Institute for Atmospheric and Climate Science

Cloud-resolving modeling project

The focus of this activity is to better understand and represent small-scale processes and physical feedbacks that are relevant for weather and climate in Central Europe and the Alpine region in particular. Real-case and idealized high-resolution (km-scale) simulations are conducted that include an explicit (rather than parameterized) treatment of moist convection and a better representation of the topography of the underlying surface. The research addresses the predictability of weather phenomena at cloud-resolving scales, as well as case and process studies, partly using long simulations (months to seasons). Results demonstrate improvements over lower-resolution simulations, for example, in terms of the diurnal cycle of precipitation. In a recent study the feedbacks between soil-moisture and precipitation were demonstrated to be highly sensitive to the representation of convection (Hohenegger et al. 2009). In the next years, we intend to further develop this methodology towards a cloud-resolving climate modeling capability.

Towards kilometer-scale climate modeling

Moist convection during summertime is explored for a large Alpine region with an explicit treatment of convective processes at kilometer-scale resolutions. The mesoscale nonhydrostatic COSMO-CLM model is utilized in order to conduct simulations on timescales reaching from days to years. Besides numerical model components also physical aspects of moist convection are studied to gain further knowledge of the involved highly nonlinear processes and related climate feedbacks. Research also addresses feedback processes between the grid-scale and the Alpine scale, and the resolution requirements for such simulatoin is investigated.

Fig.1: Kilometer scale simulation of convective summertime precipitation over the Alps.Coloured contours show cloud water content and gray contours indicate accumulated precipitation

Idealized studies of convective summer precipitation

The goal of this activity is to understand the key parameters that determine the characteristicsof moist convection in the European summer climate and to investigate the sensitivity of these parameters to changes in future climates. This is done by performing idealized studies of summer precipitation using a cloud resolving model. Different settings covering present-day and future climates are addressed. The sensitivity of convective precipitation to different atmospheric profiles and for specific large-scale weather regimes are examined.

Studies of the valley wind system

Diurnal slope and valley winds are an essential component of the fair-weather mountain atmosphere. They strongly influence the weather and climate in mountain valleys and together with turbulent processes they control the land surface-atmosphere exchanges in mountainous regions and thus influence the development of clouds and deep convection. The focus of this activity is on the basic physical mechanisms governing the evolution of the valley wind system and on its numerical simulation.

Selected publications:

Hohenegger, C., P. Brockhaus, and C. Schär, 2008: Towards climate simulations at cloud-resolving scales. Meteor. Z., 17, 383-394.

Hohenegger, C., P. Brockhaus, C.S. Bretherton, C. Schär, 2009: The soil moisture-precipitation feedback in simulations with explicit and parameterized convection. J. Climate, 22, 5003-5020.

Schmidli, J., G.S. Poulos, M.H. Daniels and F.K. Chow, 2009: External
influences on nocturnal thermally driven flows in a deep valley. J.
Appl. Meteorol. Climatol., 48, 3-23.


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