Premium
Sensitivity of thin cirrus clouds in the tropical tropopause layer to ice crystal shape and radiative absorption
Author(s) -
Russotto R. D.,
Ackerman T. P.,
Durran D. R.
Publication year - 2016
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2015jd024413
Subject(s) - cirrus , ice crystals , tropopause , atmospheric sciences , radiative transfer , ice cloud , cloud physics , absorption (acoustics) , environmental science , relative humidity , stratosphere , physics , meteorology , cloud computing , optics , computer science , operating system
Subvisible cirrus clouds in the tropical tropopause layer (TTL) play potentially important roles in Earth's radiation budget and in the transport of water into the stratosphere. Previous work on these clouds with 2‐D cloud‐resolving models has assumed that all ice crystals were spherical, producing too few crystals greater than 60 μm in length compared with observations. In this study, the System for Atmospheric Modeling cloud‐resolving model is modified in order to calculate the fall speeds, growth rates, and radiative absorption of nonspherical ice crystals. This extended model is used in simulations that aim to provide an upper bound on the effects of ice crystal shape on the time evolution of thin cirrus clouds and to identify the physical processes responsible for any such effects. Model runs assuming spheroidal crystals result in a higher center of cloud ice mass than in the control, spherical case, while the total mass of ice is little affected by the shape. Increasing the radiative heating results in less total cloud ice mass relative to the control case, an effect which is robust with more extreme perturbations to the absorption coefficients. This is due to higher temperatures reducing the relative humidity in the cloud and its environment, and greater entrainment of dry air due to dynamical changes. Comparisons of modeled ice crystal size distributions with recent airborne observations of TTL cirrus show that incorporating nonspherical shape has the potential to bring the model closer to observations.