Process Studies

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).

Ship tracks, which are characterized by recurring streak-line patterns of increased cloud albedo are often used as an illustrative example for aerosol-cloud effects. They are predominantly observed in stratocumulus regions with relatively shallow boundary layers along the worlds major shipping corridors. Although the radiative effect of ship emissions on a global scale is likely to be insignificant, they provide an ideal test bed for studying aerosol-cloud effects of stratiform boundary layer clouds. The ship exhaust particles are injected into pristine regions of very low aerosol concentrations, providing a large perturbation to the system where aerosol effects are distinguishable. At the regional 2-km scale, all involved processes rely on sub-gridscale parameterisations. Therefore this work contributes towards evaluating the contributing parameterisations (e.g. microphysics, turbulence, or radiation) and the interplay between them during track formation and maintenance.

Animation of Ship Tracks over the Gulf of Biscay (Possner et al. 2015) — Vertically integrated cloud droplet number concentration (CDNC) and optical thickness as simulated in COSMO between 03 UTC – 23 UTC on the 26th of January. 3 distinct ship tracks are formed and advected during this time period (Possner et al, 2015).

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

Possner, A.,E. Zubler., U. Lohmann, and C. Schär (2015) : Real-case simulations of aerosol–cloud interactions in ship tracks over the Bay of Biscay, Atmos. Chem. Phys., external pagedoi:10.5194/acp-15-2185-2015

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