Abstract
Analysis of the hourly Canadian Prairie data for the past 60 years has transformed our quantitative understanding of land–atmosphere–cloud coupling. The key reason is that trained observers made hourly estimates of the opaque cloud fraction that obscures the sun, moon, or stars, following the same protocol for 60 years at all stations. These 24 daily estimates of opaque cloud data are of sufficient quality such that they can be calibrated against Baseline Surface Radiation Network data to yield the climatology of the daily short-wave, long-wave, and total cloud forcing (SWCF, LWCF and CF, respectively). This key radiative forcing has not been available previously for climate datasets. Net cloud radiative forcing changes sign from negative in the warm season, to positive in the cold season, when reflective snow reduces the negative SWCF below the positive LWCF. This in turn leads to a large climate discontinuity with snow cover, with a systematic cooling of 10 °C or more with snow cover. In addition, snow cover transforms the coupling between cloud cover and the diurnal range of temperature. In the warm season, maximum temperature increases with decreasing cloud, while minimum temperature barely changes; while in the cold season with snow cover, maximum temperature decreases with decreasing cloud, and minimum temperature decreases even more. In the warm season, the diurnal ranges of temperature, relative humidity, equivalent potential temperature, and the pressure height of the lifting condensation level are all tightly coupled to the opaque cloud cover. Given over 600 station-years of hourly data, we are able to extract, perhaps for the first time, the coupling between the cloud forcing and the warm season imbalance of the diurnal cycle, which changes monotonically from a warming and drying under clear skies to a cooling and moistening under cloudy skies with precipitation. Because we have the daily cloud radiative forcing, which is large, we are able to show that the memory of water storage anomalies, from precipitation and the snowpack, goes back many months. The spring climatology shows the memory of snowfall back through the entire winter, and the memory in summer, goes back to the months of snowmelt. Lagged precipitation anomalies modify the thermodynamic coupling of the diurnal cycle to the cloud forcing, and shift the diurnal cycle of the mixing ratio, which has a double peak. The seasonal extraction of the surface total water storage is a large damping of the interannual variability of precipitation anomalies in the growing season. The large land-use change from summer fallow to intensive cropping, which peaked in the early 1990s, has led to a coupled climate response that has cooled and moistened the growing season, lowering cloud-base, increasing equivalent potential temperature, and increasing precipitation. We show a simplified energy balance of the Prairies during the growing season, and its dependence on reflective cloud.
Highlights
Understanding land–atmosphere–climate coupling is challenging, because so many coupled processes are involved: soil temperature and moisture, vegetation types, properties and coverage, near-surface temperature and humidity, the atmospheric boundary layer, the shallow and deep cloud fields which determine the surface radiation balance and surface precipitation, and the soil hydraulic properties that determine the surface and deep runoff, to name only the local components
We have reviewed progress in the quantitative understanding of the coupling between the land surface, clouds, precipitation, snow cover and the climate system that came from analysis of the long-term hourly Canadian Prairie dataset
This progress was possible because, along with conventional hourly measurements, trained observers have recorded opaque cloud fraction hourly across the Prairies for the past 60 years. These 24 daily estimates of opaque cloud are of sufficient quality such that they can be calibrated against Baseline Surface Radiation Network (BSRN) data to obtain the climatology of the daily short-wave, long-wave, and total cloud forcing
Summary
Understanding land–atmosphere–climate coupling is challenging, because so many coupled processes are involved: soil temperature and moisture, vegetation types, properties and coverage, near-surface temperature and humidity, the atmospheric boundary layer, the shallow and deep cloud fields which determine the surface radiation balance and surface precipitation, and the soil hydraulic properties that determine the surface and deep runoff, to name only the local components. Earlier reviews [1,2] looked at hydrometeorology from the global modeling perspective using model reanalysis data, which showed how net long-wave and short-wave radiation, cloud cover, surface fluxes, diurnal temperature range, soil moisture, and cloud-base height were coupled on daily timescales over river basins [3]. Because there are 17 years of Baseline Surface Radiation Network (BSRN) data just 25 km south of Regina, SK, we were able to calibrate the opaque cloud data in terms of the LW and SW cloud forcing (Section 3.3) This is transformative, as it meant that we were able to determine quantitatively the climate coupling between the cloud radiative forcing, and the diurnal and seasonal cycle.
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