Uncertainties in numerical predictions of weather and climate are often linked to the representation of unresolved processes that act relatively fast compared to the resolved general circulation. These processes include turbulence, convection, clouds, and radiation. Single-column model (SCM) simulation of idealized cases has become an often-used and relied-upon method to obtain insight at process-level into the behavior of such parameterization schemes; benefits are the enhanced model transparency and computational efficiency.
Although having achieved demonstrable success, over the years some shortcomings of using idealized case studies have been identified:
- their statistical significance and relevance might be questioned
- the use of observational data sets has typically been relatively limited.
The KNMI Parameterization Testbed (KPT) is part of a general move towards more statistically significant process-level evaluation. With the emphasis on the representation of atmospheric boundary-layer processes, KPT has two main goals that are designed to address the shortcomings of single idealized case-studies as mentioned above:
- To reproduce with the SCM the same statistical level at which a GCM climate is typically evaluated, by means of generating continuous series of SCM simulations that cover long (that is, multi-year) periods of time
- To evaluate the complete parameterized system at multiple time-scales against as many independent observational data sets as possible: for example, as available at permanent meteorological sites.
The application of such continuous long-term SCM simulation, in combination with comprehensive evaluation at multiple time-scales, enables the reproduction of typical long-term mean behavior of fast physics in GCMs, while on the other hand still preserving the benefits of single-case studies (such as model transparency). This strategy facilitates the tracing and understanding of errors in parameterization schemes, which should eventually lead to a reduction of related uncertainties in numerical predictions of weather and climate. Two examples of long-term SCM evaluation in KPT are shown in Figs. 1 and 2.
Figure 1. Impacts of a subgrid-scale (SGS) overlap function for cumuliform boundary-layer clouds on the short-wave downward radiation at the surface as produced by the SCM at Cabauw. The scatterplot shows monthly-mean observations at 12 UTC (abscissa) against the equivalent SCM results (ordinate), for the period 2007-2009. The blue dots represent SCM simulations with an experimental statistical cloud scheme ignoring SGS overlap, while the green dots represent simulations including SGS overlap. Accounting for SGS overlap results in a significantly lower bias and root-mean-square error.
Figure 2. Composite hodograph of the nocturnal 200-m wind for one month of SCM simulations (July 1999) for the Southern Great Plains Central Facility site of the Atmospheric Radiation Measurement (ARM) program. The full lines indicate various SCM versions; the dashed line indicates averaged observations of the 915-MHz wind profiler. Numbers indicate local time. While still underestimating the amplitude of the low-level jet, the TKE scheme clearly performs better than the REF version. The label ‘enKF’ refers to SCM experiments that are assimilated towards local measurements.
Currently participating SCM codes include various of the major operational European general circulation models, such as :
- the Integrated Forecasting System (IFS) of the European Centre for Medium-range Weather Forecasts (ECMWF)
- the ECHAM climate model as operated by the Max Planck Institute for Meteorology (MPI-M) in Hamburg
- the HARMONIE model as applied in high-resolution numerical weather forecasting at KNMI.
Observational datasets are available from among others the various European CloudNet sites, including the Cabauw Meteorological site in the Netherlands, and the Southern Great Plains site of the Atmospheric Radiation Measurement (ARM) program of the U. S. Department of Energy (DOE). The KPT project takes part in the ongoing European Union Cloud Inter-comparison, Process Study and Evaluation (EUCLIPSE) project, as well as the Fast-physics System Testbed and Research (FASTER) project as funded by the Earth System Modeling (ESM) program of the DOE.
Link to KPT server (in progress)
)
)
