We are grateful to Fabio D’Andrea for providing a code of the T21 QG model and information on its performance and also Prof. Steven Feldstein for his beneficial discussions. In addition, we thank two anonymous reviewers for their helpful comments on this paper. Our study is supported by the National Key Basic Research and Development (973) Project (Grant 2012CB417204) and the National Natural Science Foundation of China (Grants 41230420 and 40905023).
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