1 P1.19 Retrieval of Cloud Liquid Water Content Profiles with Radar and Lidar: Application to Multi-annual Data Sets and Comparison with Microphysical Cloud Simulation. Oleg Krasnov, Herman Russchenberg, Delft University of Technology, Delft, Netherlands; Alexander Khain, and Mark Pinsky, Hebrew University of Jerusalem, Israel 1. INTRODUCTION The parameterization of the microphysical characteristics for low-level stratiform water clouds can be developed in terms, among others, of the effective radius of droplets and the liquid water content (LWC). These parameters can be directly measured using aircraft mounted in-situ probes observations. The instruments used to perform these measurements, however, have an extremely small sample volume. The remote sensing methods are less direct but give much better coverage and are less expensive. As it was noted in many studies (e.g. Fox and Illingworth (1997)), there are some problems in applicability of the radar measurements alone for the retrieval of the mentioned above parameters. Small number of big particles (so- called drizzle) can produce the major part of the cloud's reflectivity Z without strong contribution in the LWC and effective radius. A few Z-LWC relations were published in literature, but all of them are noted as applicable only in absence of drizzle. From other point of view, in many studies were noted that the presence of the drizzle fraction in water clouds is more usual than its absence (e.g. Gerber (1996), Fox and Illingworth (1997)). All these facts give the motivation for the efforts to find the combination of the remotely measurable parameters, which can be used for the detection of drizzle fraction, its parameterization and taking into account in cloud's microphysics retrieval algorithm. In this paper a retrieval technique based on the possibilities to characterize drizzle fraction in water clouds using the ratio between simultaneously measured radar reflectivity and lidar’s optical extinction profiles is presented. This parameter is using for the detection of the presence of drizzle particles in water clouds and the classification of water cloud cells into three classes – “the cloud without drizzle”, “the cloud with light drizzle” and “the cloud with heavy drizzle”. Different relationships between the radar reflectivity and liquid water content then can be applied for different types of cloud cells to retrieve actual liquid water content. The paper is organized as follow. Section 2 describes the study of in-situ measured spectra of water drops in low level clouds. It includes the description of used datasets from a few experimental campaigns, gives the statistically based definition of drizzle in water clouds, characterizes the drizzle influence on remote sensing measurables and LWC, shows the possibility to use radar reflectivity to lidar optical extinction ratio for drizzle detection and clouds categorization into three classes - “the cloud without drizzle”, “the cloud with light drizzle” and “the cloud with heavy drizzle”. Different relationships between the radar reflectivity and liquid water content then can be applied for different types of cloud cells to retrieve actual liquid water content. Section 3 addresses the possible implementation of retrieved relationships as background for the remote sensing retrieval technique. Sections 4 and 5 presents the details and results of the proposed technique applied to the multiyear radar and lidar data from the Cloudnet dataset, including validation using LWP from microwave radiometers. In section 6 briefly formulated conclusions are given. 2. THE ANALYSIS OF THE IN-SITU DROPSIZE DISTRIBUTIONS 2.1. Observational data used The CLARE’98 campaign. The Cloud Lidar and Radar Experiment (CLARE) took place near Chilbolton (United Kingdom) in October 1998. This extensive cloud campaign included airborne and ground-based radar and lidar observations as well as in-situ aircraft measurement of the drop-size distributions (DSD) (see ESA (1999) for details). During CLARE'98 campaign the particle size spectra in clouds were measured from the UK MRF's C-130 aircraft with a Forward Scattering Spectrometer (FSS) and a Two-Dimensional Cloud (2DC) probes in the size ranges between 1 μm and 23.5 μm radius and between 6.25 μm and 406.25 μm radius, respectively. The available data have a 5-sec interval of averaging. The DYCOMS-II campaign. The DYCOMS- II field campaign took place in July 2001 in Pacific Ocean near California (Stevens et al. (2002)). It was directed to collect data to study nocturnal marine stratocumulus. The main measuring part of campaign was made during 10 research flights of the NCAR's RAF EC- 130Q. On this aircraft cloud droplet spectrums were measured using a set of probes: the PMS - PCASP 100; the PMS-FSSP-100; the PMS- * Corresponding author address: Dr. Oleg Krasnov, IRCTR, TU Delft, Mekelweg 4, 2628 CD Delft, The Netherlands; e-mail: o.krasnov@irctr.tudelft.nl