JMA„MRI„Climate Research Dep.„Yuhji Kuroda„

Role of the stratosphere on the predictability of medium-range weather forecast
-A case study of winter 2003/04-



The role of the stratosphere on the predictability of medium-range weather forecast during the northern hemisphere winter is examined using numerical experiments with a middle atmosphere climate model of the Meteorological Research Institute. It is found that in the winter of 2003/04 when the stratosphere exhibited a large variability called the Polar-night Jet Oscillation (PJO), the predictability of the model tended to be very good for large-scale zonal variability if the prediction is performed just before the occurrence of stratospheric sudden warmings (SSWs). The role of the stratosphere is examined by comparing experiments for the 2002/03 year and using the model with and without the stratosphere included. The results of the study suggest that taking account of the role of stratospheric variability (PJO) is crucial for improving the predictability of medium-range weather forecast in certain winters.

Figure 1, Anomalous zonal wind averaged from 55‹ to 65‹N, from December 1, 2003 to April 1, 2004 for observations (upper panel) and forecast (lower panel). The lower panel indicates the ensemble mean (contour) of the predictions and its statistical significance (shading), obtained using initial times between 06Z December 27 to 00Z January 1. Contours are shown for every 5 m/s from }5 m/s and zero, and for }1 m/s. Dashed lines indicate negative values. In the forecast plot, shading indicates statistical significance and light, middle and heavy shading indicate 95%, 99.9% and almost 100% significance (Studentfs t greater than 2, 4 and 6), respectively. Arrows indicate the peak day of observed major SSW. (From Kuroda, 2008)
Figure 2, Forecasts by the model without the stratosphere included (upper panel) and using the initial value of the climatological lower boundary condition (lower panel). Otherwise the same as the lower panel of Figure 1. (From Kuroda, 2008)


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