The impact of mesoscale convective-system potential-vorticity anomalies on numerical-weather-prediction forecasts

The effect of the potential vorticity (PV) anomalies formed by mesoscale convective systems (MCSs) on the downstream how is investigated using a numerical weather prediction (NWP) model. Four case studies of North American MCSs from June and July 1999 are selected in which the MCS PV anomalies are not present in the model analysis. Idealized MCS PV anomalies are introduced into the model analyses and their impact on the NWP model forecast ascertained. In two of the four cases the presence of the MCS PV anomalies leads to significant perturbation in the forecast evolution. Sensitivity studies show that it is the presence of the mid-level cyclonic vortex (MCV) that has th: biggest impact on the NWP forecast. This positive PV anomaly is shown to cause areas of ascent and descent, as has been postulated following other studies. This can lead to the development of a growing mesoscale disturbance if the MCV becomes situated in a strong baroclinic gradient. These disturbances are shown to have direct and indirect impacts on the depths of synoptic-scale depressions. The presence of the MCS PV anomalies leads to some reduction in the forecast errors. It is suggested that attempting to assimilate the MCS PV anomalies into the NWP model analysis may be of some benefit to forecast accuracy. However, uncertainties in the properties of the MCV may mean that a more practical application may be in perturbing the initial states of ensemble forecasts.