The Arctic Facility for Atmospheric Remote Sensing—AFARS

After recently joining the Atmospheric Sciences Group of the University of Alaska Fairbanks, Professor Kenneth Sassen is in the process of establishing a major new research facility at the Geophysical Institute. The Arctic Facility for Atmospheric Remote Sensing, AFARS, will be applied to the research of the peculiar clouds and aerosols of the Interior and North Slope of Alaska. Of special interest are the frigid clouds of the upper-troposphere, cirrus clouds and aircraft condensation trails (contrails), because these clouds can have a profound effect on the Earths’ climate, and yet are almost completely unstudied in polar regions. Also of interest are the aerosols of this region, the wintertime Arctic haze, the springtime Asian dust storm particles, and the summer smokes from western forest fires.

The instruments comprising AFARS are drawn largely from the facility Prof. Sassen established over the past 15-years at the University of Utah to study midlatitude clouds; for more information on the instruments and this data record visit the Web site at, http://www.met.utah.edu/ksassen/fars.html.

Current plans at UAF include the establishment of a multiple remote sensor facility on campus on the roof of the Elvy Building,, and an additional facility, the mobile Polarization Diversity Lidar (PDL), to be located initially at the Poker Flat lidar site and then at the Department of Energy Atmospheric Radiation (ARM) site at Barrow.

Located at the campus AFARS site are the Polarization Cloud Lidar (PCL) based on ruby laser (0.694 mm wavelength) transmitter, a 3.2-mm wavelength polarimetric Doppler W-band radar, and a midinfrared narrow-beam radiometer. The current specifications of the PCL and the W-band radar are given in Table 1 and 2 , respectively. These instruments point into the zenith direction, which along with other radiometers to be added soon, uniquely characterize the heights and types of clouds and aerosols present above the site. This research is being supported by the National Science Foundation for the basic study of Arctic cloud and aerosol properties, and by NASA, which has interests in compiling data that helps validate and improve the algorithms used by satellite-based radiometers to interpret the clouds and aerosols in the atmosphere. This information also helps in the development of ground-based and upcoming satellite-based approaches to characterize the atmosphere for improved climate research. Prof. Sassen is involved in two NASA satellite development programs for which the AFARS instruments are well-suited to support: the CloudSat satellite with a W-band radar, and the CALIPSO satellite bearing a polarization lidar.

The remaining major remote sensor, the PDL (see Table 3 for specifications), was developed as a testbed for polarization lidar techniques within the initial instrument development phase of the ARM program. Funding has been obtained from the Department of Energy to station the PDL at the North Slope of Alaska ARM site for an extended period, so that periodic mini-field campaigns can be conducted throughout the year under changing Arctic weather conditions. Special attention will be given to cirrus and mixed-phase clouds, cloudless precipitation that develops from open leads in the nearby Arctic Ocean, and the exotic aerosols ranging from Arctic haze to Asian dust storm particles.

Before barging the lidar van up to Barrow, the PDL will be improved through the addition of a nitrogen Raman receiver channel under support from the National Science Foundation. Raman lidar technology permits the direct determination of the extinction coefficients produced by aerosols and clouds, and so is an important supplement to the normal elastic, or Mie lidar research capabilities. Gathering a large sample of backscatter-to-extinction ratios under Arctic conditions will aid in improving radiative transfer calculation in climate models, and particularly be a boost to better interpreting the data from the CALIPSO satellite.

TABLE 1: Polarization Cloud

Lidar (PCL) System*

Current Specifications

____________________________________________________

Operational

Wavelength (Ruby) 0.694 mm

Peak Energy 1.5 J

Maximum PRF 0.1 Hz

Pulse Width 27 ns

Beamwidths - Transmitter 1.0 mrad

Receiver 1.0-3.0 mrad

Receiver Diameter 25 cm telescope

Detectors - Visible 2 PMT's

Scan Rate Manual

Data Handling

Number of Channels 2 (simultaneous)

Sample Width (resolution) 7.5 m maximum

Range gates 4 k maximum

Pulses Averaged 1

Digitizer Resolution 8 bits

Storage 8 mm tape

Polarization Properties

Transmitted Vertical

Received Vert. + Horiz.

______________________________________________________

* Additional Equipment

a. All-sky video imager with time-lapse VCR

b. PRT-5 narrow-beam (0.14°) mid-IR (9.5-11.5 mm) radiometer +

c. Camcorder camera +

(+ aligned parallel to transmitter on lidar table)

TABLE 2: 95 GHz Polarimetric

Doppler Radar

Current Specifications

____________________________________________________

Operational

Wavelength (W-band) 3.2 mm

Peak Power 1.2 KW

PRF 10 Hz – 80 KHz

Pulse Width 27 ns

Beamwidth 0.25 °

Receiver Diameter 90 cm dish

Receiver Gain 57 dB

Maximum Scan Rate 5.0° s -1

Data Handling

Number of Channels 6, two Doppler

Sample Width (resolution) 75 m maximum

Range gates 600 maximum

Pulses Averaged Programable

Digitizer Type Logarithic

Storage 8 mm tape

Polarization Properties

Transmitted Vert. + Horiz.

Received Vert. + Horiz.

______________________________________________________

TABLE 3: Two-Color Polarization

Diversity Lidar (PDL) System*

Current Specifications

____________________________________________________

Operational

Wavelength (Nd:YAG) 0.532 + 1.06 mm (simultaneous)

Peak Energy 0.35 J each color

Maximum PRF 10 Hz

Pulse Width 9 ns

Beamwidths - Transmitter 0.5 mrad

Receiver 0.2-3.8 mrad high-speed shutter

Receiver Diameter 30 cm (2 telescopes)

Detectors - Visible 2, Gated PMT's

IR 2, SAPD's

Maximum Scan Rate 5.0° s -1

Data Handling

Number of Channels 4 (simultaneous)

Sample Width (resolution) 1.5 m maximum

Range gates 8 k maximum

Pulses Averaged 1 - 10

Maximum throughput 164 k samples/second

Digitizer Resolution 8 bits

Storage 8 mm tape

Polarization Properties

Transmitted Vert. (Vis) + Horiz. (IR)

Received Vert. + Horiz. (Vis. + IR)

______________________________________________________

* Additional Equipment

a. Camcorder camera

b. X-band safety laser-shutdown radar