Matthias Zahn

PhD Thesis

#1: Climate mode simulation of North Atlantic Polar Lows in a limited area model

The main finding in an ensemble of case studies is: polar lows are reasonably simulated in climate mode (i.e., independent of intial conditions), if the regional climate model ( CLM in this case) is large-scale-constrained with global re-analysis.

#2: Tracking Polar Lows in CLM

The tracking algorithm based on the bandpass filterd mslp fields from CLM is described and applied to four two years lasting simulations. The main finding is: temporal and spatial distributions of detected Polar Lows are consistent with observational evidence.

#3: A longterm climatology of North Atlantic Polar Lows

The tracking algorithm is applied to longterm simulations (59 years) and a database of polar low occurrences is compiled. The climatology exhibits strong year-to-year variability but weak decadal variability and a negligible trend. Years of strong polar low activity are characterised by a mean flow regime, which favours cold air outbreaks and upper air troughs.

#4: Subsequent study: Decreased frequency of North Atlantic polar lows associated with future climate warming

The tracking algorithm is applied 3 IPCC future scenarios and results are compared to 20iest century conditions. We found a considerable decrease in the frequency of polar lows per winter season and a northward shift of their genesis region.

Trends and variability in extratropical mesoscale cyclones

The weather and climate of the extratropical latitudes are dominated by synoptic scale cyclones. Also mesoscale cyclonic systems of various dimensions and intensity are observed over the extratropical oceans, some of which are of large impact on the weather and climate of the European coastal regions and even on continental Europe. These systems include polar lows, post frontal lows, comma clouds, and others. Due to observational constraints over the open oceans, not much is known about their phenomenology, origin, and life cycle. Even standard satellite information (e.g. from Meteosat or NOAA/AVHRR) does not provide ready information for the detection and analysis of such mesoscale features. But as they are often connected with intense precipitation, they are fairly reliably detected through microwave based precipitation algorithms. Using the HOAPS climatology ( it is possible to detect, classify and count such exceptional precipitation events over the North Atlantic since 1987. To study and classify their life cycle, a new tracking algorithm for precipitation fields has to be developed, which is complicated by the spatially non-continuous nature of precipitation fields and their availability only every 12 hours. Complementary information contained in standard weather analysis fields may help in developing an appropriate algorithm and will provide insight in the preferred environment of their life cycle development. Finally, conditional analysis of satellite radar data near extratropical mesoscale features will provide evidence of their impact on sea state. This might in turn hint at alternative techniques to detect such features through systematic satellite radar surveys.

[Zusammenfassung] (pdf in deutsch)