The tunable diode laser (TDL) absorption instrument consists of a very high resolution scanning near-infrared diode laser spectrometer. The laser diode is a 3 mW single-mode distributed feedback (DFB) InGaAsP/InP laser that is cooled and temperature stabilized via a Peltier cooler. The laser is scanned in frequency by varying the injection current linearly. The resulting frequency scan covers the entire CH4 R(3) ro-vibrational transition in the 2ν3 overtone band at 1.653 μm.

Because the line strengths are very weak for this overtone transition, the laser beam is multipassed through a custom designed low volume astigmatic Herriott cell yielding a total optical pathlength of 245 m. The transmitted light is detected by a dc-coupled InGaAsP detector and digitized by a custom 20-bit A/D converter. This ADC is synchronized to the 16-bit software generated laser scan waveform running in continuous DMA mode. The laser scans continuously over the methane absorption at a rate of 0.25 - 0.5 KHz and coadds typically 100 scans in a 2 second integration time.

By use of the Beer-Lambert law, the methane number density is calculated from the direct absorption measurements. This calculation is performed by a non-linear least squares Voigt fitting program. The program constraints include the measured cell temperature and pressure in addition to the known absorption line strengths and pressure broadening coefficients associated with the three transitions that make up the R(3) lineshape.

The NOAA frost point instrument was designed to run unattended under the wing of NASA’s WB-57. An aircraft rated Stirling cooler provides cooling to 100 K. The cooler avoids consumables and provides a large temperature gradient that improves the response time. The vertical pylon houses the optics and provides aerodynamic pumping of the sample volume. At the bottom of the pylon there is a boundary layer plate and a vertical inlet that separates particles larger than 0.2 microns from the sampled air. There are two channels that use blue LEDs and scattered light to detect frost on the mirrors. Diamond mirrors are used for low thermal mass and high conductivity. The two channels are to be used to understand frost characteristics under flight conditions. High flow rates are used to decrease the shear boundary layer to facilitate diffusion through the boundary layer to the mirrors.

In order to make an accurate temperature and pressure measurement, a Weston digital pressure temperature transducer is used to measure both static and ram pressure. These transducers are accurate to within +/- 0.01 % of full-scale or +/- 0.1 mbar. When the aircraft was manufactured, two ports on either side of the aircraft were placed at positions where the air moving across the skin is perpendicular to the port. These ports are connected together and to the static pressure transducer. The ram pressure measurement consists of a forward-looking tube with a wideangle opening connected to the ram pressure transducer. The ram pressure is calculated by subtracting the static pressure from this measurement.

The temperature probes consist of a slow and fast responding type 102 probe from Goodyear Aerospace Corporation. The platinum wire temperature sensor in the type 102 probe is calibrated to less than +/- 0.1 degree.

Data is gathered once every second from these probes using a custom data system. The Weston pressure transducers are held at a constant temperature of 50 degrees Celsius in order to reduce temperature effects on the measurement and in order to prevent condensation within the sensor. The analog to digital converters are also held at a relatively constant temperature, and a thousand samples from each channel is averaged each second. This over sampling results in a precision of 0.03 degrees in temperature and 0.03 mbars in pressure. We estimate the total accuracy of these measurements in flight to be +/- 0.5 degrees for temperature and +/- 0.5 mb for pressure.

The JPL Laser Hygrometer (JLH) is an autonomous spectrometer to measure atmospheric water vapor from airborne platforms. It is designed for high-altitude scientific flights of the NASA ER-2 aircraft to monitor upper tropospheric (UT) and lower stratospheric (LS) water vapor for climate studies, atmospheric chemistry, and satellite validation. JLH will participate in the NASA SEAC4RS field mission this year. The light source for JLH is a near-infrared distributed feedback (DFB) tunable diode laser that scans across a strong water vapor vibrational-rotational combination band absorption line in the 1.37 micrometer band. Both laser and detector are temperature‐stabilized on a thermoelectrically-cooled aluminum mount inside an evacuated metal housing. A long optical path is folded within a Herriott Cell for sensitivity to water vapor in the UT and LS. A Herriott cell is an off-axis multipass cell using two spherical mirrors [Altmann et al., 1981; Herriott et al., 1964]. The laser beam enters the Herriott cell through a hole in the mirror that is closest to the laser. The laser beam traverses many passes of the Herriott cell and then returns through the same mirror hole to impinge on a detector.

The Cloud Physics Lidar, or CPL, is a backscatter lidar designed to operate simultaneously at 3 wavelengths: 1064, 532, and 355 nm. The purpose of the CPL is to provide multi-wavelength measurements of cirrus, subvisual cirrus, and aerosols with high temporal and spatial resolution. Figure 1 shows the entire CPL package in flight configuration. The CPL utilizes state-of-the-art technology with a high repetition rate, low pulse energy laser and photon-counting detection. Vertical resolution of the CPL measurements is fixed at 30 m; horizontal resolution can vary but is typically about 200 m. The CPL fundamentally measures range-resolved profiles of volume 180-degree backscatter coefficients. From the fundamental measurement, various data products are derived, including: time-height crosssection images; cloud and aerosol layer boundaries; optical depth for clouds, aerosol layers, and planetary boundary layer (PBL); and extinction profiles. The CPL was designed to fly on the NASA ER-2 aircraft but is adaptable to other platforms. Because the ER-2 typically flies at about 65,000 feet (20 km), onboard instruments are above 94% of the earth’s atmosphere, allowing ER-2 instruments to function as spaceborne instrument simulators. The ER-2 provides a unique platform for atmospheric profiling, particularly for active remote sensing instruments such as lidar, because the spatial coverage attainable by the ER-2 permits studies of aerosol properties across wide regions. Lidar profiling from the ER-2 platform is especially valuable because the cloud height structure, up to the limit of signal attenuation, is unambiguously measured.