Cloud-microphysical sensors intercomparison at the Puy-de-Dôme Observatory, France

Abstract

Clouds play an important role on the radiative budget of the earth (Boucher et al., 2013). Since the late 70s, several instrumental developments have been made in order to quantify the microphysical and optical properties of clouds, for both airborne and ground-based applications. However, the cloud properties derived from these different instrumentations have rarely been compared. In this work, we discuss the results of an intercomparison campaign, performed at the Puy de Dôme during May 2013. During this campaign, a unique set of cloud instruments were compared. Two Particle Volume Monitors (PVM-100), a Forward Scattering Spectrometer Probe (FSSP), a Fog Monitor (FM-100) and a Present Weather Detector (PWD) were sampling on the roof of the station. Within a wind tunnel located underneath the roof, two Cloud Droplet Probes (CDP) and a modified FSSP (SPP-100) were operating. The main objectives of this paper are to study the effects of wind direction and speed on ground based cloud observations, to quantify the cloud parameters discrepancies observed by the different instruments, and to develop methods to improve the quantification of the measurements. The results reveal that all instruments, except one PVM, show a good agreement in their sizing abilities, both in term of amplitudes and variability. However, some of them, especially the FM-100, the FSSP and the SPP, display large discrepancies in their capability to assess the cloud droplet number concentrations. As a result, the total liquid water content can differ by up to a factor of 5 between the probes. The use of a standardization procedure, based on data of integrating probes (PVM-100 or visibilimeter) and extinction coefficient comparison, substantially enhances the instrumental agreement. During the intercomparison campaign, the total concentration agreed in variations with the visibilimeter, except for the FSSP, so corrective factor can be applied and range from 0.43 to 2.2. This intercomparison study highlights the necessity to have an instrument which provides a bulk measurement of cloud microphysical or optical properties during cloud ground-based campaigns. Moreover, we show that the orientation of the probes in the main wind flow is essential for an accurate characterization of cloud microphysical properties. In particular, FSSP experiments show strong discrepancies when the wind speed is lower than 3 m s −1 and/or when the angle between the wind direction and the orientation of the instruments is greater than 30°. An inadequate orientation of the FSSP towards the wind direction leads to an underestimation of the measured effective diameter.