The SIRTA is a multi-instrumental atmospheric observatory located 20 km south of Paris at the university campus of École Polytechnique. This location has a relatively high fog incidence, of about 30 cases per year, mostly occurring between October and March. The long time series of in-situ and remote sensing measurements of relevant fog variables such as turbulence, short and long-wave radiation, upper and lower boundaries, inner reflectivity, liquid water path, droplet size distribution and temperature, pressure and humidity profiles, continues to enable highly relevant studies on the physical processes driving the fog life cycle.

PARAFOG is a near-real time decision tool that aims to support nowcasting fog formation event at local scales. It is based upon combination sets of Automatic Lidars and Ceilometers observables together with meteorological station and visibility measurements. To date, the PARAFOG algorithm retrieves fog alert levels (low, mod, high) specific to radiation and stratus lowering conditions, minutes to hours prior to fog onset. PARAFOG is experimentally used at SIRTA and Paris airports (Roissy-Charles-de-Gaulle and Orly).

The Sirta is hosting the french unit (CCRES-FR) of the ACTRIS Centre for Cloud Remote Sensing (CCRES), a center offering operational support to ACTRIS National Facilities operating cloud remote sensing instrumentation. The CCRES-FR unit provides support and state-of-the-art expertise in Doppler Cloud Radar activities, developing new-generation technology and retrieval algorithms for cloud properties, target-based calibration procedures, and reference equipment for on-site calibration at ACTRIS national facilities. To sustain a high level of performance  and to stimulate the advancement of new techniques and methodologies within the cloud remote sensing community, the CCRES-FR contributes to expert collaboration networks.

The SIRTA is equipped with a total sky camera for the continuous visualisation of the sky state and cloud cover. The first camera was a TSI instrument, from 2008 to 2015. It was replaced by a EKO sky imager, which had been running since then. Those cameras are part of a network of sky imagers within ACTRIS-FR, from all the Instrumented Sites of the research Infrastructure. A common process of the total sky images of ACTRIS-FR is centralized at AERIS, with several harmonized products:

• Initial images dataset
• Daily mosaic of 24 images per day for a quick look of the sky for a given day
• Daily movie of all images taken per day
• Netcdf files with the cloud cover estimates, calculated with the ELIFAN algorithm

ELIFAN (Lothon et al. 2019) benefits from a CECIL lisense and a git-lab group. The latter enables the improvement of the algorithm through each new participation (adaptation to new cameras, or new collaborations).

The GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN) data processing for the Meteomodem M10 radiosondes has been developed to meet the criteria for reference measurement. Since September 2011, the M10 radiosonde has been regularly launched at Trappes (48.773°N and 2.012°E, French Met-Office station near Paris) and at Saint Denis (20.897°S and 55.495°E, French Met-Office station at La Reunion Island). Moreover, since April 2018 and October 2018, GRUAN pre-launching procedures are applied at Trappes and Saint Denis, respectively. These two French Met-Office stations are respectively associated with two Meteorological Observatory (SIRTA and OPAR, Maido, La Reunion) to provide active and passive remote sensing instrument data necessary for GRUAN site certification.