Understanding Land–Atmosphere–Climate Coupling Using Data from the Canadian Prairies
For the past 60 years, analysis of the unique hourly Canadian Prairie data has transformed our quantitative understanding of land–atmosphere–cloud coupling at northern latitudes. The Canadian Prairie data is exceptional because observers at most major airports were trained to estimate the opaque cloud fraction in tenths, by cloud level, and in total on an hourly basis. Following the same protocol for 60 years at all stations, these trained observers made hourly estimates of the opaque cloud fraction that obscured the sun, moon, or stars. These 24 daily estimates of opaque cloud data are good enough to be calibrated against the Baseline Surface Radiation Network. data to produce daily short-wave, long-wave, and total cloud forcing climatology (SWCF, LWCF and CF, respectively). This critical cloud radiative forcing was previously unavailable for surface climate datasets. When reflective snow reduces the negative SWCF below the positive LWCF, the sign of net cloud radiative forcing changes from negative to positive in the warm season. As a result, there is a significant climate discontinuity with snow cover, with a systematic cooling of 10°C or more with snow cover. Furthermore, snow cover modifies the relationship between cloud cover and the diurnal temperature range. The diurnal temperature, relative humidity, equivalent potential temperature, and pressure height of the lifting condensation level are all tightly coupled to the opaque cloud cover during the warm season. With over 600 station-years of hourly data, we are able to extract, possibly for the first time, the coupling between cloud forcing and the diurnal cycle’s warm season imbalance, which changes monotonically from a warming and drying under clear skies to a cooling and moistening under cloudy skies with precipitation. Because we have a large daily cloud radiative forcing, we can demonstrate that the memory of water storage anomalies, The memory of snowfall in spring extends back through the entire winter, and the memory of snowmelt in summer extends back to the months of snowmelt. Lagged precipitation anomalies change the thermodynamic coupling of the diurnal cycle to cloud forcing and shift the mixing ratio’s diurnal cycle, which has a double peak. The seasonal extraction of surface total water storage dampens the interannual variability of precipitation anomalies in the growing season significantly. The large shift from summer fallow to intensive cropping, which peaked in the early 1990s, has resulted in a coupled climate response that has cooled and moistened the growing season while decreasing cloud-base. increasing the equivalent potential temperature and precipitation We depict a simplified energy balance of the Prairies during the growing season, as well as its reliance on reflective cloud.
Dr. Alan K. Betts
Atmospheric Research, Pittsford, VT 05763, USA.
Raymond L. Desjardins
Agriculture and Agri-Food Canada, Ottawa, ON K1A0C6, Canada.
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