Abstract
Abstract. Visual observations of clouds have been performed since the establishment of meteorological observatories during the early instrumental period, and have become more systematic and reliable after the mid-19th century due to the establishment of the first national weather services. During the last decades a large number of studies have documented the trends of the total cloud cover (TCC) and cloudy types; most of these studies focus on the trends since the second half of the 20th century. Due to the lower reliability of former observations, and the fact that most of this data is not accessible in digital format, there is a lack of studies focusing on the trends of cloudiness since the mid-19th century. In the first part, this work attempts to review previous studies analyzing TCC changes with information covering at least the first half of the 20th century. Then, the study analyses a database of cloudiness observations in Southern Europe (Spain) since the second half of the 19th century. Specifically, monthly TCC series were reconstructed since 1866 by means of a so-called parameter of cloudiness, calculated from the number of cloudless and overcast days. These estimated TCC series show a high interannual and decadal correlation with the observed TCC series originally measured in oktas. After assessing the temporal homogeneity of the estimated TCC series, the mean annual and seasonal series for the whole of Spain and several subregions were calculated. The mean annual TCC shows a general tendency to increase from the beginning of the series until the 1960s; at this point, the trend becomes negative. The linear trend for the annual mean series, estimated over the 1866–2010 period, is a highly remarkable (and statistically significant) increase of +0.44% per decade, which implies an overall increase of more than +6% during the analyzed period. These results are in line with the majority of the trends observed in many areas of the world in previous studies, especially for the records before the 1950s when a widespread increase of TCC can been considered as a common feature.
Highlights
Clouds are the main cause of variability of shortwave and longwave radiation at the Earth’s surface and the top of the atmosphere, and play a major role in the global energy balance
The first objective of this study is to present a comprehensive review of existing studies reporting long-term trends of total cloud cover (TCC) series before the second half of the 20th century; this synthetic review configures Sect
3.1 Cloudiness datasets In Spain, very few TCC series have currently records in digital format for the period before the 1960s, as pointed out by SL09, despite the abundant observations that have been taken in Spain since the 19th century
Summary
Clouds are the main cause of variability of shortwave and longwave radiation at the Earth’s surface and the top of the atmosphere, and play a major role in the global energy balance. It is well known that clouds can contribute both to cooling and warming the Earth’s surface, e.g. due to their high albedo and capacity to absorb infrared radiation, respectively (e.g. Mace et al, 2006; Zelinka and Hartmann, 2010). Despite their importance, there are still large uncertainties in the trends of clouds since pre-industrial times and their response to anthropogenic and natural forcings. Part of the limitations regarding the analysis of decadal trends of clouds comes from the well-known difficulties to record objective observations of clouds and quantify cloudiness variations. Satellite data provide an excellent spatial and temporal coverage on global or regional scales, but the assessment of trends is limited to the last decades as systematic
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