Impact of Crystal Habit on Cirrus Radiative Properties

The impact of assumed ice crystal morphology of subtropical cirrus on the solar and thermal infrared (IR) radiative field above, within, and below the cirrus is quantified. For this purpose airborne measurements of ice crystal size distribution from the CRYSTAL-FACE campaign and a library of optical properties of nonspherical ice crystal habits are implemented into radiative transfer simulations.Two cirrus cases are studied in detail: a high (cold) cirrus cloud with small visible optical thickness (τ≈1), and a lower (warmer) cirrus cloud of relatively large visible optical thickness (τ≈7). For t+he solar wavelength range the impact of shape characteristics of the crystals was important for the reflected irradiance above the optically thin cirrus, especially for small solar zenith angles. In this case single-scattering dominated the solar radiation field. For the cirrus of large visible optical thickness the enhanced multiple scattering tended to diminish particular shape features in solar radiation caused by nonspherical single-scattering.The relative effects of ice crystal shape on thermal IR irradiance are substantial for the high, optically thin cirrus (up to 70%). Spectrally, the largest effects of ice crystal shape are identified in the atmospheric window (8 to 12 μm wavelength), especially for the upwelling irradiance above the cirrus.For the low cirrus of large visible optical thickness the thermal IR irradiance is only slightly sensitive to ice crystal habit (less than 15-20%).