Comment on acp-2021-387

Abstract

Liquid water path (LWP) is one of the most important cloud parameters. The knowledge on LWP is critical for many studies including global and regional climate modelling, weather forecasting, modelling of hydrological cycle and interactions between different components of the climate system: the atmosphere, the hydrosphere, and the land surface. Satellite observations by the SEVIRI and AVHRR instruments have already provided the evidences of the systematic difference between the LWP values derived over the land surface and over the Baltic Sea and major lakes in Northern Europe during both cold and warm seasons. The goal of the present study is to analyse the phenomenon of the LWP horizontal inhomogeneities in the vicinity of various water bodies in Northern Europe making focus on the temporal and spatial variation of LWP. The objects of investigation are water bodies and water areas located in Northern Europe which are different in size and other characteristics: Gulf of Finland, Gulf of Riga, the Neva River bay, lakes Ladoga, Onega, Peipus, Pihkva, Ilmen, and Saimaa. The input data are the LWP values of pure liquid-phase clouds derived from the space-borne observations by the SEVIRI instrument in 2011–2017 during daytime. The study revealed that in general the mean values of the land-sea LWP gradient are positive during all seasons (larger values over land, smaller values over water surface). However, the negative gradients were also detected over several relatively small water bodies during cold (winter) season. The important finding is the positive trend of the land-sea LWP gradient detected within the time period 2011–2017. The analysis of intra-seasonal features revealed special conditions on the territory of the Gulf of Finland where in June and July large and moderate positive LWP gradients prevail over negative ones while in August positive and negative gradients are much smaller (in terms of absolute values) and occur with equal frequency. This result can lead to the conclusion about possible common physical mechanisms that drive the land-sea LWP difference in the Baltic Sea region at small distances from the coastline. The diurnal cycle of the LWP land-sea gradient has been detected in June and July while there was no evidence for it in August. For several specific cases, atmospheric parameters over the mesoscale domain comprising Gulf of Finland and several lakes have been simulated with the numerical model ICON in limited area and weather prediction mode. These simulations have clearly demonstrated the LWP land-sea gradient and have pointed out less stability of the atmosphere over land surfaces.