Cloud Occurrence Frequency at Puy de Dôme (France) Deduced from an Automatic Camera Image Analysis: Method, Validation, and Comparisons with Larger Scale Parameters

Abstract

We present a simple algorithm that calculates the cloud occurrence frequency at an altitude site using automatic camera image analysis. This algorithm was applied at the puy de Dôme station (PUY, 1465 m. a.s.l., France) over 2013–2018. Cloud detection thresholds were determined by direct comparison with simultaneous in situ cloud probe measurements (particulate volume monitor (PVM) Gerber). The cloud occurrence frequency has a seasonal cycle, with higher values in winter (60%) compared to summer (24%). A cloud diurnal cycle is observed only in summer. Comparisons with the larger scale products from satellites and global model reanalysis are also presented. The NASA cloud-aerosol transport system (CATS) cloud fraction shows the same seasonal and diurnal variations and is, on average, 11% higher. Monthly variations of the ECMWF ERA-5 fraction of cloud cover are also highly correlated with the camera cloud occurrence frequency, but the values are 19% lower and up to 40% for some winter months. The METEOSAT-SEVIRI cloud occurrence frequency also follows the same seasonal cycle but with a much smaller decrease in summer. The all-sky imager cloud fraction (CF) presents larger variability than the camera cloud occurrence but also follows similar seasonal variations (67% in winter and 44% in summer). This automatic low-cost detection of cloud occurrence is of interest in characterizing altitude observation sites, especially those that are not yet equipped with microphysical instruments and can be deployed to other high-altitude sites equipped with cameras.

Classification of Clouds Sampled at the Puy de Dôme Station (France) Based on Chemical Measurements and Air Mass History Matrices

“…PUY is able to characterize air masses from various histories, coming from the boundary layer or in the free troposphere, varying as a function of the seasons and time of the day. The top of the mountain is frequently in cloudy conditions, on average 30% of the time per year, with higher occurrences during winter and autumn [50]. This makes PUY a reference site to study and sample cloud properties.…”

Localising and quantifying night-time cooling effects from sub-catchments in a mid-European low mountain area

“…Clear nights offer ideal conditions for radiative cooling and therefore promoting the formation of cold air, whereas a high degree of overcast weakens the process (Barr and Orgill 1989;Fries et al 2009;Gudiksen et al 1992;Novick et al 2016). It can therefore be assumed for temperate climate cities that cold-air drainage is weaker and less common in winter when cloudy conditions dominate (Meerkötter 2004;Baray et al 2019). This is supported by a study in Aachen, Germany, where a higher probability of cold-air drainage events in summer than in winter was found (Sachsen 2013).…”

Evaluation of the Sources, Precursors, and Processing of Aerosols at a High-Altitude Tropical Site

“…The model used is Meso-CAT. This model results from the combination of the CAT trajectory code for the advection calculation 65 and a nonhydrostatic mesoscale atmospheric model, Meso-NH 66 (http://mesonh.aero.obs-mip.fr, last accessed 12 May 2022), to provide the dynamical fields (wind, temperature, and humidity) and topography at high spatial (grid nested domains at 100 and 500 m on 72 vertical levels) and temporal resolutions (1 h). The vertical resolution decreases with height from less than 10 m at ground level to up to 1 km to the top of the domain at 25 km.…”