Comment on acp-2022-117

Abstract

In this paper, the effects of ice-supersaturated regions and thin, subvisual cirrus clouds on lapse rates are examined. For that, probability distribution and density functions of ten years of measurement data from the MOZAIC/IAGOS project and ERA-5 reanalysis data were produced, and an analysis of an example case of an ice supersaturated region with a large vertical extent is performed. For the study of the probability distribution and density functions, a distinction is made between ice-subsaturated, ice-supersaturated air masses and so-called Big Hits, which are situations of particularly high ice-supersaturation that allow the formation of optically thick and strongly warming contrails. The distribution functions show much higher lapse rates, which correspond to almost neutral stratification, for ice-supersaturated regions and Big Hits than for subsaturated air masses. The highest lapse rates are found for Big Hit situations, because of the strong interaction between radiation and high ice-supersaturation. For the examination of an example case, ERA-5 data and forecasts from ICON-EU (DWD) are compared. ERA-5 data, in particular, shows a large ice-supersaturated region below the tropopause, that was pushed up by uplifting air, while the data of ICON-EU indicates areas of saturation. The lapse rate in this ice-supersaturated region (ISSR), which is large, is associated with clouds and high relative humidity. Supersaturation and cloud formation result from uplifting of air layers. The temperature gradient within an uplifting layer steepens, both for dry and moist air, but for moist air there is an additional mechanism: it is the emission and absorption of radiation within the moist air: The upper part of this region emits longwave infrared radiation to space, while the bottom absorbs infrared radiation from lower and warmer layers, which consequently increases the lapse rate. This effect becomes even stronger, if ice crystals are involved (clouds).