Processes

The focus of this activity is to better understand and represent processes and physical feedbacks that are relevant for weather and climate in Central Europe and the Alpine region in particular. These studies investigate processes across spatial scales ranging from O(100 m) to O(1000 km).

Pseudo-global-warming (PGW) simulations are a powerful tool to study the importance of different processes associated with climate change. The method was first introduced by Schär et al. 1996. In a PGW simulation, a regional climate model is forced with a process of interest taken from a global climate simulation (Fig. 1). As an example, we used PGW simulations to show that non-homogeneous changes in the atmospheric stratification have a crucial influence on the structure of climate change in Europe during summer (e.g. Kröner et al. 2017).

PGW Figure — Fig. 1: The general principle of the Pseudo-global-warming method

In recent studies, the convergence of domain-averaged and integrated properties related to a large ensemble of convective cells (hence bulk convergence) at kilometer-scale resolutions was demonstrated in both idealized (Panosetti et al., 2018a) and real-case (Langhans et al., 2012; Panosetti et al., 2018b) simulations. These results prove that the mean flow properties and the feedbacks between convective clouds and the large-scale environment are relatively insensitive to further refinement of the mesh grid, and thus support the use of kilometer-scale resolutions in climate models.

(a) Mean diurnal cycle of domain-averaged surface rain rate [mm h-1] in a 5-day long idealized simulation over flat homogeneous terrain (CTRL). The total accumulated precipitation [mm] is displayed at the bottom left of the panel. (b) Ensemble-averaged normalized resolution increment ([NRI %]) versus the horizontal grid spacing (∆x) computed for the mean diurnal cycle of domain-averaged surface rain rate. The NRI quantifies the differences between one resolution (∆x) and the next higher (∆x/2). A systematically lower NRI at smaller ∆x is consistent with the hypothesis of bulk convergence. Bulk convergence is obtained for a variety of experiments (see Panosetti et al., 2018a for details). The bars indicate the ensemble spread for each experiment, defined as the standard deviation and based on a 5-member ensemble at each resolution.

The goal of this activity is to understand the key parameters that determine the characteristics of moist convection in the European summer climate and to investigate the sensitivity of these parameters to changes in future climates. This is done by reducing the complexity of the full earth system in idealized studies. Different settings covering present-day and future climates are addressed. In addition, the role of orography and soil-moisture inhomogeneity in triggering and organizing moist convection is investigated within idealized settings. The knowledge gained from these studies flows back into real-case climate studies.

Enlarged view: Figure from Froidevaux et al. (2014) — Conceptual scheme of convection initiation by soil moisture heterogeneity. (left) Without background wind, convection is initiated over the dry areas and over the ascending branches of local sea-breeze-like circulations in the planetary boundary layer (red circular arrows). Storms are stationary and rain falls predominantly over the dry areas. The numbers in the clouds indicate local time (right) With significant background wind (blue arrows), the superposition of the local and the background vorticity terms (small red and blue circular arrows, respectively), enhances the circulation upstream of the wet patch. The circulation downstream of the wet patch is weakened. Convection is preferentially initiated upstream of the wet patch, developing storms are propagating downwind, and rain falls preferentially over the wet patch. Figure from Froidevaux et al. (2014)

Enlarged view: Figure 4 from Imamovic et al. (2017) — Impact of (a) uniform and (b) heterogeneous soil moisture perturbation on the rain amount at a mountain as a function of mountain height. In case of the heterogeneous soil moisture perturbation in (b) only the mountain is dried / moistened. The x-axis shows the soil mositure perturbation amplitude (soil moisture saturation relative to reference soil moisture). The y-axis shows the response of rain amount relative to the run with reference soil moisture. A positive soil-moisture precipitation feedback emerges for uniform perturbations (more rain over wet soils), while a negative feedback acts for heterogeneous soil moisture perturbation (more rain over dry soils). The strength of the negative feedback strongly depends on the mountain. A relatively low mountain of 500 m height is sufficient to neturalize the negative soil moisture precipitation feedback. The Figure is adapted from Imamovic et al. (2017)

The COSMO model is validated using satellite products from GERB, SEVIRI and AVHRR sensors, a raingauge-based precipitation data set and temperature measurements from surface stations. The validation is done for 12 km (convection-parameterizing model) simulations as well as 2 km (convection-resolving model) simulations. Radiative transfer models are used to generate synthetic satellite radiances. These synthetic satellite images are then compared to the observed satellite measurements. This approach benefits from a direct comparison of model states with observed states, without any assumptions about the observed atmosphere.

Enlarged view: Figure from Keller et al. (2015) — Brightness temperatures (BT) at 10.8 μm at 13 UTC of 5 June and 4 UTC of 6 June 2007 for observations from SEVIRI and three simulations (using a grid spacing of 12 km or a grid spacing of 2 km, employing a 1-moment microphysics scheme or a 2-moment microphysics scheme). Bluish colors indicate BT < -20 ◦ C, white -20 ◦ C< BT < 0 ◦ C and brownish colors BT > 0 ◦ C. These areas are referred to as BT_HC, BT_MC and BT_LCG, respectively. Figure from Keller et al. (2015).

Frei et al. 2018, Future snowfall in the Alps: projections based on the EURO-CORDEX regional climate models. The Cryosphere.

Schär and Kröner 2017, Sequential Factor Separation for the Analysis of Numerical Model Simulations. Journal of the Atmospheric Sciences.

Kröner et al. 2017, Separating climate change signals into thermodynamic, lapse-rate and circulation effects: theory and application to the European summer climate. Climate Dynamics.

Schär, C., C. Frei, D. Luthi, and H. C. Davies, 1996: Surrogate climate-change scenarios for regional climate models. Geophysical Research Letters.

Panosetti, D., L. Schlemmer, and C. Schär, 2018a: Convergence behavior of idealized convection-resolving simulations of summertime deep convection over land. Clim. Dyn., external pagehttps://doi.org/10.1007/s00382-018-4229-9

Panosetti, D., L. Schlemmer, and C. Schär, 2018b: Bulk and structural convergence at convection-resolving scales in real-case simulations of summertime moist convection over land. Quart. J. Roy. Meteor. Soc., under review.

Imamovic, A., Schlemmer, L., & Schär, C. (2017). Collective impacts of orography and soil moisture on the soil moisture‐precipitation feedback. Geophysical Research Letters, 44, 11,682–11,691. external pagehttps://doi.org/10.1002/2017GL075657

Panosetti, D., S. Böing, L. Schlemmer, and J. Schmidli (2016): Idealized large-eddy and convection-resolving simulations of moist convection over mountainous terrain. J. Atmos. Sci., 73, 4021–4041, external pagehttps://doi.org/10.1175/JAS-D-15-0341.1

Keller, M., Fuhrer, O., Schmidli, J., Stengel, M., Stöckli, R. and Schär, C. (2015): Evaluation of convection-resolving models using satellite data: The diurnal cycle of summer convection over the Alps, Meteorologische Zeitschrift, external pagedoi: http://doi.org/10.1127/metz/2015/0715

Froidevaux, P., L. Schlemmer, J. Schmidli, W. Langhans, and C. Schär (2014): Influence of the background wind on the local soil-moisture precipitation feedback, J. Atmos. Sc., external pagedoi: http://dx.doi.org/10.1175/JAS-D-13-0180.1

Langhans, W., J. Schmidli, C. Schär (2012) Bulk Convergence of cloud-resolving simulations of moist convection over complex terrain, J. Atmos. Sc., external pagedoi: http://dx.doi.org/10.1175/JAS-D-11-0252.1

Schlemmer, L., C. Hohenegger, J. Schmidli, C. Bretherton, and C. Schär (2011): An idealized cloud-resolving framework for the study of summertime midlatitude diurnal convection over land, J. Atmos. Sci., external pagedoi: http://dx.doi.org/10.1175/2010JAS3640.1