5. PLANETARY BOUNDARY LAYER OBSERVATIONS (next update 4/30/97)

The boundary layer component of SGP97 is configured to primarily evaluate the influence of soil moisture on the local surface energy budget and the influence of mesoscale variability in the surface energy budget on the development of convective boundary layer. To the extent possible, attempts will be made to quantify the water vapor budget of the boundary layer (advection, entrainment, and evapotranspiration) using remotely sensed and in situ data. The instrumentation to be deployed during SGP97 and the data to be collected are described below. The plan is not yet complete particularly in regard to the flight plans of the two flux aircraft; an update is expected on 4/30/97.

5.1. Water vapor profiles

Aboard the Wallops P-3 aircraft together with ESTAR, the NASA Langley Research Center (LaRC) instrument, Lidar Atmospheric Sensing Experiment (LASE), will provide observations of atmospheric water vapor and aerosol profiles, and locations of cloud top along the flight track. The LASE instrument is a compact and highly engineered differential absorption lidar (DIAL) system that has completed its development and validation aboard the high-altitude ER-2 aircraft (Higdon et al., 1994; Browell et al., 1996); the lidar parameters are given in Table 7.

Differential Absorption Lidar (DIAL) is an active remote sensing technique that takes advantage of the absorption of the pulsed laser light along the beam direction to obtain the concentration of the molecular species that causes the selective absorption. In practice, two laser pulses are transmitted near simultaneously one at the peak of the absorption line called the "on-line" and another in the wing of the absorption line called the "off-line". An illustration of the DIAL principle is given in Figure 15. If P_on and P_off denote power received "on-line" and "off-line", respectively, the average molecular number density between ranges R₁ and R₂ is calculated using the relation:

The advantage of the DIAL method is that it can be used to obtain range-resolved profiles of atmospheric gases with high vertical resolution. In addition to measuring gas concentration profiles, high spatial resolution aerosol backscattering distributions are simultaneously obtained as part of the DIAL measurement using the off-line lidar signals. DIAL offers the advantage of adjusting vertical and/or horizontal resolution by averaging the lidar data that are collected at a very high resolution. With the DIAL method, lidar measurements can be made during day or night and between and above cloudy regions in the atmosphere.

In the current mode of operation LASE operates locked to a strong water vapor line and electronically tunes to any spectral position on the absorption line profile. This permits the choice of suitable absorption cross-sections for optimum measurements over a wide range of water vapor concentrations in the atmosphere. In addition, electronic tuning allows the system to rapidly take data over two or three water vapor concentration ranges. This unique method of operation permits rapid and flexible absorption cross-section sampling capability and provides water vapor measurements over the entire troposphere on one aircraft pass. This new method of using two water vapor absorption cross-sections from a single water line (one on the line center and one on the side of the line) was implemented and tested during the LASE validation experiment in September 1995; the intercomparison with a number in situ and remote sensors from the ground and other aircraft demonstrated the accuracy, reliability, and dynamic range of LASE measurements.

The LASE system has been developed as a precursor to a space-based DIAL instrument, and has operated autonomously from the ER-2 aircraft. Several modifications are being made in order to deploy LASE aboard the P-3 aircraft during SGP97; the projected capabilities are listed in Table 8. The projected performance (random error profiles, representing the precision of the water vapor measurement) of LASE aboard P-3 is compared with the LASE capability from the ER-2 in Figure 16.

Table 8. LASE Water Vapor and Aerosol Profiling Capability on P3 (SGP97 Mission)
WATER VAPOR
ALTITUDE COVERAGE	GROUND TO NEAR AIRCRAFT
MEASUREMENT CAPABILITY	DAY AND NIGHT
MEASUREMENT RANGE	0.01 G/KG TO 20 G/KG
ACCURACY (MIXING RATIO)	BETTER THAN 10% (OR 0.01 G/KG)
RESOLUTION (NOMINAL)	10 KM (HORIZ),0.3KM (VERTICAL)
AEROSOL BACKSCATTER (815-NM)
ALTITUDE COVERAGE	GROUND TO NEAR AIRCRAFT
MEASUREMENT CAPABILITY	DAY AND NIGHT
MEASUREMENT RANGE	0.2 TO >100 (AER. SCAT. RATIOS)
PRECISION	BETTER THAN 3% (OR 0.2 S/R)
RESOLUTION	0.2 KM (HORIZ,0.03 KM (VERTICAL)
*LASE DATA WILL BE REDUCED TO RETAIN HIGHEST RESOLUTION POSSIBLE IN THE PBL. ALGORITHMS ARE IN PROGRESS TO EXTEND WATER VAPOR PROFILES TO WITHIN 100M OF GROUND

.

An upgraded computer system is planned to support on-board LASE monitoring, data processing and analysis; the post-processing will be used to produce analysis products more refined than is possible with the real-time processing. The on-board data display will provide real-time information concerning the development of the convective boundary layer via images of lidar backscatter. These observations can be used to guide the flux aircraft with regard to choice of flight altitudes and the location of interesting mesoscale features.

5.2. Airborne fluxes

Two research aircraft will be deployed for the measurement of eddy fluxes of momentum, latent and sensible heat, and other scalars, along with the measurement of mean thermodynamic variables and various radiative components; one is the Twin Otter from the National Research Council (NRC), Canada, and the other the Long-EZ airplane from the NOAA Atmospheric Turbulence and Diffusion Division (ATDD). The flight plans are in the process being defined, keeping in mind the possible airspace and operational limitations. The basic objective, however, is to make boundary layer measurements at several altitudes across an area with large gradients in soil moisture that are prompted by recent contrasts in either wetting or drying conditions. At this writing, three types of flight operations are being contemplated:

(1) Reciprocal runs at various altitudes from near the surface (100 ft) to on top of the boundary layer (perhaps 5000 ft). These tracks would be approximately 10-30 km in length and would be along a gradient in soil moisture. It is proposed that several such tracks be planned well in advance of the project in cooperation with the FAA. On any given day one or two of these tracks would be activated, the selection being made at the previous evening's briefing based on data from the surface sites and the P-3.

(2) Coordinated flights with the P-3, likely flying long segments of the four primary P-3 tracks. Again, these runs would be flown at several altitudes, perhaps also coordinated with the second flux aircraft (ATDD Long-EZ). It is possible that on these tracks the Twin Otter would not fly below 500 ft.

(3) Intercomparison flights with surface-based systems. There are three areas in the overall operational area that have significant arrays of surface moisture measurements as well as occasional flux-measuring stations (see section 5.3). These intercomparison would consist of low altitude passes (down to 100 ft if possible) on relatively short tracks, perhaps 5 km or less.

5.2.1. NRC Twin Otter

The NRC Twin Otter atmospheric research aircraft is a twin-engine turboprop STOL transport with a gross takeoff weight of 11579 lb. Without the use of a supplementary oxygen system, it has a service ceiling of 10,000 feet and an endurance of about 3.5 hours (depending on installed instrumentation). In its trace gas flux-measuring role, the aircraft is flown at about 105 knots (55 mps) and can operate at altitudes as low as 100 feet. Research flights are usually flown with a crew of four.

Configured for flux measurement, the basic instrumentation aboard the aircraft measures the following:

• the three orthogonal components of atmospheric motion.
• the vertical fluxes of sensible and latent heat, momentum, turbulent kinetic energy, CO2 and ozone.
• concentrations of CO2, H2O and ozone.
• atmospheric state parameters such as pressure, temperature, dew point, and mean winds.
• aircraft position (GPS), motion and attitude (Litton-90 Inertial Reference System), pressure height, and height above ground (radio-altimeter, laser altimeter).
• radiometric surface temperature, incident and reflected solar radiation, net radiation, greenness index (NDVI), 4-channel satellite simulator (Landsat or SPOT).
• VHS videotape using under-nose and side-looking cameras with superimposed listing of time, altitude, heading, latitude and longitude.

Data are recorded digitally on DAT tape at a rate of 32 Hz, after anti-alias filtering at 10 Hz, giving an along-track resolution of about 5 meters at the usual flux measuring speed of 50-55 mps. Winds and estimated fluxes are computed in real time by the aircraft's VME-based computer system, with results immediately displayed to the cockpit and cabin crew. This allows the crew to assess the state of the boundary layer, or recognize instrumentation problems, and modify the flight plan as required.

Along with aircraft spares and maintenance equipment, a full data playback facility is transported to the field site, and is usually set up in a meeting room in the crew's hotel. Within a few hours of landing, collaborating scientists can have access to the analyzed data, which includes run-averaged fluxes, analog traces, flight tracks, videotape, tephigrams from soundings, spectra and cospectra of flux contributions. A review of these data allows scientists to compare the measurements with expectations and with data from other sensing platforms, and thus make decisions on the scientific direction of subsequent flights.

After the completion of the field phase of the experiment, the data are re-analyzed, applying adjusted calibrations, and correcting the measured horizontal wind data using a Kalman-filtering technique, which removes small drifts present in velocity measurements from inertial navigation systems. The run-averaged results and all 32-Hz data (approximately 160 variables) are archived on optical disk. At the request of collaborating scientists, these files can be accessed at a later date to strip off time histories of a selection of parameters, which are then electronically transferred by ftp. For this project, run-averaged data could also be archived in the format used in the BOREAS project and already stored at NASA. Finally, about six months after completion of the field experiment, NRC will produce a project report, which will include a description of the instrumentation used, a summary of all the flights, data presentations and preliminary analyses related to the objectives of the experiment. An example of the report from the 1994 BOREAS project is available upon request. More information can also be found at the website (http://www.cmc.ec.gc.ca/rpn/mermoz).

5.2.2. ATDD Long-EZ

The Long­EZ flux aircraft, N3R, is an experimental airplane; with its wide body and higher power, it is more capable than the standard Rutan Long­EZ, a two passenger high performance canard airplane. Its aerodynamic characteristics have many advantages for high­fidelity turbulent flux measurement. The small, laminar­flow airframe has an equivalent flat plate drag area of 0.2 m², minimizing flow distortion at the nose for high­fidelity measurements of winds, temperature and trace species. The pusher configuration leaves the nose free of propeller­induced disturbance, engine vibration, and exhaust. The canard design resists stalling and has excellent pitch stability in turbulent conditions. This, combined with its low wing loading, allows for safe low­speed (50 m s^-1), low­altitude (10 m) flux measurement. For enhanced safety, the Long­EZ is equipped with a ballistically­deployed safety parachute (deployment requires 0.9 s).

The Long­EZ has an empty mass of 430 kg and a maximum gross takeoff mass of 800 kg. Endurance significantly exceeds 10 hours, although pilot fatigue precludes routine 10­hour missions. Typical operations include two 4­hr or three 3­hr missions. The small size allows operation from relatively small airports, though requiring at least 1000m of paved runway.

The airborne flux instrumentation, and the data system with its associated software were specifically designed and built by ATDD (Crawford et al., 1990). Wind velocity and temperature fluctuations are measured with ATDD's turbulence probe (Crawford and Dobosy, 1992). The probe is mounted five chord lengths ahead of the wings, where flow distortion is small (Crawford et al., 1996). It carries pressure, temperature and acceleration sensors in a nine­hole pressure­sphere gust probe of ATDD design. This sensor suite is specifically designed for eddy­flux measurement at the higher frequencies required for low altitude flight. A thermistor in the central pressure port provides simultaneous temperature measurement, at a location symmetrical with respect to the flow, for accurate determination of true air speed and heat flux. Water-vapor and CO₂ fluctuations are measured with an open­path, infrared gas absorption (IRGA) analyzer, developed at ATDD (Auble and Meyers, 1992). This sensor responds to frequencies up to 40 Hz, has low noise and high sensitivity (for CO₂, 20 mg m^-3 v^-1). The sensor is rugged and experiences little drift.

A unique difference in the Long­EZ instrument system is its pioneering use of a mix of differentially-corrected GPS and integrated acceleration measurements to determine position, velocity, and platform attitude. Differential correction of GPS involves determining the position or velocity as a relative quantity, the difference between values at two receivers. Many GPS errors are common to all receivers in a given area and are canceled when the measurements from two separate receivers are subtracted. The receivers we use report position, velocity and attitude ten times per second. To obtain this information at higher frequencies we integrate acceleration measurements. By adding filtered signals (high pass for integrated accelerations, and low pass for GPS) information on position, velocity, and attitude of the Long-EZ can be obtained over the same range obtainable from a high-quality inertial navigation system (INS).

The data stream is dominated by high­frequency analog signals from the accelerometers, pressure sensors and the like. Analog signals are first electronically conditioned by 30­Hz lowpass Butterworth anti­aliasing filters. The conditioned signals are then sampled and digitized at 250 Hz. The 250­Hz data are digitally filtered and sub­sampled to 50 Hz. Although several other data frequencies are being written to disk, all are synchronized to a single clock frequency. Spectra and cospectra data analysis show that the 50­Hz flux data rate is adequate for measuring fluxes at the Long­EZ flight speed and altitude. The final, meteorologically relevant quantities from Long-EZ are listed in Tables 9 and 10.

Table 9. Long-EZ Measurements, Data Provided Fifty Per Second
Datum	Units	Measured/Derived
Eastward wind U	m s-1	Derived
Northward wind V	m s-1	Derived
Upward wind W	m s-1	Derived
Air Temperature (probe)	K	Adjusted for dynamic pressure
Air Temperature (hatch)	K	Adjusted for dynamic pressure
H2O mixing ratio	g(H2O) kg-1 (dry air)	Converted from vapor density (IRGA)
CO2	mg(CO2) kg-1 (dry air)	Converted from gas density (IRGA)
Ambient pressure	mb	Corrected for airspeed and angle of attack/slideslip
Laser Altitude	m	Measured
rW	kg m-2 s-1	Dry-air density times W

Table 10. Long-EZ Measurements, Data Provided Once per Second
Datum	Units	Derived/Measured
Latitude	Deg	Derived (GPS)
Longitude	Deg	Derived (GPS)
Altitude	m	Derived (GPS)
Exotech radiometer four channels	Filters to match TM, SPOT, MSS	Measured
PARdownwelling	mEinstein m-2 s-1	Measured
PARupwelling	mEinstein m-2 s-1	Measured
Net Radiation	W m-2	Measured
Surface Temperature	C	Derived (Infrared)
Radar Altitude	m	Measured
CO2 mixing ratio	mMole Mole-1	Measured (LiCor)
H2O mixing ratio	mMole Mole-1	Measured (LiCor)
Ground Speed	m s-1	Derived

5.3. Surface Flux Measurements

In addition to the ARM surface flux stations within the SGP study area, there will be a group of investigators collecting surface flux and ancillary meteorological data during the SGP intensive field campaign. These are summarized below:

NASA-GSFC/Univ of Arizona

The Univ of Arizona Eddy Correlation system measures the 3D wind vector with a weather resistant Solent sonic anemometer, and concentrations of CO2 and H2O using a Li-Cor 6262 infrared gas analyzer at 20 Hz. All the raw data is saved, and processed at a later time on a PC (but a real­time first guess is possible). Supporting measurements are standard met variables measured with a Campbell weather station: wind speed and direction, relative humidity, air temperature, solar and net radiation, soil temperature, soil heat flux, and rainfall. Plans for deployment are still TBD. The goal is to help validate the ARM EC and Bowen Ratio measurements.

USDA-ARS

The USDA-ARS plans to conduct measurements at three sites within the El Reno facility. The sites will be representative of the three main vegetation cover types: winter wheat, Bermuda grass and natural rangeland/prairie. At each site a Campbell 3D sonic anemometer along with a 1D KH2O for measuring sensible and latent heat fluxes will be deployed. Ancillary measurements will include soil temperature, soil heat flux and meteorological observations: wind speed and direction, air temperature, relative humidity, and net and solar radiation. There are also plans to co-locate a Campbell Scientific Bowen ratio system using the Li­Cor 6262 CO2/H2O gas analyzers at two of the sites. In addition, there are plans to install on a more permanent basis three SHAWMS (Soil Heat And Water Measurement Systems) nearby the flux measurement systems. These systems measure soil heat flux, soil temperatures, soil thermal conductivity and moisture in the root zone (approximately the first 1 m of the soil profile). At the three sites radiometric surface temperatures will be collected on a continuous basis using Everest 4000's.

Univ of Wisconsin

A major focus of this group will be to conduct comparisons between a "roving" eddy

correlation unit (consisting of a 3D sonic and 1D KH2O) and the different instrumentation running at the various flux stations during the SGP study period. Part of this "roving" system, will be a newly purchased Kipp & Zonen CNR-1 net radiometer to compare with net radiometer measurements being made by other net radiometer type(s). This project will help determine which flux stations may be having problems or at the very least showing large discrepancies with the "roving" system. It will also provide a means for reducing variation in net radiation observations caused by differences in net radiometer types/calibrations. They are also planning the installation of infrared radiometers at as many of the flux sites as possible for recording surface temperature on a continuous basis.

JPL

Main interest is to collect surface flux data during aircraft thermal infrared observations and compare eddy correlation measurements using different instrument types with ground-based thermal infrared observations. Instruments include; a Campbell Scientific 3­D sonic and a 1­D sonic system, a couple of KH20 for EC humidity measurements, and about 6­8 fine wire thermocouples that could be used for sensible heat flux estimation via the variance method, a TDR for soil moisture measurements, and about 60 or more soil thermocouples. Will also be able to bring along at least one Everest IR radiometer, about 5­10 CSI data loggers, a profile system with anemometers, thermocouples, and humidity sensors that can be used for estimating surface roughness as well as to compare with the eddy correlation and Bowen ratio systems. Have not decided on a location for his surface flux and ancillary meteorological measurements.

Georgia Tech

Campbell Scientific Bowen ratio system and will be siting the instrumentation with the sounding location in the Little Washita Watershed. The exact location is unknown, but plans are to locate on a pasture site. Measurements include incoming solar radiation, net radiation, ground heat flux and soil temperature, wind speed and direction, surface pressure, air temperature, and relative humidity .

NOAA/AATD

This group has been collecting energy and CO2 flux data on a continuous basis at a rangeland site in the Little Washita. The flux instrumentation includes a 3­D sonic (Gill instruments, model R2) and an ATDD open path H2O/CO2 gas analyzer. Ancillary measurements include net and solar radiation, incoming and reflected PAR, soil temperatures (at 6 levels), ground heat flux, precipitation, surface wetness, surface temperature, air temperature and relative humidity, atmospheric pressure, and soil moisture.

5.4. Atmospheric Soundings

In addition to the ARM IOP sounding schedule of 8 times per day at the CF and 4 BF's (see Section 7), a tethersonde system will be deployed from the Little Washita with emphasis on the morning hours. (A radiosonde system to the southwest corner of the SGP domain is pending.)