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Cloud Radiative Forcing of the Diurnal Cycle Climate of the Canadian Prairies

This analysis uses 40 years of hourly observations of temperature, relative humidity, and opaque cloud cover from 14 climate stations across the Canadian Prairies to analyze the diurnal cycle climate; represented by the mean temperature and relative humidity and their diurnal ranges. From April to October, when incoming shortwave radiation dominates over longwave cooling, maximum temperature and the diurnal ranges of temperature and relative humidity increase with decreasing opaque cloud cover, while minimum temperature is almost independent of cloud. During the winter period, both maximum and minimum temperature fall with decreasing cloud, as longwave cooling dominates over the net shortwave flux, which is reduced by the high solar zenith angle and surface reflection by snow. We relate the daily mean opaque cloud cover to the longwave and shortwave cloud forcing and the effective cloud albedo, using multiyear measurements of downward shortwave and longwave fluxes, and longwave fluxes under clear skies from historical weather reanalysis. We provide quadratic fits to compute effective cloud albedo and net longwave fluxes from opaque cloud cover. During the warm season, the daytime rise of temperature is related to the net radiation, and the night-time fall is related to the net longwave cooling. The diurnal range of temperature, relative humidity and all the net radiative fluxes have a quasi-linear dependence on the effective cloud albedo. This gives a seasonal climate perspective on the coupled land-surface system of temperature, relative humidity and cloud cover over the Canadian Prairies, and the winter transitions in snowy climates.

Correction - Author's Note - The hourly data in Figures 3, 4 and 5 are misplotted one hour to the right. In Local Standard Time (LST), the data should run from 0-23h not 1-24h as plotted. Note that LST is also one hour later than local solar time.

Plain English Discussion

Clouds have an opposite impact on the daily cycle of temperature in summer and winter. Clouds in summer greatly reduce the maximum temperature, because they reflect the strong sunlight, but they have little effect on the minimum temperatures. In winter when there are no clouds, minimum temperatures plunge because the Earth can cool to space; while sunlight is weak and is reflected back by snow. This analysis of 40 years of hourly data from 14 climate stations across the Canadian Prairies shows how the daily cycle of temperature and humidity changes with cloud cover over the four seasons. We relate the cloud cover to the short-wave radiation (from the sun) and long-wave radiation (from the sky) that reaches the Earth's surface.

The American Geophysical Union highlighted this importance of this paper with a Research Spotlight

Balcerak, E. (2013) Clouds' effects on daily temperature. Eos, Transactions AGU, Volume 94, Issue 35, page 312, 27 August 2013, DOI: 10.1002/2013EO350011

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Full Citation

Betts, A.K., R. Desjardins and D. Worth (2013), Cloud Radiative Forcing of the Diurnal Cycle Climate of the Canadian Prairies. J. Geophys. Res. 118, 8935–8953, doi:10.1002/jgrd.50593.