Cloud liquid water path in the sub-Arctic region of Europe as derived from ground-based and space-borne remote observations
Abstract. Tropospheric clouds are a very important component of the climate system and of the hydrological cycle in Arctic and sub-Arctic. Liquid water path (LWP) is one of the key parameters of clouds urgently needed for a variety of studies, including the snow cover and climate modelling at Northern latitudes. A joint analysis of the LWP values obtained from observations by the SEVIRI satellite instrument and from ground-based observations by the RPG‑HATPRO microwave radiometer near St.Petersburg, Russia (60N, 30E) has been made. The time period of selected datasets spans two years (December 2012–November 2014) excluding winter months, since the specific requirements to SEVIRI observations restrict measurements at Northern latitudes in winter when the solar zenith angle is too large. The radiometer measurement site is located very close to the shore of the Gulf of Finland, and the SEVIRI measurements have shown considerable differences between the LWP values over land and over water areas in the region under investigation. Therefore, special attention has been paid to the analysis of the LWP spatial distributions derived from SEVIRI observations at scales from 15 km to 150 km in the vicinity of St.Petersburg. A good agreement between the daily median LWP values obtained from the SEVIRI and the RPG-HATPRO observations has been shown: the RMS difference has been estimated as 0.016 kg m−2 for a warm season and 0.048 kg m−2 for a cold season. During seven months (February–May and August–October), the SEVIRI and the RPG-HATPRO instruments revealed similar diurnal variations of LWP, while considerable discrepancies between the diurnal variations obtained by the two instruments have been detected in June and July. The LWP diurnal cycles obtained from the RPG-HATPRO and the SEVIRI observations have been compared to the diurnal cycles derived from the reanalysis data.