12th EMS Annual Meeting & 9th European Conference on Applied Climatology (ECAC)Łódź, Poland, 10 – 14 September 2012

The estimation of cloudiness and incoming short-wave radiation in Latvia by using satellite data

Zanita Avotniece, Lita Lizuma, Andris Vīksna, Latvian Environment, Geology and Meteorology Centre, 165 Maskavas str., Riga, LV-1019, Latvia

e-mail: zanita.avotniece@gmail.com

INTRODUCTIONNowadays there is a great interest in the use of the satellite data in both meteorological and climate studies as they provide valuable information on the atmospheric conditions in sites not covered by the surface observation network. European Organisation for the Exploitation of Meteorological Satellites EUMETSAT has created eight Satellite Application Facility SAF projects with the main goal to provide satellite products that meet the users needs and requirements. One of these projects - CM SAF or The Satellite Application Facility on Climate Monitoring – has been created with the aim of providing data that can be further used for climate studies and the assessment of the current climate, climate variability and change and other applications which require climate data of high spatial and temporal resolution, spatial coverage and continuity. The aim of this study is to use the satellite data provided by the CM SAF project to estimate the seasonal characteristics of cloudiness and incoming short-wave radiation in Latvia.

A

Autumn Winter

Spring Summer

Figures 1-4. The mean intenisty of the incoming short-wave radiation (W/m2) over the period 1983-2005.

Autumn Winter

Spring Summer

Figures 5-8. The mean fractional cloud cover (%) over the period 2005-2011.

CONCLUSIONSThe use of satellite data is an important step in the climate and meteorological research in Latvia, because in this country, which is situated in the transition zone between continental and maritime climate, significant spatial and seasonal differences in the meteorological parameters are characteristic, and thus may not be completely represented by the rather few surface observation stations in the country. The most important aspect of the use of satellite data is the possibility to assess the climatic characteristics of cloudiness and solar radiation, which besides the applications in climate studies can be also used in the industry of renewable energy and solar power production.

Figure 9. Monthly mean cloud cover (tenths) in Riga over the period 2005-2010 – the comparison of satellite and ground

observations

During this study the possibility to characterise and visualise cloudiness and incoming short-wave radiation in Latvia by using the satellite observation data was estimated. The figures characterising the mean incoming short-wave radiation and fractional cloudiness were computed from the monthly satellite data provided by the CM AF by using the statistical softwares CDO or Climate Data Operators and R.

Figures 1-4 contain the information of the seasonal mean incoming short-wave radiation in Latvia over the period 1983-2005, but the figures 5-8 represent the characteristics of the seasonal mean fractional cloudiness in Latvia over the period 2005-2011. The intensity of the incoming short-wave radiation on the surface of the Earth is closely linked to the characteristics of cloudiness, so the seasonal differences in both of these parameters should be assessed unitedly. The annual mean intensity of the incoming short-wave radiation in Latvia is 110-125 W/m2 and the mean cloudiness is approximately 65-75%, however both of these parameters are subjected to significant seasonal differences. During the autumn and winter seasons due to a high cyclonic activity the fractional cloudiness increases up to 75-85%, which together with the narrow Sun angle and short sunshine duration determines the least intensity of the incoming short-wave radiation – during these seasons it is not greater than 30-70 W/m2. At the same time over the water areas of the Baltic Sea and the Gulf of Riga where the cloudiness in the cold season of the year is even greater, the incoming short-wave radiation does not exceed even 20-50 W/m2. The situation changes when spring and summer seasons are approaching – due to intense convection by day the cloudiness is greater over the land surface of the territory of Latvia, especially over the highlands, but at night more convection occurs over the coastal waters of the Baltic Sea and the Gulf of Riga. Accordingly during the warm half of the year a greater intensity of the incoming solar radiation can be observed in the territories with the least cloudiness – over the open Baltic Sea, in the coastal areas and the central regions of Latvia, where it reaches up to 200-230 W/m2.

The satellite data on fractional cloudiness used in this study were compared to the surface observation data at the meteorological observation station Riga located in the capital city of Latvia. Figure 9 shows similarities in the monthly course of cloudiness, however the cloudiness measured by the surface observation station is on average by two tenths greater than the one measured by the satellite. The reason for this difference could be the fact that in this observation station cloudiness is still observed visually, which could be a reason for the occurrence of systemic errors.