Fractal Characteristics and Seasonal Variations of Ice-SupersaturatedRegions (ISSRs) in the Tropopause Region

Ice supersaturated regions (ISSRs) are area in the upper troposphere and lower stratosphere (UTLS) where relative humidity (RH) with respect to ice exceeds 100%. These regions are critical for the formation of cirrus clouds and contrails. While the impact of ISSRs on the planetary radiation balance is small to negligible, the thin cirrus clouds and aircraft induced contrail cirrus formed within them have a large radiative impact. Understanding the characteristics of ISSRs, including their geometry and seasonal variability, is essential for improving atmospheric models. While ISSR path-length statistics have been studied, their geometric properties, particularly fractal characteristics, and their seasonal variability remain largely unexplored.

We identify ISSRs using ERA5 reanalysis data spanning from 2010 to 2020 at three pressure levels. An area-perimeter method is employed to compute fractal dimensions. The results reveal slopes equaling fractal dimensions with high coefficients of determination, strongly suggesting that ISSRs in the UTLS exhibit fractal behavior. A seasonal cycle in both dimension and total count of observed ISSRs was verified on both hemispheres. We hypothesize that this is caused by the seasonal variation of convective and frontal activity.

We further analyzed the latitudinal and longitudinal spans of ISSRs and the path lengths of modeled flights along common IAGOS flight routes. The results of the latitudinal and longitudinal spans suggest two distinct formation processes for ISSRs.