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.
Author(s) Details
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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