Abstract

Water vapour measurements from a ground-based Raman lidar and an airborne differential absorption lidar, complemented by high resolution numerical simulations from two mesoscale models (Arome-WMED and MESO-NH), are considered to investigate transition events from Mistral/Tramontane to southerly marine flow taking place over the Gulf of Lion in Southern France in the time frame September-October 2012, during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) Special Observation Period 1 (SOP1). Low-level wind reversals associated with these transitions are found to have a strong impact on water vapour transport, leading to a large variability of the water vapour vertical and horizontal distribution. The high spatial and temporal resolution of the lidar data allow to monitor the time evolution of the three-dimensional water vapour field during these transitions from predominantly northerly Mistral/Tramontane flow to a predominantly southerly flow, allowing to identify the quite sharp separation between these flows, which is also quite well captured by the mesoscale models.

Highlights

  • Mistral and Tramontane are two strong dry northerly winds which are often found to blow in the Mediterranean basin

  • This research effort is dedicated to the measurements performed by the ground-based University of BASILicata Raman Lidar system (BASIL,[4]) and the airborne differential absoption lidar (DIAL) system LEANDRE 2 [5], which are used to illustrate the high time and space variability of the water vapour field associated with the occurrence of transition events from

  • In the frame of Hydrological Cycle in the Mediterranean Experiment (HyMeX)-Special Observation Period 1 (SOP1), BASIL was deployed in Candillargues and operated between 5 September and 5 November 2012, collecting more than 600 hours of measurements, distributed over 51 measurement days and 19 intensive observation periods (IOPs)

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Summary

INTRODUCTION

Mistral and Tramontane are two strong dry northerly winds which are often found to blow in the Mediterranean basin. Mistral and Tramontane sometimes last only one or two days, frequently lasts several days, and sometimes lasts more than a week They bring cold and dry continental air over the sea and are frequently observed to extend few hundred kilometres off the coast (among others, [1]). In the Gulf of Lion, Southern France, the occurrence of Mistral and Tramontane at the coast are often observed to alternate with southerly marine flows. During HyMeX-SOP1 (September-November 2012) several events when the southerly marine flow broke through and overcame the Mistral/Tramontane flow were observed. Mistral/Tramontane to southerly marine flow in Southern France during HyMeX-SOP 1. Measurements from these two systems are used to validate the numerical simulations from the mesoscale model MESO-NH and the mesoscale numerical weather prediction model AROME-WMED

METHODOLOGY
RESULTS
19 September 2012

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