Sensitivity of a heavy-rain-producing western Mediterranean cyclone to embedded potential-vorticity anomalies

The development and track of a surface cyclone that produced heavy precipit ation in the western Mediterranean region on 28-29 September 1994 is numeri cally simulated using a mesoscale model. Diagnostic calculations reveal tha t surface-pressure falls-and upward motion-occur in response to a well-defi ned pattern of upward quasi-geostrophic forcing at all tropospheric levels. In addition, convective instability and low-tropospheric water vapour flux convergence are significant over the western Mediterranean, and therefore the environment is highly supportive for convection development. The appreciable dynamical forcing at upper levels appears to be associated with two positive potential-vorticity (PV) anomalies that are embedded with in the large-scale trough and rotate about each other. Motivated by the fac t that the small-scale features of the PV field are more prone to analysis or forecast error than the large-scale components, a sensitivity study is c onducted in order to analyse the dependence of the mesoscale forecast on th e initial intensity and position of the two embedded PV centres. This is ac complished by first applying a piecewise PV-inversion technique which allow s the balanced flow associated with each PV centre to be calculated, and th en the inverted mass and wind fields are used to modify the model initial c onditions. Eight simulations are run after doubling or removing one or both anomalies (sensitivity to intensity), and eight other simulations after di splacing the anomalies towards or away from the Iberian peninsula (sensitiv ity to position). The results exhibit a clear dependence of the track and shape of the surfac e cyclone and its associated pattern of low-tropospheric warm air advection on the characteristics of the PV anomalies. Therefore, spatial details of the mesoscale forecast are shown to be highly sensitive to the precise stru cture of the upper-level dynamic forcing. Nevertheless, for the full set of arbitrarily defined initial conditions, one or several low pressure system s develop in the western Mediterranean area and heavy precipitation always occurs. On the other hand, there is an overall tendency of the simulations to favour cyclone development leeward of the Atlas mountain range and local rainfall enhancements over the exposed mountains of eastern Spain. The rel ative roles of the orography and sea surface latent-heat flux versus the ac tion of the upper-level PV centres are examined by means of additional simu lations.