Abstract

Measurements of cloud condensation nuclei (CCN) conducted both from the ground and from an instrumented aircraft during two winter seasons, utilizing a thermal diffusion chamber, suggest that.

1. CCN spectra prove to always have, in both rainy and dry weather, continental-like characteristics. Their concentrations were shown to depend on wind direction and found to be surprisingly high, even in air masses with a partial maritime trajectory. The existence of local sources appears to have a limited and a generally minor effect on the characteristics of CCN spectra.

2. On the average, CCN concentrations tend to decrease by a factor of 2 during the first three days of a rainy spell. Consequently, cloud droplet concentrations, computed for cumulus clouds assumed to have base updraft velocities of 250 cm sec⁻¹, seem to show a similar trend but are always within the range of 180–950 cm⁻².

3. Since these variations do not seem to correlate with rainfall, they are not dominant in their effect on the relative efficiency of the rain-forming processes. However, the ever-present high concentrations of CCN, and thus, of cloud droplets, must have a significant effect on the nature of these processes in view of the resulting high colloidal stability of local clouds.

Abstract

This paper considers a linear hydrostatic model of a stable, uniform, constant rotational airflow over three- dimensional, elliptic, cross-sectional families of mountains in a z system. The surface pressure and the winds that are induced around the mountain chain are deduced using Fourier representation in both horizontal directions. The surface pressure perturbations and the induced wind intensities are linked to 1) the incoming airmass thermodynamic properties through Froude and Rossby numbers, 2) the geometrical aspect ratio of the mountain, 3) the direction of incidence of the incoming flow relative to the mountain orientation, and 4) the Coriolis effect through the Rossby number. The balance between the different factors that contribute to the morphology of the pressure and wind fields was established for northerly and southerly incoming flows that were blocked by an elliptical barrier resembling the Pyrénées mountain chain. Fair agreement was found between the results of the model and the experimental data collected during PYREX (Pyrénées experiment) intensive operational periods, with special regard to the asymmetry of the lateral flow for northerly incoming air masses.

Abstract

The Pyrénées experiment (PYREX), launched by the French and Spanish meteorological services, had provided an extensive database that was used in the present work to describe the airflow around the Pyrénées and to evaluate the applicability of the linear theory to stable flows that are blocked by a relatively large mountain range.

The direction of incidence of the incoming synoptic wind was used to classify the flow in northerly and southerly categories. In both categories, the directions of incidence bear westward from normal incidence to the main axis of the mountain range. Froude (Fr) and Rossby (Ro) numbers were used to represent the thermodynamic characteristics of the incoming air masses and the scale of interaction in terms of the mountain shape. Here, Fr and Ro were found linearly correlated, and their combination corresponding to blocked flow in all the cases.

Average cross correlations between the pressure drag across the mountain and the vertical profiles of the wind components at selected locations demonstrate that the horizontal flow is clearly separated in an upper undisturbed regime and a lower blocked regime in which regional winds are induced. The separating layer corresponding to the mean Froude number encountered during PYREX was found at an altitude of about two-thirds of the mountaintop. Due to the perturbation induced by the deflected wind, the total wind during northerly flows is stronger on the east side than on the west side, while during southerly incoming flows, the total wind distribution is almost symmetric.

Good correlation was found among the pressure field perturbation, the pressure drag, and Froude number of the incoming flow. Clear correlation rules can be deduced from the analysis of the experimental data at different altitudes for both categories of incoming flows. Therefore, since the pressure drag can be easily determined using field measurements, it becomes a powerful tool for the prediction of the regional winds around the Pyrénées.

The experimental data are in agreement with the linear theory predictions of the linear model in regarding (a) the perturbation of the surface pressure field, which resembles the predicted bipolar distribution; (b) the dependence of the drag on Fr⁻¹, which enables the assessment of the linear theory and the definition of the conditions of applicability of two models [(i) a two-dimensional model, for which it was possible to define quantitatively the effective blocked area, and (ii) a three-dimensional model, for which a scaling function that combines the direction of incidence, the mountain shape, and the Coriolis effect was found almost constant, with an average value of 0.2 for all the cases under study]; (c) the extension of the area affected by the blocking effect, estimated to be 4.5–5 times the width of the barrier and the drift of the strong deceleration point due to the Coriolis effect; (d) the dependence of the wind velocities on Fr⁻¹ at the edges of the barrier; and (e) the asymmetric flow deviation induced by the Coriolis effect and biased by the departure of the flow from normal incidence.