Retrieval of Ice-Water Content Using Near-Infrared Remote Sensing

 

Research Staff: Peter Pilewskie and Warren Gore

Field Observations

The Ames Atmospheric Radiation Group completed a major field program, the Department of Energy Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) II in February/March, 2000. A newly modified Solar Spectral Flux Radiometer (SSFR) was integrated on the Sandia National Laboratories Twin Otter which flew six successful missions over the DOE ARM Cloud and Radiation Testbed (CART) site in north central Oklahoma. An additional SSFR was operated on the ground at the CART site. These data will be used to determine the extent to which models can describe the spectral, spatial, and temporal radiative properties of clouds and, in particular, how much solar radiation is absorbed in clouds. The ARESE II flight data will be processed, quality assessed, and archived in FY2001. These data will be corrected for aircraft attitude using the onboard aircraft navigational system data. We will compare spectral irradiance with radiative transfer calculations to assess the effectiveness of our ability to model both the clear and cloudy sky radiative energy budget.

A paper by Stephens et al. (see Publication List) was published recently that included data from the joint NASA ERAST/ARM-UAV Hawaii experiment in Kauai, Hawaii, in April and May, 1999. The SSFR was integrated on the NASA Altus UAV (Uninhabited Aerial Vehicle) and the DOE Sandia Twin Otter in both zenith and nadir viewing modes. These data were used to assess the radiative characteristics of high sub-tropical cirrus and to assess its effects on the radiative energy budget.

Modeling and Algorithm Development for Cloud Ice-Water Retrieval

Radiative Transfer modeling:

We have completed work on the following aspects of a k-distribution model which will be used for forward calculations in our cloud water retrieval algorithm:

  1. generation of the k's for H2O, CO2, O2 and O3 from the latest LBLRTM (Line-By-Line Radiative Transfer Model) and the latest HITRAN database (with corrections; see the report by Giver et al.);
  2. construction of the algorithm for computing the k's for any arbitrary pressure and temperature (an interpolation in log pressure and temperature from a standard atmosphere) for the 140 spectral bands (16 sub intervals per band); the bands range from 300 nm to 1700 nm in 10 nm increments;
  3. generation of correlation of the k's and the instrument specific filter function for all of the bands. This allows the direct comparison of the predictions with the data;
  4. integration of the DISORT scattering code to the program;
  5. successful test runs of 140 bands, 2 layer H2O only atmosphere, simple aerosol properties, 16 scattering angle DISORT computations (time required was about 5 minutes per spectra on a Pentium III, 500 MHz PC running the Linux operating system);
  6. generation of the solar spectra (from Kurucz calculations) for all 140 bands accounting for the correlations of the H2O lines and the solar emission lines.

Determination of Photon Path lengths:

A large part of our effort to derive cloud water paths using near-infrared cloud hyperspectra depends upon using a permanent gas absorption band to derive photon path lengths. We have begun to analyze the ground SSFR data acquired during the ARESE II experiment at the CART facility. Data from clear conditions were used to derive Oxygen A-, B-band, and 1267 nm O2 transmission and 2000 nm CO2 transmissions.

Collaborators: Sean Twomey, Maura Rabbette and Robert Bergstrom, Bay Area Environmental Research Institute

Point of Contact: Peter Pilewskie, 650/604-0746, ppilewskie@mail.arc.nasa.gov