Köppen, Hadley and Dethloff Zwei Seiten einer Medaille: Vom Globalen und vom Regionalen

Climate = statistics of weather The genesis of climate C s = f(C l, Φ s ) with C l = larger scale climate C s = smaller scale climate Φ s = physiographic detail at smaller scale

Global climate Formation of the general circulation on an aqua planet from a state of rest (from Fischer et al., 1991)

Risbey and Stone (1996) Long term mean of - zonal wind at 200 hpa, - geopotential, height at 500 hPa, and - band-pass filtered variance of 500 hpa geopotential height („storm track“) caused by planetary scale land-sea contrast and orographic features Continental climate

Regional climates do not create global climate. Instead, regional climate should be understood as the result of an interplay of global climate and regional physiographic detail. The local processes are important for the formation of the global climate not in terms of their details but through their overall statistics. Implications: Planetary scale climate can be modeled with dynamical models with limited spatial resolution the success on planetary scales does not imply success on regional or local scales.

Dynamical processes in the atmosphere

Dynamical processes in a global atmospheric general circulation model

The concept of downscaling does NOT imply that smaller scales are irrelevant for the larger scales. Small scale processes, such as convection, play a key role in forming the global climate – However it is only the overall effect of these processes which matter, not the space-time details. Therefore parameterizations of small scale processes are sufficient for global (and regional) models. It is this fortuitous arrangement, which allows us simulate the global and continental climate well – even without simulating any regional climate adequately.

Concept of Parameterization von Storch, H., 1997: Conditional statistical models: A discourse about the local scale in climate modelling. In P. Müller and D. Henderson (Eds.): Monte Carlo Simulations in Oceanography: Proceedings of the 9th 'Aha Huliko'a Winter Workshop, 1997, University of Hawaii at Manoa, January 14-17, 1997, 49-58

Roeckner & Lohmann, 1993 No cirrus Effect of black cirrus detailed parameterization Latitude-height distribution of temperature (deg C) Difference “black cirrus” - detailed parameterization Difference “no cirrus” - detailed parameterization

Winter (DJF) geopotential difference (gpm) at 500 hPa ‘‘New sea-ice and snow albedo run minus control run’’ (a) averaged over the first 250 years of the 500 year long run and (b) averaged over the last 250 years, (c) short-term response (years 2-9) Dethloff, K., A. Rinke, A. Benkel, M. Køltzow, E. Sokolova, S. Kumar Saha, D. Handorf, W. Dorn, B. Rockel, H. von Storch, J. E. Haugen, L. P. Røed, E. Roeckner, J. H. Christensen, and M. Stendel, 2006: A dynamical link between the Arctic and the global climate system. Geophys. Res. Lett. 33 L03703, doi: /2005GL025245

1.Concepts of regional modelling 2.Intermittent Divergence in Phase Space (IDSP) 3.Added value generated by RCMs Downscaling

Concepts of regional modelling 1.In the conventional set-up, the regional model is constrained by providing time variable boundary values along lateral margins and at the surface (as long as sea surface temperatures, ice coverage etc. are not modelled themselves) 2.To avoid numerical problems, the concept of sponge zones was invented by Davis in the 1970s. This sponge zone has typically a width of several grid boxes. 3.This concept is the conventional approach. The large scale dynamical state is imposed through the lateral boundary conditions, the regional physiographic detail by the lower boundary conditions. 4.In the “large-scale constraint” approach (specifically: spectral nudging) an additional constraint on the large scales is imposed in the interior of the domain.

The Rinke & Dethloff study on regional modelling of the Arctic atmosphere Rinke, A., and K. Dethloff, 2000: On the sensitivity of a regional Arctic climate model to initial and boundary conditions. Clim. Res. 14, Ensemble standard deviation 500 hPa height [m²/s²] idsp

Regional climate dynamics are also chaotic – very different trajectories may emerge from very slightly disturbed initial conditions – long after the predictive influence of the initial state has disappeared. The phenomenon is intermittent – as soon as the influence of the boundary is recovering, the “divergence in phase space” is vanishing. idsp

IDSP problem In large area simulations, lateral control is insufficient to maintain large scale states The development in the interior of a domain is mostly but not completely determined by the lateral boundary conditions. The RCM tendency to exhibit IDSP depends on the degree of “flushing” the area, i.e. the efficiency that disturbances travel from the boundaries through the area in a relatively short time. Midlatitude marine climate with moderate longitudinal extension (4000 km): mostly ok Arctic: problem Tropics: don’t know Note: In certain cases, one may be interested in getting idsp, in order to determine the degrees of freedom in a given region.

Added Value  Dynamical downscaling does not add value to global reanalysis wind speed in open ocean areas, while it does for complex coastal areas.  Regional models show an added value in describing meso-scale variability compared to the driving global reanalysis, in particular, when the RCM is constrained at the large spatial scales  Polar lows locations and places of occurrence can be described realistically in regional model simulations while not all polar lows can be found in the global reanalysis.  RCM typhoon core pressure values and near-surface wind speed values over all closer to best track data than forcing reanalyses.  Higher resolved description of physiographic details  Higher spatial resolution  Better representation of spatially distributed processes

Summary Climate = statistics of variability, exists for all spatial (and temporal) scales. Spatial scales are ordered, so that for the determination of larger scales only a statistical description of the influence of smaller scales is needed (parameterizations – hurray for Hadley). But, large-scale models fail to specify details of smaller scale climate (need for downscaling – hurray for Köppen). Global and regional variability are siblings, which impact each other, but play a rather different role in the dynamics and impacts of climate. The Arctic is a particular active region, with a climate having strong own dynamics and strong impact on the global dynamics (hurray for Dethloff)