Impacts of spectral nudging on the simulation of present-day rainfall patterns over southern Africa

Mavhungu Muthige, of the University of the Witwatersrand, South Africa, used a “spectral-nudging technique” to improve predictions of rainfall pattern and seasonality.

Mavhungu Muthige

Mavhungu Muthige


Regional climate models (RCMs) are expected to provide finer-scale simulations than those provided by global climate models, while maintaining the large scale circulation patterns provided by the host models. In this study, we examine the influence of spectral nudging on the simulation rainfall patterns in South Africa.


We use the Conformal-Cubic Atmospheric Model (CCAM) as RCM to downscale ERAinterim reanalysis data with a resolution of 50km in the horizontal over the globe. The simulations are performed for the period 1979-2011. A scale-selective filter (spectral nudging technique) is used for nudging the CCAM simulations. The filer is applied at length scales of 4,000 km, 1,000 km and 200 km and 0 km (no nudging). Simulations are also performed without using any atmospheric nudging (only sea-surface temperatures and sea-ice are provided in this case). The filter is applied at six-hourly intervals and from 900 hPa upwards. The model simulations of rainfall are compared against CRUTS3.2 observed monthly rainfall data sets, and the merits of the different length scales of nudging are discussed.

Results and conclusions

Use of the spectral-nudging technique ensures that observed synoptic-scale circulation patterns are represented with increasing realism as the length-scale at which the filter is applied decreases. The model was able to simulate the rainfall pattern and seasonality realistically.


Love Ekenberg, Risk, Policy and Vulnerability Program, IIASA


Mavhungu Muthige, of the University of the Witwatersrand, South Africa, is a citizen of South Africa. He was funded by the IIASA South African National Member Organization and worked in the Risk, Policy and Vulnerability Program during the YSSP.

Please note these Proceedings have received limited or no review from supervisors and IIASA program directors, and the views and results expressed therein do not necessarily represent IIASA, its National Member Organizations, or other organizations supporting the work.

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Last edited: 03 February 2016


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