Matthias Zahn

Post doctoral research project

Projected Response of Extreme Precipitation and Atmospheric Radiative Energy (PREPARE)

We are interested in monitoring, understanding and improving predictions of changes in the global water and energy cycles, in particular exploting satellite datasets to assess current changes in precipitation and its extremes and the implications for future projections by climate models. (Richard Allan, Lennart Bengtsson)
See also PREPARE Homepage

Third paper in the PREPARE project on "Quantifying present and projected future atmospheric moisture transports onto land"

In this work we calculate atmospheric moisture transports across the global shorelines (i.e. from ocean to land) in a present and in a projected future climate applying high space and time resolution data from a Global Climate Model. We find land- as well as seaward moisture transports intensify with warming. A more pronounced intensification of landward transports results in increased moisture budgets for the continents. In addition we demonstrate how limitations of 4 times daily data may result in systematic biases of the moisture transports. 4 times daily data may not capture minimum and maximum transports of the diurnal cycle well enough.
[accepted version of paper]

Second paper in the PREPARE project on "Climate Warming related strengthening of the tropical hydrological cycle"

We applied the method developed in our first paper to the output fields of a high time and space resolution run of the IPCC model ECHAM5 and investigated climate warming related changes of the hydrological cycle. This time we just used the physically consistent instantaneous values to calculate the moisture transports rather than linking mean wind vectors and humidity values.
We find a significant intensification of the hydrological cycle evident in stronger lower level inward (into the wet regions) as well as in stronger mid level outward moisture transports. This intensification is most pronounced at the high transport percentiles, supplying more moisture during an expected intensification of extreme precipitation events in the tropics. The intensification is mainly found to be due to the higher amount of moisture in a warmed atmosphere, while the contribution of a weakening wind slightly counteracts this response.
[accepted version of paper]

First paper in the PREPARE project on "Changes in water vapor transports of the ascending branch of the tropical circulation"

Continuing our work on moisture transports in the tropical Hadley Cell, we estimated regions of ascending (tropical wet regions) and descending (sub tropical dry regions) air motion based on monthly mean values as well as on instantaneous values. We calculated moisture transports from the dry regions into the wet regions, again applying mean and instantaneous values for humidity and wind (resulting in four different experiments). All data were taken from homogeneous global reanalyses (ERA interim).
Overall we found a significant intensification of the lower level inward (into the wet regions) as well as an intensification of the mid level outward moisture transport. The intensification is much less distinct in the the total moisture budget (inflow - outflow or precipitation - evaporation (P-E)) of the tropical wet regions, as as strengthening of the lower level lower inflow and upper level outflow at the same time neutralize each other, at least to some extent. When mean values are applied, the total moisture budget is too high as compared to P-E, because high outward transports due to the temporal coincidence of high mid level humidity and strong outward directed wind vectors are not represented in the mean values. We thus highlight the importance of estimating moisture transports from 3-d high resolution instantaneous data.
[accepted version of paper]

First study on the moisture budget of the ascending branch of the tropical Hadley Cell

In a first study for the 5th IPWG Workshop Richard Allan and I highlight the importance of using instantaneous wind and humidity values for moisture transport calculations. If monthly mean values are applied, the resulting moisture budget is 25% too high as compared to a reference budget derived from precipitation minus evaporation.
[Conference Proceedings Contribution]