The Impact of Clouds, Land use and Snow Cover on Climate in the Canadian Prairies.

This study uses 55 years of hourly observations of air temperature, relative humidity, daily precipitation, snow cover and cloud cover from 15 climate stations across the Canadian Prairies to analyze biosphere-atmosphere interactions. We will provide examples of the coupling between climate, snow cover, clouds, and land use. Snow cover acts as a fast climate switch. With the first snow fall, air temperature falls by 10°C, and a similar increase in temperature occurs with snow melt. Climatologically, days with snow cover are 10°C cooler than days with no snow cover in Alberta. However the interannual variability has a larger range, so that for every 10% decrease in days with snow cover, the mean October to April climate is warmer by 1.4 to 1.5°C. Snow cover also transforms the coupling between clouds and the diurnal cycle of air temperature from a boundary layer regime dominated by shortwave cloud forcing in the warm season to one dominated by longwave cloud forcing with snow cover. Changing agricultural land use in the past thirty years, specifically the reduction of summer fallowing, has cooled and moistened the growing season climate and increased summer precipitation. These hourly climate data provide a solid observational basis for understanding land surface coupling, which can be used to improve the representation of clouds and land-surface processes in atmospheric models.

Plain English Discussion

This is a review of a series of six papers, which study the impact of clouds, land-use and snow on the climate in the Canadian Prairies.

Since 1953, the 15 climate stations on the Canadian Prairie have made unique observations every hour of opaque reflective cloud, as well as pressure, temperature, humidity, wind, precipitation and snow depth. These cloud observations have transformed our understanding of the daily cycle of temperature and humidity, because they tell us the heating by the sun in the daytime (clouds reflect sunlight), and the cooling of the Earth at night (clouds trap the Earth's heat). The day-night rise and fall of temperature and humidity depends strongly on clouds. But this daily cycle also varies between summer and winter, and is especially dependent on whether there is snow on the ground which strongly reflects sunlight. Snow cover acts as a climate switch which drops the mean temperature by 10C (18F). This is easy to see for months like November and March when there is snow cover for only part of the month on the Prairies. Snow cover also switches the effect of clouds. In the warm season (April to October) when there is no snow cover, the afternoon maximum temperature goes up steadily with less cloud cover (because the clouds reflect sunlight), but the minimum temperature near sunrise barely changes. The reverse is true in the cold season with snow cover. Reduced cloud cover means the sunrise minimum temperature falls sharply, because the earth cools rapidly at night under a clear sky; while the daytime maximum temperature also falls - but rather less. Agriculture has became more intensive on the Prairies. Thirty years ago a quarter of the land was left fallow. Now almost all the land is cropped. This increase in crops that evaporate water (transpire) has cooled and moistened the growing season climate and increased summer precipitation.

Related Topics

• Canadian Prairies,
• Climate,
• Clouds,
• Datasets,
• Land-surface,
• REVIEWS