2002 Annual

Tuesday, 15 January 2002: 9:00 AM
Cloudiness trends in Canada
Ewa J. Milewska, MSC, Toronto, ON, Canada
Poster PDF (112.7 kB)
Cloudiness is a visible manifestation of complex physical processes that occur in the atmosphere. Clouds, in turn, affect these processes in a ceaseless interaction. The rise in global temperature should increase evaporation and thus create more clouds. This effect would be supposedly most noticeable in high latitudes, where the largest increase of temperature is expected (Western Canada). The feedback from increased cloudiness to the climate system is hard to predict, because of the dual role of the clouds in the Earth’s energy budget: clouds reflect short wave solar radiation back to the space, which means less warming of the surface; at the same time they trap and send long wave radiation back to the ground, which results in an increase of temperature at the surface. Which effect is predominant depends on the cloud type (opacity, structure) and cloud height. It is not clear whether there will be more stratiform or more convective clouds. Another question is whether there will be enough moisture available to sustain more clouds in the warmer climate. Warmer conditions may in fact contribute to the creation of deserts.

Observational records can be studied to identify any to date trends in cloudiness and possibly relate them to temperature trends. Cloud time series are challenging to analyze, because of the elements of subjectivity contained in the reports from human observers, as well as the limited view of the cloud layers above the lowest layer by the observer who is confined to the ground. Additionally, in 1977 in Canada there was a change in the observing procedure of individual cloud layers – since then the cloud layer amounts are no longer reported in tenths of the sky dome they cover, but rather as a cloud condition code, e.g. 1 stands for -X (“partially obscured”) or 5 for –OVC (“thin overcast”), which is different than former 1 for the 1/10, or 5 for the 5/10 amount. Cloud trends in Canada until 1950s were previously studied from the observations that were done once or twice a day. There has been very little research concerning cloudiness in the second half of the twentieth century. Fifty to sixty years of hourly observations that are now available at the principal climatological sites should produce very reliable results and enable studies of changes in diurnal cycle. These fairly long time series of sub-diurnal resolution represent a unique data set - not only trends in average cloudiness can be studied but also frequency occurrence of certain cloud type and amounts, now and in the past. The preliminary analysis that will be presented in the paper is performed at five stations located in different climatological regimes across Canada. In addition, for the period of one year concurrent manned and automated observations were collected at these stations. This overlapping data provides an exceptional opportunity to study the continuity of cloud observation with automation.

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