NASA DC-8 and JPL Airborne Cloud-Profiling Radar (ACR)

The JPL Airborne-Cloud-Radar (ACR) (mounted on the NASA DC-8) took measurements of cloud bases, tops, and structure including estimates of cloud liquid water content. The ACR is a scanning Doppler, cloud-radar that operates at a frequency of 95 GHz. The radar will be operated during CLEX-1 with a PRF of approximately 5 kHz, in both a fixed (downward and upward looking, nadir parallel) and scanning mode (vertical scanning, various degree intervals off nadir). The minimum detectable signal of the radar for cloud-heights of 200 m (approximately 11 km from aircraft cruising altitude) is ~-30 dBZ. The beamwidth of the radar is 0.56, providing cross -track and along track resolutions of approximately 120 m at aircraft cruising speeds. Vertical resolution (pulse length dependent) of ~ 90 m. The ACR is mounted on the side of the NASA DC-8 fuselage.

The NASA DC-8 provided measurements of time, position (GPS, INS etc.), altitude, air temperature, dewpoint, surface temperature, flight level winds and other navigational variables. These variables were tape-archived aboard the aircraft during operations as part of the Data Archive and Distribution System (DADS) resident in the DC-8 on board computer data collection system.

Wyoming King Air, in-situ Measurement Instrumentation

Instrumentation on the University of Wyoming King Air provided in-situ microphysical measurements and measurements of other standard meteorological variables. Primary microphysical instrumentation on the King Air include: a) cloud-droplet spectra/particle imaging probes such as Particle Measurements Systems (PMS) FSSP (.5 - 45 mm), 1D-C (12.5-186 mm), and 2D-C (25- 800 mm); b) for precipitation the PMS 2D-P (200-6000 mm) probe; and c) for liquid water content, both in-house and Johnson-Williams (J-W) hot-wire probes (accuracy 0.2 g m^-3) The King Air team will also locate cloud layer tops and bases visually and via the in situ measurements. The airborne radar was not used for the CLEX experiment due to equipment problems.

An example of several parameters collected during the CLEX flights is shown in a chart that was built for a one-hour portion of the flight on June 22nd.

Desert Research Institute (DRI) Dual Frequency Mobile Microwave Radiometer

The DRI mobile radiometer is a van-mounted, scanning, dual-frequency (20.6 and 31.65 GHz) passive microwave sensor that will be used to estimate column integrated water vapor and cloud liquid water (Fig. A3). The instrument can be operated while mobile or stationary. When in scanning mode the radiometer can scan a full 360 degrees (+/- 1 accuracy), at a fixed elevation angle (+/- 0.1 accuracy). Temporal resolution in the scanning mode is approximately 3 minutes. The beam sampling angle is 2.6. The instrument has an accuracy of 5-10%, and will measure values of water vapor and liquid water over ranges of 0-5 cm for vapor and 0-5 mm for liquid water. This DRI system was based at the North Central Oklahoma (ARM) site and deployed to several nearby sites during the CLEX-1 data collection.

CSU Instrumentation - Micro Pulse Lidar

The MPL provides users with capabilities previously unavailable to researchers. It was designed with the goal of developing a system that is capable of continuous eye-safe use while having the sensitivity to detect all significant cloud and aerosol scattering layers. Also, it needed to be sufficiently user friendly to be operated by non-specialists. A brief overview of the MPL and its differences from more standard systems is given below with a complete discussion available in Spinhirne (1993).

General description and comparison with other Lidar systems

There are three basic differences between the MPL and conventional lidar systems. First, the laser pulse repetition frequency (PRF) is much higher and the pulse energies much lower in the MPL. It is this low pulse energy expanded to fill the 20 cm transmitting aperture, thus lowering the energy density, that permits the system to be eye-safe. Secondly, the solid state lasers are diode pumped rather than flashlamp pumped and are much more efficient and smaller. This results in system power requirements on the order of tens of watts rather than hundreds of watts for a nominal one watt of transmitted power. The third difference is that the signal detector is a solid state Geiger Avalanche Photon Diode, photon-counting detector rather than a photo multiplier detector. The advantage of this is that photon counting offers much higher quantum efficiency and is generally a more accurate and problem free means of signal acquisition for low level signals than is analog detection.

An example of the basic differences between the MPL and the more conventional lidars is provided in Table 1. System specifications are given for the MPL as well as lidars used in the LITE and experimental cloud lidar pilot study (ECLIPS) programs as described by McCormick et al. (1993) and Carswell et al. (1995), respectively. All systems operate in the visible portion of the spectrum. Notice the major difference in energy levels between the MPL and the lidars used in the two experiments. Although this low energy level allows the MPL to be eye safe, it does, in general, decrease the signal to noise ratio. It also makes it nearly impossible to detect stratospheric aerosol during daylight hours due to background radiative energy. To help limit background noise in the MPL, the smallest possible receiver FOV is necessary. It will be shown later, that this small receiver FOV significantly limits multiple scattering effects. Even with the MPL's low pulse energy levels, laser beam expansion is required to obtain eye safety. Therefore, beam expansion and collimation by a telescope is required. With a pulse repetition frequency (PRF) of 2,500 Hz, a pulse of energy travels approximately 120 km at the speed of light before the next pulse is emitted. Therefore, if it is assumed that only the lowest orders of scattering contribute significantly to the overall return, multiple trip echoes do not play a significant role in MPL operations.

Pennsylvania State University - Cloud Radar

The Penn. State University cloud-radar provided measurements of cloud bases, tops and liquid water content. The radar is Dopplerized and operates at a frequency of 94 GHz. The radar operated at the Oklahoma ARM site from June 20 to July 3, 1996 in vertically pointing mode only. Data was collected for altitudes between 200 m and 15 km at a temporal resolution of 2 seconds and a vertical resolution of 50 m. The Nyquist velocity will be 1.2 m s^-1. The beamwidth of the radar is approximately 0.3, providing a horizontal resolution of approximately 30m at 10 km altitude. "First Look" images of data that was collected for the CLEX experiment can be found at http://wwwarc.essc.psu.edu/datasets/clex1/clex-data.html.

Automated Surface Observing System (ASOS) Data

ASOS data at 5 minute temporal resolution was collected from locations in the general area of operations for CLEX-1. These data will consist of all meteorological information collected by the ASOS suite of sensors (Fig. A5) including laser ceilometer measurement of cloud bases (to 12,000 ft AGL) and sky cover, temperature, dewpoint, visibility, and precipitation type and amount. The ASOS data will be collected from both commissioned and non-commissioned sites via computer modem dial-up from CSU to the National Center of Atmospheric Research. ASOS locations were used to determine optimum tracks for overflights of the King Air and DC-8 in the presence of clouds.

Satellite Data

In addition to the instrumentation deployed specifically for CLEX-1 operations, a host of satellite data were collected. These data include GOES-8 and 9 data (all channels, 1-minute rapid scan when possible), DMSP overpasses (see Fig A6) including SSM/I and SSM/T2 data, and AVHRR data aboard the NOAA 11 and 12. The data collected near the surface and from aircraft was collected, as much as possible, to coincident with DMSP satellite overpasses. These data will be used to validate satellite based algorithms used to estimate cloud parameters such as cloud base, top, number of layers, microphysical structure, cloud liquid water and water vapor profiler as per the scientific objectives of the Center for Geosciences research tasks (5, 8, 10 and others).

NOAA Profiler Data

NOAA 6-minute profiler data will be used to support CLEX-1. These data were also considered when planning overflights with the King Air and DC-8 in the presence of clouds.

Radiosonde Soundings (special support launches at the ARM site)

In order to provide support for many of the CLEX sub-tasks, the normal radiosonde data collection at ARM CART was combined with radio theodolite soundings taken by CSU. The routine soundings are taken at ARM at 0600, 1200, 1500, 1800, and 2100 UTC. CSU launched rawindsondes specifically in support of satellite and/or aircraft overflights. The CSU launches attempted to obtain in-cloud measurements of the temperature, relative humidity and wind velocity synchronized as closely as possible to the time at which the satellite of interest (SSMT2, SSMI, AVHRR) passes over the site. These soundings were obtained on a non-interfering bases due to the frequency separation between the ARM sondes (404 Mhz) and the CSU radio theodolites (1680 Mhz). During extended King Air sampling in the layered clouds, the wind velocity versus height information that is provided in real time may be relayed to the King Air to assist in aligning the flight patterns at different altitudes in order to compensate for cloud parcels differentially advected by vertically sheared winds.