NASA - National Aeronautics and Space Administration

The Configurable Scanning Submillimeter-wave Instrument/Radiometer (CoSSIR) is an airborne, 16-channel total power imaging radiometer that was primarily developed for the measurement of ice clouds. CoSSIR was first flown in CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cirrus Experiment) in 2002, followed by CR-AVE (Costa Rica Aura Validation Experiment) in 2006, and TC4 (Tropical Composition, Cloud and Climate Coupling Experiment) in 2007. For CRYSTAL-FACE and CR-AVE, CoSSIR had 15 channels centered at 183±1, 183±3, 183±6.6, 220, 380±.8, 380±1.8, 380±3.3, 380±6.2, 487.25±0.8, 487.25±1.2, 487.25±3.3, and 640 GHz, where the three 487 GHz channels were dual-polarized (vertical and horizontal). For TC4, the 487 GHz channels were removed, 640 GHz was made dual-polarized, and an 874 GHz channel was added.

In 2022, CoSSIR was completely updated with new receivers under funds through the Airborne Instrument Technology Transition (AITT) to improve measurement accuracy and enable CoSSIR to be a stand-alone sensor that no longer shared a scan pedestal with its millimeter-wave sibling, CoSMIR. Frequencies were selected for CoSSIR to optimize snow and cloud ice profiling, and dual-polarization capability was added for all frequencies to provide information on particle size and shape. New channels are centered at 170.5, 177.3, 180.3, 182.3, 325±11.3, 325±3.55, 325±0.9, and 684 GHz. The updated CoSSIR flew for the first time in the 2023 deployment of IMPACTS (Investigation of Microphysics and Precipitation for Atlantic Coast Threatening Snowstorms) and operated nominally for the entire campaign, collecting a wide variety of observations over different types of clouds and precipitation.

All the receivers and radiometer electronics are housed in a small cylindrical scan head (21.5 cm in diameter and 28 cm in length) that is rotated by a two-axis gimbaled mechanism capable of generating a wide variety of scan profiles. Two calibration targets, one maintained at ambient (cold) temperature and another heated to a hot temperature of about 323 K, are closely coupled to the scan head and rotate with it about the azimuth axis. Radiometric signals from each channel are sampled at 10 ms intervals. These signals and housekeeping data are fed to the main computer in an external electronics box.

Clouds are a key element in the global hydrological cycle, and they have a significant role in the Earth’s energy budget through its influence on radiation budgets. Climate model simulations have demonstrated the importance of clouds in moderating and forcing the global energy budget. Despite the crucial role of clouds in climate and the breadth of our current knowledge, there are still many unanswered details. An improved understanding of the radiative impact of clouds on the climate system requires a comprehensive view of clouds that includes their physical dimensions, vertical and horizontal spatial distribution, detailed microphysical properties, and the dynamical processes producing them. However, the lack of fine-scale cloud data is apparent in current climate model simulations.

The Cloud Radar System (CRS) is a fully coherent, polarimeteric Doppler radar that is capable of detecting clouds and precipitation from the surface up to the aircraft altitude in the lower stratosphere. The radar is especially well suited for cirrus cloud studies because of its high sensitivity and fine spatial resolution.

The in‐situ diode laser spectrometer system, referred to by its historical name DACOM, includes three tunable diode lasers providing 4.7, 4.5, and 3.3 μm radiation for accessing CO, N2O, and CH4 absorption lines, respectively. The three laser beams are combined by the use of dichroic filters and are then directed through a small volume (0.3 liter) Herriott cell enclosing a 36 meter optical path. As the three coincident laser beams exit the absorption cell, they are spectrally isolated using dichroic filters and are then directed to individual detectors, one for each laser wavelength. Wavelength reference cells containing CO, CH4, and N2O are used to wavelength lock the operation of the three lasers to the appropriate absorption lines. Ambient air is continuously drawn through a Rosemount inlet probe and a permeable membrane dryer which removes water vapor before entering the Herriott cell and subsequently being exhausted via a vacuum pump to the aircraft cabin. To minimize potential spectral overlap from other atmospheric species, the Herriott cell is maintained at a reduced pressure of ~90 Torr. At 5 SLPM mass flow rate, the absorption cell volume is exchanged nominally twice per second. Frequent but short calibrations with well documented and stable reference gases are critical to achieving both high precision and accuracy. Calibration for all species is accomplished by periodically (~4 minutes) flowing calibration gas through this instrument. Measurement accuracy is closely tied to the accuracy of the reference gases obtained from NOAA/ESRL, Boulder, CO. Both CO and CH4 mixing ratios are provided in real-time to investigators aboard the DC‐8.

The University of Colorado closed-path tunable diode laser hygrometer (CLH) is based on the water vapor hygrometers designed by R. D. May (Maycomm, Inc.). CLH is coupled to a heated, forward-facing inlet that enhances particulate water by anisokinetic sampling. Ice water content (IWC) is derived from the measurement of enhanced total water, with knowledge of the instrument sampling characteristics, particle size distributions and ambient water vapor.

In contrast to the open-path systems of similar heritage, the CLH, which was designed for operation in the troposphere on commercial aircraft, has a single-pass absorption cell (27.62 cm long). The light source is a room-temperature solid-state laser that puts out 3-5 mW of radiation at 1.37 mm (7306.752 cm-1).

The CPI records high-resolution (2.3 micron pixel size) digital images of particles that pass through the sample volume at speeds up to 200 m/s. In older models, CCD camera flashes up to 75 frames per second (fps), potentially imaging more than 25 particles per frame. More recent camera upgrades capable of bringing frame rate to nearly 500 fps. Real time image processing crops particle images from the full frame, eliminating blank space and compressing data by >1000:1. CPI is designed for ummanned use, with AI parameters to optimize performance without supervision.

The Cloud Spectrometer and Impactor (CSI) combines the counterflow virtual impactor with a new lightweight cloud droplet probe to allow for detailed studies of total condensed water (TCW), liquid and ice, in clouds. The CSI can measure TCW from ~ 1 mg/m3 to several g/m3 depending on the configuration; in addition particle sizes from 2 to 50 μm are resolved with the droplet probe. The instrumentation can be mounted externally on most aircraft.

The NCAR counterflow virtual impactor (CVI) (Noone et al., 1988; Twohy et al., 1997) is an airborne instrument that can be used for studies of aerosol/cloud interactions, cloud physics, and climate. At the CVI inlet tip, cloud droplets or ice crystals larger than about 8 µm aerodynamic diameter are separated from the interstitial aerosol and impacted into dry nitrogen gas. This separation is possible via a counterflow stream of nitrogen out the CVI tip, which assures that only larger particles (cloud droplets or ice crystals) are sampled. Because droplets or crystals in a sampling volume of about 200 l/min are impacted into a sample stream of approximately 10 l/min, concentrations within the CVI are significantly enhanced. The water vapor and non-volatile residual nuclei remaining after droplet evaporation are sampled downstream of the inlet with selected instruments. These may include a Lyman-alpha or similar hygrometer, a condensation nucleus counter, an optical particle counter, filters for chemical analyses, or user instruments.

EDOP is an X-band (9.6 GHz) Doppler radar nose-mounted in the ER-2. The instrument has two antennas: one nadir-pointing with pitch stabilization, and the other forward pointing. The general objectives of EDOP are the measurement of the vertical structure of precipitation and air motions in mesoscale precipitation systems and the development of spaceborne radar algorithms for precipitation estimation.

EDOP measures high-resolution time-height sections of reflectivity and vertical hydrometeor velocity (and vertical air motion when the hydrometeor fall speed and aircraft motions are removed). An additional capability on the forward beam permits measurement of the linear depolarization ratio (LDR) which provides useful information on orientation of the hydrometeors (i.e., the canting angle), hydrometeor phase, size, etc. The dual beam geometry has advantages over a single beam. For example, along-track horizontal air motions can be calculated by using the displacement of the ER-2 to provide dual Doppler velocities (i.e., forward and nadir beams) at a particular altitude.

EDOP is designed as a turn-key system with real-time processing on-board the aircraft. The RF system consists of a coherent frequency synthesizer which generates the transmitted and local oscillator frequencies used in the system, a pulse modulated (0.5 to 2.0 micro-second pulse) high gain 20 kW Traveling Wave Tube Amplifier which is coupled through the duplexer to the antenna, and the receiver which is comprised of a low-noise (~1dB) GaAs preamplifier followed by a mixer for each of the receive channels. The composite system generates a nadir oriented beam with a co-polarized receiver and a 350 forward directed beam with co- and cross- polarized receivers. The antenna design consists of two separate offset-fed parabolic antennas, with high polarization isolation feed horns, mounted in the nose radome of the ER-2. The antennas are 0.76 m diameter resulting in a 30 beamwidth and a spot size of about 1.2 km at the surface (assuming a 20 km aircraft altitude). The two beams operate simultaneously from a single transmitter.

The FCAS II sizes particles in the approximate diameter range from 0.07 mm to 1 mm. Particles are sampled from the free stream with a near isokinetic sampler and are transported to the instrument. They are then passed through a laser beam and the light scattered by individual particles is measured. Particle size is related to the scattered light. The data reduction for the FCAS II takes into account the water which is evaporated from the particle in sampling and the effects of anisokinetic sampling (Jonsson et al., 1995).

The FCAS II and its predecessors have provided accurate aerosol size distribution measurements throughout the evolution of the volcanic cloud produced by the eruption of Mt. Pinatubo. (Wilson et al., 1993). Near co-incidences between FCAS II and SAGE II measurements show good agreement between optical extinctions calculated from FCAS size distributions and extinctions measured by SAGE II.

Accuracy: The instrument has been calibrated with monodisperse aerosol carrying a single charge. The FCAS III and the electrometer agree to within 10%. Sampling errors may increase the uncertainty but a variety of comparisons suggests that total uncertainties in aerosol surface are near 30% (Jonsson, et al., 1995).

Precision: The precision equals 1/ÖN where N is the number of particles counted. In many instances the precision on concentration measurements may reach 7% for 0.1 Hz data. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are processed at 0.1 Hz. However, the response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements may be processed with 1 s resolution.

Weight: Approximately 50 lbs.

The CNC counts particles in the approximate diameter range from 0.006 m to 2 m. The instrument operates by exposing the articles to saturated Flourinert vapor at 28 C and then cooling the sample in a condenser at 5 C. The supersaturation of the vapor increases as it is cooled and the vapor condenses on the particles causing them to grow to sizes which are easily detected. The resulting droplets are passed through a laser beam and the scattered light is detected. Individual particles are counted and are referred to as condensation nuclei (CN). Two CN Counters are provided in the instrument. One counts the particles after sampling from the atmosphere and the second counts particles that have survived heating to 192C. Lab experiments show that pure sulfuric acid particles smaller than 0.05 mm are volatilized in the heater. The heated channel detects when small particles are volatile and permits speculation about the composition. The CNC II contains an impactor collector which permits the collection of particles on electron microscope grids for later analysis. The collector consists of a two stages. In the first stage the pressure of the sample is reduced by a factor of two without loosing particles by impaction on walls. The second stage consists of a thin plate impactor which collect efficiently even at small Reynolds numbers. The system collects particles as small as 0.02 m at WB-57 cruise altitudes. As many as 25 samples can be collected in a flight.