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The Impact of Positive-Definite Moisture Transport on NWP Precipitation Forecasts

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  • 1 National Center for Atmospheric Research,* Boulder, Colorado
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Abstract

A positive-definite transport scheme for moisture is tested in a nonhydrostatic forecast model using convection-permitting resolutions. Use of the positive-definite scheme is found to significantly reduce the large positive bias in surface precipitation forecasts found in the non-positive-definite model forecasts, in particular at high precipitation thresholds. The positive-definite scheme eliminates spurious sources of water arising from the clipping of negative moisture values in the non-positive-definite model formulation, leading to the bias reduction.

* The National Center for Atmospheric Research is sponsored by the National Science Foundation

Corresponding author address: William C. Skamarock, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000. Email: skamaroc@ucar.edu

Abstract

A positive-definite transport scheme for moisture is tested in a nonhydrostatic forecast model using convection-permitting resolutions. Use of the positive-definite scheme is found to significantly reduce the large positive bias in surface precipitation forecasts found in the non-positive-definite model forecasts, in particular at high precipitation thresholds. The positive-definite scheme eliminates spurious sources of water arising from the clipping of negative moisture values in the non-positive-definite model formulation, leading to the bias reduction.

* The National Center for Atmospheric Research is sponsored by the National Science Foundation

Corresponding author address: William C. Skamarock, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000. Email: skamaroc@ucar.edu

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