The near-surface zero degree line (ZDL) is a key isotherm in mountain regions worldwide, but a detailed analysis of methods for determining the ZDL and their applicability in a changing climate is missing. Focusing on the Swiss Alps, a recent paper published in the International Journal of Climatology intercompares different approaches to determine the near-surface ZDL on a monthly scale, and investigates the past evolution of the ZDL in the Swiss Alps.

Atmospheric isotherms (i.e., lines of equal or constant air temperature) are a central concept in climatology. Their near-surface patterns are used in climate classification and climate zoning and inform the spatial and vertical distribution of ecosystems and of cryospheric components of the hydrological cycle. In mountainous terrain, a special isotherm is of particularly high relevance: the zero degree line (ZDL). The ZDL roughly separates regions where precipitation predominantly falls as snow or as rain. It is connected to both the snowline and the equilibrium line of glaciers.

Despite its relevance, a research gap so far exists regarding the spatial ZDL variability, its long-term changes, and its sensitivity to regional climate change. The recent work by Scherrer et al. is devoted to shedding more light on this situation, comparing and evaluating existing approaches for deriving the ZDL with the ultimate goal of enabling a climatological analysis of the temporal ZDL evolution in a climate change context. For this purpose, the Swiss Alps serve as a well-monitored testbed.

A tricky quantity

Despite its obviously simple definition, deriving the actual ZDL is often not too straightforward. The near surface ZDL, for instance, can considerably differ from the freezing level height in the free atmosphere as processes at the surface-atmosphere interface and low level dynamics play an important role. Near-surface temperature patterns can show an important spatial variability, especially in wintertime due to topographic influences, low-level fog, and cold air pooling. Elevated and persistent fog layers can cause important temperature inversions that could feature more than one zero degree level. Today's comparatively dense Alpine measurement network can partly resolve these variabilities, but there is a clear lack of spatially explicit information covering climatological time periods of 30 years and more.

A new approach

Based on previous works, Scherrer et al. present a new approach to derive the near-surface ZDL in Alpine terrain that partly overcomes the mentioned caveats and limitations (cf. Figure 1). Among others, it enables the analysis of the mean monthly ZDL evolution during the past about 150 years for larger regions such as Northern and Southern Switzerland. The method consists of fitting a flexible, non-linear vertical temperature profile to measurements obtained at a large number of meteorological stations covering an important elevation range. The new approach yields more reliable results compared to traditional methods that assume a constant temperature decrease with elevation, i.e., a linear vertical temperature profile.

Figure 1: Schematic representation of the determination of the near-surface zero degree line based on ground measurement data (left) and a flexible temperature profile (centre) as well as the temporal evolution derived from it (right).

A rising ZDL in all months and a recent acceleration

For Northern Switzerland, results based on the new approach reveal an increase of the ZDL by about 300 to 400 m in the period 1871-2019 in most months (cf. Figure 2). Smaller increases are obtained for April and September (200 to 250 m) and larger increases for October, December and January (>500 m). While the mean winter ZDL was located at the elevation of the largest Swiss cities during 1871-1900 (Zurich, Geneva), it today reaches pre-alpine locations such as Einsiedeln or Château-d'Oex. However, it is noted that the wintertime ZDL increase is subject to especially large uncertainties that require a careful interpretation of these values.

In a temporal context, today's mean March ZDL elevation corresponds to the mean April elevation in the period 1871-1900, i.e., has been subject to a temporal shift by about one month since the beginning of systematic measurements in Switzerland. The ZDL increase has shown an important acceleration in the past five decades, especially in spring and summer when increases can amount to more than 100 m per decade. Regarding the spatial ZDL variability, the winter ZDL elevation is several hundred meters higher in the southern Swiss Alps compared to Northern Switzerland, but the southern increase rate is slightly smaller than in the north in most months.

Figure 2: Mean annual variation in the elevation of the near-surface zero degree line in northern Switzerland for the near-real time period 1990-2019 (red) and the pre-industrial period 1871-1900 (blue). Values are given for January and August. The altitudes of some well-known Swiss places and peaks are shown on the right.

What next?

The new approach for deriving the ZDL has been developed and tested for the Swiss Alps but is, in principle, transferable to other regions. In addition to the ZDL it can be applied to further atmospheric isotherms which would enable a specific analysis of temperature threshold with high relevance for sectors such as mountain ecology or natural hazard assessment. It is also planned to extend the ZDL analysis into the future and to produce quantitative projections of the future ZDL evolution in Switzerland based on the CH2018 Swiss climate scenarios.

Read more

Scherrer, S. C. et al. 'The Swiss Alpine zero degree line: Methods, past evolution and sensitivities.' International Journal of Climatology (2021): https://doi.org/10.1002/joc.7228.

The authors are happy to receive any further input on their study and suggestions for future research.

This article was provided by Sven Kotlarski and Simon C. Scherrer, MeteoSwiss.