Impact of partly ice-free Lake Ladoga on temperature and cloudiness in an anticyclonic winter situation – a case study using a limited area model

Abstract

At the end of January 2012, a low-level cloud from partly ice-free Lake Ladoga caused very variable 2-m temperatures in Eastern Finland. The sensitivity of the High Resolution Limited Area Model (HIRLAM) to the lake surface conditions was tested in this winter anticyclonic situation. The lake appeared to be (incorrectly) totally covered by ice when the lake surface was described with its climatology. Both parametrisation of the lake surface state by using a lake model integrated to the NWP system and objective analysis based on satellite observations independently resulted in a correct description of the partly ice-free Lake Ladoga. In these cases, HIRLAM model forecasts were able to predict cloud formation and its movement as well as 2-m temperature variations in a realistic way. Three main conclusions were drawn. First, HIRLAM could predict the effect of Lake Ladoga on local weather, when the lake surface state was known. Second, the current parametrisation methods of air–surface interactions led to a reliable result in conditions where the different physical processes (local surface processes, radiation and turbulence) were not strong, but their combined effect was important. Third, these results encourage work for a better description of the lake surface state in NWP models by fully utilising satellite observations, combined with advanced lake parametrisation and data assimilation methods.

• lake-effect
• data assimilation
• lake ice
• NWP
• cold outbreaks
• stable boundary layer
• lake–atmosphere interaction

6. Acknowledgements

The constructive comments of three anonymous reviewers significantly helped to improve the manuscript. We thank Janne Kotro for help in the interpretation of NOAA satellite images. This research was supported by European Space Agency (ESA-ESRIN) Contract No. 4000101296/10/I-LG (Support to Science Element, North Hydrology Project). Support by the International HIRLAM-B programme is acknowledged.

Notes

¹Forecasters’ practice is to estimate cloud formation probability from the dew-point deficit, shown by the sounding observations. A rule of thumb is that Δ smaller than 2 degrees in a reasonably thick layer indicates cloud formation.