Abstract The results from a project to evaluate the use of potential vorticity in operational forecasting are presented. An analysis of the temporal and spatial coherence of analysed and forecast potential vorticity fields showed that features with space scales downn to a few hundred kilometres could be tracked for between one and two days. Of features in these fields, 90% had a corresponding signal in the vertical motion fields, showing that they were real features having an important influence on weather‐bearing systems. A comparison between potential vorticity fields and water‐vapour imagery shows a close relationship in the region of the polar front. Examples are given showing how this can be used to detect errors in model analyses and forecasts. The use of potential vorticity as a diagnostic of model behaviour is discussed. Many authors have stressed the role of potential vorticity anomalies in major cyclogenesis. In these cases the forcing is usually clear from other diagnostics. It is suggested here that it is in highlighting areas of weaker forcing or relatively small‐scale model errors where potential vorticity is most useful to the operational forecaster.

AbstractThe effect of the potential vorticity (PV) anomalies formed by mesoscale convective systems (MCSs) on the downstream flow 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 the 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.

Abstract A comparison between potential vorticity (PV) fields and water vapour imagery shows a close relationship in the region of the polar front. Examples are given showing the nature of this relationship and how a mis‐match between the PV and the imagery can be used to detect errors in model analyses and forecasts. The nature of the relationship varies according to the synoptic situation and some of the possible pitfalls encountered by not applying additional checks using model humidity or ‘pseudo imagery’ fields are illustrated. Copyright © 1997 Royal Meteorological Society

The distribution of extratropical potential vorticity (PV) on isentropic surfaces that transect the tropopause is a key feature of planetary‐scale teleconnection patterns, synoptic‐scale weather systems, and mesoscale stratosphere‐troposphere exchange. Here a Northern Hemisphere January and July climatology is presented for the mean and variability patterns of the PV using the so‐called European Centre for Medium‐Range Weather Forecasts reanalysis‐15 (ERA‐15) data set. It is derived taking into account the strong seasonal cycle of the tropopause height and the sharp quasi‐latitudinal gradient

The NASA ER-2 aircraft is used as a platform for high altitude atmospheric missions. The Meteorological Measurement System (MMS) was developed specifically for atmospheric research to provide accurate high resolution measurements of pressure, temperature, and the 3-D wind vector with a sampling rate of 5/s. The MMS consist of three subsystems: (1) an air motion sensing system to measure the velocity of the air with respect to the aircraft; (2) a high resolution inertial navigation system (INS) to measure the velocity of the aircraft with respect to the earth; and (3) a data acquisition system to sample, process, and record the measurement quantities. MMS data have been used extensively by ER-2 investigators in elucidating the polar ozone chemistry. Herein, applications on atmospheric dynamics are emphasized. Large scale (polar vortex, potential vorticity, model atmosphere), mesoscale (gravity waves, mountain waves) and microscale (heat fluxes) atmospheric phenomena are investigated and discussed.

The study explores Hurricane Michael’s impact on Hurricane Leslie’s trajectory predictability using ECMWF and NCEP ensemble systems. A clustering method focused on tropical cyclones is used to identify potential paths for Leslie: Cluster 1 accurately predicted Leslie’s direction towards the Iberian Peninsula, whereas Clusters 2 and 3 indicated a southern recurve near the Canary Islands. Analysis of potential vorticity and irrotational wind at upper levels showed a significant interaction between Michael, ridge, and trough across the jet stream from +12 h after initialization. Cluster 1 showed

The heavy rain event of November 2001 in the western Mediterranean area was synoptically characterized by the presence of a long-lived Omega blocking geopotential pattern. A set of mesoscale numerical simulations using MM5 is performed to investigate the mechanisms responsible for the convection development through several output diagnosis. A potential vorticity evolution showed how dry air masses were extruded from the stratospheric levels promoting strong cyclonic circulation at all levels. Moreover, a deep vertical column of high relative humidity over the Algerian coastline maintained the