supplied sensors into the system and to apply the ASTER library of analysis software to data from those sensors.

A server/workstation architecture has been implemented for the base station. The server provides disk storage and computational capability for data collection and routine processing. Interprocess connections are based on Remote Procedure Calls and BSD Unix sockets. This allows the ingestors and applications processes to run on any machine on the ASTER network. This architecture gives the obvious advantage of dividing the processing load among several workstations and providing the flexibility to increase system capacity as needed.

The current ASTER base station computer network consists of a Sun Sparcstation 10 with 3.0 Gigabytes of disk storage and a Sun IPX workstation, both with color graphics displays. In addition, there is an Exabyte tape drive for data and software backup, and a laser printer for text and graphic hardcopy. There are three lines available for user terminals and a modem. The system is powered through an UPS to allow for quick recovery from power outages.

The ASTER base station software is designed to give the system a considerable amount of flexibility. System configuration is managed with a small number of configuration tables contained in files that can be modified with an editor. Thus, all software is independent of the system configuration and does not require modification or recompilation for different projects or differing sensor arrays.

The heart of the base station computer software is the data ingestion task. One ingestion task is run for each ADAM in the ASTER configuration. The ingestor acts as a data switch, or pathway, between the ADAM and any given application process. Application processes can contact an ingestor and request data from a particular data channel from a given ADAM. Typical applications processes are the archivers, display tasks, and on-line diagnostic and continuous analysis procedures.

The archive process requests specified sensor channels from the ingestor and writes the data into archive files. The archivers are managed by configuration tables, allowing the investigator to organize the data set into a structure that is logically related to the planned analysis. Examples of this would be the collection of data from collocated, fast response sensors (e.g. sonic anemometer, fast temperature, and fast CO2 sensors) into a file for covariance or flux calculations or the collection of sensor data from a single tower into a file for profile analysis. To allow for continuous data collection, archived files are written to 8mm Exabyte tape on a daily basis and purged from the disk.

Note that the archival processes store raw data. However, since most analyses and useful data displays rely on calibrated data, calibration functions are used to translate the raw data to calibrated output data. Configuration tables map raw data channels to appropriate calibration functions. Thus, selection of a new calibration does not require recompilation of the analysis programs. The ASTER software provides a software tool which manages the conversion of raw data to calibrated data. The tool has facilities for data interpolation to common time intervals, filtering and phase shifting of data streams, and calculation of derived quantities which are functions of multiple data channels.

Five-minute means, variances, and covariances are calculated continuously for appropriate sensor groups, defined by configuration tables, and remain on-line for the entire experiment. These summary data or "covars" can be used to produce graphical displays or hardcopy plots for monitoring system performance, evaluating experimental results, or project decision making. A standard package of daily plots is routinely produced for these purposes. Utilities are also available to input these data into user analysis routines.

Many "off-line" display routines are available with the ASTER facility. Many of these are based on the commercially available Splus data analysis software.

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