In the period June–July 1992, four Doppler sodars were operated simultaneously in Rome and the surrounding area. The data have been used to investigate the contributions from sea breeze events to the local low-level circulation. Three days in which synoptic-scale pressure gradients were weak have been selected. A number of characteristics of the sea breeze are examined including the onset and cessation of the phenomenon, the behavior of the wind speed, and the depth of the density flow. The time propagation of the events and the influence of orography are obtained from a comparison between the data at different sites. The low-level circulation in the early morning is associated with the land breeze and appears to be enhanced by a mountain wind from the surrounding hills. The observed behavior of the vertical velocity field associated with the sea breeze is consistent with model predictions.

Observations of sea breeze events in Rome and the surrounding area by a network of Doppler sodars

In this paper we present several theoretical results concerning currents forced by the wind in coastal regions, for a shallow sea and for a very deep sea. We investigate the time behavior and the spatial structure of the stream function and the momentum components, i.e., the onset, the transient, the asymptotic, and the periodic behavior of the currents and of the upwelling, forced by winds of different spatial and time structures. Results show that the intensity of the along coast jet initially grows linearly under a δ in time wind impulse, quadratically under a Heaviside in time wind impulse, and cubically under a linearly growing wind impulse. The asymptotic state is such that the intensity of the current vanishes if the wind impulse has a finite duration, while the intensity of the sea current has a final finite amplitude if the wind intensity goes to some finite value. If the wind stress is periodic in time, there is upwelling only when the period of the forcing is longer than a characteristic time scale, which is the sum of the inertial period and the friction e-folding time. Otherwise there are waves which propagate away from the region where the wind stress is acting. The spatial structure is such that the upwelling occurs in a horizontal region of the order of the Rossby deformation radius, corrected by the effect of friction (and by the effect of periodicity, when the wind stress is periodic in time). However, the horizontal gradient of the wind stress can be more important than the Rossby deformation radius in determining the horizontal extent of the upwelling region.

G.A. Dalu

Papers

A numerical study of the marine surface layer in a sea breeze regime