Impact of ice supersaturated regions and thin cirrus on radiation in the midlatitudes

Abstract

In this study we investigate the radiative impact of ice supersaturated regions (ISSRs, i.e., cloud free air masses in the upper troposphere that are supersaturated with respect to ice) and thin cirrus. For this purpose we use corrected radiosonde data obtained from routine measurements over the meteorological observatory in Lindenberg, Germany. The radiative effect of the measured ice supersaturation is determined. By constructing an idealized profile from the measurement data the radiative properties of ISSRs and thin cirrus containing ice supersaturation were studied. The impact of ISSRs on the surface forcing is negligible but locally, within the vertical profile, changes in the heating rates up to 1 K d -1 for typical values of 130% relative humidity with respect to ice compared to the saturated profiles are found. This is also important for the local dynamics within the supersaturated layers. The outgoing longwave radiation due to the enhanced water vapor content inside ISSRs decreases up to 0.8 W m -2. The radiative impact of thin cirrus is much stronger. Thin cirrus influence the surface budget, the top of the atmosphere radiation and the vertical profile of the heating rates. Changes in the outgoing longwave radiation and in the reflected shortwave flux at top of the atmosphere up to 64 W m -2 and 79 W m -2, respectively, are possible. Changes in the surface flux (downward) up to 89 W m -2 are found. The maximal heating rate differences between thin cirrus and ISSR amount to 15 K d -1. The radiative impact of thin cirrus clouds depends strongly on cloud ice content and the size of the ice crystals. Additionally, the radiative impact of cirrus formed by parameterizations not allowing ice supersaturations in large-scale models is estimated. The errors due to artificially formed cirrus are quite large; differences up to 3 K d -1 in the vertical heating profiles and up to 38 and 40 W m -2 in the outgoing longwave radiation and the surface flux, respectively, are found. Thus we recommend using physically based parameterizations in GCMs which allow ice supersaturation. Copyright 2007 by the American Geophysical Union.