Aircraft measured microstructure of severe hailstorms in Argentina from cloud base to -45oC

The WMI Lear Jet was used in Argentina in January and February 2000 to document the cloud microstructure of the frequently occurring severe hailstorms there. Aircraft measurements were conducted in the feeders and the tops of the main updrafts of storms, which produced golf to tennis ball-size hail. The main features found in the storms were as follows: Dynamic features: Cloud base at about 10^oC, with updrafts often exceeding 7 m/sec. Updrafts increasing with height, exceeding 25 m/sec at heights of 7 km and higher. Thermal buoyancy of 5-7^oC was measured in convective towers at heights of 8-10 km. Weak shear below the 6 km level, changing to strong shear above that level.

Microphysical features: Very little coalescence was observed, with only isolated small supercooled raindrops observed by the 2DC. Nearly adiabatic cloud water was maintained up to the level of homogeneous freezing. More than 4 g/m³ was observed at -38^oC with the hot-wire DMT probe, abruptly vanishing at colder temperature. Graupel > 1 mm appeared at the tops of growing vigorous new towers above about -27^oC isotherm level, in agreement with radar first echo height of about 8 km.

The significance of these comprehensive measurements regarding the growth of hail is discussed.