Rotational Raman Lidar measurementsfor the characterization of a dry stratospheric intrusion event

A UV Raman lidar system (BASIL) is operational at DIFA-Univ. of Basilicata (Potenza-Italy). The system was recently involved in LAUNCH 2005 – the International Lindenberg campaign for assessment of humidity and cloud profiling systems and its impact on high-resolution modelling - held from 12 September to 31 October 2005. During this period BASIL collected approx. 250 hours of measurements distributed over 13 Intensive Observation Periods (IOPs) and 25 days. One specific IOP was continuously run between 1-3 October 2005, covering a dry stratospheric intrusion episode associated with a tropopause folding event and the subsequent onset of perturbed weather conditions that leaded to the development of clouds and precipitations. The use of water vapour to trace stratospheric air intrusion allows to clearly identify a dry structure (approx. 1 km thick) originated in the stratosphere and descending in the free troposphere down to ~ 3 km. A similar feature is present in the temperature field, with lower temperature values observed within the dry air tongue. Relative humidity measurements reveal values as small as 0.5-1 % within the intruded air. The stratospheric origin of the observed dry layer has been verified by the application of a Lagrangian trajectory model. The subsidence of the intruding heavy dry air is most probably responsible for the gravity wave activity observed beneath the dry layer. Lidar measurements have been compared with forecasts from a MM5 mesoscale model. Comparisons in term of water vapour reveal the capability of the model to forecast the deep penetration in the troposphere of the dry intruded layer.