Joint Centre for Mesoscale Meteorology (JCMM)


JCMM Internal Report No.71
Sensitivity of short-range weather forecasts to local potential vorticity modifications
by Meral Demirtas and Alan J Thorpe

The use of potential vorticity concepts to understand extra-tropical cyclones, anticyclones and fronts is important not only for reconciling theory with observations, but also for delineating measurements necessary to improve the accuracy of short range forecasts.  As yet, potential vorticity thinking has been insufficiently applied to weather forecasting.  This report presents a new method, to use potential vorticity ideas by combining them with satellite water vapour imagery in a manner that has not been documented before.  The aim of this method is to test sensitivity of short-range weather forecasts to local potential vorticity modifications.  It also aims to monitor incipient errors in short range weather forecasting and to provide dynamically consistent data to re-run potentially poor forecasts.  By using PV concepts one can infer additional atmospheric flow and temperature data which cannot otherwise be simply derived from the water vapour data.  In this way dynamical concepts add considerable value to the water vapour data which by themselves would probably not have as significant a forecast impact.

Case studies in which the new method is used shed light on how observations and dynamical concepts could be used together to improve our understanding and ability to forecast mid-latitude weather systems.  In cases where the analysis errors occur at upper levels of the troposphere the method leads to a significant improvement in the short-range forecast skill.  In general, it is useful in highlighting where forecast problems are arising.

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