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Current master students

Yvonne Brächter

(Lee-) Cyclogenesis in the Irminger Sea using PV-analysis

Mesoscale cyclogenesis is a frequently observed phenomenon over the Irminger Sea southeast of Greenland.
On the one hand, this could be attributed to lee processes as the Greenlandic ice sheet acts as a barrier to the airflow. On the other hand, katabatic winds also may lead to cyclogenesis.
As the cold drainage flow, which can be accelerated through channeling in fjords and through synoptic scale cyclones, arrives over the relative warm water of the Irminger current, a strong heat exchange from the ocean to the atmosphere takes place. These heat fluxes can, as described by the WISHE (Wind-Induced Surface Heat Exchange) mechanism, enhance the atmospheric circulation which in turn increases the heat fluxes. Additionally, vertical stretching of the katabatic flow leads to the generation of cyclonic vorticity.
As cyclones are characaterized as areas of cyclonic vorticity, they can be interpreted as cyclonic potential vorticity (PV) anomalies. Therefore, their genesis can be described by the so-called 'PV-thinking' which takes advantage of the conservation property of PV.

After giving an overview of the theoretical background, a climatology of mesoscale cyclogenesis, which can be attributed to katabatic flow, will be performed in order to investigate the frequency of these events.
After this case studies will be carried out to analyze which processes lead to mesoscale cyclogenesis over the Irminger Sea. With the help of piecewise vorticity inversion it will be examined how much the single processes contribute to these mesoscale cyclogenesis events.
Tamara Emmerichs

Role of dry deposition of ozone-meteorology nexus

Current global chemistry models show a significant positive bias of tropospheric ozone in the troposphere. While a major fraction of the bias is likely due to an underestimation of VOC ozonolysis in forest canopies, there is a potential that the inclusion of the vapour pressure deficit (VPD) into the dry deposition scheme reduces this bias. Therefore, a revision of the dry deposition scheme is desirable.

Two algorithms, which both consider the similarity between ozone stomatal fluxes and water vapour stomatal fluxes and which are considered to be equivalent, will be implemented in the DDEP sub-model of MESSy.
In the Pennman-Monteith equation, the VPD is calculated using air temperatures, whereas the Evaporation-Resistance approach uses the leaf temperatures for calculation.

As extreme meteorological events can significantly affect air pollution, three classes of extremes will be computed with ECHAM/MESSy to analyse the simulated ozone levels. These are heat waves, temperature inversions and atmospheric stagnation which have been shown to be clearly correlated to air quality (Hou and Wu 2016).
Finally, the global impact of the renewed parametrization for stomatal resistance will be evaluated. For this, high-resolution simulations (1°x1°) with ECHAM/MESSy will be performed. The change in the correlation with extreme events over densely vegetated areas will examined.
The final goal is an assessment of the global significance of the model improvements for the tropospheric ozone budget.
Christopher Rausch

Analysis of PV-anomalies in a North-Atlantic cyclone including PV-inversion-techniques and a Lagrangian analysis of WCB-trajectories using the COSMO-Online-Trajectory Module

While the influence of upper-tropospheric potential vorticity (PV) anomalies and of surface potential temperature anomalies on cyclogenesis are well understood, the diabatic PV anomaly of the lower and mid-troposphere still raises questions concerning its quantitative influence to the intensification of the cyclone as well as its origins related latent heat release.
The warm conveyor belt (WCB), a dynamical transport region of moist and warm air, is responsible for the majority of condensational heating throughout the cyclogenetic process and undergoes the maximum ascent. Near the cyclone center, the latent heat release of the WCB forms a strong PV dipole with its negative PV anomaly in the region of the WCB outflow.
In the last decade, studies have pointed out that the negative PV of the WCB-outflow significantly alters the upper-tropospheric wave guide by enlarging the down-flow ridge. If the diabatic heating and cooling is not simulated correctly, deficoencies in the parametrization of microphysical effects can lead to large forecast errors in numerical weather prediction models.

The aim of this study is to quantify the effect of the different PV-anomalies on a selected cyclogenesis in the North Atlantic in a COSMO simulation.
First, trajectories belonging to the WCB are assigned using the COSMO-Online-Trajectory Module and then, considering the endpoints of these trajectories, the air of the WCB in the upper ridge is spatially separated from the merely advected low PV air.
After having done so, the remaining anomalies are classified and a piecewise PV inversion is conducted with the help of an inversion tool. This analysis should enable further insights into the interaction of the PV-anomalies and the influence of the WCB-outflow on the cyclone track.

This document last modified on: 22 October, 2018.