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L. Xia, F. Zhao, Y. Ma, Z. W. Sun, X. Y. Shen, and K. B. Mao

Abstract

Cirrus clouds play an important role in the global radiation budget balance. However, the existing MODIS and Visible Infrared Imaging Radiometer Suite (VIIRS) cirrus cloud test algorithms struggle to provide accurate cirrus cloud information for the Tibetan Plateau region. In this study, the 1.38-μm cirrus cloud test was improved by adding 11-μm brightness temperature and a multiday average land surface temperature test. An algorithm sensitivity analysis indicated that the proposed algorithm lowered the threshold of the existing 1.38-μm algorithm to 0.005 in the winter and did not produce any observable misclassifications. Compared to the existing 1.38-μm cirrus test algorithm, the accuracy validation indicated that the improved algorithm detected 31.7% more cirrus clouds than the existing VIIRS 1.38-μm cirrus test and yielded 14% fewer misclassifications than the MODIS 1.38-μm cirrus test.

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S. A. Ackerman, W. L. Smith, A. D. Collard, X. L. Ma, H. E. Revercomb, and R. O. Knuteson

Abstract

This paper parts analysis of cloud observations by the High-Resolution Interferometer Sounder made from the NASA ER-2 aircraft during FIRE II. Clear and cloudy sky radiance spectra are presented in terms of differences between observations and radiative transfer model simulations.

Doubling/adding radiative transfer model simulations demonstrate that the magnitude of the brightness temperature differences (ΔBT) is a function of the cloud particle size distribution and the cloud ice water path. For effective radii greater than approximately 30 µm (size parameter of 18) there is little spectral variation in the brightness temperature (BT). An analysis of brightness temperature differences indicates that cirrus clouds over the FIRE II central site possessed a small-particle mode. The cases analyzed had similar appearances in a plot of ΔBT between 11 and 12 µm (BT11 – BT12) versus the observed ΔBT between 8 and 11 µm (BT8 – BT11), suggesting similarity in the microphysical properties of nongray cirrus. Brightness temperature differences between cirrus cloud over the central site and the Gulf of Mexico are presented to illustrate differences in the cirrus microphysical properties at the two different locations.

Cloud effective emissivities and effective radiative temperature were derived for observations over the FIRE central site using complementary lidar and radiosonde data. Small variations in these effective properties were seen on 5 December and 22 November. Although they had similar effective temperatures, the emissivities were very different. Very few clouds were observed to have an emissivity near unity.

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A. D. Collard, S. A. Ackerman, W. L. Smith, X. Ma, H. E. Revercomb, R. O. Knuteson, and S-C. Lee

Abstract

During FIRE II, cirrus clouds were observed in the wavelength range 3–19, µm with two High Resolution Interferometer Sounders as described in the Part I companion paper. One, known as AC-HIS, was mounted on the NASA ER-2 aircraft in order to look down on the clouds; these results are described in the Part II companion paper. The other, GB-HIS, also known as the Atmospheric Emitted Radiance Interferometer (AERI), was ground based. The AERI observations have been simulated, assuming scattering from spherical ice particles, using a single-layer doubling model for the cloud, for two atmospheric windows at 700–1250 and 2650–3000 cm−1. The second of these windows is affected by scattered sunlight, which has been included in the calculations. The sensitivity of the cloud signal to quantities such as the ice water path (IWP) and effective radius (r eff) have been determined. Using the cloud model, best fits have been derived for IWP and r eff, for both windows individually and together. Possible errors in these derivations have been investigated.

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