NASA - National Aeronautics and Space Administration

The University of Colorado Closed-path Laser Hygrometer, version 2 (CLH2) is an infrared absorption instrument designed to measure so-called “total water”, the sum of water vapor and particulate water. It is a second-generation sensor that derives from the original CLH and was developed for the NSF DC3 campaign in 2011 as an alternative to the NCAR CVI for measurements of cloudwater contents. It has flown on the NASA DC-8 and the NSF/NCAR G-V and C-130. The most recent campaign was NSF SOCRATES in 2018. CLH-2 uses a fiber-coupled tunable diode laser at 1.37 μm to measure by absorption the water vapor resulting from the evaporation of cloud particles. The spectrometer will be housed in a modified PMS canister and coupled to a heated forward-facing inlet. Sampling of particles is deliberately sub-isokinetic, which results in enhancements of particle mass relative to ambient by factors ranging between 30 and 70. Therefore, condensed water even in very thin clouds can be measured with high precision and accuracy.

Total water is measured in situ as vapor with a Lyman-Alpha hygrometer. High ambient sample flows through a closed cell minimize the effect of trapped water. Lyman-a light (121.6 nm) photodissociates water to produce an excited OH radical. The fluorescence from this radical at 309 nm is detected with a phototube and counting system. At aircraft pressures the fluorescence signal is quenched by air which gives a signal that is proportional to mixing ratio. The Lyman-Alpha radiation produced with a DC-discharge lamp is monitored with an iodine ionization cell that is sensitive from 115 nm to 135 nm. Calibration occurs in flight by injecting water vapor directly into the ambient sample flow.

The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses.

The sensitivity of the probe is estimated to be approximately 0.003– 0.005 g m23. The accuracy of LWC measurements in nonprecipitating liquid clouds is estimated as 10%–15%. Tests at the NRC high-speed icing tunnel have provided verification of the TWC measurement for small frozen droplets to an accuracy of approximately 10%–20%, but verification in snow and natural ice crystals has not yet been possible due to the absence of any accurate standards. The TWC measurement offers not only the possibility of direct measurements of ice content but also improved liquid water contents in drizzle situations. Airborne measurements have provided data on the baseline drift and sensitivity of the probe and have provided comparisons to other conventional instruments. Several cases have been documented that exhibit the unique capabilities of the instrument to separate the ice and liquid components of supercooled clouds.

The MAMS is a modified Daedalus Scanner flown aboard the ER-2 aircraft. It is designed to study weather related phenomena including storm system structure, cloud-top temperatures, and upper atmospheric water vapor. The scanner retains the eight silicon-detector channels in the visible/near-infrared region found on the Daedalus Thematic Mapper Simulator, with the addition of four channels in the infrared relating to specific atmospheric features.

The scanner views a 37 kilometer wide scene of the Earth from the ER2 altitude of about 20 kilometers. Each MAMS footprint (individual field of view) has a horizontal resolution of 100 meters at nadir. Since the ER2 travels at about 208 meters per second, a swath of MAMS data 37 by 740 kilometers is collected every hour. The nominal duration of an ER2 flight is 6 hours (maximum of about 7 hours).

The design of the newly developed total water instrument is based on the same principles as the water vapor instrument, and is intended to fly in conjunction with it. Conceptually, the total water instrument can be thought of as containing four subsystems:
1. An inlet through which liquid and/or solid water particles can be brought into an instrument duct without perturbing the ambient particle density.
2. A heater that efficiently evaporates the liquid/solid water before it reaches the detection axis.
3. Ducting through which the air flows to the detection axis without perturbing the (total) water vapor mixing ratio.
4. A water vapor detection axis that accurately and precisely measures the total water content of the ambient air.

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 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.