Dr. Mirjam Hirt

Dr. Mirjam Hirt

Postdoctoral Research Fellow at Ludwig-Maximilians University in Munich

I am working in the Theoretical Meteorology group of Prof. G. C. Craig on numerical weather prediction, focusing on the prediction of thunderstorms and the predictability of weather at different scales.

My research

My current research is focused on numerical weather prediction, and the limits of predictability.

In my PhD, I studied the representation of convective initiation (i.e. the developement of thunderstorms) in current weather prediction models and how we can improve it.

For my Master thesis, I analysed Atmospheric Blockings with an idealized, point vortex model.

CV

I studied Meteorology at the Freie Universität Berlin and have worked as PhD student and PostDoc at the LMU Munich since then. I spent a couple of months at the Max-Planck Institute for Meteorology in Hamburg, at the University Center of Svalbard, and at the Munich Re in Munich.

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Publications

• Hirt, M, Craig, GC, Schäfer, SAK, Savre, J, Heinze, R. Cold‐pool‐driven convective initiation: using causal graph analysis to determine what convection‐permitting models are missing. Q J R Meteorol Soc. 2020; 146: 2205– 2227. https://doi.org/10.1002/qj.3788
• Hirt, M., S. Rasp, U. Blahak, and G. C. Craig, 2019: Stochastic Parameterization of Processes Leading to Convective Initiation in Kilometer-Scale Models. Mon. Wea. Rev., 147, 3917–3934, https://doi.org/10.1175/MWR-D-19-0060.1.
• Hirt, M., Lisa Schielicke, Annette Müller & Peter Névir, 2018: Statistics and dynamics of blockings with a point vortex model, Tellus A: Dynamic Meteorology and Oceanography, 70:1, 1-20, https://doi.org/10.1080/16000870.2018.1458565
• Müller, A., Névir, P., Schielicke, L., Hirt, M., Pueltz, J. & Sonntag, I., 2015: Applications of point vortex equilibria: blocking events and the stability of the polar vortex, Tellus A: Dynamic Meteorology and Oceanography, 67:1, https://doi.org/10.3402/tellusa.v67.29184

My Theses

Hirt, Mirjam (2020): Convective initiation - relevant processes and their representation in convection-permitting models. Dissertation, LMU München: Faculty of Physics

Cold pool driven convective initiation: using causal graph analysis to determine what models are missing.

Hirt, M., Craig, G.C., Schäfer, S.A.K., Savre, J. and Heinze, R.

Convective cold pools are volumes of negatively buoyant air that originate from precipitating downdraughts. Evaporation of precipitation in the sub-cloud layer creates negative buoyancy and accelerates the downdraught. When these cold, moist air masses hit the surface, they spread in near-circular patterns, often led by a gust front, where lifting fosters the initiation of new deep convection (i.e. thunderstorms). Cold pools are essential for organizing convection and play a particular role in convective initiation in the afternoon and evening. Both aspects are deficient in current convection-permitting models and a better understanding and representation of cold pools is likely necessary to overcome these deficiencies.

In a recent publication (Hirt et al., 2020), we detected cold pools and convective initiation within hectometer simulations for four days over Germany (ICON-LEM, HDCP2 simulations) and identified several sensitivities of cold-pool-driven convective initiation to model resolution. Mostly, cold pools are more frequent, smaller and less intense in lower-resolution simulations and their gust fronts are weaker and less likely to trigger new convection. Using a causal graph analysis, we were able to estimate the relevance of several indirect effects, as displayed in the figure. We identify one single dominant pathway (the RGP path): a reduced model resolution (R) directly causes gust front upward massflux (G) to be weaker which then also reduces convective initiation (P). Pathways involving cold pool intensity or buoyancy anomaly were found to be unimportant. It is particularly the cold pool gust fronts that are too weak in km-scale models to trigger sufficient, new convection.

This gained understanding can now help in developing a cold pool parameterization scheme to reduce the identified deficits in the models. To do so, we currently develop a parameterization for the convection-permitting COSMO model to improve the representation of cold pool gust fronts, the CPP scheme. A target vertical velocity scale for cold pool gust fronts is derived using dimensional analysis and approximated based on local buoyancy gradients. Vertical velocity perturbations were then imposed so that the model vertical velocity at cold pool gust fronts converges towards this target scale. Our results show that cold pool gust fronts are indeed strengthened by CPP (Fig. 2) and - to all appearances - enhances cold pool driven convective initiation. As anticipated, the late afternoon/evening precipitation is improved for weakly forced situations and indications for enhanced organization were found. Overall, the results from the CPP scheme are promising. A corresponding publication is in preparation.

Reference: Hirt, M., Craig, G.C., Schäfer, S.A.K., Savre, J. and Heinze, R. (2020), Cold pool driven convective initiation: using causal graph analysis to determine what convection permitting models are missing. Q J R Meteorol Soc. doi:10.1002/qj.3788

Stochastic Parameterization of Processes Leading to Convective Initiation in Kilometer-Scale Models

Hirt, M., S. Rasp, U. Blahak, and G. C. Craig

Predicting convective precipitation (e.g. typical summer thunderstorms) with numerical weather prediction models is limited by their ability to initiate convection. For this initiation small-scale processes in the atmospheric boundary layer are particularly important. Yet due to the limited grid sizes of the numerical models, these processes are often not adequately represented in the models. One such process is boundary-layer turbulence. The representation of this process can be improved by approximating the variability of subgrid-scale turbulence using stochastic parameterizations. One such example is the Physically based stochastic perturbation scheme (PSP), that was developed by Kober and Craig (2016). Here, we improved the PSP scheme to be physically more consistent, resulting in a revised, PSP2 scheme.

Reference: Hirt, M., S. Rasp, U. Blahak, and G. C. Craig,2019: Stochastic Parameterization of Processes Leading to Convective Initiation in Kilometer-Scale Models. Mon. Wea. Rev., 147, 3917–3934, https://doi.org/10.1175/MWR-D-19-0060.1.