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An Estimate of Systematic Error and Uncertainty in Surface Cyclone Analysis over the North Pacific Ocean: Some Forecasting Implications

Steven L. MullenInstitute of Atmospheric Physics, The University of Arizona, Tucson, Arizona

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Abstract

The systematic error and uncertainty of surface analyses produced by the National Meteorological Center (NMC) over the North Pacific Ocean is estimated. Differences in cyclone position, central pressure, and 12-h deepening rate between an operational manual analysis for the Northern Hemisphere (NH) and the automated initial analysis for the aviation run (AVN) of NMC's Global Forecast Model are compared, and their statistical significance is judged.

The AVN central pressures are, on average, 2.2 mb higher than the NH values. The AVN initial maps underestimate cyclone intensity at all central pressures values, but the error is biggest (3.9 mb) for deep systems (central pressures ≤980 mb). The absolute displacement error averages 210 km with standard deviation of 200 km. Position errors are largest for weak systems (central pressures ≥1000 mb), averaging 300 km and being more than double the 140 km value for deep lows. The aggregate mean vector displacement of 10 km is negligible.

Cyclone positions for the AVN initial maps agree to within 130 km of the NH locations for rapid deepening lows (12 h pressure change ≤ −12 mb), the smallest mean displacement error for any category examined. The life cycle of cyclones, as portrayed by the AVN initial charts, is too slow, with the AVN maps underestimating the 12-h deepening rate for rapid deepening lows by 5.4 mb, for all other deepening lows by 1.5 mb, and the 12-h filling rate for all cyclolytic lows by 2.4 millibars.

Uncertainty in the central pressure and 12-h deepening rate, as measured by the width of confidence intervals, is largest for deep lows and for rapidly deepening cyclones, respectively, but the uncertainty in cyclone position is smallest for such systems. Weak lows or those undergoing little 12-h pressure change exhibit the greatest uncertainty in their position but they are more consistently defined in terms of central pressure.

Implications of the uncertainty estimates on forecast verification and the generation of initial perturbations for ensemble forecasts are discussed.

Abstract

The systematic error and uncertainty of surface analyses produced by the National Meteorological Center (NMC) over the North Pacific Ocean is estimated. Differences in cyclone position, central pressure, and 12-h deepening rate between an operational manual analysis for the Northern Hemisphere (NH) and the automated initial analysis for the aviation run (AVN) of NMC's Global Forecast Model are compared, and their statistical significance is judged.

The AVN central pressures are, on average, 2.2 mb higher than the NH values. The AVN initial maps underestimate cyclone intensity at all central pressures values, but the error is biggest (3.9 mb) for deep systems (central pressures ≤980 mb). The absolute displacement error averages 210 km with standard deviation of 200 km. Position errors are largest for weak systems (central pressures ≥1000 mb), averaging 300 km and being more than double the 140 km value for deep lows. The aggregate mean vector displacement of 10 km is negligible.

Cyclone positions for the AVN initial maps agree to within 130 km of the NH locations for rapid deepening lows (12 h pressure change ≤ −12 mb), the smallest mean displacement error for any category examined. The life cycle of cyclones, as portrayed by the AVN initial charts, is too slow, with the AVN maps underestimating the 12-h deepening rate for rapid deepening lows by 5.4 mb, for all other deepening lows by 1.5 mb, and the 12-h filling rate for all cyclolytic lows by 2.4 millibars.

Uncertainty in the central pressure and 12-h deepening rate, as measured by the width of confidence intervals, is largest for deep lows and for rapidly deepening cyclones, respectively, but the uncertainty in cyclone position is smallest for such systems. Weak lows or those undergoing little 12-h pressure change exhibit the greatest uncertainty in their position but they are more consistently defined in terms of central pressure.

Implications of the uncertainty estimates on forecast verification and the generation of initial perturbations for ensemble forecasts are discussed.

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