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LIRAD Observations of Tropical Cirrus Clouds in MCTEX. Part I: Optical Properties and Detection of Small Particles in Cold Cirrus

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, and CSIRO Atmospheric Research, Aspendale, Victoria, Australia
  • | 2 CSIRO Atmospheric Research, Aspendale, Victoria, Australia
  • | 3 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, and CSIRO Atmospheric Research, Aspendale, Victoria, Australia
  • | 4 CSIRO Atmospheric Research, Aspendale, Victoria, Australia
  • | 5 Desert Research Institute, Reno, Nevada
  • | 6 Naval Research Laboratory, Monterey, California
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Abstract

This paper presents further results on the optical properties of tropical and equatorial cirrus using the light detecting and ranging (lidar) radiometer (LIRAD) method. The results were obtained from observations in the Maritime Continent Thunderstorm Experiment (MCTEX). Values were obtained of cirrus cloud backscatter coefficient, infrared (IR) emittance, optical depth and absorption coefficient, cloud height and depth, and backscatter-to-extinction ratio. The values agree well with previous results obtained on equatorial cirrus in the Pilot Radiation Observation Experiment (PROBE) and extend those results to lower temperatures. Observations made of lidar linear depolarization ratio show similar trends to those observed in PROBE, extending those results to lower temperatures.

Regressions of cloud IR emittance and absorption coefficients are performed as a preliminary tropical dataset for both cloud-resolving and climate models. These regressions are compared with previous regressions on midlatitude and tropical synoptic cirrus clouds. The IR absorption coefficients in tropical and equatorial cirrus appear to be larger than in midlatitude cirrus for temperatures less than −40°C, with the difference increasing toward low temperatures. Thus, a significantly different relationship may be appropriate for tropical cirrus compared to midlatitude cirrus clouds.

Effective diameters of small particles in the colder tropical clouds are also measured using the ratio of visible extinction to infrared absorption. A new treatment of multiple scattering is used to correct the ratios. Effective diameters range from 6 to 9.3 μm at the colder temperatures.

Supplemental information related to this paper is available at the Journals Online Web site: http://dx.doi.org/10.1175/JAS2843supl1

Corresponding author address: Dr. R. T. Austin, Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523-1371. Email: austin@atmos.colostate.edu

Abstract

This paper presents further results on the optical properties of tropical and equatorial cirrus using the light detecting and ranging (lidar) radiometer (LIRAD) method. The results were obtained from observations in the Maritime Continent Thunderstorm Experiment (MCTEX). Values were obtained of cirrus cloud backscatter coefficient, infrared (IR) emittance, optical depth and absorption coefficient, cloud height and depth, and backscatter-to-extinction ratio. The values agree well with previous results obtained on equatorial cirrus in the Pilot Radiation Observation Experiment (PROBE) and extend those results to lower temperatures. Observations made of lidar linear depolarization ratio show similar trends to those observed in PROBE, extending those results to lower temperatures.

Regressions of cloud IR emittance and absorption coefficients are performed as a preliminary tropical dataset for both cloud-resolving and climate models. These regressions are compared with previous regressions on midlatitude and tropical synoptic cirrus clouds. The IR absorption coefficients in tropical and equatorial cirrus appear to be larger than in midlatitude cirrus for temperatures less than −40°C, with the difference increasing toward low temperatures. Thus, a significantly different relationship may be appropriate for tropical cirrus compared to midlatitude cirrus clouds.

Effective diameters of small particles in the colder tropical clouds are also measured using the ratio of visible extinction to infrared absorption. A new treatment of multiple scattering is used to correct the ratios. Effective diameters range from 6 to 9.3 μm at the colder temperatures.

Supplemental information related to this paper is available at the Journals Online Web site: http://dx.doi.org/10.1175/JAS2843supl1

Corresponding author address: Dr. R. T. Austin, Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523-1371. Email: austin@atmos.colostate.edu

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