Study of a convective storm series and of precipitated hail in south Argentina

A study of the period 10–19 February 1990, when frequent hailstorms occurred in the High Valley of Rio Negro and Neuquén (40°S, 65–70°W), is performed. The synoptic situation responsible for the observed convective events is characterized by the presence of a blocking high on the Atlantic Ocean (40°S, 50°W) that results in prevailing easterly winds and consequent high surface humidity. Information about precipitated hail is obtained from a network of 110 hailpad stations and from the crystal structure of hailstone samples. It is shown that, though kinetic energies up to about 1000 Jm −2 were evaluated in some stations, rather small hail with diameter D ∼ 10 mm prevailed in the hail size distributions while particles with 25–30 mm diameter were relatively few. Crop damage estimations are related to kinetic energy and stone number density on the ground, derived from hailpad analysis. It is found that, due to the type of fruit cultivation, an energy of about 100 Jm −2 was enough to determine total crop destruction. A one dimensional time independent (1DTI) model is applied to morning radiosonde data to stimulate cloud development. It is shown that the atmosphere instability, though moderate, was favorable to convection on most days of the period. For days when hail falls occurred, an accretion growth model is used to simulate the vertical motion and the development of ice particles growing in the environmental conditions characterizing the modeled cloud. It is found, that the size attained by particles displaying an up-and-down trajectory in the model could be favorably compared with the largest sizes registered on the ground and that most simulated hailstone growth occurred at rather low cloud levels, in agreement with the observed hailstone structure. Relations between the hailstone size and the simulated updraft intensity at middle levels, usually located below the updraft maximum, are suggested. The importance of the ice phase on the cloud depth and on the updraft profile shape is discussed.