1Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland
2Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
3Geophysical Institute, University of Alaska, P.O. Box 757320, Fairbanks, AK 99775-7320, USA
4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract. Recent laboratory experiments showed that at conditions resembling those near the tropopause region, small quasi-spherical ice particles can be coated by a liquid H2SO4/H2O over-layer formed after the freezing of diluted sulfuric acid/water aerosol drops. Here, idealized radiative transfer tests are conducted to evaluate the impact that such an over-layer would have on the radiative effects produced by sub-visible cirrus clouds (SVCs). The calculations show that the over-layer increases both the shortwave (SW) and longwave (LW) cloud radiative effects (CRE), but the impact is small: ~0.02 W m-2, or even less, for the total (LW+SW) CRE at the top of the atmosphere. For the smallest ice particles, for which the over-layer is thickest, the fractional change in CRE can, however, reach ~20% for the SW CRE and over 50% for the LW CRE. The dependence of LW and SW CRE on particle size is also considered in the paper. Calculations for spherical uncoated ice particles show that even for high, optically thin cirrus clouds, the total CRE can be negative, if the diameter of the particles is smaller than about 3–4 µm. Apart from the SVCs, this result could be relevant for contrail cirrus clouds, which are believed to consist of large numbers of very small ice particles.