P-3 Orion - WFF

The NASA Langley airborne High Spectral Resolution Lidar (HSRL) is used to characterize clouds and small particles in the atmosphere, called aerosols. From an airborne platform, the HSRL science team studies aerosol size, composition, distribution and movement.

The HSRL-1 instrument is an innovative technology that is similar to radar; however, with lidar, radio waves are replaced with laser light. Lidar allows researchers to see the vertical dimension of the atmosphere, and the advanced HSRL makes measurements that can even distinguish among different aerosol types and their sources. The HSRL technique takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals and thereby measure aerosol extinction and backscatter independently.

The HSRL-1 instrument provides measurements of aerosol extinction at 532 nm and aerosol backscatter and depolarization at 532 and 1064 nm. The HSRL measurements of aerosol extinction, backscattering, and depolarization profiles are being used to:

1) characterize the spatial and vertical distributions of aerosols
2) quantify aerosol extinction and optical thickness contributed by various aerosol types
3) investigate aerosol variability near clouds
4) evaluate model simulations of aerosol transport
5) assess aerosol optical properties derived from a combination of surface, airborne, and satellite measurements.

The APR-2 is a dual-frequency (13 GHz & 35 GHz), Doppler, dual-polarization radar system. It has a downward looking antenna that performs cross track scans, covering a swath that is +/- 25 to each side of the aircraft path. Additional features include: simultaneous dual-frequency, matched beam operation at 13.4 and 35.6 GHz (same as GPM Dual-Frequency Precipitation Radar), simultaneous measurement of both like- and cross-polarized signals at both frequencies, Doppler operation, and real-time pulse compression (calibrated reflectivity data can be produced for large areas in the field during flight, if necessary).

Measures concentration and records images of cloud particles from approximately 50-1600 microns in diameter with a resolution of 25 microns per pixel. Measures cloud droplet and aerosol concentrations within the size range of 0.5-50 microns.

The three DMT instruments included in the CAPS are the Cloud Imaging Probe (CIP), the Cloud and Aerosol Spectrometer (CAS), and the Hotwire Liquid Water Content Sensor (Hotwire LWC).

The CIP, which measures larger particles, operates as follows. Shadow images of particles passing through a collimated laser beam are projected onto a linear array of 64 photodetectors. The presence of a particle is registered by a change in the light level on each diode. The registered changes in the photodetectors are stored at a rate consistent with probe velocity and the instrument’s size resolution. Particle images are reconstructed from individual “slices,” where a slice is the state of the 64-element linear array at a given moment in time. A slice must be stored each time interval that the particle advances through the beam a distance equal to the resolution of the probe. Optional grayscale imaging gives three levels of shadow recording on each photodetector, allowing more detailed information on the particles.

The CAS, which measures smaller particles, relies on light-scattering rather than imaging techniques. Particles scatter light from an incident laser, and collecting optics guide the light scattered in the 4° to 12° range into a forward-sizing photodetector. This light is measured and used to infer particle size. Backscatter optics also measure light in the 168° to 176° range, which allows determination of the real component of a particle’s refractive index for spherical particles.

The Hotwire LWC instrument estimates liquid water content using a heated sensing coil. The system maintains the coil at a constant temperature, usually 125 °C, and measures the power necessary to maintain this temperature. More power is needed to maintain the temperature as droplets evaporate on the coil surface and cool the surface and surrounding air. Hence, this power reading can be used to estimate LWC. Both the LWC design and the optional PADS software contain features to ensure the LWC reading is not affected by conductive heat loss.

The Center has been developing a wideband radar altimeter that operates over the frequency range from 13 to 17 GHz. The primary purpose of this radar is high precision surface elevation measurements over polar ice sheets. The data collected with this radar can be analyzed in conjunction with laser-altimeter data to determine thickness of snow over sea ice. The radar has been flown on a NASA DC-8 aircraft, and the NSF provided a Twin Otter aircraft.

TSI Integrating Nephelometers are designed specifically for studies of direct radiative forcing of the Earth’s climate by aerosol particles, or studies of ground-based or airborne atmospheric visual air quality in clean areas. They may also be used as an analytical detector for aerosol particles whenever the parameter of interest is the light-scattering coefficient of the particles after a pretreatment step, such as heating, humidification, or segregation by size. The light-scattering coefficient is a highly variable aerosol property. Integrating Nephelometers measure the angular integral of light scattering that yields the quantity called the aerosol scattering coefficient, which is used in the Beer-Lambert Law to calculate total light extinction.

The Differential Absorption Lidar uses the backscatter of two simultaneous laser wavelengths through zenith and nadir windows to measure the vertical profiles of H2O and aerosols/clouds.

NASA's Lidar Atmospheric Sensing Experiment (LASE) system is an airborne DIAL (Differential Absorption Lidar) system used to measure water vapor, aerosols, and clouds throughout the troposphere. LASE probes the atmosphere using lasers to transmit light in the 815-nm absorption band of water vapor. Pulses of laser light are fired vertically below the aircraft. A small fraction of the transmitted laser light is reflected from the atmosphere back to the aircraft and collected with a telescope receiver. The received light indicates the amount of water vapor along the path of the laser beam.

Langley Aerosol Research Group Experiment (LARGE).  The "classic" suite of instrumenation measures in-situ aerosol micrphysical and optical properties. The package can be tailored for specific science objectives and to operate on a variety of aircraft. Depending on the aircraft, measurments are made from either a shrouded single-diffuser "Clarke" inlet, from a BMI (Brechtel Manufacturing Inc.) isokinetic inlet, or from a HIML inlet. Primary measurements include:

1.) total and non-volatile particle concentrations (3nm and 10nm nominal size cuts),
2.) dry size distributions from 3nm to 5µm diameter using a combination of mobilty-optical-aerodynamic sizing techniques,
3.) dry and humidified scattering coefficients (at 450, 550, and 700nm wavelength), and
4.) dry absorption coefficients (470, 532, and 670nm wavelength).

LARGE derived products include particle size statistics (integrated number, surface area, and volume concentrations for ultrafine, accumulation, and coarse modes), dry and ambient aerosol extinction coefficients, single scattering albedo, angstrom exponent coefficients, and scattering hygroscopicity parameter f(RH).

SPEC previously built the Version 1 and Version 2 HVPS probes that have now been discontinued due to obsolete parts and significant advances in technology. The HVPS-3 uses the same 128-photodiode array and electronics that are used in the 2D-S and 2D-128 probes. The optics are configured for 150 micron pixel resolution, resulting in a maximum field of view of 1.92 cm (i.e., particles up to 1.92 cm are completely imaged, although even larger particles can be sized in the direction of flight).

Sample volume of the HVPS-3 is 400 L s-1 at 100 m s-1. The 2D-S or 2D-128 and HVPS make an excellent pair of probes that completely image particles from 10 microns to 1.92 cm.