The Diagnosis of Vertical Motion within Dry Intrusions

Abstract

Dry intrusions play an important role in modulating precipitation patterns both in the midlatitudes and in the Tropics. The lifting of unsaturated air aloft often leads to destabilization and the enhancement of precipitation rates, and may occasionally contribute to the occurrence of severe weather. A method for qualitatively diagnosing vertical motion in a region of elevated dry advection is presented. The procedure measures the rate of propagation of relative humidity isopleths relative to the flow and deduces the sign of the vertical velocity. Changes in static stability are inferred, leading to the possibility of improved short-term forecasting of precipitation associated with dry intrusions. The procedure is illustrated with a case study involving heavy snowfall associated with a dry intrusion in the mid-Atlantic region. A diagnosis of ascent within the dry intrusion is obtained from satellite imagery and confirmed using numerical model output.