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).
Highlights
The existence of ice-supersaturation in the atmosphere is known for more than 100 years (Wegener, 1914; Gierens 20 et al, 2012), the investigation of physical and dynamical characteristics of ice-supersaturated regions (ISSRs) only commenced about 25 years ago, mainly because the -new MOZAIC project (Measurement of OZone and water vapor by Airbus Inservice airCraft, Marenco et al, 1998) started delivering large amounts of humidity data in the upper troposphere where ISSRs are quite frequent
The transition from general ISSRs to the smaller class of Big Hits that require higher degrees of supersaturation leads to a narrowing of the lapse-rate distribution to the highest values, whereby the peak stays almost the at the same value
From a combination of ten years of measurement data from the MOZAIC/In-service Aircraft for a Global Observing System (IAGOS) project, from which we take the information on relative humidity at aircraft positions, with ERA-5 reanalysis data, from which we take the information on temperatures on adjacent pressure levels, we determine probability distribution and density functions, conditioned on three 290 situations: 1) subsaturated airmass, 2) ice-supersaturation and 3) situations with strongly warming contrails (Big Hits, a subset of 2)
Summary
The existence of ice-supersaturation in the atmosphere is known for more than 100 years (Wegener, 1914; Gierens 20 et al, 2012), the investigation of physical and dynamical characteristics of ice-supersaturated regions (ISSRs) only commenced about 25 years ago, mainly because the -new MOZAIC project (Measurement of OZone and water vapor by Airbus Inservice airCraft, Marenco et al, 1998) started delivering large amounts of humidity data in the upper troposphere where ISSRs are quite frequent. The European Centre for Medium Range Weather Forecast (ECMWF) was the first weather centre to introduce ice-supersaturation in their forecast model in 2006 (model version C31R1 of the integrated forecast system, IFS, Tompkins et al, 2007) and ice supersaturation was later represented in its reanalysis products ERA-Interim (Dee et al, 2011) and the most recent one ERA-5 (Hersbach et al, 2018a, b, 2020). These data made it possible to investigate the distribution of vertical velocity, divergence and relative vorticity inside and outside of ISSRs (Gierens and Brinkop, 2012). Characteristic differences were found in a study covering Europe and the Tropics and four months of data (June, September and December 2011 and March 2021): Ice supersaturation occurs predominantly in uplifting divergent airmasses and in anticyclonic circulation.
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