Intercomparison of Water Vapor Retrieval Between Three Raman LIDAR Stations

ABSTRACT A study involving three groups operating Raman lidar in the Centre-South of Italy is in progress. The aim is evaluating the accuracy of the water vapor retrieval from Raman lidar signals. The purpose of the activity is defining the dependence of the resulting water vapor profiles on the retrieval method adopted by the different groups. The consistency among the results is an essential prerequisite for the creation of a reliable water vapor database in the upper-troposphere and for the constitution of a Raman lidar network devoted to this purpose. In this study, each group has applied its retrieval code, independently developed and currently applied in the analysis of its own data, on the raw H 2 O and N 2 signals provided by the respective Raman lidar involved. We report preliminary outcomes of the comparison, showing some differences in the resulting water vapor profiles and discussing the origin of these appearing discrepancies. 1. INTRODUCTION An accurate determination of the water vapor concentration still represents a challenging issue, mainly in the dried upper troposphere, owing to its very low concentration in this region and the generally insufficient sensitivity of the standard measurement techniques [1]. Nevertheless, the knowledge of distribution and variability of this component has fundamental implications both in meteorological and in climatological applications. Lidar measurements have been demonstrated being able to observe dynamical processes, as front passages or stratosphere-troposphere exchanges [2], but, at the moment, the lack in spatial coverage of this kind of measurements represents a limitation for these applications. Conversely, a network of high-feature Raman lidars for systematic water vapor measurements would be a powerful tool to monitor the water vapor content in the middle-upper troposphere, allowing the identification of possible trends and providing fundamental hints for the assessment of the stratospheric water vapor evolution. Furthermore, having simultaneous and comparable lidar water vapor data would constitute a useful support for dynamical studies and wheatear forecast improvement. The creation of a water vapor database for climatological purposes requires all the conditions affecting the final data were strictly fixed. Within such a context a primary aspect concerns the reliability of the retrieval methods. The aim of the inter-comparison presented here is determining the sensitivity of resulting water vapor profiles to the peculiarities of the different retrieval codes. The comparison has been performed using data from three Raman lidar systems operating in Italy, respectively at the University of L’Aquila (AQ), at ISAC-CNR in Rome (RM) and at IMAA-CNR in Potenza (PZ), see Tab. 1. This first exercise can be seen as a very preliminary work to establish a network of Raman lidars for the water vapor monitoring, taking also into account recent statements of the Network for Detection of Stratospheric Change (NDSC) [3]. One of the most relevant problems concerning the water vapor retrieval by Raman lidar technique is related to the non-independent nature of the retrieval procedure. In fact, despite the attempt to define an independent calibration method, the comparison with co-located radiosounding is still the most used method to transform relative lidar profiles in absolute water vapor content [4]. In this sense, in the first phase of the inter-comparison, the first task was establishing the uncertainty in the calibration constant estimate and, consequently, up to which extent it affects the measurement accuracy. Furthermore, the effects of the different choices in the retrieval procedure have been investigated.