ClimaTea Lecture: "Understanding land-atmosphere-climate coupling"

Speaker: Dr. Alan Betts (Atmospheric Research)

Abstract:

The recent release of a remarkable dataset of hourly climate observations of temperature, RH, precipitation, opaque cloud cover and snow depth from 13 climate stations across the Canadian Prairies (1953-2011) is transforming our understanding of land-atmosphere-climate coupling. By calibrating the opaque cloud data to give the shortwave and longwave radiative fluxes, we are able for the first time to understand using observations (rather than models with parameterized clouds and boundary layers) how precipitation and net radiation drive the coupled heat and moisture budgets, and the climate on daily, monthly, seasonal and multi-decadal timescales. This talk will review the first four of an ongoing series of papers, available at http://alanbetts.com. Some highlights are: 

1) The cloud forcing of the diurnal climate has distinct warm and cold season behavior. From April to October, when incoming shortwave radiation dominates, maximum temperature and the diurnal ranges of temperature and relative humidity increase as cloud decreases, while minimum temperature is almost independent of cloud. There is a fast transition with snow to a winter climate, when longwave cooling dominates, and both maximum and minimum temperature fall with decreasing cloud.

2) The agricultural land-use conversion from summerfallow to annual cropping on 5 MHa (15-20% of the land area in Saskatchewan) in recent decades has cooled and moistened the summer climate due to increased transpiration.

3) The fall-winter and winter-spring transitions in November and March between warm and cold seasons occur within five days of snowfall and snowmelt. With lying snow, 2-m temperature drops 10^oC. As a result of this fast climate switch, winter temperatures have a linear relation to the fraction of days with snow cover.

4) On monthly timescales, anomalies of opaque cloud and precipitation explain 60-80% of the variance in the diurnal temperature range, afternoon relative humidity and lifting condensation level. We can construct simplified heat and moisture budgets for the growing season that couple the surface fluxes to these same observables. 

References at http://alanbetts.com

Betts, A.K., R. Desjardins and D. Worth (2013), Cloud Radiative Forcing of the Diurnal Cycle Climate of the Canadian Prairies. J. Geophys. Res. Atmos., 118, 1–19, doi:10.1002/jgrd.50593. 

Betts, A. K., R. Desjardins, D. Worth, and D. Cerkowniak (2013), Impact of land use change on the diurnal cycle climate of the Canadian Prairies, J. Geophys. Res. Atmos., 118, 11,996–12,011, doi:10.1002/2013JD020717.

Betts, A.K., R. Desjardins, D. Worth, S. Wang and J. Li (2014), Coupling of winter climate transitions to snow and clouds over the Prairies. J. Geophys. Res. Atmos., 119, 1118-1139, doi:10.1002/2013JD021168.

Betts, A.K., R. Desjardins, D. Worth and B. Beckage (2014), Climate coupling between temperature, humidity, precipitation and cloud cover over the Canadian Prairies. J. Geophys. Res. Atmos., 2014JD022010, in final revision.